Exploration practices of and recent production breakthroughs in deep middle-rank coalbed methane in the Daniudi gas field,Ordos Basin

Breakthroughs have been achieved in the exploration and production of middle-rank coalbed methane (CBM)at a depth of 2800 m and even deeper in the Daniudi gas field within the Ordos Basin. Notably, well YM-1HF exhibits a maximum single-well production of over 104000 m³/d, an average stable production of 63000 × 10⁴ m³/d over one consecutive year, and an accumulative gas production exceeding 23 million m³. By analyzing the geological characteristics of deep middle-rank coals and CBM in the Daniudi gas field, we propose and practically apply a production capacity improvement approach that integrates play selection, interval-based drilling, volume fracturing, and fine coalbed gas production. For play selection, we assess the Yuyang area from the perspectives of gas content, coal seam thicknesses and characteristics, roof conditions, and thermal maturity. The results indicate that coals in the Yuyang area are dominated by semi-bright coals with primary texture, exhibiting gas content ranging from 16.50 to 26.00 m³/t, free gas accounts for 25 % to 30 % of the total gas content, an average porosity of 4.68 %, and an average permeability of 0.24 × 10^-3 μm². Cleats and fissures are well-developed in the coals and are partly filled with calcite. The substantial total thickness of bright and semi-bright coals and high gas content provide the foundation for high gas production, while high injection rates of fracturing fluids and large-scale hydraulic-fracture stimulation are critical to high gas production. To achieve volume fracturing, it is recommended to employ effective technical approaches including multi-cluster closely spaced perforation, high injection rates of fracturing fluids, and high-intensity proppant injection based on a work philosophy that integrates saturated proppant injection, fracturing fluid control for creating large-scale primary fractures near wellbores and more secondary fractures far away from wellbores, and effective propping of induced fractures. In addition, proper control of the depressurization rate is vital for stable, high production of deep CBM wells, and the production concept of preventing proppant production while promoting coal fines is adopted. Bare casing should be utilized in the fluid drainage and rapid production stage, and in the flowing production stage, it is necessary to employ production strings to achieve continuous and stable production under pressure control. This production concept has proven effective in the pressure-controlled production of well YM-1HF.