【作者】 张旭东；

【导师】 孙峰；

【作者基本信息】 中国石油大学(华东) ， 力学， 2023， 硕士

【摘要】 深层能源开发利用过程中伴随着持续的高压流体注入,如低渗油田高压注水、增强型地热系统（EGS）注水循环、碳捕集封存（CCS）CO₂注入地层等,引起地层孔隙压力变化、导致水力/天然裂缝扩展,诱发断层活动,产生地震等地质灾害。研究高压流体流动产生的近井筒破裂、裂缝起裂、扩展力学机理,具有重要的理论与工程指导意义。本文通过岩石物理实验与数值模拟方法,对比分析向低渗透砂岩注入水和液态CO₂引起的近井筒破裂特征差异,探讨地质因素（地应力、孔渗性质）与工程因素（注入介质、排量）对裂缝特征的影响,主要研究内容如下:（1）开展低渗透砂岩的岩石力学、岩石渗透特性测试。通过巴西劈裂、三轴压缩实验获取岩样的拉压强度、弹性模量、泊松比等力学参数;开展孔隙度、渗透率测试分析岩样孔渗参数特征;利用声发射仪器测试不同岩样的波速变化,为高压流体注入破裂实验及数值模拟研究提供了基础参数。（2）利用三轴高压流体致裂试验系统,开展（?）100×200 mm尺寸全直径岩样高压注水、注液态CO₂近井筒破裂特征的实验分析。综合评价了地层围压、砂岩渗透性、注入排量及注入方式等因素影响,对比注入压力曲线、裂缝形态、声发射响应等特征参数变化差异,确定了影响近井筒裂缝形态的主控因素。（3）针对传统裂缝扩展模拟存在的网格依赖性问题,将损伤演化方程中的等效应变非局部化,建立了隐式梯度型非局部损伤的流固耦合裂缝扩展模型;开发了数值模拟程序;对比“L”型脆性材料致裂实验验证了程序有效性;分析高压流体注入过程中非局部内部尺度参数及网格尺寸对水力裂缝扩展形态的影响规律,为高压流体注入裂缝扩展形态描述提供了低网格敏感性的数值模拟方法。（4）考虑岩石的非均匀性,在隐式梯度型非局部损伤流固耦合模型中,利用Weibull分布实现了岩石力学与渗流参数的非均质赋值,模拟结果与试验对比,吻合较好。研究了岩石非均质性、地应力、岩石渗透性、流体粘度、注入排量等因素对破裂压力、裂缝形态及损伤带扩展范围的影响,明确了高压流体注入对近井破裂特征的影响机理。更多还原

【Abstract】 The development and utilization of deep energy is accompanied by continuous high-pressure fluid injection,such as high-pressure water injection in low-permeability oilfields,enhanced geothermal system（EGS）water injection cycle,carbon capture and storage（CCS）CO₂injection into the formation,which will cause changes in formation pore pressure,lead to hydraulic/natural fracture expansion,induce fault activity,and produce geological disasters such as earthquakes.It is of great theoretical and engineering significance to study the mechanical mechanism of near wellbore fracture,fracture initiation and propagation caused by high pressure fluid flow.In this paper,through rock physics experiments and numerical simulation methods,the differences in near-wellbore fracture characteristics caused by injecting water and liquid CO₂into low-permeability sandstone are compared and analyzed.The effects of geological factors（in-situ stress,porosity and permeability properties）and engineering factors（injection medium,displacement）on fracture characteristics are discussed.The main research contents are as follows:（1）The rock mechanics and rock permeability characteristics of low permeability sandstone were tested.The mechanical parameters such as tensile and compressive strength,elastic modulus and Poisson’s ratio of rock samples were obtained by Brazilian splitting and triaxial compression experiments.Carry out porosity and permeability tests to analyze the characteristics of pore permeability parameters of rock samples;the acoustic emission instrument was used to test the wave velocity changes of different rock samples,which provided basic parameters for high pressure fluid injection fracture experiment and numerical simulation research.（2）Using the triaxial high-pressure fluid fracturing test system,the experimental analysis of the fracture characteristics of high-pressure water injection and liquid CO₂injection near wellbore of（?）100×200 mm full-diameter rock samples were carried out.The effects of formation confining pressure,sandstone permeability,injection displacement and injection mode were comprehensively evaluated.The main controlling factors affecting the fracture morphology near wellbore were determined by comparing the changes of characteristic parameters such as injection pressure curve,fracture morphology and acoustic emission response.（3）Aiming at the problem of mesh dependence in the process of simulating crack propagation by damage mechanics method,the equivalent strain in the damage evolution equation was non-localized,and a Hydro-Mechanical coupling crack propagation model based on implicit gradient nonlocal damage was established.A numerical simulation program for non-local damage crack propagation was developed.The effectiveness of the program was verified by comparing the cracking experiments of’L’brittle materials.The influence of nonlocal internal scale parameters and mesh size on the propagation morphology of hydraulic fractures during high-pressure fluid injection was analyzed,which provided a numerical simulation method with low mesh sensitivity for the description of fracture propagation morphology during high-pressure fluid injection.（4）Considering the heterogeneity of rock,the Weibull distribution was used to realize the heterogeneous assignment of rock mechanics and seepage parameters in the implicit gradient nonlocal damage Hydro-Mechanical coupling model.The simulation results were in good agreement with the experimental results.The effects of rock heterogeneity,in-situ stress,rock permeability,fluid viscosity and injection displacement on fracture pressure,fracture morphology and damage zone expansion range were studied,and the influence mechanism of high-pressure fluid injection on near-well fracture characteristics was clarified.更多还原