Improving Parameter Uncertainty Constraints for Coal Seam Gas Reservoir Simulation using Single Well Models with Particular Emphasis on Relative Permeabilty Assumptions

Abstract

This paper presents elements of an extensive single well modelling (SWM) programme to improve confidence in the parameters used to condition the full-field reservoir simulation model. The impacts of local grid refinement (LGR), fine vs upscaled layering, and a range of static and dynamic properties have been investigated. Values for some fundamental input parameters for coal seam gas (CSG) reservoir simulation, such as coal fracture porosity and relative permeability, are extremely difficult to constrain by laboratory test results and are typically derived from history matching studies. Also, the stress and shrinkage dependent permeability effects exhibited by many coal cleat systems can be very difficult to characterise and other parameter variations could be invoked to produce similar trends in simulated water and gas production and flowing bottom-hole pressure (FBHP) behaviour. Many of QGC's CSG well production histories exhibit some very large pressure drawdowns, and obtaining a good history match to water and gas production histories in addition to shut-in bottom hole pressure (SIBHP) and flowing bottom hole pressure can be problematic for many of these wells. Simulation input parameters such as permeability, porosity and cell to cell transmissibility have been investigated using a wide range of values. Up to now, it has generally been possible to match water and gas rates and either SIBHP or FBHP, but not both pressures. Relative permeability and coal gas saturation have been the most effective parameters used to obtain an acceptable history match (HM) in recent modelling efforts. The full-field modelling assumptions for these parameters have now been validated and informed by the SWM work reported in this paper, and the range of uncertainty narrowed for these critical parameters. For the wells exhibiting the highest drawdowns, initially a very good match was obtained using a very ‘aggressive’ suite of relative permeability parameters and an irreducible water saturation of around 90%. This relies on an assumption of very low or zero critical gas saturation and extremely rapid reductions in water relative permeability as gas begins to desorb from the coal matrix and enter the coal fractures. The validity of these assumptions has been tested by considering wellbore modelling effects, to reducethe reliance on aggressive relative permeability parameters to achieve a history match. The full-field model (FFM) gridding assumptions have also been validated by this work.