基于离散元法的压裂裂缝特征研究

Fracture characteristics based on discrete element method

为了探究弱面发育页岩压裂裂缝特征,基于三维块体离散元方法,建立考虑层理弱面和天然裂缝弱面的页岩储集层压裂裂缝扩展模型,分析了不同施工排量、压裂液黏度、层理抗拉强度和天然裂缝内聚力下的压裂裂缝特征。研究表明:高排量泵注和高压裂液黏度能够减少近井筒层理对水力裂缝的限制,增加水力裂缝穿层能力,当压裂液黏度达到10 mPa·s时,水力裂缝能够连续穿过6条层理;与天然裂缝连通的层理,其抗拉强度不是影响自身开启的主要因素;天然裂缝内聚力越大,其抗剪强度越大,开启程度越低。当井筒周围发育层理和天然裂缝时,通过提高前置液阶段排量、增大压裂液黏度,可以促使水力裂缝充分延伸;对于容易形成简单双翼裂缝的页岩储层,在前置液阶段泵注适量酸液,可以溶解天然裂缝填充物,从而达到降低天然裂缝内聚力,增加其开启程度,提高裂缝复杂度的效果。

In order to explore the characteristics of fracturing fractures of shale with weak plane development,a fracture propagation model of shale reservoirs taking the weak plane of bedding and natural fractures into account is established by the three dimension discrete element method to analyze characteristics of fracturing fractures under different injection rates,fracturing fluid viscosity,bedding tensile strength and natural fracture cohesion.The research results show that the high-displacement injection and high fracturing fluid viscosity can reduce the restriction of near-wellbore bedding on hydraulic fractures and increase the ability of hydraulic fractures to penetrate layers.The hydraulic fractures can continuously pass through six beddings when the fracturing fluid viscosity is increased to 10 mPa·s.The tensile strength of the bedding connected to the natural fracture is not the main factor affecting its own opening.The greater the natural fracture cohesion is,the greater the natural fracture shear strength and the lower the degree of opening of natural fracture will be.When bedding and natural fractures develop near the wellbore,the hydraulic fractures can be fully extended by increasing the injection rates and the fracturing fluid viscosity in the early stage.For shale which is easy to form simple double-wing fractures,pumping an appropriate amount of acid in the early stage can dissolve the natural fracture filler,so as to reduce the natural fracture cohesion,increase its opening degree,and improve the complexity of fractures.