Geological characteristics and key technologies for exploration and development of the Yanchuannan coalbed methane field, Ordos Basin

ObjectiveChina boasts abundant coalbed methane (CBM) resources, which serve as a crucial replacement for the reserve growth and production addition of natural gas resources. In recent years, CBM exploration and production have gradually expanded into deep, thin coal seams, which, however, are characterized by strong heterogeneity, ultra-low permeability, high in situ stress, and complex enrichment patterns. Therefore, the exploration and exploitation of deep, thin coal seams face challenges such as inadequate geological theories, poor adaptability of key technologies, and low investment returns, which hinder large-scale commercial CBM production. MethodsFocusing on the exploration and exploitation practice of middle-deep, thin coal seams in the Yanchuannan CBM field within the Ordos Basin, this study systematically analyzed the geological characteristics of the CBM field, summarized the primary factors controlling CBM enrichment and high productivity, and established a series of geology-engineering integrated technologies for efficient exploration and exploitation of middle-deep, thin coal seams. ResultsThe Yanchuannan CBM field contains two structural belts, namely Tanping and Wanbaoshan, which exhibit significantly different sedimentary environments, lithotypes and coal quality, reservoir quality, preservation conditions, and in situ stresses. Nevertheless, this CBM field generally shows middle-deep, undersaturated, low-temperature, low-pressure, thermogenic high-quality CBM reservoirs. The production characteristics of the CBM field are governed by fracturing performance. Specifically, gas wells subjected to conventional guided fracturing exhibit late gas shows and production addition combined with limited single-well productivity and recoverable reserves. In contrast, gas wells subjected to fracturing with fractures effectively propped demonstrate rapid production addition and high single-well productivity and recoverable reserves. By integrating dynamic and static analyses, this study gained a geological understanding of four-element coupling for the high productivity and enrichment of CBM in medium-deep coal seams, highlighting sedimentation-controlled coal distribution, preservation-controlled enrichment, in situ stress-controlled permeability, and effective stimulation-controlled productivity. An indicator system for the quantitative evaluation of geology-engineering “dual sweet spots” was developed to guide play fairway selection. Multi-scale pore-fracture characterization technology was established, enabling the quantitative characterization of reservoir spaces on the centimeter, millimeter, micrometer, and nanometer scales. The key technology based on geological modeling and numerical simulation integration ascertained the types and distribution patterns of residual gas. This technology can guide well pattern adjustments in residual gas enrichment zones, thus improving the production ratio and recovery of reserves. By highlighting the suitability of the well pattern and fracture networks, this study established a well pattern – fracture network – productivity – economic benefit integrated strategy tailored to varying geological conditions. To address challenges posed by thin coal seams and great structural fluctuations, this study established the horizontal well guidance – fracturing – production integrated technology for well completion in thin coal seams while considering the requirements of well drilling and completion, fracturing, and production. Through multiple rounds of research and iterative optimization based on a deepened understanding of coal properties, this study developed optimized fracturing with fractures effectively propped characterized by the preflush of high-volume fracturing fluids for longer fractures, variable injection rates of fracturing fluids for fracture height control, and multi-sized proppants for propping multi-scale fractures. The advancement in fracturing technologies shifted the production philosophy from slow and long-term drainage to optimal rapid production addition, leading to the formation of the production system characterized by four stages, two pressuring, and three controlling factors. A "node-region-center" three-level pressure boosting model was developed to maximize productivity. ConclusionsGuided by these advancements, the Yanchuannan CBM field has achieved stable production growth and significantly increased single-well productivity, with the daily production of a single directional well increasing to 1×104 m³/d from 0.1 m³/d and that of a single horizontal well rising to (2.5‒6.0) × 104 m³/d from (0.5‒0.6) × 104 m³/d. These suggest effective fracturing performance and commercial production. This study serves as a valuable reference for the commercial production of similar deep, thin CBM resources in China.