Performance Evaluation of a Reservoir Under EOR Recovery: Intisar ‘D’ Reef, Concession 103, Libya.

Abstract

Abstract The Intisar "D" reservoir is a carbonate reef of Paleocene origin with no appreciable flow barrier. This is an under saturated reservoir with an oil of 40 °API and solution gas oil ratio of 595 SCF/STB. The reservoir, having 452 ft of net pay, is at a datum of 9,000 ftSS with an initial OOIP of 1.76 billion stock tank barrels and initial reservoir pressure of 4,257 psia. A series of very successful reservoir management strategies have been applied in order to optimize and increase the production capacity and reserves. So far 69.2 % (1.2 MMSTB) of OOIP has been recovered mainly due to implementation of an EOR project. From the commencement of production in 1969 up to 1981, Intisar "D" was under bottom water injection for reservoir pressure maintenance and crestal hydrocarbon gas injection for pressure maintenance, gas conservation and enhanced oil recovery; then water injection was halted, but gas injection has been continued successfully until now. Against common knowledge, oil recovery to gas (immiscible) was more effective than water flooding. For analysis purposes the reservoir was divided in two portions: Water Swept Zone (WSZ, from the base of the reservoir up to the level of WOC in 1981) and Gas Swept Zone (GSZ, from the level of WOC in 1981 to the top of the reservoir) to separately evaluate the displacement sweep efficiencies. Gas injection at the crest of the reservoir has been an extremely successful drive mechanism, although this process has been immiscible most of the time. To December 2005 cumulative oil production was 1,218 MMSTB, of which 823 MMSTB were driven due to gas injection as secondary and tertiary recovery methods while 395 MMSTB were produced due to waterflooding. This is one of the biggest EOR processes worldwide. This paper discusses the strategy used to produce this reservoir and also to compute the produced oil recovery from the different drive mechanism employed.