ABSTRACT:

During production, pore pressure decreases as reservoir becomes increasingly depleted. Because of reduction in pore pressure, the far-field stresses change. Consequently, formation fracture pressure decreases and therefore the safe mud window becomes narrower. Therefore, it is of paramount importance to predict horizontal in-situ stress changes during production and injection. Most often, it is assumed that a reservoir deforms uniaxially and the overburden stress is constant during production and injection. However, measured stress paths in the lab under these conditions have large discrepancies with field stress measurements from fracturing tests. These discrepancies might be due to several factors such as scale effects, damaged cores, stress arching, and faulting contributions. However, we identified that another controlling parameter is the dominant boundary conditions in the reservoir. A test matrix is designed and several laboratory experiments are conducted on Caslegate sandstone specimens using a triaxial facility. The depletion and injection tests were performed under uniaxial-strain, plane-strain, generalized-plane-stress, and unconstrained conditions. According to the results, stress paths during pressure drawdown and build-up stages are different and a permanent stress change is observed during production and injection cycles in all boundary conditions. Interestingly, the permanent stress change is highest for those BC’s where the lateral deformation is permissible (e.g., plane strain, generalized plane stress, and unconstrained conditions). In fact, the stress changes during production are irrecoverable upon repressurization for these cases. According to results, depletion-induced horizontal stress changes under plane strain and unconstrained conditions are lower than generalized-plane-stress assumptions. The highest change in horizontal in-situ stress is for the case of uniaxial strain condition.

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