A practical workflow for characterizing stress-dependent behaviour of coal from changes in well productivity

• Addresses existing knowledge gaps in the coal bed methane (CBM) industry.
• Provides an efficient workflow for characterizing stress dependent perm in CBM.
• Through the application of new analytical equations.
• Other practical applications of the new equations also highlighted.

Stress (or pressure) dependence of coal permeability is a commonly observed and generally accepted dynamic behaviour that is often ignored from production performance forecasting. Reasons for this omission typically include (a) the difficulties in reliably characterizing stress dependent effects from a limited number of pressure buildup (PBU) tests, and (b) large uncertainties in our understanding of both the porosity and compressibility of coals. This paper demonstrates a new analytical workflow, and proposes a new set of equations that overcomes some of those limitations. Engineers can use this paper to analytically translate expected permeability changes with pressure, to productivity changes with pressure differences.This paper utilizes a slightly modified form of Palmer-Mansoori (P&M) model in a workflow that includes.1Estimation of coal cleat volume compressibility using permeability-depth trends.2Characterization of mechanical skins from interpreted apparent skins.3Calculation of stress dependent pseudo pressure (SDPP) – converting permeability changes with pressure to productivity changes with pressure differences – enabling the use of well productivities over time as part of a SDP characterization process.4The matching through regression of relative changes in well productivity indices in groups of wells - utilizing the SDPP approach – with a single stress dependent controlling parameter, in a way that is suited to extrapolating away from well control.Theoretical support for this approach is provided via derivations from published models. A methodology is outlined – sharing the results of a field example – to demonstrate the relative ease with which the analytical process can be applied. Further, pitfalls are highlighted of using well productivities as a direct proxy for permeability changes, or even utilizing coarse grid numerical simulation in the matching process. Finally, further applications and limitations of its application are also discussed.This paper addresses existing knowledge gaps in the coal bed methane (CBM) industry by providing a simple, yet efficient, workflow for characterizing and incorporating stress-dependence of permeability in CBM Reservoir Engineering. This is achieved through the application of new analytical equations to characterize stress dependent pseudo pressure, enabling the direct use of well productivity changes, and can be used standalone, or as a means to accelerate a numerical history matching workflow.