Research Article Acoustic Velocity Log Numerical...

Hindawi Publishing CorporationThe Scientific World JournalVolume 2013 Article ID 101459 13 pageshttpdxdoiorg1011552013101459

Research ArticleAcoustic Velocity Log Numerical Simulationand Saturation Estimation of Gas Hydrate Reservoir inShenhu Area South China Sea

Kun Xiao12 Changchun Zou12 Biao Xiang12 and Jieqiong Liu12

1 School of Geophysics and Information Technology China University of Geosciences Beijing 10083 China2 Key Laboratory of Geo-detection (China University of Geosciences Beijing) Ministry of Education Beijing 100083 China

Correspondence should be addressed to Changchun Zou zoucccugbeducn

Received 19 March 2013 Accepted 21 May 2013

Academic Editors A Billi and U Tinivella

Copyright copy 2013 Kun Xiao et alThis is an open access article distributed under theCreative CommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Gas hydratemodel and free gasmodel are established and two-phase theory (TPT) for numerical simulation of elastic wave velocityis adopted to investigate the unconsolidated deep-water sedimentary strata in Shenhu area South China Sea The relationshipsbetween compression wave (P wave) velocity and gas hydrate saturation free gas saturation and sediment porosity at site SH2are studied respectively and gas hydrate saturation of research area is estimated by gas hydrate model In depth of 50 to 245mbelow seafloor (mbsf) as sediment porosity decreases P wave velocity increases gradually as gas hydrate saturation increases Pwave velocity increases gradually as free gas saturation increases P wave velocity decreases This rule is almost consistent with theprevious research result In depth of 195 to 220mbsf the actual measurement of P wave velocity increases significantly relative tothe P wave velocity of saturated water modeling and this layer is determined to be rich in gas hydrate The average value of gashydrate saturation estimated from the TPT model is 232 and the maximum saturation is 315 which is basically in accordancewith simplified three-phase equation (STPE) effective medium theory (EMT) resistivity log (Rt) and chloride anomaly method

1 Introduction

Gas hydrate mainly exists in the seafloor and polar per-mafrost [1] and it owns the cage structure of solid crystalwhich is formed by water molecules and natural gas (usuallydominated by methane) The formation of gas hydrate needsa low-temperature and high-pressure environment and theconcentration of methane must exceed its solubility in thepore water So gas hydrate is commonly distributed in thewater depth greater than 300m in the continental slope belt[2 3] The submarine gas hydrate reserves mainly dependon the distribution of gas hydrate area the thickness of gashydrate stability zone the porosity of sedimentary layer thesaturation of gas hydrate and so on However the accurateestimation of gas hydrate reserves is very difficult due tothe lack of research on the determination of gas hydratedistribution and gas hydrate saturation As gas hydrate isrich in methane it is associated with a series of scientificissues [4] including the global carbon cycle [5] global

temperature changes [6] the sea-level rise [7] and futureenergy supply [8] Therefore researches on determining gashydrate distribution and estimating gas hydrate saturationhave become the focus of the scientists all over the worldIn May 2007 the gas hydrate samples and various log dataof gas hydrate zone were firstly obtained in Shenhu areaSouth China Sea which made a significant breakthrough inexploration of gas hydrate in China Meanwhile it provideda great convenience for investigating the properties of the gashydrate reservoir [9]

Geophysical logging is an important tool for evaluatinggas hydrate saturation and valuable information can beobtained by studying the resistivity and acoustic velocity logdata According to the log data in Shenhu area South ChinaSea Chinese scholars have utilized some theoretical modelsand empirical formulas to estimate gas hydrate saturation[10ndash14] and the results of these researches have greatlypromoted the process of studying gas hydrate saturation byusing log data in China However previous studies mainly

2 The Scientific World Journal

focused on the estimation of the gas hydrate saturation andthe discrepancy of the estimation of gas hydrate saturationwas large because of the application of various methodsBesides previous studies did not systematically study therelationship between any two of the gas hydrate saturationelastic wave velocity and sediment porosity which led toincomplete understanding of the log data

Foreign scholars have carried out researches on the eval-uation of the marine gas hydrate saturation earlier A varietyof theoretical models or experimental models have beenproposed to estimate gas hydrate saturation such as Wyllieet al [15] time average equation with the seismic velocity[16ndash18] the effectivemedium theory [19ndash23] Biot-Gassmanntheorymodel [24ndash27] compression wave (P wave) velocity ofthermal-elastic theory [28] the three-phase equation (TPE)[29ndash34] and velocity model theory based on the two-phasetheory (TPT)model [35]Moreover Tinivella et al [36]madea research to compare the TPTmodel with the TPEmodel forevaluating gas hydrate saturations in marine sediments andthe comparison showed that the two theoretical approacheswere in very good agreement Based on this the TPT modelhas been applied to verify the TPE model and estimate thegas hydrate and free gas saturations in several different areas[37ndash39]

In this work based on the log data at site SH2 in ShenhuArea we first establish gas hydrate model and free gas modelby applying elastic wave velocity numerical model of the TPTmethod then study the dependence of the P wave velocityon gas hydrate saturation free gas saturation and sedimentporosity and finally choose the gas hydrate model to estimategas hydrate saturation at site SH2

2 Numerical Simulation of Elastic WaveVelocity by the TPT Model

The TPT model [41 42] which supposes that the rock solidpart is composed of rock matrix and gas hydrate and thatthe rock pore fluid is composed of free gas and water can beused to study the elastic wave velocity model about the elasticcharacteristics of marine sand-shale reservoirs Based on thistheory and using the reported in Tinivella [35] the velocityrelation between Pwave velocity (119881p) and shearwave (Swave)velocity (119881s) is as follows

119881p = [(1

119862m+4

3120583)

+(120601eff119896) (120588m120588f) + (1 minus 120573 minus 2 (120601eff119896)) (1 minus 120573)

(1 minus 120601eff minus 120573)119862b + 120601eff 119862f]

sdot1

120588m (1 minus (120601eff119896) (120588f120588m))

12

119881s = 120583

120588m [1 minus (120601eff sdot 120588f)(119896 sdot 120588m)]

12

(1)

Where 120601eff is the effective porosity 120583 is the average rigidityof the skeleton 120588m is the average density 120588f is the density ofthe fluid phase 120573 is the proportionality coefficient 119896 is thecoupling factor and 119862b 119862f and 119862m are the compressibility ofthe solid phase the fluid phase and the matrix respectively

3 Geological Setting

Shenhu area is considered as one of the occurrences of gashydrate which is located in the middle of the northernslope of the South China Sea between the Xisha Trough andthe Dongsha Islands and Baiyun Sag of Zhu II depressionof the Pearl River Mouth Basin (Figure 1) Shenhu areaexperienced a geological evolution process similar to thenorthern margin of the South China Sea and eventuallyformed the regional sedimentary sequences in which marinesediments were the dominant composition [43] Taking theCenozoic sedimentary strata in the Shenhu area for exampleit is about 1000ndash7000m and the organic matter content is046ndash19 [44ndash46] which can provide the material basefor gas hydrate In recent years some studies have confirmedthat mud volcanoes seafloor slips mud diapers and otherspecial structural units beneficial to the formation of gashydrate are widely being developed in Shenhu area [47]and the bottom simulating reflections (BSRs) have also beenidentified using various geophysical methods in the northernSouth China Sea by Guangzhou Marine Geological SurveyChina Geological Survey [48]

During AprilndashJune 2007 eight sites were drilled inShenhu area (see Figure 2) among which sites of SH2 SH3and SH7 in water depth of 1105 to 1423m were determined tocontain gas hydrate in recovered core samples The thicknessof gas hydrate stability zone was about 10 to 25m [9 49] andthe sediment lithology in and above the zone was silt and siltyclay respectively according to the core data

4 Data Set Used in the Study

As previous researchers have done much work on site SH2[10ndash14 40 50] many valuable references can be used toobtain better results and verify the reliability of our researchso we select site SH2 as the research object The waterdepth of site SH2 is 1232m and the maximum drillingdepth is 245mbsf Figure 2 shows the conventional logsof site SH2 The measurement depth range is from 50 to245mbsf and the measurement projects include caliperdensity natural gamma ray acoustic and resistivity log-ging The occurrence of gas hydrate reservoir in this siteappears to be ldquoresponse characteristics of two high and twolowrdquo in the log curve namely high resistivity high naturalgamma ray and low density low acoustic time especiallyfor resistivity and acoustic logs Besides when the layer ofwell diameter changes caliper curve can be used as theeffective parameter to identify gas hydrate reservoir becausethe abnormality of other logs has nothing to do with the wellcondition

Based on the previous researchmethods for the thicknessof gas hydrate stability zone [51ndash53] and combined with the

The Scientific World Journal 3

113

18∘

120∘E

22∘N

Figure 1 Areas of gas hydrate exploration and drilling area with drilling sites in the northern part of the South China Sea [40] (Red dotsgas hydrate samples obtained dark purple dots no gas hydrate samples obtained)

GR (API)

20 60

AC (usft)

130 200 1 4

CAL (in)

155

60

80

100

120

140

160

180

200

220

240

Depth15 22

DEN (gcm3) Rt (Ωm)1 m 1500 m

Figure 2 The conventional logs in site SH2 [10] (The area delineated by a pink line is the occurrence of gas hydrate reservoir and the depthrange is 195 to 220mbsf)

analysis of conventional log data the gas hydrate stabilityzone of site SH2 is determined to be at the depth of 195 to220mbsf [40]

5 Methodology

Seafloor sediments containing gas hydrate are generallycomposed of rock grain gas hydrate water and naturalgas In order to research the characteristics of gas hydratereservoir gas hydrate model and free gas model have beenestablished in this section and based on these two modelsthe numerical simulation method and the TPT are used tostudy the dependence of the elastic wave velocity on sedimentporosity gas hydrate saturation and free gas saturation

51 Gas Hydrate Model

511 Establishment of Gas Hydrate Model The gas hydratemodel assumes that the sediments are composed of rockgrain gas hydrate and water and gas hydrate is in thepore space which is regarded as a part of the rock matrixSupposing that 120601s 120601h 120601w and 120601g represent the volumepercentage of rock grain gas hydrate water and free gasin the sediments respectively the gas hydrate model can beexpressed as

120601s + 120601h + 120601w = 1 (2)

120601 = 120601h + 120601w (3)

where 120601 is sediment porosity


Gas hydrate saturation (119878h) and water saturation (119878w) canbe written as

119878h =120601h120601

119878w =120601w120601

(4)

The volume percentages of rock grain in solid phase(1198781015840s) and gas hydrate in the solid phase (1198781015840h) can be writtenrespectively as

1198781015840

s =120601s

(120601s + 120601h) (5)

1198781015840

h =120601h

(120601s + 120601h) (6)

512 The Parameter Determination for Numerical Simulationof Gas HydrateModel Based on the TPT In order to apply theTPT to gas hydrate model some parameters in (1) should beknown The parameters can be determined by Tinivella andSchonrsquos derivation formula [35 54]

(1) Effective porosity (120601eff) can be written as

120601eff = (1 minus Sh) 120601 (7)

(2) Average density of sediments (120588m) density of the solidphase (120588b) and density of the fluid phase (120588f) can bewritten as

120588m = (1 minus 120601eff) 120588b + 120601eff120588f (8)

120588b = 1198781015840

s120588s + 1198781015840

h120588h (9)

120588f = 120588w (10)

where 120588s is density of the rock grain 120588h is gas hydratedensity and 120588w is water density

(3) Assume that the solid compressibility lies between theVoigt and Reuss averages [54] 119862b can be written as

119862b =1

2(1198781015840

s119862s + 1198781015840

h119862h) +1

2(1198781015840

s119862s+1198781015840

h119862h)

minus1

(11)

where 119862s is rock grain compressibility and 119862h is gashydrate compressibility

(4) Compressibility of the fluid phase (119862f) is

119862f = 119862w (12)

where 119862w is water compressibility(5) Compressibility of the matrix (119862m) indicates the

compressibility of sediments without water and it canbe calculated by the following equation

119862m = (1 minus 120601eff) 119862b + 120601eff119862p (13)

where 119862p is pore compressibility The algorithm [35]to calculate 119862p is

119862p =(1 minus 120601eff1206010)

119875d (14)

where 1206010is the sediment porosity at the sea bottom

and 119875d is differential pressure

(6) Proportional coefficient (120573) is

120573 =119862b119862m (15)

(7) Shear modulus (120583) indicates average rigidity of theskeleton and it can be calculated by the followingequation

120583 = (120601s + 120601h) [120601s1198781015840

s120583sm+1198781015840

h120583h]

minus1

(16)

where 120583h is gas hydrate rigidity and 120583sm is the shearmodulus of solid matrix with gas hydrate [55] whichcan be calculated by the following equation

120583sm = (120583smKT minus 120583sm0) [120601h

(1 minus 120601s)]

38

+ 120583sm0 (17)

where 120583smKT is Kuster and Toksozrsquos shear modulus[56] and 120583sm0 is the shear modulus of solid matrixwithout gas hydrate

513 Implementation Steps of Numerical Simulation of GasHydrate Model Based on the TPT Consider the following

(1) Given the 120601 or calculate it by the empirical formula

(2) Given the 119878h and 119878w calculate 120601s 120601h 120601w 1198781015840

s and 1198781015840

haccording to (2)ndash(6)

(3) According to (7) calculate 120601eff

(4) According to (8) and (10) calculate 120588b 120588f and 120588m

(5) According to (11)ndash(14) calculate 119862b 119862f and 119862m

(6) According to (15) calculate 120573

(7) According to (16) and (17) calculate 120583

(8) According to (1) calculate 119881p

52 Free Gas Model

521 Establishment of Free Gas Model Free gas modelassumes that the sediments are composed of rock grainwater and free gas and it can be expressed as

120601s + 120601w + 120601g = 1 (18)

120601 = 120601w + 120601g (19)


Water saturation (119878w) and free gas saturation (119878g) can beexpressed as

119878w =120601w120601 (20)

119878g =120601g

120601 (21)

522 The Parameter Determination for Numerical Simulationof Free Gas Model Based on the TPT Consider the following

(1) 120601eff can be written as

120601eff = 120601 (22)

(2) 120588m 120588b and 120588f can be written as

120588m = (1 minus 120601eff) 120588b + 120601eff120588f = (1 minus 120601) 120588b + 120601120588f (23)

120588b = 120588s (24)

120588f = 119878w120588w + 119878g120588g (25)

where 120588g is free gas density(3) 119862b can be written as

119862b = 119862s (26)

(4) Assume that the fluid compressibility lies between theVoigt and Reuss averages [54] 119862f can be written as

119862f =1

2(119878w119862w + 119878g119862g) +

1

2(119878w119862w+

119878g

119862g)

minus1

(27)

where 119862g is the compressibility of free gas(5) 119862m is calculated by the same equation as that used in

gas hydrate model(6) 120573 is calculated by the same equation as that used in

gas hydrate model(7) 120583 can be written as

120583 = 120583s 120583s =120583sm01 minus 120601

(28)

where 120583119904is rock grain rigidity

(8) Variation range of 119896 is 1ndashinfin

523 Implementation Steps of Numerical Simulation of FreeGas Model Based on the TPT Consider the following

(1) Given the 120601 or calculate it by the empirical formula(2) Given the 119878g and 119878w calculate120601s120601w and120601g according

to (18)ndash(21)(3) According to (22) calculate 120601eff(4) According to (23) and (25) calculate 120588b 120588f and 120588m(5) According to (11)ndash(14) calculate 119862b 119862f and 119862m(6) According to (15) calculate 120573(7) According to (16) and (17) calculate 120583(8) According to (1) calculate 119881p

53 Estimation of Sediment Porosity Sediment porosity is akey parameter for the estimation of gas hydrate saturation forboth gas hydrate and free gas models Therefore appropriatelog data should be selected to estimate the sediment porosityat site SH2 The log data that can be applied to determine thesediment porosity include density acoustic resistivity andneutron logging

When it comes to the determination of sediment porosityby acoustic log the data need to be corrected by the regionalcore data because seafloor sediments are always loose siltand silty clay However the core data is always insufficientand it is difficult to determine the compaction correctioncoefficient so acoustic log is not available When it comesto the determination of sediment porosity by resistivity logthe Archie formula should be used to calculate porosity andthe Archie constants and formation water resistivity need tobe known for the Archie formula [57] However the previoustwo parameters are generally determined by some empiricalequations and the estimation error of the sediment porosityis significant As for the determination of sediment porosityby neutron log it usually cannot be realized because of thelack of the neutron log data

Compared with the resistivity the acoustic and thedensity logs are less affected in gas hydrate reservoirs andcan generally reflect the situation of sediment porosity so weselect the density log data to estimate the sediment porosityin this study The intensity of scattering gamma ray can bemeasured by the density log which reflects electron densityof the strata and volume density of rock (120588b) The estimationof 120601 by density log data can be expressed as [58]

120601 =(120588ma minus 120588b)

(120588ma minus 120588f) (29)

where 120588ma is matrix density and 120588f is fluid density Consider-ing the effect of shaly sediments (29) can be written as [58]

120601 =(120588ma minus 120588b)

(120588ma minus 120588f)minus 119881sh

(120588ma minus 120588sh)

(120588ma minus 120588f) (30)

119881sh =(2

GCURtimesSHminus 1)

(2GCUR minus 1) (31)

SH =(GR minus GRmin)

(GRmax minus GRmin) (32)

where119881sh is the volume content of the shale SH is the contentindex of the shale 120588sh is the density of the shale GR is thevalue of natural gamma log in the research interval GRminis the value of natural gamma log in pure sandstone intervalGRmax the is value of natural gamma log in puremud intervaland GCUR is the Hilchie index which is 37 in the Tertiaryof North America and 2 in old stratum [59]

Equations (29) and (30) are used to calculate the sedimentporosity at site SH2 and processing parameters can be set asfollows 120588ma = 265 gcm

3 120588f = 104 gcm3 120588sh = 270 gcm

3

[12 35] The porosity calculated by (29) and (30) is close toeach other (Figure 3) which varies in the range of 30 to55 and the average value is 45 The result indicates thatsediments at site SH2 are of high porosity


0 100

0 100

60

80

100

120

140

160

180

200

220

240

Depth Density porosity ()

Correction porosity ()1 m 1500 m

Figure 3 Result of sediment porosity calculated by density log data at site SH2 (Red line is the sediment porosity estimation used densitylog data bright green line is the sediment porosity estimation which considered the effect of argillaceous sediments)

6 Results

Gas hydrate model of the TPT is used to forward stimulate Pwave velocity of sediment formation in different gas hydratesaturation conditions in depth of 50 to 245mbsf at site SH2Table 1 shows the values of the main parameters used toevaluate the velocity When Sh = 0 we can get the P wavevelocity of water saturated sediments forward stimulated bygas hydrate model based on the TPT From Figure 4 thetendency between actual curve of P wave velocity log andP wave velocity curve of the saturated water condition isalmost consistent in the gas hydrate interval (above 195mbsf)so the model and its parameters are rational for numericalsimulation in this study The difference between actual Pwave velocity of the log and P wave velocity of saturatedwater condition reflects the value of gas hydrate or free gassaturation which can be used to qualitatively identify the gashydrate reservoir The specific response characteristics are asfollows the possibility of containing gas hydrate is dominantwhen the actual P wave velocity of the log is higher than Pwave velocity of saturated water condition the possibility ofcontaining free gas is dominant when actual P wave velocityof the log is lower than the P wave velocity of saturated watercondition [35] In depth of 195 to 220mbsf actual P wave

velocity of the log is significantly higher than the P wavevelocity of saturatedwater condition so this interval is the gashydrate stability zone In depth of 220 to 245mbsf the actualPwave velocity of the log has an increase relative to the Pwavevelocity of saturated water condition However Figure 2 doesnot indicate the increase of resistivity Without the coringanalysis data in this interval whether the abnormality iscaused by gas hydrate or not can not be ascertained andshould be researched in further study

When Sh gradually increases P wave velocity made byforward stimulation also increases when Sh gt 15 P wavevelocity increases significantly when Sh = 30 P wavevelocity curve is located at the right of the actual P wavevelocity logging curve The above results indicate that thebasic range of gas hydrate saturation is 0ndash30 in depth of 50to 245mbsf at site SH2

In order to study the dependence of P wave velocity onsediment porosity gas hydrate saturation it is assumed thatvalues of gas hydrate saturation increase from 0 to 1 in theinterval of 01 Using gas hydrate model of the TPT to modelthe corresponding Pwave velocity of the previous gas hydratesaturations the relation surface of previous three propertiescan be formed as Figure 5 shows With the increase of thesediment burial depth in depth of 50 to 245mbsf at site


Downhole logging velocity1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

Theoretical velocity (Sh = 0)







1 m 1500 m

Figure 4 Forward stimulating P wave velocity of sediment formation at site SH2 (The red line is the actual log P wave velocity at site SH2 theblue line sky blue line bright green line pink line green line dark blue line and black line are assumed to P wave velocity made by forwardmodeling of the gas hydrate model when gas hydrate saturations are 0 5 10 15 20 25 and 30 resp)

SH2 the porosity presents a decreasing trend except for theabnormality caused by borehole conditions in some intervalsand P wave velocity of forward stimulation (Sh = 0) slowlyincreases from 1743 to 1795ms But with the increase ofgas hydrate saturation the increase rate of P wave velocity

is obviously accelerated and P wave velocity (burial depthis 51mbsf) increases from 1743 to 3961ms From the aboveanalysis the general rule between P wave velocity of forwardstimulation and sediment porosity gas hydrate saturation atsite SH2 is the smaller the sediment porosity the greater


Table 1 Values of the main parameters of the gas hydrate and freegas model of the TPT

Parameters Details References119862s 27 times 10

minus11 Paminus1 [60]119862h 179 times 10

minus10 Paminus1 [1]119862w 479 times 10

minus10 Paminus1 [61]119862g 424 times 10

minus8 Paminus1 [62]120588s 2650 kgm3 [60]120588w 1040 kgm3 [1]120588g 8848 kgm3 [62]120588h 767 kgm3 [1]

119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]

Gas hydrate saturationsDepth below seafloor (km)

152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)

Figure 5 Relation between P wave velocity and sediment porositygas hydrate saturation at site SH2 (Because of the effect of variationof borehole conditions and actual sediments the sediment porosityin some intervals does not reduce with the increasing depth andcauses the curved surface unsmooth growth In depth of 195 to220mbsf of gas hydrate reservoir P wave velocity surface of forwardsimulation subsides as the sediment porosity relatively increases)

the P wave velocity the higher the gas hydrate saturationthe greater the P wave velocity This result is basically inaccordance with the research result made by Tinivella [35]

Similarly it is assumed that values of free gas saturationincrease from 0 to 1 in the interval of 01 in order to studythe relation between P wave velocity and free gas saturationUsing free gas model of the TPT to model the correspondingP wave velocity of the previous free gas saturations therelation surface of P wave velocity sediment porosity andfree gas saturation can be formed as Figure 6 shows In depthof 50 to 245mbsf at site SH2 with the increase of free gassaturation P wave velocity (burial depth is 51mbsf) decreasesfrom 1773 to 597ms (The velocity 597ms is obtained

05

1

15

2

Free gas saturations

Depth below seafloor (km)005

01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)

Figure 6 Relation between P wave velocity and sediment porosityfree gas saturation at site SH2 (P wave velocity is easily affected bythe free gas saturation When the free gas saturation increases Pwave velocity of forward stimulation by the free gas model of theTPT decreases rapidly)

supposing 100 free gas saturation) and the decrease rate issignificant Considering the depth effect the decrease rate ofP wave velocity slows down with the increase of burial depthFrom the above analysis the general rule between P wavevelocity of forward stimulation and free gas saturation at siteSH2 is the higher the free gas saturation the lower the P wavevelocity This result is also basically in accordance with theresearch result made by Tinivella [35]

The estimation of gas hydrate saturation for gas hydratereservoir evaluation has an important significance In orderto estimate gas hydrate saturation in sediments it is necessaryto associate the P wave velocity of the log with the P wavevelocity of gas hydrate model based on the TPT Given aninitial gas hydrate saturation based on gas hydrate model ofthe TPT the difference between P wave velocity of forwardsimulation and the actual P wave velocity of the log inthis saturation can be acquired If the difference is in therange of allowable error the saturation can be treated as theactual saturation if the difference does not satisfy the errorrequirement the value of gas hydrate saturation should bemodified until meeting the error precision

Using gas hydratemodel of the TPT to inverse gas hydratesaturation at site SH2 the values of the main parameters arelisted in Table 1 and the inversion result is shown in Figure 7In the interval of 50 to 90mbsf at site SH2 the range ofgas hydrate saturation is 0ndash175 and the average value is48 As the shallow sediments are influenced by variationof borehole conditions the estimation error of gas hydratesaturation in this interval is significant which should benoticed during the analysis In the interval of 90 to 195mbsfthe range of gas hydrate saturation is 0ndash189 and the averagevalue is 7 In the interval of 195 to 220mbsf the range of gashydrate saturation is 7ndash315 and the average value is 232With the increase of burial depth gas hydrate saturationgradually increases and finally reaches the peak value of 315in 208mbsfThen the gas hydrate saturation decreases slowly


Gas hydrate saturations ()0 50

60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m

Figure 7 Estimation of gas hydrate saturation at site SH2

with the increase of burial depth and the range of gas hydratesaturation is 0ndash258 in the interval of 220 to 245mbsf withan average value of 155

7 Discussion

It is very important to determine the porosity for theevaluation of gas hydrate saturation In order to analyze theaccuracy of porosity estimation by density log data we useresistivity log data and combine the Archie formula [57]to make a comparison between the estimation results andthe comparison results are shown in Figure 8 The porosityestimated by resistivity log data generally changes in the rangeof 30 to 50 and the average value is 43 [12 64] In theinterval of 50 to 195mbsf the curves of density porosity andresistivity porosity are approximately coincident while theformer fluctuates due to the borehole effect In the interval of195 to 220mbsf the sediment contains gas hydrate and thecurve of resistivity porosity decreases significantly comparedwith the curve of density porosity due to the significantincrease of resistivity (Figure 2) so the porosity calculated byresistivity log data needs to be corrected to exclude the influ-ence of the increase of skeleton components In the interval of220 to 245mbsf the curves of density porosity and resistivityporosity are approximately coincident again These resultsindicate that using density log data to estimate the porosity

in the gas hydrate stability zone at site SH2 is relativelymore reliable According to previous studies the range ofcore porosity by laboratory analysis in this interval was 40ndash55 which was almost coincident with the estimation ofporosity by density log data in this study [13] As Figure 8shows the core porosity distribution corresponds with thecurve of density porosity of the well and this result provesthat the porosity estimated by density log data can meet therequirement for evaluating gas hydrate saturation at site SH2

In order to verify the accuracy of gas hydrate saturationestimated by the TPT we compare gas hydrate saturationin this study with that estimated by Wang et al [50] inthe occurrence of gas hydrate (195 to 220mbsf) at site SH2(Figure 9) The curve of gas hydrate saturation made bythe TPT first increases and then decreases as the burialdepth increases which reaches the peak value of 315 inthe 208mbsfs and then decreases gradually The peak valueof gas hydrate saturation by the TPT is slightly smallerthan the resistivity log (Rt) method (405) the simplifiedthree-phase equation (STPE)method (41) and the effectivemedium theory (EMT) method (385) However the curvetrend of gas hydrate saturation estimated by the TPT andother threemethods is basically consistent and the differenceonly lies in the amplitude of the curve which indicates thatusing the TPT method to estimate gas hydrate saturation atsite SH2 is available


0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240


Correction porosity ()

Resistivity porosity ()

Core porosity ()

1 m 1500 m

Figure 8 Comparison of estimation of sediment porosity made by different methods at site SH2 (The red curve is the porosity estimatedby density log data the bright green curve is the porosity considering the effect of shaly sediments blue curve is the porosity estimated byresistivity log combined with Archie formula the black dots are the porosity measured by core laboratory)

The average value of gas hydrate saturations calculatedby chloride anomaly method is 25 and the peak value is45 [9] The peak value of gas hydrate saturation estimatedby the TPT is relatively lower than that estimated by chlorideanomalymethod but the average values estimated by the twomethods are basically same and most distribution dots ofgas hydrate saturations obtained by chloride anomalymethodcorrespondwith the curve of gas hydrate saturation estimatedby the TPT method (Figure 9) This also indicates that usingthe TPTmethod to estimate gas hydrate saturation at site SH2is available

8 Conclusions

In summary the relationships between P wave velocity andgas hydrate saturation free gas saturation and sedimentporosity at site SH2 are studie respectively by virtue of elasticwave velocity of numerical stimulation based on the TPT

and gas hydrate model and free gas model are established toestimate the sediment porosity in order to determine the gashydrate saturation of the research area Some conclusions canbe drawn as follows(1) Using the difference between P wave velocity of

saturated water condition and actual P wave velocity of thelog whether the sediment contains gas hydrate or not canbe identified quickly In the interval of 195 to 220mbsf atsite SH2 the actual P wave velocity of the log increasessignificantly relative to Pwave velocity of forward stimulationin saturated water condition so this interval is determined tocontain gas hydrate(2) By virtue of elastic wave velocity of numerical stim-

ulation based on the TPT combined with log data thedependence of P wave velocity on gas hydrate saturationfree gas saturation and sediment porosity at site SH2 canbe analyzed respectively In the interval of 50 to 245mbsfas sediment porosity decreases P wave velocity gradually


220

0

0

STPE gas hydrate saturations ()0

0

Chloride gas hydrate saturations ()0

200

205

210

215

Depth50

50

50

50

50

TPT gas hydrate saturations ()

EMT gas hydrate saturations ()

1 m 200 m

Rt gas hydrate saturations ()

Figure 9 Comparison of estimation of gas hydrate saturation made by different methods in depth of 195 to 220mbsf at site SH2 (The redcurve black curve blue curve and violet curve represent gas hydrate saturations estimated by the TPT the Rt the STPE and the EMTmethod respectively the bright green dots represent gas hydrate saturations calculated by chloride anomaly method)

increases as gas hydrate saturation increases P wave velocityincreases as free gas saturation increases P wave velocitygradually decreases(3) The log data can be used to calculate gas hydrate

saturation of the whole well and the availability is better thanthe coring data The average value of gas hydrate saturationestimated by the TPT is 232 and the peak value is 315which is basically in accordance with the values estimated by

the STPE model the EMT model the Rt model and chlorideanomaly method

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (no 41274185) and the FundamentalResearch Funds for the Central Universities (no 2652011189


no 2652012096) The authors wish to thank three reviewers(A Billi U Tinivella and the other anonymous reviewer) fortheir valuable and constructive comments and suggestionswhich helped them to improve their paper

References

[1] E D Sloan Clathrate Hydrate of Natural Gases Marcel DekkerNew York NY USA 2nd edition 1990

[2] G R Dickens ldquoThe potential volume of oceanic methanehydrates with variable external conditionsrdquo Organic Geochem-istry vol 32 no 10 pp 1179ndash1193 2001

[3] G Bhatnagar W G Chapman G R Dickens B Dugan andG J Hirasaki ldquoGeneralization of gas hydrate distribution andsaturation in marine sediments by scaling of thermodynamicand transport processesrdquo American Journal of Science vol 307no 6 pp 861ndash900 2007

[4] K A Kvenvolden ldquoMethane hydratemdasha major reservoir ofcarbon in the shallow geosphererdquoChemical Geology vol 71 no1ndash3 pp 41ndash51 1988

[5] G R Dickens ldquoRethinking the global carbon cycle with a largedynamic andmicrobiallymediated gas hydrate capacitorrdquo Earthand Planetary Science Letters vol 213 no 3-4 pp 169ndash183 2003

[6] D Archer ldquoMethane hydrate stability and anthropogenic cli-mate changerdquo Biogeosciences vol 4 no 4 pp 521ndash544 2007

[7] M Maslin M Owen R Betts S Day T D Jones and A Ridg-well ldquoGas hydrates past and future geohazardrdquo PhilosophicalTransactions of the Royal Society A vol 368 no 1919 pp 2369ndash2393 2010

[8] T S Collett ldquoEnergy resource potential of natural gas hydratesrdquoAAPG Bulletin vol 86 no 11 pp 1971ndash1992 2002

[9] H Q Zhang S X Yang and N YWu ldquoGMGS-1 Science TeamSuccessful and surprising results for Chinarsquos first gas hydratedrilling expedition Fire in the Icerdquo Methane Hydrate Newslet-ter National Energy Technology Laboratory USDepartment ofEnergy 2007

[10] J A Lu S X Yang N Y Wu et al ldquoWell logging evaluation ofgas hydrates in Shenhu area South China Seardquo Geoscience vol22 no 3 pp 447ndash451 2008 (Chinese)

[11] J Liang H B Wang and Y Q Guo ldquoStudy of seismic velocityabout gas hydrates in the northern slope of the South ChinaSeardquo Geoscience vol 20 no 1 pp 123ndash129 (Chinese)

[12] X J Wang S G Wu X W Liu et al ldquoEstimation of gashydrate saturation based on resistivity logging and analysis ofestimation errorrdquo Geoscience vol 24 no 5 pp 993ndash999 2010(Chinese)

[13] Y Q Guo S H Qiao and W J LV ldquoVertical distribution of gashydrate in Shenhu area of the South China based on acousticvelocityrdquo Marine Geology Frontiers vol 27 no 7 pp 7ndash12 2011(Chinese)

[14] H Y Gao G F Zhong J Q Liang et al ldquoEstimation ofgas hydrate saturation with modified Biot-Gassmann theorya case from Northern South China Seardquo Marine Geology ampQuaternary Geology vol 32 no 4 pp 83ndash89 2012 (Chinese)

[15] M R J Wyllie A R Gregory and G H F Gardner ldquoAnexperimental investigation of factors affecting elastic wavevelocities in porous mediardquo Geophysics vol 23 no 3 pp 459ndash493 1958

[16] W T Wood P L Stoffa and T H Shipley ldquoQuantitativedetection of methane hydrate through high-resolution seismic

velocity analysisrdquo Journal of Geophysical Research vol 99 no 5pp 9681ndash9695 1994

[17] T Yuan R D Hyndman G D Spence and B Desmons ldquoSeis-mic velocity increase and deep-sea gas hydrate concentrationabove a bottom-simulating reflector on the northern Cascadiacontinental sloperdquo Journal of Geophysical Research B vol 101no 6 pp 13655ndash13671 1996

[18] J Korenaga W S Holbrook S C Singh and T A MinshullldquoNatural gas hydrates on the southeast US margin constraintsfrom full waveform and travel time inversions of wide-angleseismic datardquo Journal of Geophysical Research B vol 102 no 7pp 15345ndash15365 1997

[19] C Ecker J Dvorkin and A Nur ldquoSediments with gas hydratesinternal structure from seismic AVOrdquoGeophysics vol 63 no 5pp 1659ndash1669 1998

[20] M B Helgerud J Dvorkin A Nur A Sakai and T CollettldquoElastic-wave velocity in marine sediments with gas hydrateseffective medium modelingrdquo Geophysical Research Letters vol26 no 13 pp 2021ndash2024 1999

[21] M Jakobsen J A Hudson T A Minshull and S C SinghldquoElastic properties of hydrate-bearing sediments using effectivemedium theoryrdquo Journal of Geophysical Research B vol 105 no1 pp 561ndash577 2000

[22] R Ghosh K Sain andMOjha ldquoEffectivemediummodeling ofgas hydrate-filled fractures using the sonic log in the Krishna-Godavari basin offshore eastern Indiardquo Journal of GeophysicalResearch B vol 115 no 6 2010

[23] U Shankar and M Riedel ldquoGas hydrate saturation in theKrishna-Godavari basin from P-wave velocity and electricalresistivity logsrdquo Marine and Petroleum Geology vol 28 no 10pp 1768ndash1778 2011

[24] C Ecker J Dvorkin and A M Nur ldquoEstimating the amount ofgas hydrate and free gas from marine seismic datardquo Geophysicsvol 65 no 2 pp 565ndash573 2000

[25] MW Lee and T S Collett ldquoIntegrated analysis of well logs andseismic data to estimate gas hydrate concentrations at KeathleyCanyon Gulf of Mexicordquo Marine and Petroleum Geology vol25 no 9 pp 924ndash931 2008

[26] MW Lee ldquoBiot-Gassmann theory for velocities of gas hydrate-bearing sedimentsrdquo Geophysics vol 67 no 6 pp 1711ndash17192002

[27] M W Lee ldquoWell log analysis to assist the interpretation of 3-D seismic data at Milne Point North Slope of Alaskardquo USGeological Survey Scientific Investigation Report 2005-50482005

[28] X Wang and X Liu ldquoA method for estimating gas hydrate andfree gas saturations in marine sedimentsrdquo in Proceedings of theSPGSEG 2004 International Geophysical Conference pp 852ndash855 2004

[29] M W Lee D R Hutchinson W P Dillon J J Miller W FAgena and B A Swift ldquoMethod of estimating the amount ofin situ gas hydrates in deep marine sedimentsrdquo Marine andPetroleum Geology vol 10 no 5 pp 493ndash506 1993

[30] M W Lee D R Hutchinson T S Collett and W P Dil-lon ldquoSeismic velocities for hydrate-bearing sediments usingweighted equationrdquo Journal of Geophysical Research B vol 101no 9 pp 20347ndash20358 1996

[31] M W Lee ldquoVelocities and attenuations of gas hydrate-bearingsedimentsrdquo US Geological Survey Scientific InvestigationsReport 2007-5264 2007


[32] M W Lee ldquoModels for gas hydrate-bearing sediments inferredfromhydraulic permeability and elastic velocitiesrdquo US Geolog-ical Survey Scientific Investigations Report 2008-5219 2008

[33] M W Lee and W F Waite ldquoEstimating pore-space gas hydratesaturations from well log acoustic datardquo Geochemistry Geo-physics Geosystems vol 9 no 7 2008

[34] M W Lee and T S Collett ldquoIn-situ gas hydrate hydratesaturation estimated from various well logs at the Mount ElbertGas Hydrate Stratigraphic Test Well Alaska North SloperdquoMarine and Petroleum Geology vol 28 no 2 pp 439ndash449 2011

[35] U Tinivella ldquoA method for estimating gas hydrate and freegas concentrations in marine sedimentsrdquo Bollettino di GeofisicaTeorica ed Applicata vol 40 no 1 pp 19ndash30 1999

[36] U Tinivella F Accaino and A Camerlenghi ldquoGas hydrate andfree gas distribution from inversion of seismic data on the SouthShetland margin (Antarctica)rdquo Marine Geophysical Researchesvol 23 no 2 pp 109ndash123 2002

[37] U Tinivella and J M Carcione ldquoEstimation of gas-hydrateconcentration and free-gas saturation from log and seismicdatardquo Leading Edge vol 20 no 2 pp 200ndash203 2001

[38] U Tinivella and E Lodolo ldquoTheBlake Ridge bottom-simulatingreflector transect tomographic velocity field and theoreticalmodel to estimate methane hydrate quantitiesrdquo Proceedings ofthe Ocean Drilling Program Scientific Results vol 164 pp 273ndash281 2000

[39] E Lodolo A Camerlenghi G Madrussani U Tinivella and GRossi ldquoAssessment of gas hydrate and free gas distribution onthe South Shetland margin (Antarctica) based on multichannelseismic reflection datardquo Geophysical Journal International vol148 no 1 pp 103ndash119 2002

[40] X Wang S Wu M Lee Y Guo S Yang and J Liang ldquoGashydrate saturation from acoustic impedance and resistivity logsin the shenhu area South China Seardquo Marine and PetroleumGeology vol 28 no 9 pp 1625ndash1633 2011

[41] S N Domenico ldquoElastic properties of unconsolidated poroussand reservoirsrdquo Geophysics vol 42 no 7 pp 1339ndash1368 1977

[42] F Gassmann ldquoElasitcity of porous mediardquo Viertelijahrsschriftder Naturforschenden Gesselschaft vol 96 pp 1ndash23 1951

[43] N-Y Wu S-X Yang H-B Wang et al ldquoGas-bearing fluidinflux sub-system for gas hydrate geological system in ShenhuAreaNorthern SouthChina SeardquoChinese Journal of Geophysicsvol 52 no 6 pp 1641ndash1650 2009

[44] P Wang W L Prell and P Blum ldquoOcean drilling programleg 184 scientific prospectus South China Sea site 1144rdquo inProceedings of the Ocean Drilling Program Initial Reports PWang W L Prell and P Blum Eds vol 184 pp 1ndash97 OceanDrilling Program College Station Tex USA 2000

[45] N Wu S Yang H Zhang et al ldquoPreliminary discussion on gashydrate reservoir system of Shenhu Area North Slope of SouthChina Seardquo in Proceedings of the 6th International Conference onGas Hydrates pp 6ndash10 Vancouver British Columbia Canada2008

[46] S LMcDonnellMDMax N Z Cherkis andM F CzarneckildquoTectono-sedimentary controls on the likelihood of gas hydrateoccurrence near Taiwanrdquo Marine and Petroleum Geology vol17 no 8 pp 929ndash936 2000

[47] S-G Wu D-D Dong S-X Yang et al ldquoGenetic model ofthe hydrate system in the fine grain sediments in the northerncontinental slope of South China Seardquo Chinese Journal ofGeophysics vol 52 no 7 pp 1849ndash1857 2009 (Chinese)

[48] H B Wang G X Zhang M Z Yang et al ldquoStructuralcircumstance of gas hydrate deposition in the continentmarginthe South China Seardquo Marine Geology amp Quaternary Geologyvol 23 no 1 pp 81ndash86 2003 (Chinese)

[49] S X Yang H Q Zhang N Y Wu et al ldquoHigh concentrationhydrate in disseminated forms obtained in Shenhu area NorthSlope of SouthChina Seardquo inProceedings of the 6th InternationalConference on Gas Hydrates pp 6ndash10 Vancouver BritishColumbia Canada 2008

[50] X Wang D R Hutchinson S Wu S Yang and Y GuoldquoElevated gas hydrate saturation within silt and silty claysediments in the Shenhu area South China Seardquo Journal ofGeophysical Research B vol 116 no 5 2011

[51] P R Miles ldquoPotential distribution of methane hydrate beneaththe European continental marginsrdquo Geophysical Research Let-ters vol 22 no 23 pp 3179ndash3182 1995

[52] YH Rao ldquoC-program for the calculation of gas hydrate stabilityzone thicknessrdquo Computers and Geosciences vol 25 no 6 pp705ndash707 1999

[53] C Zou and L Pan ldquoComment on ldquoC-program for the calcu-lation of gas hydrate stability zone thicknessrdquo by HanumanthaRao Computers amp Geosciences vol 25 pp 705ndash707 1999rdquoComputers and Geosciences vol 34 no 12 pp 1956ndash1957 2008

[54] J H Schon Physical Properties of Rocks Fundamentals andPrinciples of Petrophysics Handbook of Geophysical Explo-ration Seismic Exploration 18 Elsevier Amsterdam TheNetherlands 1996

[55] P Leclaire Propagation acoustique dans les milieux poreuxsoumis augel modelisation et experience (These de Doctorat enPhysique) Universite Paris 7 1992

[56] G T Kuster and M N Toksoz ldquoVelocity and attenuation ofseismic waves in two-phase mediamdashpart I theoretical formu-lationsrdquo Geophysics vol 39 no 5 pp 587ndash606 1974

[57] G E Archer ldquoThe electrical resistivity log as an aid in determin-ing some reservoir characteristicsrdquo Transactions of the AIMEvol 146 no 1 pp 54ndash62 1942

[58] O Serra Fundamentals of Well-Log Interpretation (vol1) TheAcquisition of Logging Data Developments in Petroleum ScienceElsevier Amsterdam The Netherlands 1984

[59] D W Hilchie Applied Openhole Log Interpretation Douglas WHilchie Golden Colo USA 2nd edition 1982

[60] R W Zimmerman and M S King ldquoThe effect of the extentof freezing on seismic velocities in unconsolidated permafrostrdquoGeophysics vol 51 no 6 pp 1285ndash1290 1986

[61] X J Wang S G Wu X Guo et al ldquoEstimation of gas hydratesaturation in the continental slope the South China SeardquoMarine Geology amp Quaternary Geology vol 25 no 3 pp 89ndash95 2005 (Chinese)

[62] R C Weast Handbook of Chemistry and Physics ChemicalRubber Co Press 1992

[63] E L Hamilton ldquoShear-wave velocity versus depth in marinesediments a reviewrdquo Geophysics vol 41 no 5 pp 985ndash9961976

[64] Y B Sun S G Wu D D Dong et al ldquoGas hydrates associatedwith gas chimneys in fine-grained sediments of the northernSouth China SeardquoMarine Geology vol 311 pp 32ndash40 2012

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of


EarthquakesJournal of


Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining


Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of


Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering


GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of


OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in


MineralogyInternational Journal of


MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Geological ResearchJournal of


Geology Advances in


focused on the estimation of the gas hydrate saturation andthe discrepancy of the estimation of gas hydrate saturationwas large because of the application of various methodsBesides previous studies did not systematically study therelationship between any two of the gas hydrate saturationelastic wave velocity and sediment porosity which led toincomplete understanding of the log data

Foreign scholars have carried out researches on the eval-uation of the marine gas hydrate saturation earlier A varietyof theoretical models or experimental models have beenproposed to estimate gas hydrate saturation such as Wyllieet al [15] time average equation with the seismic velocity[16ndash18] the effectivemedium theory [19ndash23] Biot-Gassmanntheorymodel [24ndash27] compression wave (P wave) velocity ofthermal-elastic theory [28] the three-phase equation (TPE)[29ndash34] and velocity model theory based on the two-phasetheory (TPT)model [35]Moreover Tinivella et al [36]madea research to compare the TPTmodel with the TPEmodel forevaluating gas hydrate saturations in marine sediments andthe comparison showed that the two theoretical approacheswere in very good agreement Based on this the TPT modelhas been applied to verify the TPE model and estimate thegas hydrate and free gas saturations in several different areas[37ndash39]

In this work based on the log data at site SH2 in ShenhuArea we first establish gas hydrate model and free gas modelby applying elastic wave velocity numerical model of the TPTmethod then study the dependence of the P wave velocityon gas hydrate saturation free gas saturation and sedimentporosity and finally choose the gas hydrate model to estimategas hydrate saturation at site SH2

2 Numerical Simulation of Elastic WaveVelocity by the TPT Model

The TPT model [41 42] which supposes that the rock solidpart is composed of rock matrix and gas hydrate and thatthe rock pore fluid is composed of free gas and water can beused to study the elastic wave velocity model about the elasticcharacteristics of marine sand-shale reservoirs Based on thistheory and using the reported in Tinivella [35] the velocityrelation between Pwave velocity (119881p) and shearwave (Swave)velocity (119881s) is as follows

119881p = [(1

119862m+4

3120583)

+(120601eff119896) (120588m120588f) + (1 minus 120573 minus 2 (120601eff119896)) (1 minus 120573)

(1 minus 120601eff minus 120573)119862b + 120601eff 119862f]

sdot1

120588m (1 minus (120601eff119896) (120588f120588m))

12

119881s = 120583

120588m [1 minus (120601eff sdot 120588f)(119896 sdot 120588m)]

12

(1)

Where 120601eff is the effective porosity 120583 is the average rigidityof the skeleton 120588m is the average density 120588f is the density ofthe fluid phase 120573 is the proportionality coefficient 119896 is thecoupling factor and 119862b 119862f and 119862m are the compressibility ofthe solid phase the fluid phase and the matrix respectively

3 Geological Setting

Shenhu area is considered as one of the occurrences of gashydrate which is located in the middle of the northernslope of the South China Sea between the Xisha Trough andthe Dongsha Islands and Baiyun Sag of Zhu II depressionof the Pearl River Mouth Basin (Figure 1) Shenhu areaexperienced a geological evolution process similar to thenorthern margin of the South China Sea and eventuallyformed the regional sedimentary sequences in which marinesediments were the dominant composition [43] Taking theCenozoic sedimentary strata in the Shenhu area for exampleit is about 1000ndash7000m and the organic matter content is046ndash19 [44ndash46] which can provide the material basefor gas hydrate In recent years some studies have confirmedthat mud volcanoes seafloor slips mud diapers and otherspecial structural units beneficial to the formation of gashydrate are widely being developed in Shenhu area [47]and the bottom simulating reflections (BSRs) have also beenidentified using various geophysical methods in the northernSouth China Sea by Guangzhou Marine Geological SurveyChina Geological Survey [48]

During AprilndashJune 2007 eight sites were drilled inShenhu area (see Figure 2) among which sites of SH2 SH3and SH7 in water depth of 1105 to 1423m were determined tocontain gas hydrate in recovered core samples The thicknessof gas hydrate stability zone was about 10 to 25m [9 49] andthe sediment lithology in and above the zone was silt and siltyclay respectively according to the core data

4 Data Set Used in the Study

As previous researchers have done much work on site SH2[10ndash14 40 50] many valuable references can be used toobtain better results and verify the reliability of our researchso we select site SH2 as the research object The waterdepth of site SH2 is 1232m and the maximum drillingdepth is 245mbsf Figure 2 shows the conventional logsof site SH2 The measurement depth range is from 50 to245mbsf and the measurement projects include caliperdensity natural gamma ray acoustic and resistivity log-ging The occurrence of gas hydrate reservoir in this siteappears to be ldquoresponse characteristics of two high and twolowrdquo in the log curve namely high resistivity high naturalgamma ray and low density low acoustic time especiallyfor resistivity and acoustic logs Besides when the layer ofwell diameter changes caliper curve can be used as theeffective parameter to identify gas hydrate reservoir becausethe abnormality of other logs has nothing to do with the wellcondition

Based on the previous researchmethods for the thicknessof gas hydrate stability zone [51ndash53] and combined with the


113

18∘

120∘E

22∘N


GR (API)

20 60

AC (usft)

130 200 1 4

CAL (in)

155

60

80

100

120

140

160

180

200

220

240

Depth15 22




5 Methodology




120601s + 120601h + 120601w = 1 (2)

120601 = 120601h + 120601w (3)




119878h =120601h120601

119878w =120601w120601

(4)


1198781015840

s =120601s

(120601s + 120601h) (5)

1198781015840

h =120601h

(120601s + 120601h) (6)



120601eff = (1 minus Sh) 120601 (7)


120588m = (1 minus 120601eff) 120588b + 120601eff120588f (8)

120588b = 1198781015840

s120588s + 1198781015840

h120588h (9)

120588f = 120588w (10)



119862b =1

2(1198781015840

s119862s + 1198781015840

h119862h) +1

2(1198781015840

s119862s+1198781015840

h119862h)

minus1

(11)



119862f = 119862w (12)



119862m = (1 minus 120601eff) 119862b + 120601eff119862p (13)


119862p =(1 minus 120601eff1206010)

119875d (14)




120573 =119862b119862m (15)


120583 = (120601s + 120601h) [120601s1198781015840

s120583sm+1198781015840

h120583h]

minus1

(16)


120583sm = (120583smKT minus 120583sm0) [120601h

(1 minus 120601s)]

38

+ 120583sm0 (17)





s and 1198781015840








52 Free Gas Model


120601s + 120601w + 120601g = 1 (18)

120601 = 120601w + 120601g (19)



119878w =120601w120601 (20)

119878g =120601g

120601 (21)



120601eff = 120601 (22)



120588b = 120588s (24)

120588f = 119878w120588w + 119878g120588g (25)


119862b = 119862s (26)


119862f =1

2(119878w119862w + 119878g119862g) +

1

2(119878w119862w+

119878g

119862g)

minus1

(27)




120583 = 120583s 120583s =120583sm01 minus 120601

(28)









120601 =(120588ma minus 120588b)

(120588ma minus 120588f) (29)


120601 =(120588ma minus 120588b)


(120588ma minus 120588sh)

(120588ma minus 120588f) (30)

119881sh =(2

GCURtimesSHminus 1)






3 120588f = 104 gcm3 120588sh = 270 gcm

3



0 100

0 100

60

80

100

120

140

160

180

200

220

240




6 Results







1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


113

18∘

120∘E

22∘N


GR (API)

20 60

AC (usft)

130 200 1 4

CAL (in)

155

60

80

100

120

140

160

180

200

220

240

Depth15 22




5 Methodology




120601s + 120601h + 120601w = 1 (2)

120601 = 120601h + 120601w (3)




119878h =120601h120601

119878w =120601w120601

(4)


1198781015840

s =120601s

(120601s + 120601h) (5)

1198781015840

h =120601h

(120601s + 120601h) (6)



120601eff = (1 minus Sh) 120601 (7)


120588m = (1 minus 120601eff) 120588b + 120601eff120588f (8)

120588b = 1198781015840

s120588s + 1198781015840

h120588h (9)

120588f = 120588w (10)



119862b =1

2(1198781015840

s119862s + 1198781015840

h119862h) +1

2(1198781015840

s119862s+1198781015840

h119862h)

minus1

(11)



119862f = 119862w (12)



119862m = (1 minus 120601eff) 119862b + 120601eff119862p (13)


119862p =(1 minus 120601eff1206010)

119875d (14)




120573 =119862b119862m (15)


120583 = (120601s + 120601h) [120601s1198781015840

s120583sm+1198781015840

h120583h]

minus1

(16)


120583sm = (120583smKT minus 120583sm0) [120601h

(1 minus 120601s)]

38

+ 120583sm0 (17)





s and 1198781015840








52 Free Gas Model


120601s + 120601w + 120601g = 1 (18)

120601 = 120601w + 120601g (19)



119878w =120601w120601 (20)

119878g =120601g

120601 (21)



120601eff = 120601 (22)



120588b = 120588s (24)

120588f = 119878w120588w + 119878g120588g (25)


119862b = 119862s (26)


119862f =1

2(119878w119862w + 119878g119862g) +

1

2(119878w119862w+

119878g

119862g)

minus1

(27)




120583 = 120583s 120583s =120583sm01 minus 120601

(28)









120601 =(120588ma minus 120588b)

(120588ma minus 120588f) (29)


120601 =(120588ma minus 120588b)


(120588ma minus 120588sh)

(120588ma minus 120588f) (30)

119881sh =(2

GCURtimesSHminus 1)






3 120588f = 104 gcm3 120588sh = 270 gcm

3



0 100

0 100

60

80

100

120

140

160

180

200

220

240




6 Results







1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



119878h =120601h120601

119878w =120601w120601

(4)


1198781015840

s =120601s

(120601s + 120601h) (5)

1198781015840

h =120601h

(120601s + 120601h) (6)



120601eff = (1 minus Sh) 120601 (7)


120588m = (1 minus 120601eff) 120588b + 120601eff120588f (8)

120588b = 1198781015840

s120588s + 1198781015840

h120588h (9)

120588f = 120588w (10)



119862b =1

2(1198781015840

s119862s + 1198781015840

h119862h) +1

2(1198781015840

s119862s+1198781015840

h119862h)

minus1

(11)



119862f = 119862w (12)



119862m = (1 minus 120601eff) 119862b + 120601eff119862p (13)


119862p =(1 minus 120601eff1206010)

119875d (14)




120573 =119862b119862m (15)


120583 = (120601s + 120601h) [120601s1198781015840

s120583sm+1198781015840

h120583h]

minus1

(16)


120583sm = (120583smKT minus 120583sm0) [120601h

(1 minus 120601s)]

38

+ 120583sm0 (17)





s and 1198781015840








52 Free Gas Model


120601s + 120601w + 120601g = 1 (18)

120601 = 120601w + 120601g (19)



119878w =120601w120601 (20)

119878g =120601g

120601 (21)



120601eff = 120601 (22)



120588b = 120588s (24)

120588f = 119878w120588w + 119878g120588g (25)


119862b = 119862s (26)


119862f =1

2(119878w119862w + 119878g119862g) +

1

2(119878w119862w+

119878g

119862g)

minus1

(27)




120583 = 120583s 120583s =120583sm01 minus 120601

(28)









120601 =(120588ma minus 120588b)

(120588ma minus 120588f) (29)


120601 =(120588ma minus 120588b)


(120588ma minus 120588sh)

(120588ma minus 120588f) (30)

119881sh =(2

GCURtimesSHminus 1)






3 120588f = 104 gcm3 120588sh = 270 gcm

3



0 100

0 100

60

80

100

120

140

160

180

200

220

240




6 Results







1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



119878w =120601w120601 (20)

119878g =120601g

120601 (21)



120601eff = 120601 (22)



120588b = 120588s (24)

120588f = 119878w120588w + 119878g120588g (25)


119862b = 119862s (26)


119862f =1

2(119878w119862w + 119878g119862g) +

1

2(119878w119862w+

119878g

119862g)

minus1

(27)




120583 = 120583s 120583s =120583sm01 minus 120601

(28)









120601 =(120588ma minus 120588b)

(120588ma minus 120588f) (29)


120601 =(120588ma minus 120588b)


(120588ma minus 120588sh)

(120588ma minus 120588f) (30)

119881sh =(2

GCURtimesSHminus 1)






3 120588f = 104 gcm3 120588sh = 270 gcm

3



0 100

0 100

60

80

100

120

140

160

180

200

220

240




6 Results







1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


0 100

0 100

60

80

100

120

140

160

180

200

220

240




6 Results







1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

1500 2500

60

80

100

120

140

160

180

200

220

240

Depth(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)

(ms)








1 m 1500 m











119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in








119881s

116 + 465119911ms237 + 128119911ms332 + 058119911ms

[63]

120583sm0 120588m1198812

s Pa [35]120583h 37 times 10

9 Pa [35]119896 23 [61]


152

253

354

45

02502

01501

005 002 04

0608

1

4

35

3

25

2

Vp

(km

s)




05

1

15

2



01015

02025

0 0204 06

081

06

08

1

12

14

16

18

Vp

(km

s)







60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



60

80

100

120

140

160

180

200

220

240

Depth1 m 1500 m



7 Discussion





0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


0 100

0 100

0 100

0 100

60

80

100

120

140

160

180

200

220

240




Core porosity ()

1 m 1500 m



8 Conclusions






220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


220

0

0


0


200

205

210

215

Depth50

50

50

50

50



1 m 200 m






Acknowledgments




References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



References












































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in

Research Article Acoustic Velocity Log Numerical...

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