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SPE 95545 Log-Based Pore Volume Compressibility Prediction- A Deepwater GoM Case Study. Chris Wolfe (Baker Atlas) Contributing Authors: Charles Russell (ENI Petroleum) Nicola Luise (ENI Petroleum) Richin Chhajlani (Chevron). Outline. Study objectives

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spe 95545 log based pore volume compressibility prediction a deepwater gom case study

SPE 95545 Log-Based Pore Volume Compressibility Prediction- A Deepwater GoM Case Study

Chris Wolfe (Baker Atlas)

Contributing Authors:

Charles Russell (ENI Petroleum)

Nicola Luise (ENI Petroleum)

Richin Chhajlani (Chevron)

slide2

Outline

  • Study objectives
  • Geological setting/operational procedure
  • Basics of reservoir compressibility & compaction
  • Log-based Methodology
  • Results
  • Discussion
slide3

Value of Obtaining PVC Early in the Project

  • Allows for quick calculation of expected compaction
  • Reservoir drive determination
  • Reserves estimates
  • Reservoir pressure maintenance
  • Production forecasting
  • Casing collapse issues
  • Aid in calculating the overall value of the project
slide4

This was done 8 months after the log-based PVC was computed.

Study Objectives

  • Compute the log-based pore volume compressibility (PVC) immediately after logging of the well
  • Compute the expected compaction using the log-based PVC results
  • Compare the PVC results obtained from the log-based method to the lab results
slide5

Geologic Setting/Operational Procedure

Salt

Sand “X”

original hole

sidetrack

Sand “Y” top

Sand “Y” bottom

~50’

slide6

Modes of Compaction

focus of this study

elastic or reversible

  • “Elastic region”:
    • Low effective stress levels
    • Gradual porosity decrease
    • Relatively low compressibility
    • Small displacements
    • reversible

Porosity

post-pore collapse

pore collapse stress

pore collapse

Effective Stress Increase

(pore pressure decrease)

slide7

Elastic Compressibilities- Definitions

Compressibility is defined as change in volume for a given change in pressure.

  • Bulk compressibility, Cbx
  • Pore volume compressibility, Cpx
  • Grain compressibility, Cgx

Subscripts:

The first subscript (b=bulk, p=pore, g=grain) denotes the compressibility under changing x, where x is either p (changing pore pressure) or c (changing confining pressure)

These are traditionally obtained from lab testing!

slide8

Typical Lab Testing

Uniaxial Strain Test

Hydrostatic Test

Triaxial Test

s1

s1

s1

d1

d1

d1

s1

l

s3

s3

l

l

d3= 0

d

d

d3/2

d

d3/2

log based rock mechanical properties logging of mechanical properties lmp
Log-based Rock Mechanical Properties- Logging of Mechanical Properties (LMP)

sa

sc

Log Inputs

Dtc, Dts, rb, volume fractions

Produce stress-

strain curves

Produces a “virtual

core sample”

sa

Apply

“virtual”

stresses

to the “virtual core

sample”

ea

er

Static Mechanical Properties:

rock strength, elastic moduli

Poisson’s ratio, compressibilities

log derived compressibility methodology
Log-Derived Compressibility – Methodology
  • Calculate bulk compressibility (Cbc) from LMP→
  • Calculate pore volume compressibility (PVC) →
  • Convert pore volume compressibility [Cpp] into uniaxial strain ‘equivalent’ pore volume compressibility [Cpp(u)]
  • Repeat the process at each stage of drawdown (assume a value for Ko)

NOTE: Cgc (quartz) ≈ 1.6 x 10-7 psi-1

NOTE: f is the effective porosity, n is static Poisson’s ratio, E is static Young’s modulus and Ko = Dsmin/Ds max

pvc comparison lmp vs lab
PVC Comparison – LMP vs. Lab

Sand “X”

Ko (log-based) → assumed to be 0.5

Ko (lab) → calculated to be 0.18

pvc comparison lmp vs lab13
PVC Comparison – LMP vs. Lab

Sand “Y”- top

Ko (log-based) → assumed to be 0.5

Ko (lab) → calculated to be 0.18

pvc comparison lmp vs lab14
PVC Comparison – LMP vs. Lab

Sand “Y”- bottom

Ko (log-based) → assumed to be 0.5

Ko (lab) → calculated to be 0.18

pvc comparison lmp vs lab15
PVC Comparison – LMP vs. Lab

Sand “X”

Ko (log-based) = Ko (lab) = 0.18

slide17

Discussion

  • The log-based calculations were completed two weeks after the well was logged (8 months prior to lab testing)
  • Log-based RMP & PVC results matched favorably with the lab-determined results
  • Due to the low PVC values, the expected compaction is low
      • compaction drive cannot be relied upon
      • potential permeability reduction due to compaction should be minimal
  • The log-based approach allows for quick & reliable assessment of the PVC
      • project sanctioning decisions can be made upfront
  • Due to the low PVC, Ko ramifications are minimal. For weakly consolidated formations, this may not be the case.