Effects Of Reservoir Compaction

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Effects Of Reservoir Compaction In Deep Water
Environment
N. Yusuf Harold Vance Department of Petroleum
Engineering. Texas AM University
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Outline

• Objectives
• Linear Elastic properties of a solid material
• Elastic properties applied to reservoir rock
• Review of SPE papers

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Objectives

• Understand Reservoir compaction
• Determination of parameters affecting dk, df, dh
• Review industry works to obtain, dk, df, dh
• Develop a criteria / selection tool

4
Outline

• Linear Elastic properties of a solid material

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Youngs modulus, E
F
dz
A
Z
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Poisson Ratio, ?
F
dz
A
Z
dx
X
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Bulk modulus, K
F
F
F
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Uniaxial modulus, M
F
Constrained
Constrained
Constrained
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Outline

• Elastic properties applied to reservoir rock

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Strain relationships for Porous Rock
Vb bulk volume Vp pore volume Vg grain volume
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Strain relationships for Porous Rock
With respect to change in pressure
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Rock compressibility
Elastic Compressibilities- Definitions
Volume changes Bulk volume Pore Volume
Pressure changes Overburden Pressure
Pore Pressure
Bulk Volume Compressibilities
Pore Volume Compressibilities
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Compressibility relationships for a Porous Rock
Zimmerman, 1991
Bulk strain due to pore pressure changes Vs bulk
strain due to lab pressure changes
Pore strain due to pore pressure changes Vs pore
strain due to lab pressure changes
BULK STRAIN due to PORE PRESSURE changes related
to PORE STRAIN due to CONFINING PRESSURE changes.
SIGNIFICANCE 1 psi change in Confining (or
Lab) pressure produce more strain on the bulk
pore volume than 1 psi change in pore pressure by
an amount equal to the matrix compressibility. If
matrix compressibility is negligible, equal
changes in either pore or confining (lab)
pressure produce the same bulk and pore volume
changes.
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Compressibility relationships for a Porous Rock
Zimmerman, 1991 OTHER FORMS OF THE RELATION

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Compressibility relationships for a Porous Rock
Nur Byerlee, 1971 EFFECTIVE STRESS LAW
a is the Biots constant

SIGNIFICANCE To simulate an equivalent
strain in a porous rock the laboratory stress
should be less than the pore pressure depletion
by a 1- a.
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Uniaxial compressibility
Overburden

• Loading conditions
• constrained strain
• Uniaxial compressibility Cm

Constrained
Constrained
Constrained
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Typical Lab Testing
Uniaxial Strain Test
Hydrostatic Test
Triaxial Test
s1
s1
s1
d1
l
d
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Uni-axial Strain Test / Oedometer Test
Simulates the reservoir boundary condition of
zero lateral displacement. Provide direct
measurement of Cm, uniaxial compaction
coefficient. Difficulty Requires size of core
to fit Exactly in the cell. Gap in cells
Lateral strain, disintegration of sample,
measurement errors.
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Tri-axial Test
s1
Generally used to determine strength under
various conditions of stress i.e. vary s1,
s3 Modifications s3 -gt to balance lateral
effect of s1 Calculation of n
Difficulty Requires size of core to fit
Exactly in the cell.
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Hydrostatic Test
s1
Easiest to conduct, Hydrostatic confining
pressure Measure Pore compressibility Cpc
Bulk compressibility Cbc Uni-axial
compressibility (Cm) can be calculated using the
uni-axial correction factor
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Reservoir compaction
Constrained Deformation in the reservoir
Axial Compaction co-efficient Ca
Overburden
Total reduction in reservoir height
Constrained
Constrained
Constrained
Neglecting variation of Cm with pressure
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Reservoir compaction
Factors leading to Reservoir compaction

• Large reduction in pore pressure
• Large vertical extent of the zone of pressure
  reduction
• Large order of magnitude of Cm

• Controlling magnitude of compaction
• Pressure maintenance, Limiting Perf interval
• Study of Cm

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Outline

• Review of SPE papers

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Paper Review 1
SPE 3730 Land Subsidence Above Compacting
Oil and Gas Reservoirs J Geeertsma

• Early work to understand the subject
• Causes of compaction / subsidence
• Prediction method
• Approach
• Observations from field data
• Mathematical derivation
• Compaction / Subsidence prediction

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Paper Review 1
SPE 3730 Land Subsidence Above Compacting
Oil and Gas Reservoirs J Geeertsma

• Susceptible reservoirs from observed field Data
• Loose / weakly cemented formation
• Low depth of reservoir burial (ave 1000m)
• Significant dp (e.g depletion type reservoir)
• dp over large interval
• Consolidated reservoirs with large dp H

• Factors affecting Cm
• Rock type (hard, consolidated, friable, loose)
• Degree of cementation
• Porosity (high f-gt high Cm)
• Depth of burial

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Paper Review 1
SPE 3730 Land Subsidence Above Compacting
Oil and Gas Reservoirs J Geeertsma

• Prediction tool Calculation of Cm
• Sandstone
• 3 rock types, 2 depths
• Input variables
• Rock type
• Porosity / Stress level
• Depth

• Interpretation
• Low Cm 1-3 X 10-5
• High Cm gt 10 X 10-5

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Paper Review 1
SPE 3730 Land Subsidence Above Compacting
Oil and Gas Reservoirs J Geeertsma

• Limestone
• 2 rock types, 1 depth
• Prediction tool
• Limited variables

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Paper Review 2
SPE 66479 Compaction Effects on Porosity and
Permeability in Deepwater Gulf of Mexico-R.M.
Ostermeier

• Peculiarity in Deep water Environment
• High developmental cost
• Typical sand is unconsolidated
• Limited Aquifer support
• Problematic pressure maintenance
• Approach
• Core samples collection over 4 yrs
• Laboratory measurement of stress Vs k, f
• Development of permeability model

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Paper Review 2
SPE 66479 Compaction Effects on Porosity and
Permeability in Deepwater Gulf of Mexico-R.M.
Ostermeier

• Results Varying responses with 2 extreme cases
• Case A
• Geologically younger softer sands
• Exhibiting higher PV compressibility (25)
• Compr increases to a max value (120)
• Case D
• Geologically older sands harder
• Exhibiting lower initial PV Compr (12)
• Slight decrease(10)

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Paper Review 2
SPE 66479 Compaction Effects on Porosity and
Permeability in Deepwater Gulf of Mexico-R.M.
Ostermeier

• Porosity Permeability variations Case A
• Loading Conditions
• Step increases of pressure to 7,000 psi
• Time period of 1,500 days
• Results
• Porosity 0.32 0.24 (25),
• Permeability 1.3 0.2 D (84.6)

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Paper Review 2
SPE 66479 Compaction Effects on Porosity and
Permeability in Deepwater Gulf of Mexico-R.M.
Ostermeier

• Porosity Permeability variations Case D
• Loading Conditions
• Step increases of pressure to 8,000 psi
• Time period of 300 days
• Results
• Porosity 0.265 - 0.250 (5.7),
• Permeability 0.65 - 0.475 D (26.9)

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Paper Review 2
SPE 66479 Compaction Effects on Porosity and
Permeability in Deepwater Gulf of Mexico-R.M.
Ostermeier

• Permeability model
• Limitations
• High error range /- 30
• Requires Stress Vs f as input