Controls of Coupling between Gas Content and Permeability of Coals on the Enrichment of Coalbed Methane

Yan Song^1,2, Shaobo Liu¹, Yiwen Ju³, Feng Hong¹, Lin Jiang¹, and Xingzhi Ma^1
1Research Institute of Petroleum Exploration and Development, Petrochina, Beijing, China
²State key Laboratory of Petroleum Resource & Prospecting, China University of Petroleum, Beijing, China
³University of Chinese Academy of Sciences, Beijing, China

High abundant coalbed methane (CBM) enrichment area (or “fairway”) should not only have an economic CBM geological resource, but also have a profitable condition for CBM effective well production. Gas content and recoverability of coal seams were identified as the primary control factors for the formation of high abundant CBM enrichment area. The core of gas content and recoverability of coal seams is coupling between water potential and permeability. These two control factors are controlled by water potential and permeability together. Different coal bearing basin has respective geological characteristic. The dominated factors which can affect gas content and permeability of coal seams are also different from each other. However, the coupling between gas content and permeability is illustrated to be an enrichment mechanism of CBM. Two CBM enrichment modes are expressed to be the definite showing for the enrichment mechanism including (1) the favorable slope zones with optimal superposition between gas content and permeability of coal reservoir, (2) the zones whose coal seams were experienced both brittle and ductile deformation.

1. The favorable slope zones with the optimal superposition between gas content and permeability of coal seams

Gas content of coal seams usually increases gradually with increasing burial depth. However, when the burial depth increases to a certain depth, the gas content then begins to show a decreasing trend, which is a combined function triggered by temperature and pressure. The permeability of a coal seam is identified to decrease exponentially with increasing burial depth. A high abundant CBM enrichment area can be developed in a favorable overlay region with relatively high gas content and modest permeability. Therefore, the key to study on the mechanism of high abundant CBM enrichment areas in gentle slope zones is to identify the boundaries of the coupling between gas content and permeability.

In this paper, according to the analysis of geological data (e.g. gas content, burial depth) and the production data of CBM wells in the area of the southern Qinshui Basin and the Hancheng area. The study areas are typical high abundant CBM enrichment areas for middle to high-rank coal seams. With increasing depth, the production data and gas content of the CBM wells show an increasing and then decreasing trend. The permeability of the coal seams decreases exponentially with depth. The main coal seams which CBM production exceeds 2000 m³/d per well are nearly distributed in the depth range of 300 m to 1000 m. Based on the geological features and physical simulation results, the boundary of coupling between gas content and permeability in sloped coal seams is ascertained to have a different range in depth due to different basin. Finally, we establish a mode of sloped coal seams for high abundant CBM enrichment area.

2. The zones whose coal seams were experienced both brittle and ductile deformation

Different tectonic deformation mechanism, deformation degree and deformation type have an influence on the pore - fracture system of coal reservoir through changing the behavior of coal sorption, diffusion and flow of CBM. As a result, gas content and permeability of coal seams were affected.

In order to study on the influence of brittle and ductile deformation on gas content and permeability of coal seams, three experiments of coals deformation had been performed in State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Seismological Bureau. According to the results of experiments, permeability and total surface area measurement, in comparison of original coals, the permeability of coals which experienced brittle deformation has an increasing trend. However, the total surface area has little change, which may indicate no change in gas content. After the ductile deformation, the permeability of coals became smaller than origin, and total surface area increased. When the coals deformation is dominated by the brittle-ductile deformation, the micro-fracture has an increasing trend and plastic crumple is appeared. The coal samples which experienced brittle-ductile deformation were characterized by relative high permeability and stronger sorption capability. The last deformation type coal is benefit for the enrichment and production of CBM.

Associated with the geological conditions and gas production data of Huaibei CBM area, the CBM enrichment mode in coal seams which were experienced both brittle and ductile deformation has been illustrated. Coal seams are strongly compressed and sheared from the thrusting-nappes structures. In the region of the anticline axis, brittle deformation is caused by dominated tensile stress. The syncline-axis area is primarily under compressive stress, resulting in ductile deformation. In the syncline/anticline wings, the coal seam appears to undergo the superposition of brittle and ductile deformations. The gas content from the syncline axis to the wing has a trend of decrease, whereas the permeability increases.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013