页岩超临界CO2压裂起裂压力与裂缝形态试验研究

摘要

The clay content of shale gas reservoirs in China is high,and most shale reservoirs are located in water-deficient areas.The SC-CO2 fracturing is expected to be a new non-aqueous fracturing method because of its characteristics of low viscosity and low surface tension.In this paper,the comparison tests of SC-CO2 fracturing and hydraulic fracturing in shales and sandstones were carried out under triaxial conditions.The fracture morphology was investigated by CT scanning and the effect of temperature on fracturing was studied.The results show that the initiation pressure of SC-CO2 fracturing in shale is about 50.9％ lower than that of hydraulic fracturing,and the initiation pressure of SCCO2 fracturing in sand is about 57.1％ lower than that of hydraulic fracturing.Compared to hydraulic fracturing,the pressurization rate of SC-CO2 fracturing is slow due to the compressibility of CO2.Furthermore,the SC-CO2 fracturing in shale has repeated cracking phenomenon,and it is easier to induce complex cracks.With the increase of temperature,the initiation pressure by SC-CO2 fracturing decreases.The CT scanning shows that SC-CO2 fracturing in shale is easier to form multiple network cracks and achieves the similar effect of stimulated reservoir volume.%我国页岩气储层普遍黏土含量高,且多富集于缺水地区,超临界CO2(SC-CO2)因具有低黏度、低表面张力及对储层无伤害的特性,有望成为一种新型无水压裂方法.采用页岩露头与砂岩开展了真三轴SC-CO2与水力压裂对比试验,结合工业CT扫描分析裂缝形态,并研究了温度对起裂压力的影响.结果表明:SC-CO2压裂页岩时较水力压裂的起裂压力低约50.9％,压裂砂岩时起裂压力低约57.1％;相比水力压裂,SC-CO2压裂升压过程中,由于CO2的压缩性,增压速率较慢,由于页岩本身的层理特征,页岩SC-CO2压裂有多次起裂的现象,更易形成复杂的裂缝;随着温度的升高,SC-CO2压裂起裂压力呈下降趋势;CT断面扫描显示超临界CO2压裂页岩时更容易形成多条网络化裂缝,达到类似体积压裂的效果.