The need for higher oil recoveries and longer production plateaus have led to the large scale implementation of Enhance Oil Recovery mechanisms across carbonate reservoirs in the world, especially in brown fields. The success of these mechanisms relies heavily on the accurate description of geological phenomenon and their characterization in static models. This paper summarizes the challenges of successful development of a mature, highly heterogeneous carbonate reservoir in a brown field in the Middle East with presence of Bitumen in the reservoir intervals, using CO2WAG mechanism.
This paper discusses different aspects of Bitumen characterization, beginning with a brief summary of the geological concept behind the preferential Bitumen accumulation within highly cemented intervals using high resolution core & thin section descriptions in the area. The lateral distribution of these intervals was then mapped by integrating core, signatures from logs (reduced porosity), high seismic amplitude signatures in 3D volume and production/injection data from nearby development wells. To capture this phenomenon in the static models, Bitumen was modeled as a discrete property guided by the geological concept. The porosity model includes the impact of Bitumen as the logs capture the degradation. The permeability model was modified by reducing the permeability in cells with Bitumen with a multiplier, since core RCA is subject to cleaning which may result in non-representative measurements.
The major findings & conclusions of the project are attributed to the detailed appraisal campaign in this area of the field with focus on identifying and refining presence & distribution of Bitumen using nuclear magnetic resonance logs. MDT data with Vertical interference tests at points above and below the Bitumen confirm no communication. This has impacted the placement of wells within Bitumen area, since CO2WAG mechanism relies on sweep from upward rising CO2 plume which is obstructed by presence of heavy continuous Bitumen accumulations. Improved saturation distribution in models is achieved by using dielectric saturation logs, which results in reduced uncertainty for STOOIP quantification within Bitumen rich regions of the field. An injector-producer pair of Early Production Scheme wells is planned in which will confirm performance with current placement scenarios based on above understanding of Bitumen.
The case study identifies and significantly demonstrates the impact of geological phenomenon on the recovery & sweep efficiency of CO2WAG mechanism. Development scenarios must consider the inherent reservoir complexities that are recognized by detailed geological studies, in order to provide representative forecasts that in turn influence the economic viability of the project.