Geomechanical Challenges in Petroleum Reservoir Exploitation
Oil and gas exploitation involves geosciences, transport sciences, thermodynamics and geomechanics. Geomechanics has become increasingly important because more and more new projects involve viscous or immobile oils, higher temperatures, pressures and depths, and reservoir materials that are weak, intensely fractured, or highly compressible. For example, of an initial liquid petroleum resource base of about 14‐15 Tb (USGS), about 9.5‐10 Tb are viscous oils (>100 cP in situ); of this viscous oil, ≈75% is found in high‐porosity weak sandstones, and 15% in naturally fractured carbonates. Thermal viscous oil exploitation planning requires better geomechanics understanding so that better mathematical simulations and predictions can be made, so that problems can be anticipated and avoided, and so that high recovery processes with reduced environmental impact can be implemented.
The two major reservoir litho types are carbonates and sandstones, and the two materials presenting the largest challenges are unconsolidated sandstones (UCSS) and naturally fractured carbonate reservoirs (NFCR). Including conventional oil, over 70% of the world’s liquid petroleum reservoirs are found in these two challenging materials. Other materials that present unique challenges include high porosity materials such as North Sea Chalk and California Diatomite, rocks such as intensely fractured granites or vesicular basalts, and oil shale (usually high porosity kerogenous marls). With the exception of oil shale, these additional rock types represent a small fraction of the world’s petroleum; oil shale resources are not included with liquid oil resources because the organic material is not liquid but a semisolid substance.
However, perhaps 4‐5 Tb of liquid product could eventually be recovered by in situ pyrolysis of shale using temperatures of 350‐450°C (with huge geomechanics effects). Major petroleum geomechanics areas of interest include borehole stability, hydraulic fracture stimulation, sand influx management, reservoir stress evolution, subsidence, casing shear, thermal stimulation geomechanics effects, and massive liquid and solid waste injection. Each of these processes involves at least flow‐stress coupling, many of them THM coupling, and some of them (e.g. shale stability) involve full THMC coupling. Some challenges in each of these areas are highlighted, though special emphasis is given to geomechanical understanding of thermal exploitation.