Changes in permeability due to changes in the state of stress in the reservoir during depletion arise because of the strain that results from the stress changes. This influences permeability around the producing well, as well as permeability throughout the depleted reservoir that affects future wells. Various models exist for coupling stress, strain, and permeability. The simplest models relate changes in stress relative to some reference value to changes in permeability. More complex models invoke the Kozeny equation in accounting for changes in porosity, tortuosity, and surface area. Previous work formulated a model that relates changes in permeability to changes in tortuosity due to grain rearrangement during consolidation that may be applied to any stress path with appropriate stress-strain coupling. In some cases, the hoop stress concentrations around a wellbore may cause the reservoir rock to fail in shear. This can cause either an increase or a decrease in permeability depending on if the failure is dilatant or compactive. A decrease in pore pressure should lead to compactive failure so we will assign a Mohr-Coulomb fracture criterion to the rock and model strain in the failed rock.
(above) Schematic figure of question to be answered. (a) A single well produces from a reservoir, causing pressure drawdown and changes in stress, strain, and permeability. (b) When a new well is drilled, the near wellbore stresses and permeability are affected by the alteration in rock properties as a result of the pressure drawdown. These effects will be addressed in three dimensions with a coupled poromechanical model.
PI: Hugh Daigle
Co-PI: K. E. Gray
PhD Candidate: to be recruited