Pore structure and differential fluid occurrence of deep coal seams

Objective Breakthroughs have been achieved in deep coalbed methane (CBM) exploration along the eastern margin of the Ordos Basin, exhibiting the characteristics of local breakthroughs but regional differential CBM production in multiple blocks. The pores and fractures in deep coal reservoirs, serving as the material spaces for fluid occurrence and occurrence, are critical to the differential development across deep CBM blocks.

Methods In this study, coal samples were collected systematically from the deep part of the Shenfu block located along the eastern margin of the Ordos Basin. Using conventional tests on porosity, permeability, and physical properties, as well as CO₂ adsorption, low-temperature N₂ adsorption, mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR), this study explored the Nos. 8 and 9 coal seams as examples to systematically summarize the pore structures and fluid occurrence patterns of deep coal seams.

Results and Conclusions  Key findings are as follows: (1) Deep coal seams exhibit greatly different pore structures. Coals from the Shenfu block contain both mesopores and macropores, most of which display ink-bottle and open morphologies. It is comprehensively suggested that the medium-rank coals feature pore structures with a slightly weakened cross-scale effect, which is relatively conducive to diffusion and seepage. (2) The medium-rank coals have a reduced adsorption capacity, with isothermal adsorption curves being gentle in the high-pressure part and exhibiting a low desorption efficiency initially. These coals demonstrate elevated water saturation. The high bound water content in mesopores and macropores leads to a decrease in the movable fluid porosity, reducing the storage spaces for free gas. (3) Two gas and water occurrence patterns are observed in the study area: free gas-free water occurrence primarily governed by macropores and microfractures and adsorbed gas-bound water occurrence primarily governed by micropores, mesopores, and macropores. The presence of both occurrence patterns leads to significantly differential CBM production. Specifically, the former is identified as a pattern of rapid and high-yield deep CBM production with characteristics of early gas production, medium to high gas yield, and low water yield. Given the severe damage of stress to medium and large pores and microfractures, it is recommended that appropriate CBM production rates be adopted for gas wells under this pattern to minimize damage to reservoirs and prevent a sharp decline in yield. The second pattern demonstrates the production characteristics of short-term drainage and slow gas production. Slow and continuous CBM production is preferred under this pattern to ensure stable production of gas wells. Additionally, due to a predominance of micropores and mesopores in pores and a high bound water content under this pattern, it is difficult to achieve a high CBM yield of gas wells in the short term. Therefore, it is necessary to explore the possibility of further expanding the fracturing scale to improve the CBM yield of such gas reservoirs.