Study On In-Situ Stress Evolution and Infill Well Fractures Propagation in Unconventional Reservoir

Abstract

Abstract Fluid transport and rock deformation are the core scientific issues in oil and gas development. The development of natural fractures in unconventional reservoirs, the variety of fluid flow mechanisms in the formation, and the non-homogeneity and anisotropy of rock mechanics parameters make the coupling of seepage and geomechanics in unconventional reservoirs exceptionally complex. During the production process, the reservoir around the wellbore generates different degrees of pressure drop, which disturbs the original ground stress in the pressure drop area, and the reservoir stress evolves with the extraction time, i.e., four-dimensional dynamic ground stress. Accurately predicting the reservoir's four-dimensional dynamic ground stress field is a prerequisite for the design of fracturing and repetitive fracturing of infill wells. Therefore, this paper systematically summarizes the numerical simulation methods of oil and gas reservoir seepage - geomechanical coupling and fracture extension of infill wells and discusses the progress of multi-field coupling simulation and the latest research results in depth. Currently, there are various models of oil and gas reservoir seepage - geomechanical coupling, which can be classified as full coupling, sequential coupling, one-way coupling, and proposed coupling according to the form of coupling solution. Due to the complex geological characteristics of the reservoir, the current four-dimensional stress evolution model has been improved based on the traditional model, which is mainly a continuous medium model and a discrete fracture model based on the full coupling method, as well as an iterative coupling model. The ground stress decreases with the decrease of pore pressure during the development process, and the stress direction will be deflected. Fractures can affect the reservoir stress distribution pattern and trend compared to continuous media. This evolution of the ground stress state will cause the fracture extension of the infill wells to deflect and produce the "Frac-hit" phenomenon and cause the "microseismic barrier" effect. The study of reservoir seepage - geomechanical coupling and fracture expansion in unconventional gas reservoir development is a multi-physical field, multi-dimensional, and multi-scale coupling problem. In this paper, it is suggested to study the integrated geological-engineering solution and carry out research on the mechanism of complex fracture expansion during repeated fracturing of horizontal wells and fracturing of infill wells under four-dimensional ground stress evolution conditions to provide theoretical support for the sustainable and efficient development of unconventional oil and gas reservoirs in China.