Drill stem test (DST) is replacing injection falloff to be the most common used well test for CBM reservoir. It has been realised that the most difficult CBM reservoir parameter to determine is permeability. The commonly used transient test to obtain permeability is injection falloff test (IFO). Recently, drilling stem test (DST) is used widely in Australia. Flow build-up test (FBU) is also occasionally used. However, the comparison of three methods shows that the permeability values derived from different methods are much different. Therefore, interpreted permeability has to be validated prior to being used for reservoir characterisation.
In the Surat Basin and Bowen basin, the permeability interpreted from DST is higher than IFO. There is high clay mineral composition in coal ash, which could cause clay swelling when the water is injected into the reservoir. That is why few IFO test can achieve radial flow in Australia as the inject water damages the reservoir by swelling clay. The interpreted permeability from the IFO test is lower than the realistic property of the reservoir. In China, nearly all of permeability profiles of CBM reservoir is derived from IFO test. The majority of the CBM explorationists believe that the permeability of Chinese coal is relatively low. This opinion results in abandonment of the potential blocks. The FBU or DST test shows that the permeability of CBM in some Chinese basins is relatively high, for example, in the Ordos Basin, the permeability is up to 22 mD. The IFO test has underestimated the permeability by damaging the reservoir. Similarly in India and Indonesia, the permeability has been under estimated by IFO test.
DST test relying on flow from reservoir is in favour of protecting reservoir from damage by injection fliud. In particular, DST test is a good method for the faulty coal. However, in the Sydney basin, the CBM reservoir is saturated by gas. Two-phase flow reaches the wellbore when the reservoir is penetrated by the test hole. The measurement only records water phase flow. The permeability intertpreted from the test is only the permeability to water phase instead of overall permeability. Therefore, it suggests modification of the current test to suit the two-phase flow. The history match is an efficient way of interpreting permeability for the over-saturated reservoir because it takes the two-phase production into account.
In pilot wells, the flow build-up (FBU) test is recommended for use in interpreting permeability. Compared with DST and IFO, FBU method is more reliable and is good in protecting reservoir from damage.