Rock Typing as an Effective Tool for Permeability and Water-Saturation Modeling: A Case Study in a Clastic Reservoir in the Oriente Basin

Abstract

Abstract In clastic reservoirs in the Oriente basin, South America, rock-quality-index (RQI) and flow-zone-indicator (FZI) prove to be an effective technique for rock type classifications. It has long been recognized that excellent permeability-porosity relationships can be obtained once the conventional core data are grouped according to their rock types. Furthermore, it was observed from this study that the capillary pressure curves as well as relative permeability curves also show close relationships with the defined rock types in the basin. These results lead us to believe that if rock type is properly defined, a realistic permeability model, a unique set of relative permeability curves as well as a consistent J function can be developed for a given rock type. The procedure for implementing this technique in our reservoir modeling is described in the paragraphs below. Conventional core data were first used to define the rock types for the cored intervals. The wireline log measurements at the cored depths were extracted, normalized, and subsequently analyzed together with the calculated rock types. A mathematical model was then built to predict the rock type in uncored intervals and in uncored wells. This allows the generation of a synthetic rock type log for all wells with modern log suites. Geostatistical techniques can then be used to populate the rock type throughout a reservoir. After rock type and porosity are properly populated, the permeability can be estimated using the unique permeability-porosity relationship for a given rock type. The initial water saturation for a reservoir can subsequently be estimated using the corresponding rock type, porosity, and permeability models as well as the rock-type-based J functions. It was observed in our study that a global permeability multiplier became unnecessary in our reservoir simulation models when the permeability model is constructed using this technique. Consistent initial water saturation models (i.e., calculated and log measured water saturations are in excellent agreement) can be obtained when the proper J function is used for a given rock type. As a result, the uncertainty associated with volumetric calculations is greatly reduced as a more accurate initial water saturation model is used. The true dynamic characterisitcs (i.e., the flow capacity) of the reservoir are captured in the reservoir simulation model as a more reliable permeability model is used.