Longwall horizontal gas drainage through goaf pressure control

Abstract

Gas drainage in Australian longwall mining is increasingly challenging because of complex gassy conditions, multi-seam environments beneath previously extracted goafs, and environments where drilling a large number of conventional surface vertical gas drainage boreholes is not practical. This paper presents an alternative approach using horizontal boreholes for longwall goaf gas drainage. Recent experience shows that this horizontal drainage approach has the potential to capture substantially large and consistent volumes of gas. However, there are significant risks involved in this approach, including inappropriate borehole placement and insufficient goaf gas capture and control.Recently, extensive research has been carried out by Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia, to understand longwall induced geomechanical changes in the surrounding strata, goaf gas flow dynamics and the mechanisms of horizontal gas drainage. The research results show that horizontal drainage boreholes targeted at the bottom of the roof fractured zone above the caved zone are well connected to the goaf gas. Drainage boreholes located towards the longwall tailgate are able to create low pressure sinks that protect the workings from goaf gas ingresses by changing goaf gas flow directions in this critical area. This insight has resulted in a new longwall gas drainage strategy through controlling goaf gas pressure distribution and flow pattern. A conceptual model has therefore been developed to provide simple and practical guidance for horizontal gas drainage design.