Progress and prospect of exploration and development of deep coalbed methane in the Junggar Basin

The Junggar Basin in Xinjiang is one of the six continental super oil and gas bearing basins in China. It is characterized by various types of tectonic units, the development of Jurassic middle and low rank coal seams, and the abundance of deep coalbed methane (CBM) resources and great development potential. The exploration and development of deep coalbed methane have gone through the early stage of oil and gas vertical well trial mining (1992−2019) and the recent stage of coalbed methane horizontal well exploration (since 2020). Breakthroughs have been made in deep coalbed methane exploration and development, but there is still a gap compared to the exploration and development effects of mid to high rank deep coalbed methane in the Ordos Basin, which is related to the special geological resources and development characteristics. The special characteristics of geological resources are mainly reflected in: Coal reservoirs are mainly composed of medium to low metamorphic bituminous coal, low moisture, low to medium ash content, and ultra-low sulfur coal, with developed pores and fractures, high adsorption and gas storage capacity, good pore permeability but strong wettability; There is diversity in the reservoir formation patterns and resource types of deep CBM, with coexistence of “ancient source-younger reservoir gas system” and “young source-young reservoir gas system”, with the characteristics of conventional unconventional gas spatial symbiosis and orderly succession, and it is speculated to have geological features of a fully gas bearing system; Under the depth effect, the distribution of CBM occurrence has regular changes. The critical depth of adsorbed gas of low middling coal rank is about 1000−1500 m, and the critical depth of total gas content is about 2500−3000 m. After 2000 m, the proportion of free gas is close to 50%, and the higher the coal rank, the deeper the corresponding depth of free gas proportion of 50%. The particularity of development is mainly reflected in: The adaptability of coal reservoir fracturing, where the gel and guar gum systems are superior to the active water clean fracturing fluid system, and large-scale volume fracturing in high structural traps is prone to fracturing fluid filtration and rebound fracturing effects. The production characteristics of deep coalbed methane horizontal wells show the characteristics of “fast gas breakthrough, peak gas production−moderate stable gas production, low water production, and low backflow rate”. The production behavior is mainly characterized by free gas in the early and middle stages, and adsorbed gas in the later stages. In response to the above particularities, it is necessary to carry out selection of geologically favorable areas/sweet spots for different resource types throughout the basin in the future. Optimization of the adaptability of fracturing and extraction development processes; Establishing an efficient, low-cost, and large-scale development model. Finally, the geological evaluation and development technology in line with the exploration and development of deep CBM in the Junggar Basin will be formed to promote the large-scale and cost-effective development of deep CBM.