Attic Oil Zone Redevelopment to Improve Oil Recovery in Almost Watered-out Reservoirs

Abstract

The A&B reservoir is a giant, complex, very mature carbonate reservoir undergoing waterflood. Given long history of production and waterflood, the main focus in the development philosophy of this reservoir was to maximize the oil recovery from existing wells. Many of these wells had zones with significant oil saturation behind casing, especially in the attic zone. The key to improve the recovery factor of the mature reservoir was through the redevelopment of the attic zone by shift-up and change-layer workovers. The campaign was started with a saturation logging job which was successful in locating the attic oil zone which then gave high oil rate after the zone was opened. This success was then followed by a comprehensive geological, petrophysical, and engineering review of hundreds of the existing wells. To boost the oil gain, ESP upsizing was programmed for wells with sufficiently high productivity. Several wells have been identified for the campaign. To date, seventeen jobs have been conducted successfully. Overall, the campaign could unlock the attic oil and resulted in significant oil gain. One well with saturation logging prior to the workover experienced notable watercut reduction. The production of the well with bigsize ESP was boosted four times, while the watercut was reduced rather than increased. In other wells, oil production increase was obtained while the watercut being relatively unchanged. Further evaluation is needed especially to determine the minimum attic zone thickness and minimum up-shifting distance from the existing producing intervals that would result in economical oil gain. This paper discusses the results of the past two years of the attic oil redevelopment campaign conducted in the A&B reservoir. The approach presented in this paper is applicable to mature fields at very high watercut stage.