Multiphysical field coupling in unconventional oil and gas reservoirs

Research in the field of oil and gas development has focused on the production of unconventional reservoirs all over the world. Unconventional oil and gas reservoirs have poor flow conditions, and the interaction of flow, stress, and temperature fields is very complex. Therefore, multiphysical field coupling is essential. The previous application of multiphysical field coupling theory has defects such as oversimplification and inadequate adaptability. Furthermore, the lack of adaptive production practices and effective development plans limits large-scale and efficient development, and there is an urgent necessity to investigate the adaptive multiphysical field coupling theory. Currently, the core rheology in fluid–solid coupling settings can often be measured by a triaxial test system under high temperature and pressure conditions combined with flow experiments. Moreover, the changes in pores and fractures can be tested by micro-CT and SEM. In addition, adsorption is considered an exothermic process, and desorption is deemed a heat-absorbing process, so the reservoir temperature decreases at the location where desorption occurs. Therefore, the production of unconventional oil and gas triggers a series of interactions. As the fluid flows into the wellbore through the fractures, the pressure drop increases the effective stress, decreasing the average pore radius and altering the inherent permeability. Moreover, the change of pressure causes a variation in the micro-flow effect, significantly impacting the apparent permeability, and the heat variation during desorption and adsorption also changes the flow condition as well as the physical properties of the fluid. As a result, these physical fields are closely related. A series of fully coupled partial differential equations are necessary to define the production process by modeling the dynamic porosity and permeability in various flow sectors to distinguish the interactions between different zones and physical fields. These complex interactions generally need to be solved by numerical methods. Thus, a simulator is needed that satisfies the accuracy requirements to match the actual situation. Moreover, adaptability to the decoupling process and acceptable speed requires research for high-performance computing solutions that can perform distributed or cloud computing for a large-scale unconventional reservoir simulation. Future research is necessary for laboratory measurements under realistic stress and temperature environmental conditions of the formation and hydrocarbon adsorption experiments. There should be further understanding of scientific issues such as the plastic strain of the porous rocks, changing stress environment after refracturing, and mixed hydrocarbon transport models with varying stress and temperature. This article further clarifies the dynamics and determines effective production methods of unconventional reservoirs in China to promote the development of flow mechanics.