Permeability Damage Mechanism and Evolution of Gas-Bearing Coal Seams Induced by Drilling Fluid

Gas drainage is critical for underground coal mining in coal seams that contain high coalbed methane. We propose an underground pressurized drilling method to overcome the difficulty of drilling drainage hole along such coal seams, whereby water-based drilling fluid is used instead of pure water or air to clean the borehole. However, drilling fluid leakage can cause a serious reduction in coal seam permeability and correspondingly changing its gas production characteristics. Therefore, it is important to clarify the damage characteristic for estimating the gas production capacity of drainage hole along such coal seams. In this paper, the permeability damage characteristics of contaminated coal sample were studied using a core flow test method. Results confirmed that the permeability of the contaminated coal samples decreased significantly because of water-sensitive damage, water locking, and bentonite and polymer molecule plugging. In addition, the longer infiltration time and higher effective stress were harmful to permeability. Because higher gas pressure more likely opens the blocked pores and fractures, the permeability of the contaminated coal sample increased significantly with the improvement of inlet gas pressure. Meanwhile, the intrusion of water and solid phase into coal sample decreased the influence of stress on its permeability. This study has theoretical and practical significance in supporting the reformation of the underground pressurized drilling methods and drilling fluid design.