Progress and prospects in optimal design for differentiated three-dimensional development of deep coalbed methane reservoirs: Taking Ordos Basin as an example

Gas reservoir development optimization design is a key approach to achieving efficient development of deep coalbed methane (CBM), enhancing primary recovery, and reducing costs while improving efficiency. Given the strong heterogeneity of geological conditions in deep coal seams, this study takes the Ordos Basin—with the highest degree of CBM development—as an example. Based on detailed geological investigations of more than 3,000 wells across the basin, it systematically reveals the heterogeneity of key parameters including the distribution characteristics of deep coal reservoirs, reservoir-caprock assemblages, coal seam structures, and gas-bearing properties, and proposes a differential stereoscopic development design technology for gas reservoirs.The results show that: ① The No. 8 coal seam in the basin has a thickness of 6–8 m with stable distribution, characterized by thicker and shallower in the east, thinner and deeper in the west. The coal seam is intact in the northeast and bifurcated in the central area. Zonation of coal seam structure is distinct: the northeast is dominated by intact coal (50%–70%), while the central area is dominated by single-interlayer coal (about 50%). Three types of reservoir-caprock assemblages are identified: coal-ash, coal-mud, and coal-sand, with coal-ash and coal-mud as the main types, and coal-sand dominant in the northeast. The sealing capacity follows the order: coal-mud >coal-ash >coal-sand, with gas logging peaks exceeding 90%, over 80%, and around 30% respectively. Gas content ranges from 10.6 to 36.4 m³/t (average 19.8 m³/t), higher in the east and lower in the west (average 22.3 m³/t in the east), and increases vertically with improved coal quality. ② Based on the above geological characteristics, a differential gas reservoir design concept is proposed:Adjust strategies according to gas reservoir variability, formulate plans based on different strata; commingle production for strata with similar pressure coefficients, and staggered development for those with large pressure differences; adapt to local conditions, match horizontal/vertical wells to reservoirs—horizontal where suitable, vertical where optimal. A stereoscopic development deployment mode of “large platform – multi-strata – multi-type – multi-well pattern” is established. Three development schemes and 14 major application scenarios are constructed: large well clusters of horizontal wells, clustered vertical/directional wells, and clustered vertical-horizontal combined wells, all supporting stereoscopic development of multi-type gas reservoirs. The applicable geological conditions, advantages, and typical blocks for each mode are clarified. ③ Taking advantage of relatively large, continuous, stable thickness and considerable productivity of deep coal seams, in thick coal seam areas, a single/multi-layer horizontal well pad model with "primary single-row same-direction and secondary double-row bidirectional/quadrant" was adopted, featuring 4-10 wells per single-layer platform. Horizontal well targets were designed based on coal seam structural characteristics and sweet spot locations, while well trajectory azimuths were optimized according to in-situ stress features. Horizontal well spacing was determined by fracture development extent, and horizontal section lengths and production allocation were designed considering reserves abundance. In thin coal seam areas, a combined straight/directional well pad and straight-horizontal hybrid development model was employed, enabling 10~19 wells per large platform in the straight-horizontal hybrid zone. This approach maximized reserves utilization, platform efficiency, and economic benefits. It supported China's largest deep coalbed methane field—the Daji Block of PetroChina—achieving an annual production capacity exceeding 4 billion cubic meters, with average horizontal well EUR indicators surpassing 42 million cubic meters. This significantly advanced the development of China's first 3-million-ton deep coalbed methane field. ④ Looking ahead, a new-generation gas reservoir differentiated intelligent design technology system is proposed, centered on a four-dimensional collaborative technology framework including "intelligent optimization of differentiated segment clusters, fracture propagation and time-dependent stress simulation, intelligent optimization of differentiated well patterns, and integrated geological-engineering-economic intelligent decision-making." This system achieves refined design and intelligent decision-making for the entire lifecycle of gas reservoir development. It provides crucial technical support and replicable, scalable technical paradigms for the efficient development of deep coalbed methane and coal-measure reservoirs, driving the large-scale, profitable, and high-efficiency development of the deep coalbed methane industry to ensure national energy security.