Mitigating water blockage in low⁃permeability coal seam by acid⁃based fracturing fluid

Fracturing fluid is easy to imbibe into coal matrix，which significantly increases the water saturation within the hydraulic fracture⁃coal matrix interface and severely reduces the gas permeability. Therefore，it is impor⁃ tant to remove water blockage for improving gas production in hydraulically fractured coal seams. Coal samples，rich in fracture⁃filling calcite ( The volume fraction is 4. 5%)， were collected from Liujia District of Fuxin Basin to evaluate the effect of 6% (mass fraction) acetic acid induced calcite dissolution on imbibition water removal and gas permeability recovery. The flow channels and capillary pressure alteration caused by calcite dissolution were analyzed by SEM， NMR characterization and CT imaging. The results show that: 1 The pore surfaces of original coal samples tend to be strongly hydrophilic，and thus the water is easy to invade. The storage space of coal matrix is domi⁃ nated by micro and small pores of less than 100 nm in diameter，which leads to high capillary resistance in coal matrix and is not conducive to the recovery of imbibition water. The test shows that only a small amount of water is returned under the displacement pressure difference of 0.1 and 0.5 MPa，and the saturation of retained water is greater than 90%，resulting in a more than 90% reduction of gas permeability. When the displacement pressure difference increa⁃ ses to 2.0 MPa (close to the maximum flowback pressure gradient of Fuxin Group coal seam)，the water lock damage rate of some coal samples is still greater than 60%.2 The injection of 6% acetic acid can effectively remove the frac⁃ ture blockage caused by calcite，so that the acid dissolution channels with low capillary force and high permeability can be formed in the coal core samples. Then the imbibition water in the main flow channels can be effectively returned under the displacement pressure difference of 0.1 and 0.5 MPa. The gas permeability during the imbibition water re⁃ covery reaches ( 6. 9 - 82. 3 ) × 10-15 m2 ， which is 10 - 79 times higher than that before acidizing， effectively achieving the purpose of mitigating water lockage and increasing permeability.3 With the help of numerical simulation of acid dissolution pore extension and core⁃scale seepage experiments，the acetic acid concentration in the fracturing fluid and acid dissolution time should be optimized to form a reasonable number of seepage channels with low capillary force and high permeability in the coal seam water lock damage zone， so as to prevent water lock damage within the hydraulic fracture⁃matrix interface to the maximum extent and reduce the influence of acid dissolution on coal strength and coal fines migration. For calcite⁃rich low⁃permeability coal seams，the use of acetic acid⁃modified fracturing fluid can mitigate water blockage in coal matrix without coal quality pollution， which is beneficial to the co⁃mining of coal and methane.