多岩性组合层状储层一体化压裂裂缝扩展试验研究

Experimental research on hydraulic fracture propagation in integrated fracturing for layered formation with multi-lithology combination

多岩性组合层状储层在纵向上交替发育多类含气产层,储层纵向岩性复杂多变、岩石力学性质及地应力条件差异大,采用纵向一体化穿层压裂改造时,水力裂缝垂向扩展形态复杂及延伸穿层规律不清。鉴于此,基于实际典型多岩性组合层状储层特征,通过室内浇筑灰岩、页岩与煤岩组合的大尺寸分层压裂试样,开展真三轴水力压裂物理模拟试验,研究垂直井与水平井2种井型条件下,多因素对水力裂缝垂向扩展行为的影响,并分析泵压曲线响应特征。试验结果表明:水力裂缝在纵向上呈现非对称、非平面扩展特征;依据水力裂缝与岩性界面不同的作用方式,具有停止、转向、分叉、穿透等多种复杂扩展模式;直井压裂时,主水力裂缝呈“十”字型,水力裂缝先后在水平和纵向上发生转向,充分沟通煤层割理;水平井压裂时,扩展至弱结构面时易发生分叉、转向扩展;垂向应力系数越大,水力裂缝越容易穿透岩性界面而沟通相邻产层;试样内部天然裂缝系统增加了水力裂缝转向、分叉概率及水平展布范围,但亦可耗散水力能量,限制缝高延伸;水力裂缝穿透岩性界面时,泵压曲线呈现增-降-增特征,沟通煤岩割理时曲线持续小幅度波动。建议一体化穿层压裂时避免选择在天然弱面发育的页岩层与煤层实施压裂作业,适当提高压裂液排量可增加穿层扩展概率以及提高水力裂缝分叉扩展的可能性。研究结果可为认识多岩性组合层状储层一体化压裂水力裂缝形态与优化施工参数提供指导。

Different gas-bearing zones are alternately developed vertically in the layered formation with multi-lithology combination.When it is developed by integrated fracturing,the vertical morphology of hydraulic fracture is rather complicate and the extension law is unclear due to the change of rock lithology,rock mechanics properties and in-situ stress situation.Therefore a triaxial hydraulic fracturing experiment was carried out with samples,which was combined with limestone,shale and coal sheet.These sheets were made of different types of cements according to the actual mechanical characteristics of cores in coal measure shale reservoir.The law about the effect of each factor on fracture vertical propagation was studied under vertical well and horizontal well.The results were as follows:the hydraulic fracture height geometry was complex,which showed strong asymmetric propagation characteristics.There were many typical morphologies,including arresting,bypassing,branching and crossing,when hydraulic fracture propagated closely to the interface.In vertical well fracturing,the main fracture presented“+”shape and turned in horizontal and vertical direction successively,which contributed to hydraulic fracture fully communicating coal seam cleats.Under the condition of horizontal well,the fracture propagated vertically to weak planes and then branched or turned.Hydraulic fracture penetrated the lithological boundary more easily and then connected with the adjacent layer with vertical stress coefficient increasing.Complex natural fracture network increased the possibility of fracture turning and branching and horizontal fracture distribution.However,it could dissipate the hydraulic energy and limit the fracture height growth.When hydraulic fracture penetrated the interface,the pump pressure curve exhibited the process of increasing-decreasing-increasing,and the curve continued to fluctuate slightly when it propagated closely to cleats.When integrated fracturing is carried on,rocks with natural weak planes,such as coal and shale,are not suggested to be as fracturing layers.Increasing the displacement properly can increase the ability of fracture crossing lithological boundary and the possibility of fracture bifurcation.The research results are expected to provide some insights for understanding the hydraulic fracture morphology of layered formation with multi-lithology combination and guidance for optimizing con-struction parameters.