Near-wellbore permeability alteration in depleted, anisotropic reservoirs

Pore pressure depletion from hydrocarbon production causes an increase in effective stress in the reservoir and can result in significant compaction. The change in stress state with depletion induces a change in permeability both in the far-field and the near wellbore region. Modeling the depletion induced permeability alteration is crucial in forecasting hydrocarbon production and designing a drilling strategy. Porosity-based permeability models are typically used in which the permeability changes are assumed to be isotropic. However, most rocks are anisotropic, and the stress changes due to depletion and drilling a well are also anisotropic. As an example, we compared isotropic and anisotropic permeability predictions around a hypothetical well drilled in the Tor formation in Valhall field in the North Sea. The results show that isotropic permeability models tend to underestimate the degree of permeability reduction in the near-wellbore region, but even when anisotropy is considered, the permeability in the radial direction still only decreases by about 10%. These results may still need to be considered when designing a development strategy in depleted reservoirs.