层理特征对油页岩水力压裂裂缝扩展规律影响的数值模拟研究

Numerical simulations of the effects of bedding planes on hydraulic fracture propagation law in oil shale

目前水力压裂是油页岩储层开发的主要技术手段之一,油页岩层理特征的差异对压后裂缝形态起主要影响作用,目前研究大多聚焦在层理发育程度对裂缝扩展的影响,忽略层理厚度本身对水力裂缝扩展的影响。以鄂尔多斯盆地旬邑地区油页岩为研究对象,基于线弹性断裂力学理论,构建应力-损伤-渗流的水力压裂裂缝扩展模型,并采取全局FEM-CZM的数值模拟方法,分析层理厚度、层理间距、地应力场对水力压裂裂缝扩展的影响规律,对比不同影响因素下裂缝的破坏类型、裂缝长度和层理沟通面积。结果表明:(1)层理厚度影响层理面对水力裂缝的拦截能力,在层理厚度较大时,会导致裂缝在层理面上扩展的倾向更强,发生张拉破坏,所对应的裂缝长度和层理沟通面积更大。(2)层理间距影响水力裂缝到达层理面的时间,较小的层理间距水力裂缝会直接穿透层理面,较大的层理间距增加裂缝扩展的阻力,伴随层理间距越大,发生张拉破坏,水力裂缝长度和层理沟通面积越大。(3)地应力场决定水力裂缝扩展方向,垂向地应力差较大时,垂向应力会对层理有压实作用,导致更容易穿透层理面扩展,垂向地应力差较小时,水力裂缝在层理面扩展多发生弯曲、分支情况,所对应裂缝长度和层理沟通面积均增加。建议在压裂施工选址方面选择层理厚度较大、层理间距较大、垂向地应力场较小的区域,更有利于形成高效渗流传热通道,该研究可为旬邑地区油页岩水力压裂施工提供指导。

Hydraulic fracturing serves as a major technical means for the exploitation of oil shale reservoirs presently,and the hydraulic fracture morphology in oil shales is primarily affected by differences in the characteristics of bedding planes.However,current studies mostly focus on the influence of the developmental degree of bedding planes on fracture propagation,overlooking the effects of the bedding plane thickness itself on hydraulic fracture propagation.This study delves into the oil shale in the Xunyi area,Ordos Basin.Based on the theory of linear elastic fracture mechanics,this study developed a stress-damage-seepage model of hydraulic fracture propagation.Using the global numerical simulation method termed finite element method-cohesive zone method(FEM-CZM),this study analyzed the effects of the insitu stress field and bedding planes’thickness and spacing on hydraulic fracture propagation,followed by comparing fractures’failure type and length and the area connected by bedding planes under different influencing factors.Key findings of this study are as follows:(1)The thickness of bedding planes affects their capacity to intercept hydraulic fractures.A large thickness will lead to a strong tendency of fracture propagation along laminae planes,thus inducing tensile failure.Accordingly,long fractures and a large area connected by bedding planes will be formed.(2)The bedding plane spacing affects the time for hydraulic fractures to reach bedding planes.In the case of a small spacing,hydraulic fractures will directly penetrate bedding planes.The spacing will increase the resistance to fracture propagation.A larger bedding plane spacing is associated with longer fractures and a larger area connected by bedding planes due to tensile failure.(3)the in-situ stress field determines the direction of the hydraulic fracture propagation.In the case of a large vertical in-situ stress difference,the vertical stress will compact bedding planes.As a result,hydraulic fractures are more prone to penetrate bedding planes.When the vertical in-situ stress difference is small,hydraulic fractures will bend and branch in the process of propagation on bedding planes,leading to increased fracture length and area connected by bedding planes.Based on these findings,it is recommended that hydraulic fracturing should be performed in areas with large thicknesses and spacings of bedding planes and a small vertical stress field,which are more favorable for the formation of efficient seepage and heat transfer channels.This study can provide guidance for the hydraulic fracturing construction for oil shale in the Xunyi area.