页岩水平井多段分簇压裂起裂压力数值模拟

页岩储层孔隙度和渗透率极低，天然裂缝和水平层理发育，常规压裂增产措施无法满足页岩气的开发要求，水平井多段分簇压裂是页岩气开发的关键技术之一，该技术能够大幅度提升压裂改造的体积、产气量和最终采收率。为确定页岩储层水平井多段分簇射孔压裂的起裂点和起裂压力，采用有限元方法建立了水平井套管完井（考虑水泥环和套管的存在）多段分簇射孔的全三维起裂模型。数值模型的起裂压力与室内试验结果吻合较好，证明了数值模型的准确性和可靠性。利用数值模型研究了页岩水平井多段分簇射孔压裂的起裂点和起裂压力的影响因素，研究发现：射孔孔眼附近无天然裂缝或水平层理影响，起裂点发生在射孔簇孔眼的根部；射孔簇间距越小，中间射孔簇的干扰越大，可能造成中间的射孔簇无法起裂；射孔密度和孔眼长度增大，起裂压力降低；天然裂缝的存在，在某些情况能够降低起裂压力且改变起裂位置，主要与天然裂缝的分布方位及水平主应力差有关；水平层理可能会降低起裂压力，但与垂向主应力与水平最小主应力的差值有关。获得的起裂压力变化规律，可作为进一步研究水平井多段分簇射孔条件下的裂缝扩展规律的基础，可以为压裂设计和施工的射孔参数确定及优化给出具体建议。

Numerical simulation of fracturing pressure in multiple clusters staged hydraulic fracture of shale horizontal well

General fracturing techniques are extremely difficult to satisfy requirements of the development of shale gas, due to the highly low porosity and permeability and well developed natural fractures and horizontal beddings in shale reservoirs. Therefore, one of and key technologies is the multiple clusters staged hydraulic fracture of horizontal well, which can greatly increase the stimulation volume, the production rate of gas and final recovery efficiency. In order to locate the fracturing point and fracturing pressure of multiple clusters staged horizontal well of shale gas reservoir, a three-dimensional breaking model of multiple clusters staged horizontal well is established with finite element method, which considers cement mantle and casing. Comparing the simulation results with laboratory experiment of fracturing pressure, the good agreement indicates that the numerical model is reliable and validity. This paper further studies the effect of parameters on fracturing points and fracturing pressure of multiple perforation clusters within multi-stage hydraulic fracturing in shale gas horizontal well using the simulation model. The research results show that fracturing points are found near the roots of perforation clusters, when there is no influence of natural fractures and horizontal bedding plane around perforations. The smaller perforation cluster spacing is, the greater interference between perforation clusters is, which may induce that the perforations cluster in the middle is unable to fracture. Moreover, the fracturing pressure is reduced with the increase of perforation density and perforation length. The existence of natural fracture can reduce the fracturing pressure and change the fracturing initiation position under certain circumstance, mainly depending on the natural fracture distribution and orientation and horizontal principle stress difference. In addition, the bedding plane may reduce the fracturing pressure, but relating to the difference between vertical principal stress and minimum horizontal principal stress. The regularity of fracturing pressure can provide basis for further research on the fracture propagation in multiple clusters staged horizontal well of shale gas reservoir. This study also offer concrete advises on perforating parameters in hydraulic fracture design and hydraulic operation in shale gas reservoir.