Geological conditions of deep coalbed methane in the eastern margin of the Ordos Basin, China: Implications for coalbed methane development

Abstract

Deep coalbed methane (CBM) resource potential is enormous in China, and has become a new field for unconventional natural gas exploration and development. This work discusses the geological conditions (reservoir pressure, formation temperature and ground stress) of deep coal reservoirs in the Eastern margin of the Ordos Basin and their implication on CBM development. Various field test data of CBM wells, including injection/drawdown test data, well temperature test data, and hydraulic fracturing test data were collected from this work and literature to describe the geological conditions of the deep CBM in the study area. From the results, it is found that deep CBM in this area is characterized by high reservoir pressure, high formation temperature, and high ground stress. However, there are diverse geological particularities in the different depth range: (1) Having a wide range of pressure gradient, vast majority of coal reservoirs in the study area are under abnormally low-pressure state, which is more significant in deeper coal seams. (2) Due to the impact of surface runoff, the distribution of geothermal gradient is discrete when the burial depth is less than 700 m, and relatively concentrated when the burial depth is greater than 700 m. (3) In shallow coal reservoirs, ground stress is strongest in the horizontal direction; while in deep coal reservoirs, the strongest ground stress is in the vertical direction. Because of the complex geological conditions associated with deep burial, the balance between CBM adsorption-desorption-seepage and the rheological behavior of coal reservoirs is complex, which has significant influence on the exploration and development of deep CBM in the study area. High pressure in deep coal reservoir often leads a long inefficient desorption stage and a long draining and depressurizing process, which increases production costs. Moreover, the negative temperature effect on gas adsorption indicates that CBM content decreases with increasing depth in deep conditions, and thus the evaluation of deep CBM resources needs to be reconsidered. In addition, different stress states govern fracture patterns, and in deep environments, high ground stress greatly reduces the fracturing improvement of the coal reservoir and significantly affects the deep CBM development.