Research progress on 4-dimensional stress evolution and complex fracture propagation of infill wells in shale gas reservoirs

Abstract

Fluid migration and rock deformation occur throughout oil and gas development, and they are the core scientific problems. The coupling of flow and geomechanics in shale gas reservoirs is extremely complicated due to natural fractures, complex flow mechanisms and the heterogeneity and anisotropy of rock mechanical parameters. Because of the pressure drop in the shale gas reservoir near the wellbores during production, in-situ stress is disturbed and changed over time, that is, 4D stress evolution. Accurate prediction of stress evolution of a shale gas reservoir is the prerequisite of optimal design of parent well re-fracturing and infill well fracturing. In this paper, research progresses and results of simulation methods of flow and geomechanical coupling and fracture propagation are reviewed, especially in shale gas reservoirs. At present, there are various flow and geomechanical coupled models of oil and gas reservoirs. According to the types of coupling solutions, these can be classified as a fully coupled approach, iteratively coupled approach, partial coupled approach and quasi-coupled approach. Complex coupling calculation can be realized by combining one or more software algorithms, but there are some differences in the calculation timeliness and applicability of various calculation methods. Due to the complex geological characteristics of shale gas reservoirs, the current four-dimensional stress evolution models have been improved on the basis of traditional models, which are mainly continuous medium models and discrete fracture models based on the full coupled approach, as well as iterative coupling models. In the process of shale gas development, as pore pressure decreases, the magnitude of three principal stresses decreases as well, and the stress direction will be deflected. Compared to a continuous medium, fractures affect the stress distribution and change trends. This stress state evolution will cause deflection of hydraulic fracture propagation of infill wells and Frac-hits, and induce a “Microseismic Events Barrier” effect. The study of flow and geomechanical coupling in a shale gas reservoir and hydraulic fracture propagation during shale gas field development is a multi-physical, multi-dimensional and multi-scale coupling problem, which needs to explore the integrated geological and engineering solutions. Therefore, further research into the mechanism and simulation methods of complex fracture propagation during re-fracturing of horizontal wells and hydraulic fracturing of infill wells in shale gas reservoirs during stress evolution should be continued. And we suggest to focus on other research, such as the mechanism of spatial interference of complex fractures during the three-dimensional development of a shale gas reservoir, the optimization of fracturing timing in re-fracturing of parent wells and hydraulic fracturing of infill wells, and the mechanism of casing damage in horizontal wells during hydraulic fracturing. These are of great significance to the efficient development of shale gas reservoirs in China.