页岩油水平井多段压裂裂缝高度扩展试验

针对吉木萨尔凹陷芦草沟组页岩油储层压裂垂向改造程度低的问题,基于真三轴水力压裂模拟试验研究CO₂与胍胶复合压裂相比于常规水基和超临界CO₂压裂缝高扩展的优势。创新性建立一套针对天然页岩的水平井多段压裂模拟试验方法,并通过试样剖分、CT扫描和声发射监测等方法综合确定多段压裂裂缝形态和破裂机制。结果表明低黏度滑溜水和超临界CO₂压裂缝高受限,且超临界CO₂压裂缝高受限更严重;高黏度胍胶向层理中滤失较弱,可提高裂缝垂向扩展程度,但开启的层理较少;CO₂-胍胶复合压裂时胍胶的隔离作用可有效降低CO₂的滤失,从而促使CO₂突破层理对裂缝高度的限制,同时CO₂的高压缩性在破裂瞬间释放大量弹性能,促使层理和天然裂缝发生剪切破裂,从而形成复杂裂缝网络;提高注入排量,破裂压力升高10.1%,剪切事件比例升高4.2%,CO₂-胍胶复合压裂形成的裂缝更复杂。进而提出并论证一种适合于层理性页岩储层的CO₂-胍胶复合压裂新方法,即先采用高黏度胍胶压裂液启缝,在近井区域突破层理,然后大排量注入CO₂进一步提高储层压裂改造体积。

Experimental study on fracture height propagation during multi-stage fracturing of horizontal wells in shale oil reservoirs

In order to solve the problem of low degree vertical stimulation in shale oil reservoir of Lucaogou Formation in Jimsar Sag,the advantages of fracture height propagation using CO₂-gel hybrid fracturing over conventional water-based fracturing and supercritical CO₂ fracturing were investigated by conducting laboratory tri-axial hydraulic fracturing experiments.A multi-stage fracturing experimental method for horizontal wells with real rock samples was firstly established.The fracture geometry induced in the multi-stage fracturing was comprehensively determined by a rock sample splitting technique along with computed tomography(CT)scanning and acoustic emission monitoring.The experimental results indicate that the fracture height formed using a low viscosity"slick-water"was restricted,though the performance of the"slick water"was better than supercritical CO₂.High-viscosity gel fracturing fluids can effectively increase the fracture height propagation,but with fewer bedding planes activated.During CO₂-gel hybrid fracturing,the blocking effect of gel can effectively reduce the leak of CO₂,so that CO₂ can break through the confinement of bedding plane on the fracture height.Meanwhile,high compressibility of CO₂ can release a large amount of elastic energy during fracturing,which can promote shear failure of the bedding planes and natural fractures and generate a complex fracture network.Increasing the injection rate of CO₂ during CO₂-gel hybrid fracturing,the fracturing pressure can be increased over 10.1%,and the shear failure event can be increased by 4.2%,which can induce a more complex fracture network.Consequently,a novel CO₂-gel hybrid fracturing method for stimulating multi layered shale oil reservoirs was proposed and verified.Firstly,high-viscosity gel is injected into the wellbore to initiate fractures vertically,and then CO₂ with a large injection rate is pumped to further increase the stimulated reservoir volume.