Investigation on coal seam gas formation of multi-coalbed reservoir in Bide-Santang Basin Southwest China

This paper presents a comprehensive study on the geological characteristics of a multi-coalbed reservoir using coal cores collected from well drilling tests in Bide-Santang Basin, China. The study focuses on the investigation of the coalbed methane (CBM)-bearing property, porosity and permeability, sealability of capping rocks, and groundwater characteristics in coal seams, providing the experimental information on the control function of sequence strata for the coalbed methane. This reservoir is comprised of a large number of coalbeds and exhibits many distinctive geological characteristics of a CBM reservoir, such as high CBM content, complex vertical volatility of CBM content, vertical discontinuous distribution of porosity and permeability, strong sealing capping property and many vertical water-bearing systems, etc. The investigation shows that the unattached multiple superposed CBM-bearing system (UMSCS), which formed the unique multi-coalbed CBM reservoir, can be identified in the study area. Depending on the development scale of vertically unattached CBM-bearing coal seams, the CBM reservoir in the study area can be classified as simple UMSCS and complex UMSCS. The former, typically appearing in Zhuzang syncline and Agong syncline, exhibits the continuity of geological reservoir-forming characteristics in terms of less coal seams, much weaker sealability of capping rocks, higher CBM content, and many vertical water-bearing systems. The latter, typically represented by Shuigonghe syncline and Santang syncline, shows the continuity of geological reservoir-forming characteristics with a large number of coal seams, strong sealing capping property, and reasonably high CBM content. For simple UMSCS reservoirs, a sublevel exploitation method should be adopted for CBM exploitation in order to minimize the effect caused by larger variation of the reservoir energy in different systems. For complex UMSCS reservoirs, one effective way for CBM exploitation is to fracture virtual reservoir (mainly key sandstone member) to depressurize it uniformly and thus achieve large-scale discharging and mining in several adjacent CBM-bearing subsystems.