Spontaneous imbibition model for micro–nano–scale pores in shale gas reservoirs considering gas–water interaction

Abstract

Spontaneous imbibition is the main factor that reduces the fracturing efficiency and damages gas production in shale gas wells, which is regulated by the gas–water interaction. Water (fracturing fluid) is imbibed spontaneously due to the characteristics of the water–rock interface, especially in shale with micro–nano–scale pores, while imbibition is not impeded but prevented by the pore structure resistance and original free gas compression as well as the increased fraction from the desorption of adsorbed gas. To accurately predict the fluid loss in a shale gas reservoir during fracturing, the imbibition pressures of the gas–water phase were comprehensively analyzed in this study, and the increased gas pressure from the desorption of adsorbed gas was first considered with other imbibition pressures, such as gas pressure from free gas compression, displacement pressure, capillary pressure, and osmotic pressure. By substituting the gas–water phase pressures into the Lucas–Washburn equation with the fractal characteristic of the capillary bundle, an imbibition model for micro–nano-scale pores in shale gas reservoirs considering gas–water interaction is established. The proposed model is compared with the traditional imbibition models and verified through imbibition experiments, and a case application to a shale gas well in the Sichuan Basin is undertaken. The results prove that the new fractal imbibition model performs better than the other models for imbibition estimation in reservoir shale, and it can accurately predict the fluid loss during the hydraulic fracturing process for shale gas wells.