Effect of aqueous phase trapping in shale matrix on methane sorption and diffusion capacity

Abstract

Large amounts of fracturing fluid could not flow back, inducing aqueous phase trapping (APT) in shale matrix, which restricts the desorption and diffusion of adsorbed gas severely. In this work, the changes of methane sorption and diffusion capacity after the APT were measured. Then the evaluation indexes of APT based on shale gas sorption and diffusion were proposed and the relevant mechanisms were analyzed. Results show that, before the APT, the methane adsorption quantity ranged from 1.6 cm3/g to 2.6 cm3/g and the desorption quantity ranged from 1.0 cm3/g to 1.6 cm3/g, while the APT reduced the adsorption quantity by 44.04% and the desorption quantity by 54.21%. Meanwhile, the retention effect of methane adsorption became more significant after the APT. For the methane diffusion before the APT, the diffusion coefficients in pores with diameter larger than 10 nm ranged from 4 × 10−12 cm2/s to 5 × 10−12 cm2/s, and the diffusion coefficients in pores with diameter smaller than 10 nm ranged from 4 × 10−17 cm2/s to 7 × 10−17 cm2/s. The APT reduced methane diffusion capacity by 57.84% in pores with diameter larger than 10 nm, and reduced the diffusion capacity by 57.42% in pores with diameters smaller than 10 nm. The fracturing fluid is easy to enter but difficult to flow back in shale matrix. The short-term retention of fracturing fluid might have certain benefits, such as displacing methane and even inducing cracks due to the hydration of clay minerals, but it is not conducive to supplying gas from matrix to fracture network in the long run.