Effect of fluid–solid coupling on shale mechanics and seepage laws

Abstract

In this paper, the cores of outcropped black shale of Lower Silurian Longmaxi Fm in the Yibin area, Sichuan Basin, were taken as samples to investigate the effects of extraneous water on shale mechanics and seepage laws during the production of shale gas reservoirs. Firstly, the development of fractures in water saturated cores was observed by using a VHX-5000 optical superdepth microscope. Secondly, water, formation water and slick water, as well as the damage form and compression strength of water saturated/unsaturated cores were investigated by means of a uniaxial compression testing machine and a strain testing & analysis system. Finally, the effects of fluid–solid coupling on shale gas flowing performance in different water saturations were analyzed by using a DYQ-1 multi-function displacement device. Analysis on core components shows that the Longmaxi shale is a highly crushable reservoir with a high content of fragile minerals, so fracturing stimulation is suitable for it. Shale compression strength test reveals that the effects of deionized water, formation water and slick water on shale are different, so the compression strength of shale before being saturated is quite different from that after being saturated. Due to the existence of water, the compression strength of shale drops, so the shale can be fractured easily, more fractures are generated and thus its seepage capacity is improved. Experiments on shale gas seepage under different water saturations show that under the condition of fluid–solid coupling, the higher the water saturation is, the better the propagation and seepage capacity of micro-fractures in shale under the effect of pressure. To sum up, the existence of water is beneficial to fracturing stimulation of shale gas reservoirs and helps to achieve the goal of production improvement. Keywords: Shale gas, Core, Fluid–solid coupling, Water, Compression strength, Permeability, Seepage characteristic, Sichuan Basin, Yibin area, Early Silurian