深层页岩气压裂多裂缝的竞争起裂及扩展

The deep shale gas resources in China are huge. With the properties of high strength, plasticity and complex geological environment, deep shale is faced with problems such as high initiation pressure, hard sand injection and limited stimulated volume during staged multi-cluster fracturing which proved to be an effective method in shallow to mid-deep shale. The key scientific issues involved in plastic breaking and the interaction of bedding/natural fractures need to be solved urgently. This project investigates the deep shale multi-fracture competitive initiation and propagation by the combination of physical and numerical simulation based on the geological characteristics of deep shale. Reveals the deep shale rock strength characteristics and fail mechanism by conducting brittle-ductile transition and fracturing experiments with downhole cores and outcrops. According to the experiments results, establishes deep shale perforation hole competitive initiation model by considering the rock plasticity, seepage-stress damage and multi-fracture flow dynamic distribution and investigates the initiation pressure and location. Reveals the deep shale horizontal well multi-fracture competitive propagation mechanism by establishing phase field fracture propagation model considering deep shale rock mechanics and geological characteristics based on the initiation results by utilizing fracturing mechanics, fluid mechanics and fluid-solid coupling. Preliminarily generates the deep shale perforating and fracturing parameters optimization technic which enhances the fracture network, improves the hydraulic fracturing performance deep shale of deep shale horizontal wells in China.

我国深层页岩气资源巨大，深层页岩强度高、塑性大以及地质环境复杂等特征导致在中深层页岩取得良好效果的分段多簇压裂出现起裂压力高、加砂困难、改造体积小等现象，其中涉及的塑性破裂和层理/天然裂缝交互扩展等关键科学问题亟待解决。本课题基于深层页岩地质特征，采用物理与数值模拟相结合的方法，研究深层页岩气压裂多裂缝竞争起裂和扩展机理。采用井下岩心或露头，开展深层页岩脆性-延性转变特征及压裂物模实验，揭示深层页岩强度特征和破坏机理；根据实验结果，考虑岩石塑性、渗流-应力损伤等特征，耦合多裂缝流量动态分配，建立深层页岩压裂孔眼竞争起裂模型，研究簇内起裂压力和位置；基于起裂结果，利用断裂力学、流体力学以及流固耦合等理论，建立考虑深层页岩力学和地质特性的相场法复杂裂缝扩展模型，揭示深层页岩水平井多裂缝竞争扩展机理。形成促使深层页岩形成复杂裂缝网络的压裂参数优化技术，提高我国深层页岩气水平井压裂效果。

项目紧密围绕深层页岩气压裂多裂缝竞争起裂及扩展基础理论开展了深入研究。充分利用实验与理论相结合的手段，考虑深层页岩原地应力、井眼诱导应力和水平井井眼轨迹等，建立了耦合岩石力学、套管/水泥环力学特征、流体扰动等综合影响下的水平井井周应力预测模型、渗流-应力-损伤破裂压力预测模型和基于变分原理的扩散型裂缝扩展模型；基于岩石力学和真三轴深层页岩破裂实验结果，揭示了深层页岩脆性-延性转变特征、破裂形态和水力裂缝与天然裂缝动态交互延伸机制；建立了不同应力、逼近角下水力裂缝张开天然裂缝、穿过天然裂缝、被天然裂缝阻断的破坏准则；实现了任意井斜角、井筒方位角、射孔方位角、施工排量、压裂液粘度下破裂压力预测和裂缝竞争起裂扩展路径的自适应求解；明确了簇内孔眼起裂位置、孔数、施工排量和簇间距对裂缝转向、均衡扩展程度的影响，揭示了深层页岩多簇裂缝竞争起裂扩展机理。项目研究成果发表论文19篇，其中SCI收录12篇、EI收录2篇、核心期刊收录2篇；授权发明专利33项，其中美国发明专利3件、中国发明专利30件；参加学术会议11次；项目负责人获省部一等奖2项、中国专利优秀奖1项，获孙越崎能源大奖、天府杰出科学家等荣誉称号；项目组成员获省学术和技术带头人及后备人选、中石油创新基金资助等；主要研究成员1人晋升教授，1人晋升副教授；培养毕业博士研究生3人，硕士研究生10人，在站博士后2人，在读博士19人；培养的研究生获国家级学科竞赛一等奖、创新创业大赛金奖等14人次，国家奖学金、优秀研究生、优秀学生干部等15人次。本项目研究成果在海相深层页岩气得到了直接应用，其中在威远、涪陵深层页岩气现场应用后，平均单井产能提高了63.1%、30%。项目研究成果发展了页岩气水平井压裂形成复杂缝网的改造理论，为形成深层页岩气藏储层高效改造技术奠定了理论基础。