Effect of an invaded zone on pressure-transient data from multiprobe and packer-probe wireline formation testers

This paper examines the effect of a mud-filtrate-invaded zone on pressure transients from multiprobe/packer-probe wireline formation testers (WFTs). Invasion zones are modeled as composite zones concentric with the wellbore that have different rock and fluid properties (permeability, porosity, viscosity, and compressibility) from those of the native uninvaded formation. The results show that for multiprobe wireline testers, the sink(production) and horizontal-probe pressure responses are highly affected by the invaded-region properties, while the vertical-probe pressures are influenced mainly by the properties of the uninvaded zones. For the packer-probe configuration, similar results are obtained (i.e., the vertical-probe pressures are influenced mainly by the properties of the uninvaded zones, while the packer-interval pressures at early times are influenced by the invaded-zone properties). It is shown that if the invaded zones are incorporated into the interpretation process with a 3D r-theta-z single-phase, finite-difference model like the one developed in this work, simultaneous matching of spatially available WFT pressure-data sets using nonlinear regression can provide estimates of both invaded- and uninvaded-zone parameters. A synthetic example of a multiprobe test is presented to confirm the theory and procedures developed in this work.