Gas/condensate relative permeability of a low permeability core: Coupling vs. inertia

Flow around the wellbore of gas/condensate systems, when pressure drops below dewpoint, is controlled by complex interaction of capillary, viscous, and inertial forces. Hence, it is challenging to determine accurately gas/condensate relative permeability (k_r) that is affected by the relative impact of these competing forces. There are a number of reports demonstrating k _r of high per-meability rocks affected by both coupling [the increase of k_r as velocity increases and/or interfacial tension (IFT) decreases] and inertia (i.e., the reduction of k_r as velocity increases) for these low IFT systems. However, there is little information on this subject for low-permeability rocks. In this work, different series of steady-state k_r values for a 3.9-md sandstone reservoir rock with a porosity of 6% are reported. These k_r data sets have been measured experimentally at three IFT levels below 1 mNm^-1 and five velocity levels below 200 md^-1 The results indicate that at the highest IFT, inertia is dominant at low condensate-/gas-flow-rate ratio(CGFR) at test conditions, whereby k_r reduces with increasing velocities. At higher CGFR, an increase in k_r is observed because of the dominant effect of coupling. At lower IFT, the negative impact of inertia on k_r as velocity increases is observed at all CGFR. Jamiolahmady et al. (2008) have recently reported k_r of highly conductive propped fractures showing a more pronounced effect of inertia at lower IFT, but this is the first report of such behavior for low permeability rocks. These k_r measurements are also compared with the corresponding predicted kr using the generalized k _r correlation recently developed by Jamiolahmady et al. (2009). The correlation expresses the combined effect of coupling and inertia with universal parameters. The unique contribution of inertia, as observed in the experiments and predicted by the correlation, is attributed mainly to the high single-phase inertial factor of the rock and fluid properties of the flowing phases. © 2010 society of Peroleum Engineers.