TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractManagement of mature oil fields requires accurate formation evaluation information. The ability to acquire essential data through casing adds a new dimension to reservoir characterization and affects the reservoir development of a mature oilfield like the Bahrain field. New logging tools and technology fill the gap in evaluation, especially for monitoring water saturation changes over time and for detecting bypassed oil pockets across the zone.In this paper we present the results and validity of a logging through casing run in a depleted well completed in a sandstone reservoir in the Bahrain oilfield. The reservoir has a complex lithology of high-permeability sandstone with thin interbeds of siltstone and shale, erratically deposited but prolific in oil production. The objectives of the data acquisition were to monitor oil depletion through casing and to study the effect of lithology on the water encroachment pattern across the zone.In this paper we describe the interpretation procedures and data validation of the information obtained from an advanced suite of logs run through casing. The paper also focuses on how this new information has resulted in a review of potential further development of this pay in one part of the field.
Management of mature oil fields requires accurate formation evaluation information. The ability to acquire essential data through casing adds a new dimension to reservoir characterization and affects the reservoir development of a mature oilfield like the Bahrain field. New logging tools and technology fill the gap in evaluation, especially for monitoring water saturation changes over time and for detecting bypassed oil pockets across the zone. In this paper we present the results and validity of a logging through casing run in a depleted well completed in a sandstone reservoir in the Bahrain oilfield. The reservoir has a complex lithology of high-permeability sandstone with thin interbeds of siltstone and shale, erratically deposited but prolific in oil production. The objectives of the data acquisition were to monitor oil depletion through casing and to study the effect of lithology on the water encroachment pattern across the zone. In this paper we describe the interpretation procedures and data validation of the information obtained from an advanced suite of logs run through casing. The paper also focuses on how this new information has resulted in a review of potential further development of this pay in one part of the field. Introduction The AC zone in Bahrain field is an erratically deposited sandstone formation that has not been completely delineated in the Bahrain field but is often prolific in oil production. The deposition is so erratic that the sand can develop to 25ft thickness in between two wells separated by only 1000ft, though both wells record no development. Thus it has become a challenge to develop the reservoir and to recover oil efficiently. Figure 1 is an effective thickness map of the zone across the field. New and improved logging tools and technology with encouraging economic factors are making it possible to look at the reservoir more closely than before. Geological Review. The AC zone, some times called the first pay "C", refers to the sandstone facies of the Wara formation. In the Bahrain field, the Wara varies from 60' to 95' in thickness and the sand interval from 0' to 60'. Where the sandstone facies is present it is white to tan, fine to medium grained, friable, well sorted, sub-rounded glauconitic and locally becomes silty. Porosity and permeability are usually high. The sandstone "thickens" in a NE trend direction but is randomly distributed. There are two theories regarding the environment of deposition:The sands were deposited in stream channels cut into a fairly broad, low relief coastal plain or shelf. Most of the sections drawn across the sand show them to be concave up with a more or less flat top.The sands were deposited as offshore bars in a shallow marine environment. The presence of carbonaceous material and lignite just below the sand unit indicate shallow lagoonal environment bordering on non-to marginally marine environment. The sands probably represent channel and bar sands and thus add to the uncertainty of the trends of depositional areas. A layer presumed to be shale, with thickness of 4' to 6', separates the upper sand from the lower sands. This is distinctly seen in the Southern part of the field.
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