The success of any development drilling project depends heavily on how quickly the drilling team is capable of optimising the drilling programme. After gaining drilling experience from only one well in the area, British Gas, a new operator to the east coast of Trinidad, successfully developed a gas field. In order to effectively develop this field, highly deviated (55 to 70 degree) wells were drilled through young, highly reactive shales and unconsolidated sandstone formations from the Dolphin Platform. Drilling efficiencies were quickly optimised and in just over one year from startup of the first well, a total of eight wells were successfully drilled and completed to an average depth of 11,000 ft MD.
The Dolphin field is located approximately 55 miles east of the island of Trinidad. The field is situated in the Columbus Basin which has a long history of oil and gas production in Trinidad. The basin contains Miocene to Pleistocene-aged, lower delta plain sediments sourced from the Orinoco delta which is located to the south west in Venezuela.
The Dolphin field consists of an elongated structure which trends from the north-west to the south-east with hydrocarbon bearing reservoirs which lie at subsea vertical depths of 5,000 ft to 8,000 ft. The reservoirs are all extremely fine-grained, unconsolidated sandstones interbedded with soft, dispersible shales.
The lease on the Dolphin field was originally awarded to a consortium comprising Tenneco Trinidad Inc. (50%) and Texaco Trinidad Inc. (50%) in February 1974 with Texaco as the operator. The group completed the first discovery well (Dolphin 1) in September 1976 and was followed with the drilling of the Dolphin wells 2, 3 and 4 between October and December 1980. Following the drilling of the Dolphin wells 1 through 4 commerciality of the Dolphin field was declared in August 1981.
As part of the Joint Operating Agreement between Tenneco and Texaco, the operatorship was assumed by Tenneco in October 1981. Tenneco's holdings were then acquired by British Gas in 1989.
In September 1993, British Gas and Texaco successfully executed a gas sales contract with the National Gas Company of Trinidad and Tobago. The contract involved a 20 year supply of gas to the National Gas Company of Trinidad and Tobago with a first quarter 1996 delivery on a predetermined ramp-up schedule.
In order to develop the reservoir model and field development plan it was necessary to drill one more offset well to further appraise the Dolphin structure prior to embarking on a full scale development of the Dolphin field.
The Dolphin 5 well was drilled on the northern exposure of the Dolphin field. It was designed to test the series of Pleistocene aged sands already encountered in the Dolphin wells 1 to 4. Unlike the previous Dolphin wells, the Dolphin 5 well was a directional well.
The well was spudded in January 1995 and reached a TD of 10,000 ft in March 1995 after 67 days of drilling operations. After cementing 20 inch casing at 2,000 ft-MD, the well was kicked off in the 17 1/2 inch hole section and built to a terminal inclination of 45 degrees utilising a 9.8 ppg Low Solids Non Dispersed (LSND) drilling fluid system. On achieving section TD, a well control situation developed because of the exposure of an abnormally pressured gas sand. This necessitated the redrilling of this entire hole section with an increased mud weight of 10.4 ppg.
The 12 1/4 inch hole section was drilled to a TD of 10,000 ft using a 10.4 ppg polymer drilling fluid. A total of six cores were cut and recovered in this hole section.
