Wellhead Innovations for Hot High-Pressure Wells

Abstract

Summary Mobil Oil Indonesia's (MOI's) Arun wells produce gas A rates up to 150 MMscf/D (4.25 × 10 6 M 3 /d). The gas contains 14 % CO2, and up to 100 ppm H2S. The Rowing wellhead temperature is about 325deg. F (163deg.C). The original shut-in surface pressure was 5,800 psi (40 MPa). Failure of the resilient wellhead casing seals occurred in about 40% of the wells. This was attributed primarily to the effects of high flowing wellhead primarily to the effects of high flowing wellhead temperatures and to temperature cycling between flowing and shut-in conditions. MOI decided that a metal to metal seal, between the 9%-in. (24.5-cm) casing long string and wellhead, was required to ensure a long-term effective wellhead seal for Arun producing conditions. Wellhead metal-to-metal seals of the size and type required for 9 5/8%-in. (24.5-cm) casing did not exist anywhere in the industry. We worked with the manufacturers of the wellheads used in Arun who designed, tested, and manufactured metal-to-metal wellhead seals. Special tools were developed to field-machine the casing so that an effective metal-to-metal seal is obtained between the casing and the wellhead. Arun Field Arun field is one of the largest gas fields in the world. It is in the rice-growing area of North Sumatra about 8.7 miles (14 km) from the coast (Fig. 1). The field was discovered by MOI in 1971 in the Bee Block Contract Area in Aceh Province, North Sumatra. The structure is about 11.5 miles long and 3. 1 miles wide (18 1/2 km long and 5 km wide) wide a productive area of about 23,400 acres (94.7 × 10 6 M 2 ). The reservoir is a carbonate reef. The top of the reef averages about 9,800 ft (2990 m) subsea. Both the original reservoir pressure of 7,100 psi (49 MPa) and the reservoir temperature of 352deg. F (178deg.C) are abnormally high. The hydrocarbon well stream contains 13.8 mol% CO2, 5.9 mol% water, 50 to 100 ppm H2S, and traces of mercury. About 45 to 50 bbl/MMscf (0.00028 m 3 /std M3) of condensate are produced with the gas. produced with the gas. Producing wells, production facilities, and compression facilities are placed in clusters where the produced fluids are cooled, dehydrated, and separated (Fig. 2). Excess produced gas is compressed and injected into a number of downdip injection wells. Gas and condensate flow through separate pipelines to Pertamina's liquefied natural gas (LNG) plant on the Pertamina's liquefied natural gas (LNG) plant on the coast near Lho Seumawe for processing, liquefaction, and export. Design of Arun Wells The Arun formation is a thick, highly productive carbonate reef. To take advantage of this productivity, 7-in. (17.8-cm) tubing was chosen. A diagram of the original producing wells is shown in Fig. 3. The casing strings of producing wells is shown in Fig. 3. The casing strings of 20-, 13%-, and 9%-in. (50.8-, 34.0-, and 24.5-cm) are run and cemented to the surface. Original Design of Arun Wellheads (Fig. 4) The Arun well producing conditions are among the most severe in the world. The original reservoir pressure was 7,100 psi (49 N4Pa) at an average depth of 10,000 ft (3050 m), and the reservoir temperature is 352deg. F (178deg.C). The wells are designed to produce at rates of 150 to 175 MMscf/D (4.25 × 106 M3 /d to 5.0x 106 M3/d) with 45 to 50bbl/MMscf 0.00028 M3 /std M3) Of condensate. The original surface wellhead pressures were 5,800 psi (40 MPa) shut-in and 4,900 psi (34 MPa) flowing. The surface temperatures range from 100deg.F (38deg. C) when the well is shut in, to 325deg. F (163deg. C) when the well is flowing, to 80deg. F (27deg. C) when the well is being treated. JPT p. 284