Research On Local In-Situ Stress Field Disturbance And Fault Stability During Water Injection In Shale Gas Reservoirs

Shale gas is an unconventional oil and gas resource with huge reserves.Due to the compactness of shale itself,hydraulic fracturing technology has become an effective technical means for shale gas exploitation.Hydraulic fracturing breaks tight shale by injecting a large amount of fracturing fluid under high pressure,the potential impact of this process on the surrounding environment（such as hydraulic fracturing induced earthquakes）has received extensive attention from society.In order to develop shale gas more safely and reasonably,optimize hydraulic fracturing construction design and reduce its impact on the surrounding environment,it is necessary to carry out research on the evolution of reservoir in-situ stress and fault stability during the water injection process of hydraulic fracturing.Relying on the engineering background of a shale gas exploitation block,this paper firstly establishes a three-dimensional geomechanical model of the shale gas reservoir in the study area;Based on the three-dimensional in-situ stress field,numerical simulation studies on the influence of local in-situ stress changes and different water injection conditions on fault stability during quasi-dynamic and dynamic water injection are carried out.The main research contents and conclusions of the paper are as follows:（1）A three-dimensional geomechanical model of a shale gas block is established,which takes into account the actual formation structure and non-uniform distribution of pore pressure field;Aiming at the problem of discontinuous distribution of rock mass at natural faults,based on the equivalent stiffness theory,the mechanical parameters and corresponding stress field variation characteristics of rock mass cut by faults are described in this paper.The effectiveness of the model is verified by comparing the calculated stress results with the field measurement and laboratory test data.（2）Based on the three-dimensional geomechanical model,the disturbance of the local in-situ stress field and the stability of adjacent faults caused by the water injection pressure and injection distance in the quasi-dynamic water injection process are simulated.The calculation results show that:the disturbance size and scope of the water injection process to the local stress field of the reservoir are limited,which will not cause large-scale instability of the fault.At the current stress level,the minimum escape distance between the hydraulic fracturing well and the fault is about 650 m,and the critical ratio between injection pressure and formation pore pressure is about 1.3.In addition,based on the failure of fault element and three-dimensional shear strain distribution,the moment magnitude response of fault area during water injection is calculated and analyzed.The results show that the induced moment magnitude of fault area is about level 1 after water injection in this block.（3）It is aimed at the uplift of reservoir conductivity after the hydraulic fracture network communicates with the natural fracture system.Based on the fluid-structure coupling model,this paper analyzes the stability of faults under different permeability conditions.The calculation results show that:when the equivalent permeability of the reservoir reaches above1.0×10^-15 m² after hydraulic fracturing,with the progress of the water injection process,the pore pressure field in the fault rupture zone and fault core adjacent to the injection well increases obviously（increased about 1.1 times）,resulting in local instability of the adjacent fault,but the overall instability of the fault does not occur.