A Stimulation and Production Experiment in a Vertical Coal Seam Gas Drainage Well (1996)

R.G. Jeffrey, SPE, CSIRO Petroleum, J.R. Enever, SPE, CSIRO Petroleum, J.H. Wood, BHP Collieries Division, J.P. Connors, BHP Collieries Division, S.K. Choi, CSIRO Petroleum, K.T.A. Meaney, CSIRO Petroleum, D.A. Casey, SPE, D.A. Casey and Associates, and R.A. Koenig, SPE, In-Situ Inc. Abstract An integrated program of characterisation, stimulation, and post-stimulation trial production has been completed in three coal seams intersected by two HQ-size (96 mm diameter) boreholes located adjacent to Appin Colliery in NSW. Data on stress, seam permeability and pressure, formation mechanical properties, and gas content and desorption pressure were collected during field and laboratory testing. Production well testing was used to establish gas desorption pressures and, by history matching using a numerical reservoir simulator, the relative permeability of the seams. Three seams were then stimulated by hydraulic fracturing using a water and sand system. A 25 day production test was then carried out in each well to establish an inflow performance curve and to collect data for use in refining the relative permeability curves by history matching the data. Based on the history match, production forecasts were made that also evaluated changes in gas content in the seam over the term of the forecast. The small hydraulic fracture stimulations produced a large increase in the gas production rate compared to the initial, unstimulated rates. Introduction The problem of handling seam gas emissions is an important issue for many Australian underground coal mining operations. In-seam drilling from underground mine headings has become the accepted practice for pre-draining the gas from the seam being worked prior to extraction. This approach has proved effective in most situations. There are, however, three drivers that continue to dictate that other options be pursued:–increases in the mine production rate requires that gas is drained more quickly ahead of the mining activities,–the desire to be able to remove gas drainage from the mining cycle by using surface holes for pre-drainage, with the possible added incentive of selling the gas produced,–the relative difficulty encountered when trying to pre-drain seams beneath and above the seam being worked, in some instances, using conventional in-seam drilling. The activities described here were conducted at Appin Colliery, NSW, motivated by the above considerations. Appin Colliery is located in the Southern Coalfield of the Sydney Basin, 20 kilometres west of Wollongong and mines the Bulli coal seam at depths from 400 to 500 metres. Appin has successfully used in-seam drilling to pre-drain gas from the Bulli seam for about 15 years. In some areas of the workings, however, the amount of advance drilling required and difficulties in pre-draining gas from the unworked seams underlying the Bulli seam (mainly the Balgownie and Wongawilli) have prompted interest in using hydraulic fracturing to enhance gas drainage. The small-scale production trial described here was undertaken to collect data on seam conditions, hydraulic fracture growth behaviour, and to compare pre- and post-stimulation production of the Bulli, Balgownie, and Wongawilli seams. Fig. 1 shows the locations of the two vertical boreholes employed in this trial and the extent of the mine workings at the time the work described here was conducted. Both holes were drilled from the surface and cased to just above the coal sequence. The locations of some of the horizontal holes drilled from the mine in the Bulli seam are also shown in the figure. The in-seam drainage holes that end near DDH 53 were drilled after the work in that well was completed. DDH 53 was cored through the top four coal seams (Bulli, Balgownie, Cape Horn and Hargraves). DDH 54 was cored through the complete sequence (Bulli, Balgownie, Cape Horn, Hargraves, Wongawilli, American Creek, Tongara). The field activities involved: In DDH 53:–pre-stimulation characterisation of the Bulli and Balgownie seams using CSIRO's multi-phase well testing capability,–small-scale hydraulic fracture stimulations in the Bulli and Balgownie seams, which included pre-stimulation stress measurement in adjacent rock layers and mechanical testing of rock core in the laboratory,–post-stimulation multi-phase well testing of both seams, P. 183