Comparison of DFIT, DST and IFT Permeabilities in CBM Reservoirs

Abstract

Abstract The Diagnostic Fracture Injection Test (DFIT) is a welltesting technique used to assess the fracture growth, fracture closure and permeability from reservoirs; recently, it has been applied successfully to unconventional reservoirs such as: coal seams, tight sands and shale gas. The conventional DFIT procedure shares similarities with the author's Step Rate Test (SRT) procedure; the author adapted the SRT for coal seams application in Australia more than 15 years ago, since then it has generated a data base of more than 700 tests. During this period, the SRT was primarily used to assess stress conditions in coal seam reservoirs. Because of the similarities between the DFIT and STR procedures, we decided to apply the DFIT analysis technique to the SRT data with the aim to assess the coal seam permeability; furthermore, compare these permeability results with Drill Stem Test (DST) and Injection Falloff (IFT) permeabilities of tests undertaken in the same coal seam. The permeability results from the three methods are not always similar and sometimes differ appreciably; this generates a dilemma for reservoir engineers because they need to choose a representative permeability for their production forecast and reserves recovery assessments. The explanation of these permeability differences relies on the interpretation of coal stress parameters provided by the DFIT analysis. Furthermore, the appropriate integration of the DFIT, DST and IFT analysis results with wellbore storage, wellbore calliper and leak-off type, will assist the engineers to choose the representative reservoir permeability for their production forecast and reserves estimation; for this reason, running these three tests at the early stages of the exploration project will assists substantially with the property valuations and the planning of future work.