Evolution of pore structure and adsorption-desorption in oil shale formation rocks after compression

Abstract

As the extensive application of reservoir stimulation technologies such as hydraulic fracturing (including CO2 fracturing) and in-situ conversion, identifying the developing rules of the pore structure in shale formations rocks after compression becomes all the more important. Here we carry out experimental studies to reveal the evolution mechanism of pore structure of oil shale formation rocks after compression primarily via adsorption-desorption isotherms. The results show that the BET specific surface area of the oil shale rocks first increases and then decreases after the confined compression, while the second dominant peak of pore size distribution shows the opposite behaviors. The pore volume experiences the stage of decrease, increase and continuous decrease with the increasing applied stress, and at last decreases by ∼30% under 700 MPa compression. The increase of fractal dimension of shale formation rocks after compression indicates that the pore structure gets rougher and more heterogeneous. It reveals that the pore structure and pore-system of shale rock experience a very complexed evolution during the compression, including compressing, cementing, splitting and vanishing. This work provides some guidelines for the rational design of stimulation technologies to improve shale oil and gas recovery, and the carbon sequestration in deep earth formations.