Fracturing fluid retention in shale gas reservoirs:mechanisms and functions

Abstract

Shale gas reservoirs are characterized by nano-Darcy permeability, rich organic matter, ultralow water saturation, developed bedding, and micro-fractures, leading to multi-scale transportation of shale gas. Multistage fracturing of horizontal wells is the main development technology to stimulate shale gas reservoirs, and tens of thousands of cubic meters of fluids are injected into the reservoir. Retention and flowback of fracturing fluids on gas transfer plays an important role on the effect of hydraulic fracturing. Through the analysis of geological and engineering factors controlling water imbibition and diffusion in shale gas reservoirs, the mechanisms of low recovery of fracturing fluids and the reasons that some shale gas wells with low recovery of fracturing fluids have high gas production rate were revealed. This paper carried out a series of experiments of the fluid imbibition and fracture initiation in the shale selected from the Longmaxi Formation in Sichuan basin. In detail, this paper investigated imbibition and dispersion the fluid in shale and analyzed the mechanisms of the shale-water interactions causing the reduction of shale strength and fracture initiation and propagation. Based on the field case studies of well shut-in after fracturing, its influence on the well production was assessed. Formation damage caused by fracturing fluid is multi-scale, and the evaluation of formation damage caused by fracturing fluids should take the influence on the gas transmission capacity into consideration, and residual fracturing fluids may have a positive effect on gas transfer. It should take the long-term stable production of shale gas well and the effect of fracturing fluids transmission on gas multi-scale transport into consideration when determining the timing and pressure difference of fracturing fluids flowback from shale gas reservoirs.