Mechanism of pore pressure variation in multiple coal reservoirs, western Guizhou region, South China

Abstract

Pore pressure is an important parameter in coalbed methane (CBM) exploration and development; however, the distribution pattern and mechanism for pore pressure differences in the Upper Permian CBM reservoirs are poorly understood in the western Guizhou region of South China. In this study, lateral and vertical variations and mechanisms for pore pressure differences are analyzed based on 126 injection-falloff and in-situ stress well test data measured in Permian coal reservoirs. Generally, based on the pore pressure gradient and coefficient in coal reservoirs of the western Guizhou region, five zones can be delineated laterally: the mining areas of Zhina, northern Liupanshui, northern Guizhou, northwestern Guizhou and southern Liupanshui. Vertically, there are two main typical patterns: i) the pore pressure gradient (or coefficient) is nearly unchanged in different coal reservoirs, and ii) the pore pressure gradient (or coefficient) has cyclic variations in a borehole profile with multiple coal seams, which suggests the existence of a “superimposed CBM system”. The mechanism analysis indicates that coal permeability, thermal evolution stage and hydrocarbon generation contribute little to pore pressure differences in coal reservoirs in the western Guizhou region. The present-day in-situ stress field, basement structure and tectonic activity may be the dominant factors affecting lateral pore pressure differences. The sealing capacity of caprocks and the present-day in-situ stress field are significant parameters causing vertical pore pressure differences in coal reservoirs. These results are expected to provide new geological references for CBM exploration and development in the western Guizhou region.

Graphical abstract

Keywords

pore pressure difference / influencing factor / coalbed methane reservoir / Upper Permian / western Guizhou region

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