In situ determination of relative permeability and capillary pressure curves using multiphase flow rates and pressures of wireline formation testers.

Abstract

Relative permeability and capillary pressure curves are crucial inputs for reservoir modeling and simulation. Measuring these quantities on core samples in a laboratory is a necessary, but lengthy process. Wireline Formation Tester (WFT) logging is routinely applied in field operations for reservoir evaluation purposes. Since a historical record of WFT data often exists for a field, we propose in our study to estimate multiphase flow properties in situ from the acquired WFT data in a relatively short period of time. The WFT tool records, among other measurements, bottomhole pressures, water-cuts and flow rates by pumping out fluids from a hydrocarbon bearing formation during the cleanup process in order to reduce the contamination level near the wellbore. The contamination, which is water based mud (WBM) filtrate in this study, invades from the openhole during drilling and changes the saturation of hydrocarbon and water in an invaded zone near the wellbore. The proposed methodology estimates multiphase flow properties occurring near the wellbore, such as relative permeability, capillary pressure, damage skin and mud-filtrate invasion by inversion of water-cut and bottomhole pressure recorded during the WFT cleanup. Although the mud-filtrate invasion and the WFT cleanup replicate, respectively, secondary processes of imbibition and drainage in the reservoir, our study is designed under the assumption that a single set of multiphase flow curves represents both imbibition and drainage processes. The study shows that the most important parameters are the curvatures of relative permeability and capillary pressure curves during the processes. Although they can be optimized, prior knowledge of saturation endpoints and depth of mud-filtrate invasion improves the results. Noise levels of WFT logging, and heterogeneities of reservoir properties should be integrated correctly in the study. The methodology uses a detailed numerical model of the invasion and cleanup processes, and WFT tool geometry coupled with an optimizer for the inversion. The model investigates reservoir and fluid properties, and represents the events accurately occurring during drilling, logging, invasion and cleanup. The results from synthetic and field examples demonstrate that relative permeability and capillary pressure, mud-filtrate invasion, damage skin can be estimated successfully in the inversion process.