Well test is an efficient approach in determining permeability of coalbed methane and conventional gas reservoir. The permeability is the key parameters in evaluating deliverability of CBM reservoir. One of the primary goals of exploration wells is to obtain the permeability by performing pressure transient tests. The easy and common used type of well test is injection fall off test (IFO). However, the IFO test always presents discouraged permeability values. Based on the interpretation, the simulation forecasting gas production delivers uneconomic profile. On the contrary, the actual production is much higher than the prediction and has brought large amount of profit to the CBM operators. The later history match by actual production data gives higher permeability than the interpretation from IFO. This experience proves that the IFO test could underestimate the permeability of the CBM reservoir.
In order to assess the accuracy of transient test, the authors have conducted comparison between IFO test and DST (Drilling stem Test). The flow build up data are occasionally used if it is available. The data used by this study were collected from the Bowen Basin, Surat Basin, Galilee Basin and Sydney Basin in Australia, and Ordos Basin and Qinshui Basin in China.
In the Bowen Basin, there 135 sets of test data have been analysed by this study, which include 70 sets of IFO and 65 of DST. The target seams are Seam P and GM in Permian coal measures. In the Suart Basin, there are 50 sets of IFO data and 38 DST data, where the target seam are Walloon Coal Measures, Jurassic. In the Galilee Basin, there 100 sets of DST data and 5 sets of FBU data are used in the analysis. The target seams in the Galilee Basin are Betts Creek and Aramic Coal Measures in Permian. In the Sydney Basin, there are 10 sets of IFO and 12 DST with target Permian coal seams.
Two CBM bearing Basins in China are chosen for the study, which are Qinshui Basin and Ordos Basin. In China, IFO test is dominated over the CBM pressure transient tests. However, DST tests were conducted in two wells under instruction of the authors in the Ordos Basin.