A Semianalytical p/z Technique for the Analysis of ... · Natural Gas Engineering | 26 May - 30 May...

Natural Gas Engineering | 26 May - 30 May 2014 (U. Kavala/GREECE) Tom BLASINGAME | [email protected] | Texas A&M U.

T.A. Blasingame, Texas A&M U.Department of Petroleum Engineering

Texas A&M UniversityCollege Station, TX 77843-3116

+1.979.845.2292 — [email protected]

Natural Gas Engineering

A Semianalytical p/z Technique for the Analysis

of Abnormally Pressured Gas ReservoirsSPE 71514 | R. Gunawan Gan | M.S. Thesis (2001)

Slide — 1


ObjectiveTo present a new technique that can beused to:●Calculate gas-in-place for an abnor-

mally pressured gas reservoir using only average reservoir pressure and cumulative production data.

●Calculate pore volume compressibi-lity as a function of reservoir pressure.

Slide — 2


Presentation Outline

● Introduction● Overview of Existing Methods● New Method ● Field Examples● Conclusions

Slide — 3


Introduction●p/z schematic for a normally-pressured

volumetric gas reservoir.

G

p/z

Gp

GG

zp

zp p

i

i 1

Slide — 4


Introduction●p/z schematic for an abnormally-pressured

gas reservoir.

p/z

Gp G

GG

zppp

zp p

i

ii 1)(1

Gapp

Slide — 5


Introduction

● Reasons for the non-linear p/z behavior:■ Rock and water compressibility effects —

"rock collapse theory" (Hawkins, 1969).■ Shale water influx (Bourgoyne, 1989).

Slide — 6


Existing Methods

Methods based on presumed knowledge ofsystem compressibility:

Hammerlindl (Constant Compressibility), 1971

Ramagost (Constant Compressibility), 1981

Yale et al. (Variable Compressibility), 1993

GG

zp

SccSp

zp p

i

i

w

fww 1)1(

)(1

Slide — 7


Methods based on presumed knowledge ofsystem compressibility (continued):

Fetkovich, Reese, and Whitson - 1991- Derived General Material Balance Eq.- Define cumulative effective compressibility,

wi

ftwftwwie S

pcpcMpcpcSpc

1)]()([)()(

)(

- ce represents the cumulative change in hydrocarbon PV caused by compressi-bility effects (and water influx).

Slide — 8


Methods which do not require a priorknowledge of system compressibility: Roach - 1981

- very sensitive to initial pressure.- method sometimes doesn’t exhibit

a negative intercept (which is not possible).

Bernard - 1985- using Least Squares approach.- very sensitive to data scatter.

Ambastha - 1991: Type Curve Approach- non-uniqueness problems.

Slide — 9


New Method

Develops 2 new plotting functions:

1. )/)/(/(versus)( iiie zpzpppc

2. /GGzpzp pii versus)/)/(/(

Requires production data only (p and Gp).

Satisfies both "rock collapse" and "shale water influx" theories.

Slide — 10


New Method Uses general material balance equation

(proposed by Fetkovich, et al.).

GG

zpppc

zp p

i

iie 1)(1

Rearranging, we obtain:

GG

zpzpppc pii

ie 1 //1)(

Slide — 11


New Method Calculate the ce(pi-p) function for

each p/z versus Gp trend.

ce(pi-p) = ???

ce(pi-p) = ???

Gp

p/z

G GappSlide — 12


New Method For early time data (1st straight line):

GG

GG

ppc app

zp

zp

app

zp

zpie

i

i

i

i

)/(1

)/(11)(

For late time data (2nd straight line):

GG

ppc pA

zpzpie

iiA

111)()/(

)/(

where: A is the inflection pointSlide — 13


New Method

Plot of log ce(pi-p) versus (p/z)/(pi/zi):

(p/z)/(pi/zi)

G/Gapp=0.7

G/Gapp=0.6

G/Gapp=0.8

inflection point

Slide — 14


Plot of log ce(pi-p) versus (p/z)/(pi/zi) :

(p/z)/(pi/zi)

inflection point

New Method

Slide — 15


New Method

/GGzpzp pii versus)/)/(/(

Gp/G

(p/z

)/(p i

/zi)

0 1

1

Infl. Point: GpA/G, (p/z)A /( pi /zi )

GG

GG1

/zpp/z p

appii

GG

GGzpzp

/zpp/z p

pAii

A

ii )/1)(/()/(

Slide — 16


New Method


Gp/G

(p/z

)/(p i

/zi)

0 1

1

G/Gapp=1

G/Gapp= 0.8

G/Gapp=0.6Inflection point

Slide — 17


New Method


Gp/G

(p/z

)/(p i

/zi)

0 1

1

Inflection point

G/Gapp=0.8

Slide — 18


New Method

/GGzpzp pii versus)/)/(/( Dynamic Type Curve Matching. Automatic Matching using SOLVER

(Excel function for non-linear regression).

Slide — 19


New Method

Data required for analysis: Fluid property data Initial Reservoir p and T p and Gp data

Slide — 20


New Method

Computer program: Visual Basic Application in MS Excel

Easy to use - especially for analysis Only requires MS Excel

Slide — 21


Data Analysis Sheet

Slide — 22


Example 1: G is too low

Slide — 23


Example 1: G is too high

Slide — 24


Example 1: Correct G

Slide — 25


Example 2: Long transition period

Slide — 26


Example 3: Early time data

Slide — 27


Example 4: Synthetic Dry Gas Case

Slide — 28


Example 4: Backcalculated cf

Procedure to calculate cf vs. p from production data:

1. Get )(pce from type curve matching

3. Calculate cf (p):

jfnif pcppcn

jj

1)(

wi

ftwftwwie S

pcpcMpcpcSpc

1)]()([)()(

)(

2. Use the following equation to calculate )(pcf :

Slide — 29


Example 4: Backcalculated cf

Slide — 30


Conclusions We have developed a straightforward

approach for analyzing p/z versus Gpbehavior for abnormally pressured gasreservoirs — the approach considersthat two straight-lines must be ob-served on the p/z plot.

The proposed method determinesgas-in-place without using system compressibility data. Only p, Gp, and fluid property data are required.

Slide — 31


Conclusions (continued)

Our approach of using ce(pi-p) versus(p/z)/(pi /zi) and (p/z)/(pi /zi) versus Gp/Gas dynamic type curve matching func-tions has been shown to work extreme-ly well.

Using our new method, it is possible to calculate rock compressibility as a

func-tion of pressure from p and Gp data

Slide — 32


Conclusions (continued)

The "dynamic type curve matching technique" used for calculating gas-in-place from production data is more representative (and more stable) than the non-linear optimization method provided by SOLVER.

Slide — 33


T.A. Blasingame, Texas A&M U.Department of Petroleum Engineering

Texas A&M UniversityCollege Station, TX 77843-3116

+1.979.845.2292 — [email protected]

Natural Gas Engineering

A Semianalytical p/z Technique for the Analysis

of Abnormally Pressured Gas ReservoirsSPE 71514 | R. Gunawan Gan | M.S. Thesis (2001)

(End of Presentation)

Slide — 34

A Semianalytical p/z Technique for the Analysis of ... · Natural Gas Engineering | 26 May - 30 May...

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