Abstract

Joint work by MOI reservoir engineers and production geologists has identified layers of relatively low permeability extending over a wide area in the Arun Field. These layers appear to be acting as a restriction, controlling vertical communication within the reservoir.

Production geologists developed a field-wide correlation based on porosity trends and divided the reservoir into four zones. From conventional core data a permeability versus porosity transform was established for each zone and used to estimate permeability in uncored wells. An outcome of this work was the recognition of tight subzones which were mappable over a wide area.

Permeability-thickness values calculated by reservoir engineers from pressure build-up data for total thickness compared favorably with permeability-thickness derived from the transforms. However, when the same calculation was applied to perforations below the tight zone after running liner, a much lower permeability-thickness resulted which was found to match closely the transform-derived permeability-thickness for the reservoir section below the tight zone. Similar contrasts were observed in other wells where both lower zone and total section test data were available.

The conclusion is that the Arun reservoir contains a low permeability layer which restricts flow between the upper and lower levels. This concept will be a valuable aid in managing the reservoir. Such a tight zone will have prevented injection gas from sweeping the underlying section, which consequently should still contain condensate-rich gas and can be exploited by the suitable location of injection and completion intervals below the tight zone. This project is a good example of how a better understanding of the reservoir can be gained from interdisciplinary, cooperative effort between engineers and geologists.