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.