Study on drilling and fracturing technology for deep complex structurecoal reservior in Qinshui Basin

In this paper, an efficient development technology for coalbed methane for complex coal structure in deep mines is proposed. The application of this technology was conducted on northern Shizhuang region of Qinshui basin. This technology is based on the practice of PDC bit cooperating with screw drill, low solids polymer drilling fluid, drilling with low displacement, low rotational speed, and low pump pressure in coal seam, and synchronous hydraulic fracturing in small well group during the development of directional well group with the water source drill rig. The main factors that cause coal wall to collapse when drilling deep coal seams with complex coal structure were discussed. Then the drilling and hydraulic fracturing parameters were optimized. The study shows that the main factors that cause coal wall to collapse are the drilling fluid invasion and pressure fluctuation. The drilling fluid performance in different formations and drilling parameters can be optimized when the drilling pressure is 20～30 kN, rotational speed is 25～30 r/min, and displacement is 16～18 L/s. The effective fracture extension can be formed by using synchronous hydraulic fracturing technology of small well group, and the effective fracture length was 250～350 m. The study results suggest drilling with low speed, low pump pressure and low displacement to ensure stable drilling for coal seam with complex coal structure. The best fracturing effect can be achieved by applying the combination of single well fracturing and synchronous hydraulic fracturing according to the well spacing.