On the use of Wireline Formation Testing (WFT) data: 2. Consequences of permeability anisotropy and heterogeneity on the WFT responses inferred flow modeling

Abstract

Abstract WFT tests are commonly performed to sample fluids and to assess the formation pressures in reservoirs at different depths. The interpretation of these pressure values provides the fluid gradients and the location of the interfaces between these fluids. The WFT data analysis can also be used to assess mobility values from the drawdown, the buildup or both, based on analytical solutions of the diffusivity equation initially developed for the well test analysis. Various publications, comparing the permeability values derived from WFT data with plug measurements, Nuclear Magnetic Resonance (NMR) interpretation, and log modeling, have shown that a good match is infrequently obtained. For most authors, the discrepancy between permeability values could be related to either the uncertainty on fluid viscosity or the permeability anisotropy. This work investigates a new hypothesis – the heterogeneity of the permeability field related to different investigated volumes during the WFT. We use flow modeling in synthetic reservoirs to explain why mobility values inferred from drawdown analysis often differ from mobility values inferred from buildup analysis. It is shown that drawdown mobility values are clearly correlated to the mobility of the cells near the WFT tool. These mobility values are comparable to the mobility inferred from plug measurements. On the contrary, buildup mobility values inferred from WFT hydrodynamic response appear to be controlled by the horizontal and vertical correlation ranges of the permeability field. This paper is the second part of a paper entitled “On the use of Wireline Formation Testing (WFT) data. 1. Field review of 420 WFT tests” that addresses the interpretation of 420 WFT and discusses the results.