Reservoir Compaction and Surface Subsidence Analysis
Technology Summary
The weight of sediments overlying a producing horizon is supported partially by the rock matrix and partially by the fluid pressure within the rock porespace. As fluids are withdrawn and pressure depletes, more of the load is transferred to the rock matrix and producing formation compacts. Reservoir compaction and associated bedding plane slip and overburden shear has induced damage to hundreds of wells in fields throughout the world.
Terralog Technologies has unique expertise and experience analyzing compaction, subsidence, and well casing damage in a variety of settings worldwide. We can develop geomechanical models from the reservoir scale to the individual well component scale to predict subsidence and well damage risks and to develop mitigation strategies to reduce such risks.
Terralog Services
- Simple analytical studies for initial screening of compaction, subsidence, and well damage risks.
- 3D influence function modeling coupled to reservoir simulation output to evaluate surface subsidence and induced strains along any well trajectory.
- Full 3D geomechanical modeling of both reservoir and overburden to assess compaction, subsidence, induced fault movement, and well damage risks.
- Quantitative risk assessment and decision analysis services by comparing the costs and benefits of alternative well design.
Reservoir Compaction and Surface Subsidence Analysis
The weight of overburden sediments above a producing formation is supported partially by the rock matrix and partially by the pressurized fluid within the rock porespace. When fluid pressure is reduced by oil and gas production, more of the load is transferred to the rock matrix resulting in formation compaction. Subsurface compaction can sometimes produce surface subsidence with significant displacements in the both the vertical and horizontal directions. In extreme instances surface fissures have been created, fault movement has been induced, well casings have been seriously damaged, and offshore platforms have partially submerged.
Reservoir conditions which may lead to significant compaction and subsidence problems include :
• Thick producing interval (> 500 ft).
• Large lateral extent of formation relative to depth.
• Soft formation materials (Compressibility > 10E-6/psi).
• Large pressure declines (> 5000 psi).
• Highly faulted or seismically active areas.
A variety of analytical and numerical methods have been developed and applied by Terralog to assess compaction and subsidence and their potential influence on wells, surface facilities, or on nearby faults. These techniques include:
• simple analytical estimates
• three-dimensional elastic models
• two-dimensional numerical models
• three-dimensional numerical models
Terralog Technologies has developed unique inversion software to identify subsurface source mechanisms responsible for surface subsidence by analysis of measured surface displacements.
| Simple analytical estimates: | ||
| S=2Cm(1- n )[H-(R 2+(D+H)2)0.5+(R2+D2)0.5]DP | (subsidence) | |
| ec=0.5(1+cos2q)CmDP; gc=0.5(sin2q)CmDP | (casing compression; shear) |
| Three-dimensional elastic models: |
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Reservoir production layers can be discretized into a 3-D assembly of elements, which |
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Analytical solutions are then available to estimate displacements and strains induced on any well trajectory, or on any horizon due to the combined influence of all compacting reservoir cells. |
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Sample discritization of offshore reservoir horizon Resulting shear strain distribution at 1650m depth for
for 3D displacement discontinuity modeling uniform depletion scenario
A two-dimensional geomechanical model can be used to account for structural influences in one plane, vertical layering and heterogeneity, and inelastic material behavior. The appropriate location and orientaion of such 2-D high-resolution models is often guided by insights gained from simple 3-D analytic solutions previously discussed. |
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2D Geomechanical Cross-Section Model to Investigate Subsidence and Well Damage at a San Joaquin Valley Oilfield
The final level of complexity is to fully discretize the reservoir and overburden in three dimensions. Such a model can then account for 3D structural effects and for production and injection patterns that vary across a field, thereby introducing non-uniform and non-symmetric displacement patterns. |
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SFI™, Slurry Fracture Injection™, TTI™, Terralog™, and SFI-Slurry Fracture Injection™ are trademarks of Terralog Technologies Inc.
