A maturing UAE field has multiple stacked oil and gas reservoirs experiencing differential depletion causing unequal increases in vertical and horizontal effective stresses on reservoirs and bounding rock units. The stress variation has resulted from long-term production with some intervals experiencing a reduction of reservoir pressure of more than 2,000 psi. Moreover, reservoir compaction and pore collapse become more serious with the increasing stress acting on the rock framework—particularly upon reaching a critical pressure value. Laboratory measurements of rock compressibility under simulated in-situ stress conditions were conducted to quantify production-induced changes and evaluate pore volume and permeability reduction as a function of reservoir pressure. Furthermore, integration of lab measurements and logs (well and seismic) is driving a geomechanical model covering a broad view of the stacked reservoirs to provide appropriate pore collapse mitigation measures.
Pore collapse implications were examined by conducting geomechanical laboratory testing on representative samples—honoring rock heterogeneity through well logs and continuous core measurements and including uniaxial-strain compression (far-field compaction), triaxial compression (near-wellbore compaction), hydrostatic compression (compactant cap), and constant stress path (fixed Ko, far-field compaction). Laboratory data were combined to evaluate shear failure vs. compaction failure in q-p space (i.e., shear stress vs. effective mean stress). Reservoir depletion was monitored continuously from preproduction in-situ stress to planned abandonment conditions for 10 reservoir sections with varying porosity.
Testing conducted on the reservoir intervals under study was designed to capture all possible depletion scenarios during the potential life of the reservoir. This study thus improves the appreciation of the mechanics of rock compressibility—in connection with its strong dependence on reservoir stress path (e.g., hydrostatic compression or uniaxial strain compression) and depletion rate—for addressing problems such as rapid loss in permeability, generation of fines, surface subsidence, wellbore instability, casing deformation, and loss of caprock containment. This paper starts by outlining core analysis workflows that can adequately assess potential changes to reservoirs during depletion, highlighting a workflow for constructing a 3D geomechanical model.
The results showed that rock with porosity >25% has a propensity for accelerated compaction prior to reaching abandonment pressure. These results were then integrated with reservoir simulation models for long-term field management, which is part of an ongoing modeling effort. The integration of laboratory testing with seismic-driven geomechanical modeling helps predict the impact of production-induced stress changes on field performance. This will, therefore, aid operators in making life-of-reservoir decisions that relate to compaction mitigation and formation stimulation.