The Drilling and Testing of High-Pressure Gas Condensate Wells in the North Sea

Abstract

During the 1980's exploration activity in the North Sea turned towards deep, high pressure, Jurassic gas condensate reservoirs. These reservoirs occur at 16 - 18,000 ft, are abnormally pressured with pore pressures of approximately 15,500 psi, and temperatures approaching 400F. This presented a new challenge to the drilling industry which required the development of new techniques, equipment and philosophies. By 1985 considerable experience had been gained in the drilling and testing of these wells. In mid 1985 Ranger Oil drilled well 29/5b-4 to a depth of 17,930 ft using their own semi-submersible drilling unit, the SEDCO 714. Electric logging indicated that the Upper Jurassic objective at 17,080 ft had a pore pressure of 15,800 psi. The presence of H2S and CO2 were detected and calculations showed that the maximum anticipated tubing head pressure was 13,500 psi. By following the concepts developed from past experience that wells of this type cannot safely be tested from a floating drilling unit, the well was suspended. Procedures were established to permit the safe testing of this well using engineering innovation to develop new equipment and techniques. Re-entry and testing took place in August 1986 using the jack-up GLOMAR MORAY FIRTH and produced a total of 54 mmscf/d and 4,500 BOPD. Maximum wellhead pressure was 12,500 psi, surface temperatures reached 300F and reservoir temperatures of 406F were recorded. By examining the operational procedures, equipment and techniques used to overcome the problems encountered, a drilling and testing philosophy is developed, which provides a field proven basis for the economic exploitation for reservoirs of this type. Introduction By the 1980's the North Sea was a mature area producing over two million barrels of crude oil per day, and exploration interest was turning towards deeper higher pressured prospects. Ranger Oil had been active in the North Sea since the early 1970's and, in order to play a key role in this new round of exploration, built a third generation semi-submersible drilling unit, the SEDCO 714. The SEDCO 714 is the latest of the SEDCO 700 series rigs built by Hyundai in Korea in 1983, with a total variable deck load of 3700 tons and equipped with a Hydril 16.3/4 15M BOP stack. The SEDCO 714 arrived in the North Sea in March 1984 and commenced operations on 29/2a-2. Ranger's exploration activity with the drilling unit commenced in June 1984 with the drilling of 22/27a-1, followed by wells on 22/8a-3, 4/26-1a and 22/27a-2. Each of these Jurassic prospects presented a challenging series of problems. JURASSIC WELLS DRILLED BY RANGER OIL IN THE NORTH SEA Total Max. Pore Max RFT Max. Temp Well Depth Pressure Pressure deg F ft ppg psi 22/27a-1 16,585 17.2 13,620 320 22/8a-3 15,195 17.2 12,974 340 4/26-1a 17,285 17.2 14,353 321 29/5b-4 17,930 17.3 15,849 406 22/27a-2 17,247 16.7 14,140 343 P. 399^

A Method for Handling Gas Kicks Safely in High-Pressure Wells

“…Therefore, the recommended initial kill circulation rate should be as high as the (1) choke manipulation for startup, (2) weighting up of mud (wait-and-weight method), and (3) sufficient time to react to pressure anomalies will allow.…”

“…The drilling and exploitation of these reservoirs have been described as the new challenge to the drilling industry. 1 However, serious wellcontrol incidents have compelled the industry to re-examine the equipment and procedures used to drill these wells. 2 This paper examines a major shortcoming identified recently: the ability of rig equipment to separate and vent large gas volumes from mud safely.…”