Impact assessment of strain-dependent permeability on reservoir productivity in CSS




Geomechanical analysis is essential to assess the productivity forecast of cyclic steam simulation (CSS) operations in heavy oil reservoirs. The high-temperature and high-pressure fluid injection, as well as depletion in unconsolidated and poorly cemented porous media during CSS, may generate a relevant stress–strain response at levels that can lead to irreversible changes in reservoir permeability. Therefore, it is fundamental to consider permeability dependence on rock strain to properly analyze the impact of geomechanics, pressure, and temperature on reservoir performance. This paper implements a proposed directional strain-dependent permeability model to assess the productivity and compare it with a conventional volumetric permeability model through numerical simulation, considering the effects of wellbore creation on the stress–strain initial state. An explicit coupling between CMG-STARS and the geomechanics in-house simulator GSIM is carried out to perform the simulations using the proposed model. The results of oil production rates and permeability profiles show competitiveness between dilation and compaction periods that modify the structure of the porous media. There is a significant influence of stress state, strain, and injected energy on the permeability parameter. The approximations of this study might be used for feasibility assessment and optimization of CSS when integrating reservoir flow and geomechanical behavior analysis in productivity forecast.

  • Permeability changes near the wellbore are amplified when the open-hole internal condition is implemented in reservoir simulation.
  • A flow capacity and productivity loss due to depletion are expected when CSS is implemented on unconsolidated heavy oil reservoirs.
  • The permeability loss on reservoirs subjected to CSS may be managed by selecting the adequate operating variables.

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6 Conclusions and recommendations

• The proposed directional strain-dependent permeability model assumes that each component in the strain tensor has its own effects on the changes in permeability in each direction due to the thermoporoelastic response of the porous media,

implying complex mechanisms between the permeability,rock elasticity and plasticity, grain size,shape, and sorting.

• A significant difference in near-wellbore permeability response is identified when comparing the volumetric and directional permeability approaches. The differences are attributed to the open-hole condition and the nature of the radial and tangential strains.

• To estimate the calibration parameters of the proposed directional strain permeability model, it is necessary to properly design a set of triaxial testing experiments with proper permeability measurements to calibrate and isolate strain and thermal effects when addressing permeability changes.

• Despite the overall permeability increase during injection stages, the production loss due to permeability variation is more evident in the later CSS cycles due to reservoir depletion and compressive strain accumulation. A certain competitiveness

is observed between dilation and compaction processes during CSS.

• Injection variables, such as injection rate and steam quality, report a greater effect on nearwellbore permeability. In contrast, production parameters such as the number of cycles and minimum oil rate impact the permeability value outward the thermal front. Hence, it may be possible to strategically modify operational parameters during the injection and production stages to mitigate and assess the risk of any permeability loss triggered by the thermal effects and reservoir depletion.

• For further research, it is suggested to assess the degree of irreversible permeability and strain variations to which the reservoir may be subjected in elastic/plastic regimes as a means to determine the potential permeability that can be restored through an adequate reservoir exploitation strategy. The anisotropic initial stress state should also be a sensitive scenario to analyze in thermal operations.