Uncertainty in well test and core permeability analysis: a case study in fluvial channel reservoirs, northern North Sea, Norway

Reservoir permeability is one of the important parameters derived from well test analysis. Small-scale permeability measurements in wells are commonly made using core plugs or, more recently, probe permeameter measurements. U pscaling of these measurements for comparisons with the permeability derive d from drill stem tests (DSTs) can be completed by statistical averaging me thods. DST permeability is commonly compared with one of the core plug aver ages: arithmetic, geometric, or harmonic. Questions that commonly arise are which average does the DST-derived permeability represent and over what re gion is this average valid! Another important question is how should the da ta sets be reconciled where there are discrepancies? In practice, the permeability derived from well tests is commonly assumed t o be equivalent to the arithmetic Cin a layered reservoir) or geometric Cin a randomly distributed permeability field average of the plug measures. Th ese averages are known to be members of a more general power-average soluti on. This pragmatic approach (which may include an assumption on the near-we ll geology) is commonly flawed, owing to several reasons that are expanded in this article. The assessment of in situ reservoir permeability requires an understanding of both core (plug and probe) and well test measurements i n terms of their volume scale of investigation, measurement mechanism, inte rpretation, and integration. This article presents a comparison of core and well test measurements in a North Sea case study. We undertook evaluation of three DSTs and associated core plug and probe data sets from Jurassic fluvial channel sandstones in a single field. The well test permeabilities were generally found to differ from the core estimates, and no consistent explanation could be found for t he group of wells. However, the probe permeameter data were able to further constrain the core estimates. This study highlights the uncertainty in eff ective in situ reservoir permeability, resulting from the interpretation of small (core) and reservoir (well test) scale permeability data. The techni ques used are traditional upscaling combined with the Lorenz plot to identi fy the dominant flowing interval. Fluvial sandstones are very heterogeneous , and this exercise is instructive in understanding the heterogeneity for t he guidance of reservoir models in such a system.