深层油气藏水力压裂裂缝扩展数值模拟

Numerical simulation of hydraulic fracture propagation in deep reservior

深层油气藏水力压裂过程中的岩石变形,不仅要考虑弹性变形还要考虑塑性变形的问题,以往的裂缝扩展数值模拟都没有考虑塑性变形.文章基于Drucker-Prager屈服准则和相关联的流动法则建立了弹塑性水力裂缝扩展的数学模型.模型使用润滑理论描述裂缝内流体流动,耦合岩体变形,采用内聚力模型对裂缝扩展过程进行描述.基于有限元方法编制了数值求解器,通过与经典模型计算结果比较,验证了数学模型和算法的正确性.研究结果表明:相比于弹性水力裂缝,在流体注入量相同时,弹塑性水力裂缝长度短,裂缝宽度大,扩展时需要更高的流体压力.由于塑性应变的存在,缝内流体返排后,弹塑性水力裂缝不会完全闭合.随着岩石摩擦角的增大,弹塑性水力裂缝的宽度减小,所需流体压力增大.相对于理想弹塑性,硬化岩石介质中的水力裂缝宽度较小,所需流体压力较大.

To investigate the large-scale yielding of rock in the process of the hydraulic fracturing in deep reservoir, the mathematical model of elasto-plastic hydraulic fracture propagation was established. Rock deformation was modeled with the Drucker-Prager yield criterion with associative flow rule. Fluid flow in the fracture was modeled by the lubrication theory. The cohesive zone model was used as the fracture propagation criterion. The problem was modeled numerically with the finite element method to obtain the solution. The correctness of mathematical model and algorithm were verified by comParing its results with the classic model. Research results showed that the created elasto-plastic hydraulic fracture was much shorter and wider than the elastic hydraulic fracture of the same volume of injecting fluid, and higher pressure was needed to propagate an elasto-plastic hydraulic fracture than an elastic hydraulic fracture. Elasto-plastic hydraulic fracture remained open after the applied load was released as a result of the presence of plastic strain. With the increase of the friction angle of the rock, the width of the elasto-plastic hydraulic fractures decreased and the required fluid pressure increased. The hydraulic fracture in the hardened rock was narrower than that in the ideal elasto-plastic rock, and higher pressure was needed to propagate a hydraulic fracture in the hardened rock than in the ideal elasto-plastic rock.