低渗储层砂岩渗流–应力–损伤渐裂过程的渗透特性演化研究

低渗储层采收率的准确评价是制定合理开发方案的重要理论基础,有效揭示开采过程诱发岩体渐裂的渗透性演化机理至关重要。以低渗储层砂岩为研究对象,分析不同荷载组合下岩体裂纹的发展规律,研究渗流–应力–损伤破裂过程中渗透率与裂纹状态的关联特性。试验结果表明:加载初期由于岩石内部孔隙及微裂隙的压密,渗透率减小;随着环向裂纹应变增大,岩石内部裂纹开始稳定扩展,渗透率缓慢增大,随着荷载的增大,裂纹加速扩展导致渗透率快速增大;最后断裂面发生相对滑移,岩石碎屑堵塞原有的渗流通道,渗透率下降。基于试验结果,运用理论方法研究不同荷载下的岩体损伤特征,建立损伤变量与裂纹环向变形的关联性,推导岩体渗透率与损伤变量的关系式,描述岩体渗流–应力–损伤渐裂中的渗透特性,揭示低渗储层砂岩的渗透率演化机理。其研究成果可为低渗砂岩储层开发过程的优化及产能预测提供新的研究思路和技术手段,对确保石油工业的可持续发展具有重要的实践价值。

Permeability evolution of low-permeability reservoir sandstone considering hydraulic-mechanical-damage coupling effect during gradual fracturing process

The accurate assessment of the recovery ratio is significant for drawing up a rational development scheme, so it is significant to reveal the evolution mechanism of the rock permeability considering the progressive failure induced by the reservoir development. Taking the low-permeability reservoir sandstone as the case study, the variation of the rock cracks under different loading combinations is analyzed, and the correlation between the permeability rate and the cracks considering seepage-stress-damage coupling effect is also deeply investigated. According to the results, the permeability decreases at the first stage of loading due to the progressive compaction of the rock pores and small cracks. And then, the increasing hoopstrain results in a moderate increase of permeability due to the stable development of rock cracks, subsequently the increasing load induces the cracks to spread very fast so that the permeability quickly increases. Finally, the permeability gradually decreases because the relative sliding appears in the broken planes and the corresponding rock fragments block the original fluid channels. Based on the experiments, the characteristics of rock damage are analyzed using the theoretical methods, and the relation between the damage and the hoopstrain is described to derive the formula for the permeability and damage to explain the permeability variation. The evolution mechanism of the sandstone permeability is revealed clearly. The above achievements can be an important support for the development optimization and oil capacity prediction for the low-permeability reservoir, and they are also significant for the sustainable development of oil.