Lecture 18 PGE 311

7/21/2019 Lecture 18 PGE 311

http://slidepdf.com/reader/full/lecture-18-pge-311 1/13

Conventional InterpretationTechniques

7/21/2019 Lecture 18 PGE 311


Importance loginterpretation

• The most important phase of well-logging operations is interpretation.

• During this phase, geologists, geophysicists, engineers, and log analystsuse well logs to obtain information necessary to perform their tasks.

• Logs have many uses

• Eploration geologists use logs to recogni!e deposition environments andother signi"cant geologic features.

• Development geologists use them mostly to correlate and to map

potential formations.• Logs are valuable tools for geophysicists interpreting seismic data.

• Drilling engineers use log information to detect overpressured !ones andto estimate epected pore pressure and fracture gradient, information thatis indispensable for safe and e#cient drilling operations.

• Logs are also used during completion.

• Log data are etremely valuable in reservoir engineering calculations,especially in reserve estimation.

• The most critical use of logs is the detection of hydrocarbons and theestimation of the potentials of hydrocarbon-bearing formations.

7/21/2019 Lecture 18 PGE 311


$uestions that Loganalysts are faced with

• Log analysts are faced with four main %uestions.

• &. Does a speci"c formation or !one contain hydrocarbons'

• (. )hich hydrocarbon is present, oil, gas, or both'

• *. Is the hydrocarbon saturation high enough to indicate su#cient

e+ective permeability to hydrocarbons'• . Is the hydrocarbon accumulation large enough to warrant the

completion of the well'

• If the log analyst can answer all four %uestions conclusively andpositively, the well is usually completed in the !one of interest.

•

If the answers are conclusively negative, the formation is abandoned.

7/21/2019 Lecture 18 PGE 311


Interpretation

techni%ues• The available interpretation techni%ues vary from %uick

look techni%ues, which provide relatively %uick answersusing interpretation aids likely to be on hand at a wellsite,

to sophisticated and comprehensive interpretations usingall available data.

• The conventional calculations usually use only logresponses and are based on generali!ed petrophysicalmodels developed mainly for clean formations.

• ome of these models are approimations. )hereapproimations are not appropriate, a degree of uncertaintyis introduced.

• The application of the conventional interpretation techni%ueis hence limited to development problems.

7/21/2019 Lecture 18 PGE 311


Acquiring Raw DataFrom Logs

• /bsolute-depth measurement with wireline tools is very di#cult,and depth variation between logs recorded in the sameborehole may eist.

• These variations are caused mainly by borehole irregularities

and tool type.• ome tools, such as the induction, are cylindrical, so

withdrawing them from the wellbore is easy.

• 0ther tools, such as the density, sidewall neutron, and sonic logwith caliper, have arms that drag on the side of the hole.

•

In an inclined borehole, even the cylindrical tools may drag.1ariations in hole si!e and tool shape cause variations in theamount of drag, a+ecting the degree of logging cable stretchand thus the location of a bed on the log.

Correlation

Between Logs

7/21/2019 Lecture 18 PGE 311


•)hen this happens, a bed will show at di+erent recordeddepths on di+erent logs.

• These logs are designated o+-depth.

• The o+-depth error can be consistent or inconsistent.

• The logs may be o+ a consistent ( ft, for eample, or the

error may be ( ft at one point and ft at another point upthe hole. – This inconsistency results mainly from a change in borehole conditions.

• 2ombining logging devices to produce simultaneousreadings of several di+erent logs is possible.

• 2ombination log systems have the advantage of providingdepth-matched recordings. The "rst step in anyinterpretation techni%ue that uses more than one log is thecorrelation of the logs to ensure that they are on-depthrelative to each other.

continue

7/21/2019 Lecture 18 PGE 311


• To correlate between logs, amarker has to be selected. – / marker is an anomaly or a

distinctive response that appears onall logs.

• hale and low-porosity stringers areusually good markers. 3ecause ofthe inconsistency of the error, more

than one marker should be used.• Two markers, one at the top and

one at the bottom of the logsection analy!ed, are usuallyrecommended.

• 0nce the markers are recogni!ed,

logs are put on the same depthreference.

7/21/2019 Lecture 18 PGE 311


4ones election

• /fter the logs are placed on-depth, the net step is to select the !ones ofinterest.

• In the detection of hydrocarbons, the !ones of interest are those thatdisplay permeability.

• The permeable beds are usually identi"ed using the 5 log. The 6icrolog7

is an ecellent permeability indicator.• The e+ect of mud-"ltrate invasion on di+erent resistivity tools helps

indicate permeable beds.

• hallow-investigation resistivity devices are most a+ected by invaded-!one resistivity and, in the case of freshwater-based muds, usually display

an apparent resistivity that is higher than that of the deep resistivity tool.

•

eparation between resistivity curves can be absent in permeable beds incases of etremely shallow or etremely deep invasion conditions whereboth shallow and deep devices investigate practically the same resistivitypro"le.

7/21/2019 Lecture 18 PGE 311


• Thick permeable beds seldom display a constant resistivity, porosity, orother log readings. These beds must be divided into !ones on the basis oflog-reading variations.

• 4one selection is a delicate procedure because di+erent tools havedi+erent vertical resolutions8 i.e., they measure formation properties indi+erent detail.

• Laterologs7 and porosity tools respond to beds as thin as & ft, while inresistive beds, the induction tool averages ft or more at one time.

• Thus,

• the induction conductivity curve is smoother than the log of intervaltravel time. This is illustrated hi Fig. 11.6 which is a tracing of the

conductivity curve of Eample &&.& onto the sonic log.• In addition to being smoother, the conductivity curve does not clearly

show very thin beds like those indicated by arrows in 9ig. &&.:.

7/21/2019 Lecture 18 PGE 311


ConventionalInterpretation

Technique• The conventional interpretation techni%ue makes use of the followinge%uations;

• These e%uations, derived for clean formations, were discussed in detail in2hap. &.

• 2ases of comple lithology and shaly formations, because of theircompleity and importance, are discussed in separate chapters, as are gas-bearing formations.

• The use of the conventional techni%ue re%uires resistivity logs, a porositylog <sonic, density, or neutron=, and an 5 log or formation water resistivity.

• >esistivity logs are used to determine R, and, if possible, R xo values. The

method used to determine these values is eplained in 2hap. ?.

7/21/2019 Lecture 18 PGE 311


• The porosity-log reading is used to calculate the formation porositywith E%. (.?@, *.:, *.A, or *.:(. These e%uations were discussedthoroughly in previous chapters.

• Their use assumes that the pore Buid is a li%uid and re%uiresknowledge of the matri type8 matri density, ρma; and matri travel

time, Δt ma.

• 0nce the porosity is determined, F is calculated from E%. &&.& and theappropriate values of the coe#cients a and m. election of

appropriate a and m values is eplained in 2hap. &.• Rw can be estimated from the 5 log reading with one of the ap

proaches described in 2hap. :.

• )hen the 5 log is not available or when another value is needed tocross check, Rw can be estimated from a clean water-bearing !one

believed to have the same water as the !one of interest. 3ecause

R,=R0, in water-bearing !ones, E%. &&.(, after rearranging, can beused to calculate Rw:

• where F is the formation factor of the water-bearing !one derived from itsporosity

7/21/2019 Lecture 18 PGE 311


Calculation o!Recovera"le

#$%rocar"ons• The amount of oil that may be recovered <reserves= from a well that

encountered a formation of interest can be calculated with

• where NR = recoverable oil volume in T3, /Cdrainage area in acres,

9>Crecovery factor, B0=oil 919, h¡=thickness in ft of an individual!one capable of Bowing oil at rates of economic interest within the

interval of interest, i fractional porosity, S0=fractional oil saturation,

and A,A?Cnumber of barrels per acre-foot.

• / similar e%uation can be written to estimate the standard cubic feet

of recoverable free gas, GR:

• where SgCfractional gas saturation, Bg=gas 919, and *,?:@ C

number of cubic feet per acre-foot.

7/21/2019 Lecture 18 PGE 311


Thankyou

Lecture 18 PGE 311

Documents

Transcript of Lecture 18 PGE 311