Abstract
Because permeability and skin are important parameters to assess the potential success or failure of a coal seam gas or mining project, it is necessary to implement their measurement at an early stage. For this reason, a suite of Flow Build-up (FBU), Injection Falloff Test (IFT) and Step Rate Test (SRT) tests should be run early in the project but their correct design, execution and interpretation should also be carefully planned.
It is expected that permeability and skin damage outcomes from FBU and IFT tests conducted in the same layer be equal since both employ the same fluids and the same equipment; however, these outcomes are not always the same.
In a Coal field a total 91 tests were run including: 39 FBU, 33 IFT and 19 SRT. Furthermore, in 33 zones the operator conducted sequentially both the FBU and IFT tests. The results indicate that in 42% of these tests, the permeability result from FBU tests differed from the permeability derived from IFT between 0% and 10%, and furthermore, in 28% of these cases, this difference was more than 30%. It was also observed that in 85% of these tests, the skin damage recorded from IFT was higher than from FBU. These outcomes are illustrated with statistics and real examples.
It is the author's view that coal sensitivity to stress is the major contributor to the difference in the permeability results derived from the two procedures since the stress around the borehole is different during the FBU than in IFT. These differences are more frequently seen in low pressure, low permeability seams.
It is also the authors contention that, the higher skin in the IFT test is the result of damaged material (mud cake, drilling fluids and possibly coal fines) being pushed into the formation by the pumping action during the injection period of the IFT test. This effect is noticed more frequently when the well is killed post FBU test to retrieve the testing tool from the well and re-run for the corresponding IFT test.
In summary, with knowledge of the true coal permeability, stress and skin at an early stage by measurement, the project risk of failure could be reduced. The combined effort of the operator who carries out the testing and the analyst who interprets it will help to achieve this aim.
