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Permeability is a rock property which measures the ability of a porous medium to transmit fluids and controls the fluid flow direction and rates. Permeability can be obtained from cores, wireline formation tests (WFT), nuclear magnetic resonance (NMR) logs and well tests.Typical usages of WFT include measuring formation pressures along the wellbore, taking fluid samples, identifying fluid types and evaluating reservoir structure. Occasionally, WFT is used to replace a drill stem test (DST) in an exploration well. As in well tests, WFT data are measured in situ but, whereas well tests provide an average over a larger formation volume, WFT permeabilities are generally associated with a smaller volume of investigation around the measurement depth. A limitation of pressure transient analyses with WFT, however, is that they often only yield spherical permeabilities, due to short radii of investigation resulting from small rates and short pretest durations. In such a case, anisotropy can be obtained from formation rate analysis (FRA). Pressure transients generated during fluid sampling by WFT tools can have longer radii of investigation and reach radial flow.In this study, pressure transient analysis (PTA) and FRA techniques are applied first to model generated WFT data, then to an actual gas reservoir using invaded zone and formation fluid properties. PTA derived spherical permeabilities and FRA results are then combined to obtain permeability anisotropy and estimate both horizontal and vertical permeabilities versus depth. These are then upscaled to compare with permeabilities obtained from DST’s.Various upscaling methods were investigated. It was found that (1) weighted arithmetic average upscaling yields the best results for horizontal permeability (within 20% of DST permeabilities), and (2) harmonic averaging of vertical permeabilities gives the best match over a wide range of anisotropy values.This study confirms that upscaled WFT permeabilities are representative of the formation permeabilities, provided appropriate upscaling methods and fluid properties representative of the test zone are used in the WFT analysis. |
