Distribution and effect of retained fracturing fluid in pores and fractures of shale: Experimental and field study

Abstract

Large amounts of fracturing fluid retained in the multi-scale transport channel for shale gas after fracturing, such as the hydraulic fracture networks, natural fractures, matrix pores. Therefore, the distribution and occurrence state of retained fracturing fluids greatly affect shale gas production. The flowback rate of multi-scale pores and fractures of shale and the distribution characteristics of retained fracturing fluid in multi-scale pores and fractures of shale are revealed through the imbibition and flowback experiment of shale with variable fracture width and NMR test, the occurrence state of the retained fracturing fluid during the production process is clarified through flowback experiment with shale sample in parallel and microscopic observation, and suggestions for regulating the retained fracturing fluid under reservoir conditions are put forward through rich field data analysis. The pores with diameter larger than 100 nm are the main contribution space of the flowback fracturing fluid, which provide 74.33% of the flowback fluid. 82.96% retained fracturing fluid was retained in the mesopores (2 nm < pore diameter <50 nm), which are the main existing space for retained fracturing fluid. The main occurrence states of the retained fracturing fluid are liquid bridges, continuous water films or porphyritic water films. The suggestions for regulating the retained fracturing fluid under reservoir conditions are adding drainage aids to reduce the retention of fracturing fluid during fracturing process, reasonably determining the shut-in time to improve the flowback rate in pores and fractures of diameter >0.1 μm during shut-in process, and prolonging the duration of the dominant gas seepage channel and expanding the time of pure liquid seepage by increasing pressure difference during production process.