砂砾岩循环注液水力裂缝扩展规律研究

Study on the propagation law of hydraulic fractures by cyclic injection in glutenite

循环注液作为一种新兴的水力压裂技术,在砂砾岩储层中应用前景广阔。为明确循环注液条件下砂砾岩水力裂缝扩展规律,基于真三轴水力压裂系统,对比砂砾岩在不同应力条件下的连续注液和循环注液后的水力裂缝扩展特征。研究结果表明:应力差和砂砾岩的结构特征是裂缝扩展的主要控制因素,循环注液能够显著提高砂砾岩压裂改造效果。连续注液时,随应力差的降低,裂缝扩展主控因素由应力差转为砾石和天然裂缝,易产生散射状多裂缝。起裂前循环注液对改造效果的提升存在局限性,仅能在低应力差条件下(6 MPa)降低砂砾岩的破裂压力,降低幅度约19%;起裂后循环注液对裂缝面施加的循环载荷,促进了砂砾岩的非均匀变形,强化了砾岩结构对裂缝扩展的影响,促使形成多分支裂缝。同时,起裂后循环注液可突破地应力条件限制,在高应力差(10 MPa)条件下仍能促进砂砾岩中多裂缝扩展。

As an emerging hydraulic fracturing technology,cyclic injection has a promising application in glutenite reservoirs.Based on the true triaxial hydraulic fracturing system,the hydraulic fracturing experiments by continuous and cyclic injection are carried out under different stress conditions.By comparing the propagation characteristics,the propagation law of hydraulic fractures in glutenite under cyclic injection is clarified.The results show that the fracture propagation in glutenite is mainly controlled by two factors,including the stress difference and the structural characteristics,and the fracturing effect in glutenite can be improved significantly by cyclic injection.Under continuous injection,with the decrease of stress difference,the main controlling factor of fracture propagation will be changed from stress difference to gravel and natural fractures,and the glutenite will tend to produce scattered multiple fractures.There are limitations in enhancing the fracturing effect by pre-breakdown cyclic injection.Only under low stress difference(6 MPa),the breakdown pressure of glutenite can be decreased by 19%.By applying cyclic load to the fracture surface,the non-uniform deformation of the glutenite will be promoted by post-breakdown cyclic injection,and the influence of glutenite structure on the fracture propagation will be strengthened,thus the formation of multi-branch fractures will be promoted as well.At the same time,the limit of in-situ stress conditions can be broken by post-breakdown cyclic injection,the multi-fracture propagation in glutenite can still be formed under the condition of high stress difference(10 MPa).