Assessment of reservoir conditions and engineering factors influencing coal bed methane recovery in the South Wales Coalfield

Abstract

This paper investigates the sensitivity of methane recovery from a representative deep coal bed in the South Wales Coalfield, UK, considering the influence of reservoir conditions and engineering factors. A data set of reservoir conditions is used to establish the expected ranges of coal permeability and methane content in the region. By applying a numerical model for high pressure gas transport with kinetically controlled desorption and permeability evolution, the sensitivity of methane recovery to the different reservoir conditions is studied. The role of key engineering factors, namely the well pattern and spacing, is also considered. This is achieved by comparing the results for methane recovery predicted by the model, firstly under a series of reservoir conditions for single production well recovery, and subsequently for four-spot well patterns with different spacing. From the results analysis, it is demonstrated that the permeability influences the rate of methane recovery more than the methane content, thereby presenting an engineering challenge to widespread exploration in the generally gassy yet low permeability seams found in the region. The study of a four-spot well pattern at 150 m spacing clearly demonstrated the adverse effects of well interference. In contrast, a spacing of 250 m resulted in very little interference for the 1 year simulation period considered. These findings are important since to the authors' knowledge they are the first that relate directly to some of the key reservoir conditions encountered in the South Wales Coalfield. As a result, they are useful for the development of accurate techno-economic analyses for coal bed methane recovery proposals in the region.