关键词: 二连盆地, 低阶煤, 煤岩可压性, 加砂压裂, 地质工程一体化

Abstract: The low-rank coalbed methane resources in China are estimated to be approximately 10.3×10¹² cubic meters, with the Erlian Basin Group contributing a quarter of these resources and exhibiting significant potential for large-scale industrial development. The coal reservoir within the Jiergalangtu block of the Erlian Basin is characterized by a low coal rank (vitrinite reflectance Ro of 0.35%), low temperature (26.6°C), 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 extension stress (7 MPa), and substantial vertical coal seam thickness (ranging from 40 to 128 meters), which can be summarized by the "six lows and one thick" descriptor. During the initial exploration and evaluation of 22 wells, suboptimal gas production results were observed post-fracturing and operation due to insufficient geological understanding, an immature process system, and mismatches in technical schemes. Building upon a deeper comprehension of the geological conditions in the study area, this paper identifies the key challenges in development. It is recognized that the low gas content necessitates volume fracturing to achieve industrial gas production, the low temperature requires addressing the issue of low-temperature rubber breaking technology to avert reservoir damage, the extensive coal seam thickness necessitates the selection of high-quality intervals for focused transformation, and the low pressure coefficient requires measures to reduce mud loss and fracturing fluid filtration. The strong plasticity requires overcoming the impact of proppant embedment on flow conductivity. After addressing the difficulties associated with fracturing reconstruction, targeted research was conducted, leading to the development of concentrated energy segmented volume fracturing technology for low-rank horizontal wells, based on an integrated geological-engineering fracturing concept. The horizontal well type is utilized to maximize the control of reserve volume. The mechanical specific energy model has been refined to calculate the crushing index of coal and rock, thereby evaluating the compressibility of coal and rock. The low-temperature soluble bridge plug + perforation combined fracturing process has been upgraded, and light casing pumping provides a high displacement fracturing space. Perforation parameters have been optimized, with a perforation length of 2 meters, hole density of 16 holes/meter, and a phase angle of 60° spiral perforation. The fracturing scale and intensity have been optimized, and a fracturing fluid volume of 1,500 m³ has been designed.To optimize the scale and intensity of hydraulic fracturing, a fracturing fluid volume of 1,500 cubic meters per segment, a sand addition volume of 180 cubic meters per segment, and a displacement rate of 18 cubic meters per minute were designed. Research and development efforts focused on a low-temperature, low-concentration, and low-damage guanidine gum fracturing fluid system, utilizing a combination of sands with mesh sizes of 70/140, 40/70, and 20/40. The JP1 well, located within the research area, was successfully implemented in the field. Following pressure treatment, the well achieved a stable daily gas output exceeding 4,000 cubic meters, becoming the highest-producing single well with a low-rank coal casing fractured horizontal well in China. This development effectively accelerated the process of enhancing the efficiency of low-rank coal bed methane extraction in the country.

Key words: Erlian Basin, Low rank coal, Compressibility of coal and rock, Sand fracturing, Geological engineering