含煤系产层组多气合采水力裂缝穿层致裂机理研究

The accumulated gas production and production cycle would be significantly increased by exploiting the unconventional gas resources of coalbed methane and tight gas jointly in coal measure strata (tight sandstone-coal Interbedding reservoir). However, difficulties of insufficient interaction between sandstone-coal Interbed, sand blocking as well as limited main fracture propagation might be encountered during simulation process due to the clouds on mechanisms of nonplanar propagation and penetration of fractures networks. In which case, experiments considering formation rock sample deformation and fracturing needed to be conducted to understand formation fracturing mechanism and establish mechanics constitutive relations as well as the type I, II and III fracture toughness models with fluid-solid coupling method. Mathematical model of the hydraulic fracture distortion in the sand-coal lithologic transition zone and criterion of the distortion propagation of the hydraulic fractures in the 3D space were also studied. A control theory of the non-planar multi-fractures distortion propagation was thus established. Through true triaxial fracturing experiment of sand-coal interbed, the interaction mechanism between non-plane propagation of hydraulic fractures and cementing property of layer interface is revealed, which can be used to solve the problem of uncertainty of propagation orientation during fracture interlayer penetration. A theory and an operable engineering method were thus formed to solve the problem when fracturing before jointly exploitation of coalbed methane and tight gas on the eastern margin of Ordos.

含煤系产层组（砂-煤）赋存的致密砂岩气和煤层气合采可以提高累计产量，延长生产周期，但在压裂改造砂-煤组合层时常会遇到起裂时裂缝难以沟通砂-煤互层、容易砂堵、主裂缝扩展不充分等问题，其核心问题是在地应力空间中对产层组压裂产生的非平面多裂缝起裂及穿层扩展机理认识不清。因此有必要开展产层组岩石变形破坏和断裂实验，探索流-固耦合条件下产层组损伤断裂机理及规律，建立产层组力学本构关系和I、II与III型断裂韧性模型。研究水力裂缝在砂-煤岩性过渡带中扭曲扩展的数学模型，提出三维空间水力裂缝穿层扭曲扩展的判别准则，形成非平面多裂缝扭曲扩展控制理论，通过砂-煤组合的真三轴压裂物模实验验证，揭示水力裂缝非平面扩展规律与层间胶结面的交互作用机理，解决穿层致裂时裂缝转向、扭曲并同时与层间胶结面相互作用的理论难题，形成产层组压裂裂缝可控的理论和可行性方法，解决制约鄂尔多斯东缘多气合采压裂时的瓶颈理论和工程难题。

含煤系产层组（砂-煤）在压裂改造时常会遇到起裂时裂缝难以沟通砂-煤互层、容易砂堵、主裂缝扩展不充分等问题。针对含煤系产层组不同砂岩-煤岩及砂-煤过渡带岩石结构的力学特性和断裂特征，采用GCTS-RTR-1500高温高压岩石力学试验系统开展砂-煤岩石组合体的变形破坏力学评价实验，明确了含煤系产层组不同岩性的力学特性、变形破坏与断裂特征，揭示了产层组岩石非线性变形破坏及各向异性特征；采用真三轴压裂实验模拟系统，明确了不同因素与裂缝起裂和非平面扩展形态之间的关联性，掌握了水力裂缝逼近、穿透以及在过渡带岩性内扩展过程中交界面应力变化规律。通过建立大斜度井近井水力裂缝转向扩展的数学评价模型，对不同起裂角及缝内压力条件下，裂缝的转向距离及空间转向形态进行分析，揭示了缝内净压力、起裂角对裂缝形态的影响规律，明确了垂向应力差异、过渡区各向异性及压裂液注入速率等对裂缝缝尖扩展行为的影响；基于流固耦合理论，建立了含有过渡带的产层组水力裂缝三维有限元模型，定量分析了分层段地应力场、层间应力差、压裂液排量和粘度等因素对水力裂缝垂向穿层扩展的影响，揭示了水力裂缝在过渡带的非平面穿层扩展及复杂裂缝体的形成机制。基于离散元数值模拟方法研究了水力裂缝垂向穿透层间界面的判别准则，分析了产层倾角、地应力状态、压裂液排量和压裂液粘度等参数对水力裂缝穿层致裂的影响，揭示了产层组水力裂缝穿层致裂机制，形成产层组压裂裂缝可控的理论和可行性方法，解决制约鄂尔多斯东缘多气合采压裂时的瓶颈理论和工程难题。