砂砾岩水力压裂裂缝扩展规律的数值模拟分析

Numerical simulation of propagation of hydraulic fractures in glutenite formation

砂砾岩储层一般具有岩性和渗透性变化大、孔隙度低、连通性差、孔隙结构复杂和非均匀性严重等特点,因此,在水力压裂过程中,裂缝扩展形态难以控制,大规模改造难度大。针对国内某典型砂砾岩油藏特征,采用数值计算方法对砂砾岩压裂裂缝的扩展规律进行了研究,包括地应力场、砾石含量和粒径等对裂缝扩展形态及压裂压力的影响。研究表明,砾石的存在增加了压裂裂纹扩展的复杂性,裂纹主要有止裂、偏转、穿透和吸附4种表现模式,但主应力差严格控制着裂纹的走向,随着主应力差的增大,裂纹由总体绕砾扩展转变为总体穿砾扩展,失稳压力随着主应力差的增大而明显减小;砾石含量的多少体现了砂砾岩试样宏观的非均匀性,含量越高均匀性越差,随着砾石含量的提高,裂纹与砾石的相互作用占据主导地位,失稳压力随砾石含量的增加而增大;当砾石体积含量一定时,砾石粒径对压裂压力的影响主要取决于砾石排列的随机性,失稳压力随砾石粒径的增大而略有增大。

Great variability of lithology and permeability, low porosity, poor connectivity, complex pore structure and serious heterogeneity are the common characteristics of glutenite reservoir. Therefore it is difficult to control crack propagation form of hydraulic fractures, and undertake a large-scale reconstruction. Based on the characteristics of a typical glutenite reservoir in China, the propagating mechanism of hydraulic fractures is investigated by using numerical method. Numerical simulation results show that the hydraulic fracturing is more complex due to the existing of gravels and there are four fracturing modes： terminations, deflections, penetrations and attractions. The primary fractures are strictly dependent on the difference of the maximum and minimum stress. With the increasing of principal stress difference, fractures are selected to cut through gravels rather than propagate around gravels. Consequently the propagation of primary fractures appears discrepancy and a lower breakdown pressure is needed. The higher the gravel volume content is, the worse the heterogeneity is. With the increasing of gravel volume content, the interaction between gravels and fractures is more evident. The higher gravel volume content needs a higher breakdown pressure. For the cases with a constant gravel volume content, the fracture initiation is mainly influenced by the stochastic distributing of gravels, consequently a gently higher breakdown pressure is required for the larger gravels.