Performance of Pilot Water Injection in an Oolite Reservoir

This paper describes design, implementation and performance of powered injection pilot and dumpflood injection pilot in Oolitic limestone reservoir in Kuwait. For powered water injection, a closed water injection pilot plant was developed in the field with a minimum pressure of 80 psi. Zubair aquifer was used as source water. The injection pressure in excess of 1500 psi was maintained. Decline in the Injectivity of pilot wells was not observed during the injection period and there was no water breakthrough. Pilot dumpflood from Zubair aquifer was also investigated which showed severe erosion/corrosion of tubing in one well. Based on investigations, powered water injection using aquifer water and effluent water will be implemented for full field application.

The subject reservoir is a faulted, anticlinal structure which has 350-425 ft thick oil column in Oolitic limestone section of Cretaceous age. The area of oil accumulation is about 60 sq. km and is limited by the oil-tar-water contact of varying thickness at depths ranging from 9725 to 9850 ft subsea. The wells completed in this reservoir are prolific producers. The light reservoir oil (34 API) is highly undersaturated.

Production began in late 1959. In the initial production period, rapid pressure decline was observed similar to that of fluid/rock expansion type (Fig.- 1). Following this period, there was steady pressure build-up upon field shut-in/offrake reduction. Gas injection was started at the crest in 4 wells in mid-1967 for storage of surplus gas.

Based on reservoir simulation studies, pressure maintenance by peripheral water injection above the tarmat was recommended to maximise oil recovery. Before full field application of pressure maintenance, powered pilot injection in two wells was recommended. Pilot Water Injection Plants were designed incorporating findings of laboratory study on the Zubair Aquifer water and Oolitic limestone reservoir. Two pilot plants were made operative to inject 20,000 BWPD to each of the two peripheral wells viz. Inj-1 in South in September 1986 and Inj-2/Inj-2A in North in August 1988 (Fig.-2). The pilot water injection plants were destroyed during Gulf Crisis in 1990/91. Post war powered water injection has not been resumed. Two dump water injection pilots were also evaluated in order to take benefit of larger pressure depletion of the reservoir to reduce the cost.

Deposition of carbonates of lower Cretaceous age began within a stable shallow shelf sea environment with the formation of a dense shaly limestone base upon which the Oolitic limestone was deposited. Subsequent development to deeper water facies occurred with the transgression of the marine waters. The oolites were deposited in a very shallow, high energy environment with strong wave and current action. At least three different periods of oolite deposition occurred. The earliest period is seen in the form of relatively small oolite particles in the basal 120 to 140 ft of development. The small oolite particles is reflected in low permeability but good porosity. The next interval of development of about 140 ft thick and larger oolite size has very good permeability in relation to porosity. The third interval characterised by intermediate size oolites, was also 140 ft thick but reduced permeability compared to the middle development. A type log is presented in Fig.-3.

A transgressive sea deposited dense shaly carbonates over the oolite development and so provided an effective seal for later hydrocarbon entrapment.

The structure of Oolitic limestone reservoir may be described as symmetrical anticline with uniformly dipping flanks. The structure axis trend north to south with a steeper dip on the north plunge than on the south. Based on shortened sections, numerous faults have been interpreted and these are generally thought to be in a radiating pattern. Displacement of these faults is normally less than 60 ft. Processing and interpretation of recent 3D seismic acquisition is in progress which is expected to help in confirming the faulting pattern in this reservoir.