深部裂缝性储层大斜度井水力裂缝非平面扩展机理研究

Hydraulic fracture in highly deviated well could communicate more pay formations and increase drainage area. However, hydraulic fracturing in deep reservoirs is faced with some problems, such as sand reservoir hard to be fractured, sand plug, fracture propagation insufficiently and natural fracture hard to be communicated. The reason for the problems is that there is no clear understanding on the interaction mechanism between non-planar hydraulic fracture in bending wellbore and natural fracture in deep reservoirs with high stress and high temperature. In this paper, the following studies have been done: a) rock deformation and fracturing experiments under high confining pressure and high temperature are conducted to study the mechanism and rule of rock fracturing under the condition of fluid-solid and chemo-mechanical coupling, and the fracturing toughness models of type I, II and III are established; b) three-dimensional mathematical model for the initiation and propagation of hydraulic fracture in highly deviated wellbore under the condition of thermo-fluid-solid coupling is studied, a numerical simulating model based on three-dimensional boundary element method is established, a criterion for hydraulic fracture re-orientation and twisting is proposed, and the theory for the control of non-linear fracture propagation is formed; c) true tri-axial fracturing experiments have been conducted to verify the theory and numerical simulation models. The studies above have revealed the interaction mechanism between non-planar hydraulic fracture and natural fracture, and solved the theory problems of hydraulic fracture reorientation, twisting and the interaction with natural fracture in deviated wellbore. At last, a feasible method for hydraulic fracture controlling in highly deviated wellbore is formed, and the theory bottleneck and engineering problems for deep reservoir stimulation in Tarim and Jidong oilfield have been solved.

采用大斜度井压裂可以沟通更多产层、增加油气泄油面积，但在深层压裂时常会遇到储层难以压开、容易砂堵、裂缝扩展不充分、难以沟通天然裂缝等问题，其核心问题是对弯曲井筒在地应力空间中非平面水力裂缝与天然裂缝的相互作用机理认识不清。开展深部储层岩石变形-断裂实验，探索高温下力学-化学、固体-流体双重耦合条件下岩石断裂机理及规律，建立深部储层岩石力学本构关系和I、II与III型断裂韧性模型；研究热-流-固耦合条件下大斜度井水力裂缝起裂及转向扩展三维数学模型，建立三维边界元数值模拟方法，提出三维空间水力裂缝转向扭曲判别准则，形成非平面裂缝转向扩展控制理论，并通过室内大型真三轴压裂模拟实验验证，揭示水力裂缝非平面扩展规律及与天然裂缝的交互作用机理，解决斜井筒水力裂缝转向、弯曲并同时与天然裂缝相互作用的理论难题，形成大斜度井压裂裂缝可控的理论和可行性方法，解决制约塔里木、冀东等深层压裂的瓶颈理论和工程难题。

深井超深井地层温度高、压力大，储层纵向上岩性交错分布，力学特征复杂，对压裂改造影响大。针对深层储层的力学特性和破坏特征进行研究，建立了温度对不同岩性和孔洞结构岩石力学参数的影响规律，形成了岩石的破坏形态与强度间的相互关系，发现高温（160℃）及25MPa围压条件下，细岩砂、泥质细砂岩以及含泥中砂岩等致密性岩石易发剪切破坏，且强度高，而高含泥岩石易发生张性破坏。结合裂缝缝尖三维应力分布，建立了斜井水力裂缝起裂及转向扩展的三维数学模型，分析了不同因素与裂缝转向形态之间的影响关系。当井眼方位与水平最大地应力的夹角大、水平应力差大、射孔相位角大、井斜角为30-60、起裂层相对较薄且裂缝起裂角与垂向夹角较大时，近井裂缝在水平及垂直方向转向，裂缝可能以非垂直状态逼近界面层，不利于穿层扩展。建立了一体化穿层压裂三维有限元分析和评价模型，当巨厚砂泥互层储层穿层压裂时，泥质夹层段缝宽剖面复杂，最小缝宽<0.5mm，易砂堵，应做好相应施工措施，同时，建议针对泥质夹层多以及层厚分布、储层地质力学分布和物性分布特点进行组合层及分层压裂设计和施工，合理设计射孔位置。以断裂力学为理论依据，对水力裂缝尖端应力场和作用在天然裂缝面上的应力场进行分析，建立了I型水力裂缝贯穿天然裂缝的判别模型；针对非平面水力裂缝进行应力场分析，建立了I-II复合型水力裂缝贯穿天然裂缝的判别模型。针对页岩储层中层理对水力裂缝扩展路径的影响，以边界元方法研究水力裂缝在层状介质中的扩展行为以及围岩应力场，评价了页岩岩层弹性参数的差异性对水力裂缝扩展角、裂尖应力强度因子和裂缝宽度的影响，建立了判别水力裂缝能否穿越层理面的力学模型。本研究建立的考虑井斜条件下高温高应力砂泥互层储层近井水力裂缝转向模型以及人工裂缝穿透天然裂缝的判别方法，为压裂方式和施工方案的优选做好前提基础，也为开发非常规页岩气资源体积压裂改造提供理论参考。