Discovery and delineation wells which proved the Udang Field did not yield sufficient data to indicate the complexity of the reservoir. Development drilling showed the Udang pay section to be a series of interbedded shales and sands having individuals gas caps and water/oil contacts. The initial plan, to complete the wells by perforating a 40-foot section and installing a screen with slurry pack, was quickly changed in favour of multi-zone single selective completions.

This paper describes the various types of completions and equipment used, and the success achieved by utilizing single selective completions to control excessive gas and water production.

The field commenced production in January 1979. Since then to the present, about 100 trips have been made with wireline tools to oen or close sliding sleeves with only one failure. In 3 wells the same sliding sleeve was successfully actuated six times. This has been achieved even though some wells have 65 degree inclinations and most all wells produce measurable amounts of sand. It is concluded that these type completions have substantially increased present and projected oil recovery from the Udang Field.

Introduction

Conoco, as operator for itself, Java Gulf Limited, Getty Oil International (Indonesia) Ltd. and Inpex Natuna Limited, has completed development of the Udang offshore field in the Natuna Sea, Indonesia. It is located approximately 1000 Km north of Jakarta as shown in Figure 1. Water depth at this location is 300 feet. Production commenced January 18, 1979 with completion of well A-1 and reached 40,000 BOPD after completion of development drilling in the â??Bâ?¿ platform early October 1981.

Exploration and Discovery

On October 16, 1968 a Production Sharing Contract was signed between Pertamina and conoco Indonesia to explore for and develop hydrocarbons in the Block â??Bâ?¿ contract in the Indonesia South China Sea. The Udang anticline in Figure 2 was primarily delineated by data from seismic surveys run between 1968 and 1975, with most detailed data being acquired in the period 1973 – 1975. In 1977 additional detailed seismic data were obtained in the northeast and north areas of the structure, and in 1979 in the southwest portion of the anticline.

Four exploratory wells, Udang 1, 2, 3, and 4, were drilled on the Udang anticline between July 1974 and May 1975. All wells tested hydrocarbons; however, Udang 1 and 3 had the most significant oil flows.

Development

The results of the first four exploratory wells justified the placing of a production platform (Udang â??Aâ?¿) in the central part of the anticline and the drilling of 13 development wells in the period December 1978 to November 1979. An additional platform (Udang â??Bâ?¿) was set in September 1980 from which an additional 13 wells were drilled with the last well completed on September 23, 1981. (See Figure 2).

The producing reservoir in the Udang Field in the Udang Field is the Gabus Sandstone of Oligocene age. The sand was deposited from a very large deltaic fan identified to be 1500 feet thick and extending 30-40 miles in length with a width of 3 to 5 miles. Udang production occurs in the top 300 feet of this sand/shale sequence, hence the entire field is underlain with water. Several individual sand members have been identified in the immediate field area and given nomenclature as shown on Figure 3. An example log is shown on Figure 4. Most producing zones have average permeabilities exceeding one Darcy.

The complexity of the reservoir was not known when initial development plans were formulated based on information obtained from exploratory wells, 1, 2, 3, and 4. The reservoir was described as two separate accumulations as shown by Figure 5, the Upper Gabus as a thin sand having a large gas cap and a small oil ring; the Main Gabus as a massive sand, with interbedded shales, containing undersaturated oil. The development plan called for all Main Gabus wells to be completed by perforating a 40-foot section in the 7" production string and doing a single inside gravel pack completion using 40–60 mesh Ottawa sand resieved to meet specifications of less than 5% coarse or 2% fine particles. The screen and tell-tale was 8 gauge wrapped all-welded type 316 stainless steel double annealed wire having a controlled Rockwell â??Câ?¿ hardness of less than 22.

Development well A-1, completed on 18 January 1979, did not substantiate the original concept of two main hydrocarbon bearing units, (Upper and Massive Gabus Sands), within the Gabus reservoir. It was temporarily completed as a single well without gravel pack in order to evaluate the sensitivity of sand production and check the performance of the reservoir under this producing condition. Well A-2 found an additional and distinct oil bearing sand not seen in other Udang wells; it was completed as dual single selective with the lower zone gravel packed as per original plans (Figure 6).

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