Best practices for the determination of low-pressure/permeability coalbed methane reservoirs, Yuwu Coal Mine, Luan mining area, China

Abstract

Permeability and pressure are two important parameters for characterizing coal reservoirs and for evaluating coalbed methane (CBM) production potential. In this study, the reservoir pressure and permeability of six steel-cased CBM wells at Yuwu Coal Mine (Luan coal mining area, Qinshui Basin, China) were studied using new injection falloff testing (IFOT) methodology. The tested reservoir represents coal #3 of the lower Permian Shanxi Formation, the main target coal seam for coalbed methane exploration and production in Qinshui Basin. The results show that on average coal #3 has an ultra-low permeability (0.001–0.043mD) and a low pressure gradient (4.0kPa/m). These values are significantly lower than those of previously collected data from open holes in adjacent wells. However, the validity of the new results was confirmed using two additional lines of evidence: (1) analysis of 10 CBM production wells adjacent the current test area and; (2) data from a long-term monitoring well in an Ordovician limestone aquifer, located below coal #3. On the basis of the results, it can be concluded that (1) using the IFOT method in cased wells, in which shut-in times were extended by 7–11 times that of the injection time, provides reliable data on reservoir parameters (including pressure and permeability); (2) coal #3 is characterized by very low permeability and very low pressure; (3) the low reservoir pressure is related to the Xinan Spring Group, located on the eastern edge of the Qinshui Basin, which discharges reservoir groundwater to the surface hydrological system; and (4) low permeability may be caused by low reservoir pressure and by a cleat system filled with post-structural mineralization. These conclusions were used to further optimize stimulation design and production management for this CBM play. As a result, there has been a good production rate for these wells since 2011.