Performance of Horizontal Wells in a Thin Oil Zone Between a Gas Cap and an Aquifer, Immortelle Field, Trinidad
Authors: Thakur Subhash C. , Bally Keith , Therry Dan , Simon Leslie 
Published: December 2007

Abstract
Six horizontal wells have been drilled in the Immortelle Field, offshore Trinidad since August 1994. The Immortelle horizontal well program was aimed to develop a 48-ft oil column overlain by a large gas cap and underlain by an aquifer. A key challenge in the selection of horizontal well targets was to maximize exposure to good quality pay sands while staying within a 10-ft true vertical depth window, to delay potential gas or water coning. Undulations in the horizontal section need to be minimized to facilitate passage of wire-wrapped sand screens during well completion. Production logging and early production data indicate that the horizontal well program has been highly successful, in spite of some uncertainties in reservoir characteristics. Multidisciplinary teamwork has been the key to finding real-time solutions of the problems encountered during drilling and production. Field performance monitoring and reservoir modeling work continue to provide guidance in current operations and in planning additional development of the reservoir.

Introduction
Immortelle Field is located in the Caribbean sea 40 miles southeast off the island of Trinidad (Fig. 1). Immortelle is primarily a gas field with nearly 2 tcf of original gas in place in 15 different reservoirs. Oil columns ranging from 30 to 70 feet have been identified in three different reservoirs. Immortelle "16 Sand" reservoir has a gross thickness of 250 ft with a 48-ft oil column. The oil has a gravity of 39 API. The oil column is overlain by gas cap (which varies in thickness by fault block) and underlain by water. The 16 Sand reservoir has an estimated 65 million barrels of original oil in place (OOIP) and 275 billion std cu ft of original gas in place. Since an additional demand for gas in the island of Trinidad did not exist in 1991, the development of this oil from Immortelle field appeared highly attractive in terms of monetizing this asset. After the oil production declines, the wells would be recompleted in other horizons to meet the growing gas demand.
Due to the relatively thin oil zone and likelihood of early gas and water coning, low oil recoveries (less than 10 percent of OOIP) were expected from the 16 Sand reservoir if conventional wells were used. Reservoir model studies showed that horizontal wells could produce at higher initial rates, delay high gas and water production, and recover almost twice the oil reserves compared to a vertical well. Industry literature indicates that oil recoveries in the range of 20 to 25 percent are attainable using horizontal wells in such situations. A multidisciplinary project team was formed to design and implement a horizontal well program to effectively drain the 16 Sand reservoir. Six horizontal wells have been drilled since August 1994, of which five are producing from the 16 Sand. A deviated well is also producing from the 16 sand, which provides a means of comparing the performance of a horizontal well with a deviated well. This paper describes the selection of well targets, well completions, and the early production performance of the horizontal wells. The results of a reservoir modeling study to plan future development of the field are also presented.

16 Sand Geology
The Immortelle field is a faulted anticlinal structure with hydrocarbons trapped in three-way dip closures against the normal A-4 fault (Fig. 2).
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