砂砾岩液态CO2破裂机制试验研究

Experimental study of liquid CO fracturing mechanism of glutenite

砂砾岩储层一般较为致密,且非均匀性强,常规压裂方法压裂效果不理想。液态CO2(L-CO2)压裂是近年来提出的一种压裂增产手段,压裂效果上具有明显的优势。采用井底岩芯开展不同围压下清水和L-CO2压裂试验,并结合μCT扫描和核磁共振试验,比较两种压裂方式在破裂压力、破裂特征和裂缝分布的差异,深入分析砂砾岩L-CO2压裂的破裂机制。发现相同围压下L-CO2可大幅度降低破裂压力,且增大围压,L-CO2和清水压裂的破裂压力相差越大;μCT扫描显示,L-CO2压裂形成的是不规则的多分支裂缝,与主裂缝共同构成复杂裂缝网络,L-CO2压裂的裂缝体积是远大于清水压裂。核磁共振(NMR)结果发现,L-CO2压裂中破裂以贯通砾石颗粒界面微裂纹的方式为主,剪切激活机制占主导作用,而清水压裂以形成张拉的单一裂缝方式为主。L-CO2压裂获得更好的裂缝网络主要与砂砾岩中砾石颗粒造成的非均匀性有关,并影响着裂缝网络的复杂性。相关的研究成果可为砂砾岩储层改造和增产增效工艺优化提供指导。

Glutenite formations usually characterized by their dense and heterogeneity, thus the fracturing effect of conventional fracturing methods is not ideal. Liquid CO2(L-CO2) fracturing is a fracturing stimulation method proposed in recent years, and the fracturing effect has obvious advantages. Water and L-CO2 fracturing tests are conducted on downhole cores, μCT scanning and NMR tests are used to compare the difference of breakdown pressure, fracture characteristic and crack distribution and the L-CO2 fracturing mechanism in glutenite is deeply analyzed. It is found that under the same confining pressure, L-CO2 can greatly reduce the breakdown pressure, and the difference between the breakdown pressure of L-CO2 fracturing and water fracturing increases with increasing confining pressure. μCT scanning shows that the fractures induced by L-CO2 fracturing are irregular fractures, which are more likely to deflect between gravel particles in glutenite, causing a branch fractures and complex fractures networks. The fracture volume of L-CO2 fracturing is much larger than that of hydraulic fracturing. Nuclear magnetic resonance(NMR) results show that L-CO2 fracturing mainly breaks through the micro-cracks at the interface of gravel particles, and the shear activation mechanism has a significant effect, while hydraulic fracturing mainly takes the form of a single tensile fracture rupture. The better fracture network obtained by L-CO2 fracturing is mainly related to the strong heterogeneity caused by gravel particles in glutenite, which affects the complexity of induced fractures. Relevant research results can provide guidance for glutenite reservoir fracturing and process optimization for increasing production and efficiency.