Relative permeability and capillary pressure curves are crucial inputs for a reservoir description. However, measuring these quantities on core samples in the laboratory is an extensive and time-consuming process. Wireline Formation Tester (WFT) logging is routinely applied in field operations for reservoir evaluation purposes. Often a historical record of WFT data exists for a field which can be re-interpreted, and since this interpretation takes days, rather than months to years in the case of core analysis, we propose in this paper to revisit the acquired WFT data with the purpose of estimating multiphase flow properties.
WFT logging is generally conducted in an open hole environment. By the time of logging, the near-wellbore region has been exposed to mud filtrate invasion. In the case of immiscible mud filtrate and formation fluid the invasion resembles a small scale water-flood process. During WFT sampling the mud filtrate is first cleaned and formation fluid subsequently sampled in a multiphase flow environment while measuring bottom-hole pressures and water-cuts. As shown in previous papers (Cig et al., 2014, 2015), the measured WFT data can be utilized in an inversion workflow to estimate relative permeabilities by combining a forward model of the cleanup process with an optimization engine.
The proposed methodology starts with an open hole log interpretation which provides reservoir properties including a saturation distribution. The filtrate invasion is represented as accurate as possible from the open hole logs and drilling reports. WFT tool geometry and its internal tool storage and fluid segregation effects are incorporated into the modeling. A numerical forward model is then simulated within an optimization workflow where relative permeabilities, capillary pressures, damage skin, and depth of mud filtrate invasion are estimated by minimizing a misfit function between measured and modeled bottom-hole pressures and water-cuts. Industry accepted parameterization techniques are used for the relative permeability and capillary pressure curves. Initial parameter estimates are provided from the interpretations of the open hole logs, such as resistivity, dielectric, nuclear magnetic resonance, as well as from pressure transient analysis.
Previously we have studied WFT data from a 3D radial probe (3DRP) and a dual packer (DP) (Cig et al., 2014, 2015). The proposed methodology was also validated with synthetic datasets in the same papers. In this paper we extend the methodology to a single probe (SP) inlet and to joint inversion of both capillary pressure and relative permeability curves. We demonstrate the methodology using field data from a Central Asian clastic reservoir. Interpretation of multiphase flow properties from WFT data represents a valuable complement to core measurements and will help to condition reservoir models for more reliable forecasting.
