Effects of shale swelling and water-blocking on shale permeability

In the process of producing shale gas by fracturing, a large amount of water enters shale due to shale imbibition, which affects the shale permeability because of the positive effect by cracks caused by shale swelling and the negative effect by water-blocking. However, there is still a lack of quantitative description of these two effects. In this paper, permeability tests were conducted on shale samples before and after deionized water/NaCl solution imbibition. The results show that, compared with deionized water imbibition, the decrease of shale permeability after NaCl solution imbibition is stronger and more complex because of the NaCl crystals. The permeability of shale after NaCl solution imbibition is only 0.06 times of that before imbibition, while imbibition of deionized water reduces the permeability to 0.15 times of the starting value. In deionized water imbibition, the shale swelling effect causes the increase of permeability by 2.78 times, while the water-blocking effect causes an average permeability loss as high as 99.0%. Shale swelling and water-blocking effects increase the sensitivity of shale permeability to pore pressure by 1.46 times and 1.05 times respectively. NaCl crystals can also increase the sensitivity of shale permeability to pore pressure. Except for the complex influence of NaCl solution and the influence of NaCl crystals on permeability, the relationship between permeability and true effective stress can be closely fitted with an exponential function. This study is helpful to understand the effects of water and saline solution on the permeability of shale gas reservoirs in the process of hydraulic fracturing.