Geomechanical factors affecting the installation and stability of a surface goaf well at a deep UK coal mine

Authors: D. N. Whittles, I.S. Lowndes, S.W. Kingman, C. Yates, S. Jobling

This paper describes a three dimensional computational modelling study undertaken to investigate theeffects that the geomechanical stress and strain fi elds produced by deep longwall mining operations mayhave on the installation and long term stability of a vertical, surface to goaf, gas drainage well installed
within a deep UK coal mine. The modelling programme consisted of the investigation of two modelling
scenarios. The fi rst scenario represented the installation of the well, close to the face line of an active
longwall coal face, during the early production life of the panel. The second scenario represented the
installation of the well at a position behind the coal face, and over the goaf, once the panel had retreated
some distance back from the face start line. An analytical method to predict the infl uence that the developed
shear strains within the rock mass may have on the subsequent stability of the well casing has been developed
and its derivation given. From an analysis of the results produced by these geomechanical models it
was concluded that the well was infl uenced by the geomechanical behaviour of the longwall panel at two
distinct stages, namely, during the installation of the well and during the subsequent stages of the retreat
of the panel. The geomechanical models predicted that during the installation phase, the surface goaf
wells would encounter broken ground at heights of between 30 to 75 metres above the worked horizon. It
was found that the long term stability of the well was signifi cantly infl uenced by the position of the well
in relation to the face line at the time of installation. The modelling showed that a well installed close to
an active longwall face during the early development phases of a panel would experience signifi cantly
more strain and hence potential predicted damage during subsequent panel extraction than a well installed
behind the face line directly over the goaf. The results of the modelling aided in the design of the well by
allowing the correct depth of installation of the well casing to be determined.
Keywords: goaf wells, three dimensional modelling, gas drainage