Three-Dimensional Full-Field and Pad Geomechanics Modeling Assists Effective Shale Gas Field Development, Sichuan Basin, China

Abstract A shale gas field at the southern edge of the Sichuan basin, China, started its oilfield development plan (ODP) in early 2014. The first wells drilled in this field and its adjacent blocks experienced significant challenges, such as severe mud losses, stuck tools, losses in the hole, high treating pressure, and unexpected screenout. Because it is vital to have accurate understanding of geomechanics and its roles at various scales, three-dimensional (3D) full-field and pad geomechanics models were developed for achieving both efficiency and effectiveness during the ODP. The work is based on high-resolution structural, geological, reservoir property, and multiscale natural fracture models. An extensive characterization of mechanical properties was conducted by the evaluation of cores, well logs, and seismic data. A systematic approach was implemented to build a 3D pore pressure model of the field. Finally, an advanced finite element simulator was used to compute 3D stress distribution, which fully owns all features and local changes of structural, geological, mechanical, and reservoir properties, and multiscale natural fracture models. The large model (80×80-m cell) covers the full field, and the pad model (20×20-m cell) covers a 15- to 20-km2 area. All have 0.5-m vertical resolution of the targeted sweet section to capture vertical heterogeneities measured from logs. Large-scale parallel computing technology was used to perform such massive geomechanical modeling. The models were calibrated or constrained by all available data such as mud logs, cores, borehole images, drilling data, prefracturing injection tests, hydraulic fracturing responses, microseismic events, and flowback data. All models were updated continuously when new data became available. The computed stress models match the highly compressive background and current understanding of the dominant tectonic movements of the Sichuan basin. They are sufficient to reveal orientations, magnitudes, anisotropies, and heterogeneities of in-situ stresses. Large variations of in-situ stresses can be quantified among pads and wells and along laterals. Such variations correspond to or align with changes in texture and composition at various scales, such as faults and complex multiscale natural fractures. The full-field model was used to optimize pad and well locations and well trajectories and assess geological integrity, resources in place, and instability of natural fractures. The high-resolution pad models were used for near-wellbore stability analysis, real-time drilling management, engineering hydraulic fracturing design and monitoring, and integrated post-fracturing analysis. The implemented approach was proven to be effectively integrated into the progress of drilling and completion. This is the first time such 3D geomechanics models have been built for China's shale gas development. The knowledge and experience gathered can certainly benefit other similar projects.