Locating the Remaining Oil in a Mature Complexly Channelized Clastic Reservoir Using a Combination of Geological and Reservoir Engineering Concepts

Abstract

Locating the remaining oil (LTRO) in a matured reservoir can be complex and often requires a combination of techniques to achieve the best results. LTRO becomes an uphill task if the reservoir is structurally complex, highly channelized and at a mature stage. The studied reservoir has been on production since 1964 and is at the declining stage of the reservoir life. This paper describes the approach taken to identify additional oil from the areas of un-swept bypassed oil. A methodical approach has been followed to pinpoint those areas of the reservoir where the by-passed oil volumes are expected to be concentrated. The method consists of reviewing historical reservoir performance, fluid-flow dynamics including water movement based on cased and open-hole logs from the wells drilled from time to time, and understanding of the sweep of the reservoir by creating few appropriate sector models. The goal is to comprehend the connection between reservoir actual performance, current sweep and the geological framework of the reservoir. Next step to screen for infill opportunities, EOR opportunities and establish the monetary attractiveness for implementation of these opportunities. The results of this study has shown that the sweep efficiency has not been uniform due to the complexities in development strategy of reservoir consisting of commingled and multiple layer production, peripheral to pattern water injection and water conformance issues etc. Geological complexity also contributed to the non-uniform sweep efficiency. Stratigraphically, the reservoir is subdivided into layered sand and Massive sand units. The layered sand unit consists of shale and coal layers with minimal pressure support and massive sand unit is fully supported by an aquifer. The time lag saturation maps have clearly shown some by passed oil in certain areas of the reservoir. A number of scenarios were worked out to recover these un-swept oil in terms of pore scale displacement and overall sweep efficiency using a representative sector model of the reservoir. A 3D drainage radius workflow has also been applied with simplifying assumptions for production allocation and distribution back into the reservoir. The drainage domains have been defined on the basis of sedimentology and fluid characteristics in combination with quantitative volumetric analysis. Drainage volumes assessment has shown areas where reservoir significant amount of mobile oil volumes is present. The novelty of the new workflow is in the ability to visualize and semi-quantify combined effect of production strategy, reservoir performance, sweep efficiency and underlying geological framework on the drained and undrained areas of the reservoir. This paper provides a systematic methodology for integrating different sources of data and analysis to identify bypassed oil in a mature reservoir and formulate a strategy to recover additional oil.