Study and develop on leak plugging fluid for CBM drilling in goaf of coal mine

The leakage of drilling fluid, block stuck, and casing resistance is a severe problem when constructing coalbed methane wells in coal mine gobs.Based on the characteristics of overburden of gob, the ideal adding amount of each additive is determined by comparing the effects of different amount of activator, suspension concentrate, and fluid loss additive on the performance of plugging fluid. An inorganic cementsolidifying drilling plugging solution is developed, the formula of which is 100% water, 5% activator (JHQ), 0.5% suspending agent (CMC), 3% fluid loss agent (G33S), and 54% slag. This drilling plugging fluid has fluidity of 21 cm at 30 ℃, thickening time within 61 min, and 1 d compressive strength of over 6 MPa. The performance parameters meet the requirements for plugging fluid used in shallow well in coal mines. To further analyze the mechanism of the plugging solution, the formed solidified body was tested using modern testing methods such as SEM, XRD, and thermal analysis. The results show that when contacting with water, hydration reaction occurred on the cementation material of plugging solution, producing C-S-H gelation which increases with time. The structure of the final hydration product becomes denser gradually, resulting in higher compressive strength. The whole process that the plugging fluid realizes plugging can be described as three steps-the plugging fluid with good fluidity is first pumped into the formation crack, filling the crack until full, then the plugging fluid gradually loses fluidity as the hydration reaction proceeds, and finally forms a cementation in the original fissure to realize plugging.