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Coal Geology & Exploration

Abstract

Objective Design optimization for gas reservoir development serves as a key approach to the efficient exploitation, enhanced primary recovery, and elevated efficiency with reduced costs of deep coalbed methane (CBM). Methods In this study, the most extensively developed Ordos Basin in China was investigated as an example to explore the significant heterogeneity of the geological conditions of deep coal seams. Based on a fine-scale geological investigation of over 3 000 wells across the basin, this study systematically revealed the heterogeneity of the key parameters of deep coal reservoirs, including reservoir distribution, reservoir-cap rock assemblages, coal seam structure, and gas-bearing properties. Accordingly, design optimization technologies for differential gas reservoir development with three-dimensional well patterns were presented. Advances The results indicate that the No. 8 coal seam in the Ordos Basin exhibits thicknesses ranging from 6 m to 8 m and a continuous distribution. The coal seam is thick and shallowly buried in the east, while proving thin and deeply buried in the west. It remains intact in the northeast but bifurcates in the central part. It shows structures with distinct zonation, dominated by integrated coals (50%–70%) in the northeast and by coals with a single interlayer (50%) in the central part. Three types of reservoir-cap rock assemblages are identified in the No. 8 coal seam: coal-limestone, coal-mudstone, and coal-sandstone assemblages, with the former two predominating in the main body and the latter one prevailing in the northeast. The sealing performance of the three assemblage types decreases in the order of coal-mudstone, coal-limestone, and coal-sandstone assemblages, which correspond to peak mud logging-derived values averaging ＞ 90%, ＞ 80%, and around 30%, respectively. The No. 8 coal seam exhibits gas content ranging from 10.6 m3/t to 36.4 m3/t, with an average of 19.8 m3/t. In the plan view, the gas content is high in the east (average: 22.3 m3/t) and low in the west. Vertically, it increases with the coal quality. Based on these geological characteristics, a differential design philosophy has been proposed for gas reservoir development. Specifically, development strategies should be adjusted and implemented based on specific gas reservoirs and strata; commingled production is recommended for strata with similar pressure coefficients, otherwise succeeding production should be applied; and well types (horizontal or vertical wells) should be selected based on the vertical heterogeneity of coal reservoirs in specific zones. Arrangement and development modes based on three-dimensional well patterns have been developed for gas reservoirs, involving large drilling pads, multiple strata, multiple resource types, and multiple well types. Notably, three collaborative development models that consider multiple gas reservoir types have been established, namely the models based on large horizontal-well clusters, cluster vertical/directional wells, and the combination of cluster vertical wells and horizontal wells, with 14 application scenarios having been determined. Furthermore, the applicable geological conditions, advantages, and representative suitable blocks of the three models have been identified. Given the substantial thicknesses, continuous distribution, and considerable production of deep coal seams, the development model based on large well clusters consisting of single-/multi-layer horizontal wells can be adopted for areas with thick coal seams. In this model, single-row horizontal wells with the same orientations are primarily arranged, supplemented by double-row horizontal wells with two-/four orientations, with one drilling pad typically hosting 4−10 single-row horizontal wells. In this model, horizontal well targets are designed based on both the characteristics of coal seam structures and the sweet spot locations of gold targets, the azimuths of horizontal well trajectories are designed in accordance with the characteristics of in situ stress, and the horizontal well spacing is designed based on the fracture density. Furthermore, the horizontal section lengths and production allocation are designed in combination with the reserve abundance. For areas with thin coal seams, the development modes based on cluster vertical/directional wells and the combination of cluster vertical wells and horizontal wells can be implemented. Notably, the latter mode, using 10‒19 wells per large drilling pad, enables full reserve production, the effective utilization of drilling pads, and maximum economic benefits. The abovementioned technologies have supported the Daji gas field of PetroChina—China’s largest deep CBM field—in achieving an annual productivity exceeding 4 × 109 m3 and an estimated ultimate recovery (EUR) per 1 000-m-long horizontal section of producing horizontal wells surpassing 42 × 106 m3 on average. These breakthroughs have greatly promoted the establishment of the Daji gas field as China’s first deep CBM field with 3 million tons of oil and gas equivalent. [Prospects] For deep gas reservoir development in the future, this study proposes a new generation of differential, intelligent design technology system focusing on a four-dimensional collaborative technologies: the differential, intelligent optimization of segment-cluster and fracturing parameters for horizontal wells, the simulation of fracture propagation and time-varying stresses, the intelligent optimization of differential well pattern arrangement, and the geological-engineering-economic integrated intelligent decision-making. This system enables the fine-scale design and intelligent decision-making for the full-lifecycle gas reservoir development. Overall, the results of this study provide crucial technical support and a replicable technical paradigm deserving widespread application for the efficient development of deep CBM and coal-measure gas reservoirs, thereby driving the large-scale, commercial, and efficient development of the deep CBM industry to ensure national energy security.

Keywords

deep coalbed methane (CBM), coalbed methane (CBM), ifferential design for gas reservoir development, large drilling pad, development with a three-dimensional well pattern, artificial intelligence (AI), Ordos Basin

DOI

10.12363/issn.1001-1986.26.04.0228

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