Exploration and exploitation of deep coalbed methane in the Baijiahai uplift, Junggar Basin: progress and its implications

The Jurassic coals in the Baijiahai uplift, with breakthroughs made in the deep coalbed methane (CBM) production of vertical wells in the early stage in China, serve as an important exploration target of the Junggar Basin. Their coal reservoirs feature an ultra-low ash content, a low moisture content, medium-to-high volatility, and favorable porosity and permeability systems. With gas accumulation modes, namely, the paleo-source and young-reservoir made and the self-source and self-reservoir mode, the coal seams enjoy favorable gas-bearing properties, such as a high gas content, a high proportion of free gas, and high gas saturation, which establish them as high-quality CBM reservoirs. As corroborated by many years of exploration and exploitation practice of Jurassic coals in the Baijiahai uplift, the trial production of most vertical wells can quickly yield gas (2‒5 days), without requiring drainage and pressure reduction. The early-stage trial production tends to exhibit relatively high daily gas production (2100‒9890 m³), low or even zero daily water production (<5 m³), a capacity of natural flowing, and a low backflow rate. In contrast, the prolonged pilot production experiences a uniform pressure drop and a certain period of steady production (25‒60 days). The outcomes of the vertical-well-based pilot production can be influenced by the perforation layer thickness of coal seams, the fracturing fluid system, and the proppant ratio for fracturing. For the fracturing fluid system, gelled and guanidine gel systems outperform the active water-clean systems. The newly constructed horizontal well Caitan 1H yielded high production after fracturing, with a maximum daily gas production of 57000 m³ and a small daily water production (0.5‒3.0 m³), characteristic of conventional natural gas. Subsequently, the production of this well exhibited decayed and stabilized, with adsorbed and free gas produced jointly. Compared to vertical wells, this horizontal well featured higher production, a longer period of steady production, and a lower pressure drop rate. Based on the reservoir characteristics and gas bearing characteristics of the study area, as well as the exploration and exploitation histories of the reservoirs, this study achieved implications in three aspects: (1) Regarding the further understanding of the enrichment patterns of multiple types of deep gas reservoirs, it is necessary to highlight the research on the formation and evolution of traps for hydrocarbon migration and accumulation, as well as the spatial distribution and enrichment pattern of deep gas and water. (2) Concerning the compressibility evaluation of medium- and low-rank deep coal reservoirs, a suitable fracturing process system should be developed for such reservoirs with favorable porosity and permeability systems in high-lying areas. (3) Regarding the optimization of methods for exploiting multiple types of deep gas reservoirs, for gas reservoirs with a high proportion of free gas, the drainage and production control is crucial to achieving sequential production of free and adsorbed gas and to maintaining stable expansion of a depressurization area.