Redistribution of fracturing fluid in shales and its impact on gas transport capacity

The redistribution of retained fracturing fluid plays a key effect on shale gas transport. In this work, the fracturing fluid redistribution behavior during well shut-in is modeled through some specifically designed experiments, including fracturing fluid imbibition and retention experiments, real-time measurement of resistivity and pressure decay experiments. Results show that shale imbibition is categorized into three stages: stage I (0–2 h), where the fracturing fluid mainly stays in the microcracks greater than 15 μm; stage II (2–20 h), where the fluid enters 1–15 μm pores; stage III (20–48 h), where the fracturing fluid enters pores less than 1 μm. Moreover, the change of shale resistivity reflects the redistribution of fracturing fluid, the inflection point of which indicates the end of fracturing fluid redistribution. Results obtained by the pressure decay measurement show that the more the retained fracturing fluid is, the more serious the damage degree of permeability is. Meanwhile, the permeability recovery rate decreases with the increase of retention time. When the pores less than 1 μm are filled by fracturing fluid, the permeability damage rate is 99.09%; as well, only 1.26% of the initial permeability is recovered when the fluid redistribution is completed. Considering the disadvantages as well as the potential advantages of retained fracturing fluid, if the water saturation increases more than 74%, it is meaningless to prolong the shut-in time. This paper also provides an alternative workflow for evaluating fluid distribution and its influence on gas flow capacity in shales.