Abstract:

Low-rank coalbed methane (CBM) resources in China are approximately 10.3 × 10¹² m³, with the Erlian Basin group accounting for one-quarter of the total, indicating significant potential for large-scale industrial development. The coal reservoirs in the Jiergalangtu block of the Erlian Basin are characterized by six “lows” and one “thick”: low coal rank (vitrinite reflectance R_O of 0.35%), low temperature (26.6 ℃), low Young’s modulus (1 500 to 2 000 MPa), low pressure coefficient (1.02 to 1.03), low gas content (1.8 m³/t), ultra-low tensile stress (7 MPa), and an extremely thick coal seam (vertical thickness of 40 to 128 m). In the early exploration and evaluation stage, 22 wells were fractured and put into production. However, none achieved ideal gas production output due to insufficient geological understanding, immature engineering techniques, and mismatched technical designs. Based on an enhanced understanding of the geological conditions, this study identified the key challenges to development. The low gas content necessitated volume fracturing to achieve industrial-scale gas production. The low temperature required breakthroughs in low-temperature gel-breaking technology to prevent reservoir damage. The extremely thick coal seam necessitated careful selection of high-quality main layers for concentrated stimulation. The low pressure coefficient required strategies to reduce mud loss and fracturing fluid filtration. The strong plasticity required measures to mitigate the impact of proppant embedment on fracture conductivity. After clarifying and addressing these challenges, the research established an integrated geological-engineering fracturing strategy for low-rank coal horizontal wells using energy-focused staged volume fracturing technology. The mechanical specific energy model was modified to calculate the coal rock breakage index, thereby enabling the evaluation of coal fracability and identification of geological-engineering “dual sweet spots” for focused stimulation. A combined fracturing technique of low-temperature soluble bridge plug and perforation was upgraded, and the unperforated casing pumping was used to facilitate adequate operational space for high-displacement fracturing. Perforation parameters were optimized as follows: perforation length of 2 m, hole density of 16 holes/m, and a 60° phased spiral perforation. The fracturing scale and intensity were optimized, with a designed fluid volume of 1 500 m³ per stage, sand addition volume of 180 m³ per stage, and a displacement rate of 18 m³/min. A low-temperature, low-concentration, and low-damage guar gum fracturing fluid system was developed, and a combination of sand addition schemes was used with particle sizes of 0.106-0.212 mm, 0.212-0.425 mm, and 0.425-0.850 mm. This integrated technology was successfully applied to well JP1 in the research area. After fracturing, the well achieved a stable gas production rate exceeding 4 000 m³/day, making it the highest-producing single well among low-rank CBM horizontal wells with cased-hole fracturing in China. This successful application effectively promotes the efficient development of low-rank CBM in China.

Key words: Erlian Basin, low-rank coal, coal fracability, sand fracturing, geological-engineering integration