Because the Environmental Protection Agency has identified methane as a greenhouse gas, the degasification of coal bed methane through gob wells has become more critical. It has been observed that the degree of methane emission is closely related to the geomechanical characteristics of coal bearing strata, roof, floor, and surrounding coal seams. As a result, the study of progressive gob formation becomes necessary to understand the mechanism of strata movement and methane emission through gob wells. Several numerical models were developed to simulate gob formation of a longwall mine having similar geologic conditions as the Warrior Basin in Alabama. In addition, monthly production data from 250 gob wells and corresponding face position in each panel were collected from this area to corroborate the results of numerical modeling. This paper describes the techniques for numerical modeling of progressive gob formation and correlate the results with field investigations.
Understanding the characteristics of the methane emission zone requires a comprehensive knowledge of progressive gob formation as the longwall face advances. Methane liberation is closely linked to the coal face movement in longwall mining (McCullock and Daurice, 1973) and is dependent on the development of fractures in the roof strata (McPherson, 1975). Furthermore, the permeability in the emission zone is increased because of mining and may be the principal factor controlling gas emission into the working and gob area. To investigate this concept in detail, several two-dimensional finite element models consisting of the gob area, working seam, overburden rock and coal layers, were developed. Monthly production data for 250 gob wells were collected from 53 longwall panels over an area of around 120 square kilometers (46 square miles) comprising Numbers 4, 5 and 7 Mines of Jim Walter Resources. Locations of gob wells in the panel and monthly face advancement data were collected from the latest mine maps. Overall geological information was obtained from 15 geological columns scattered over this area. Figure 1 shows a schematic diagram of this area indicating three different mine locations. A typical geological column (S-136) is shown in Figure 2. The focus of this study is to understand the mechanism of gob formation in this area for estimating the extent of the methane emission zone. Altogether, 17 finite element models were developed based on the geologic column shown in Figure 2. Hoek and Brown criteria for compressive failure and Griffith criteria for tensile failure were used. It is also assumed that in the case of tensile stresses, cracks and fractures will develop in the roof layers which may destabilize rock masses and finally break into pieces to form the gob.
In general, the study area was divided into three different geologic groups depending on the geology as shown in the Table 1. They were named Groups I, II and III based on the type of primary rock found in that layer. The shaded areas in Figure 1 approximately represent the different geologic groups in the study area.