Development of the Erawan Gas Field, Gulf of Thailand

Abstract

Summary Erawan gas field is the first major domestic source of hydrocarbons available for industrial development in Thailand. The field has multiple producing facilities that are installed simultaneously and exemplify a tailored design to accommodate current market conditions in vendor and fabrication shops. Equipment modules and their complexity are adjusted to take advantage of worldload, labor prices, and steel prices in different countries, which enables both a rapid schedule and significant cost savings. Introduction In 1979 Union Oil Co. of Thailand and its coventurer, South East Asia Petroleum Exploration Co. (SEAPEC), decided to proceed with Erawan gas field development. Located about 90 miles (145 km) from the nearest land in the Gulf of Thailand, this offshore field was discovered in 1972. Despite the discovery and the drilling of several additional successful wells in Blocks 12 and 13, there were no plans for development because there was no available commercial market for the gas.In 1978 the government of Thailand declared its intention to construct a submarine pipeline to transport the natural gas from Blocks 12 and 13 and other offshore areas into the Bangkok vicinity (Fig. 1). Union, SEAPEC, and the Natural Gas Organization of Thailand (NGOT) [which later became part of the Petroleum Authority of Thailand (PTT)] negotiated a gas-purchase contract, agreeing to deliver gas into the pipeline for eventual power generation and other domestic and industrial uses. This gas and the associated condensate would represent the first domestic source of hydrocarbons available for use in Thailand and would have a dramatic impact on that nation's industrial development and crude oil imports.The commitment to deliver gas by Oct. 1981 allowed only 30 months for planning, engineering, construction, start-up, and checkout of the field. In addition, a grassroots program was required to acquire staff, operators, offices, and other facilities to expand Union Oil Co. of Thailand into an operating unit capable of running the field. The situation represented a major technical and managerial challenge. Geology A narrow, elongated tertiary sedimentary basin occupies the central part of the Gulf of Thailand. This basin contains a number of north-south-trending horsts, grabens, and half grabens. The basin has an extensive complex series of tensional faults. The sedimentary section consists of alternating beds of shale, sandstone. and siltstone, with several coal beds in the upper Miocene and Pliocene sections. Nearly all of the hydrocarbons found have been in rocks of the late Oligocene/early Miocene Ages.The Erawan structure is on the west flank of the basin. The structure is a north-south-trending fault complex more than 17 miles (28 km) long and some 5 miles (8 km) wide.The productive reservoirs are between 4,000 and 9,400 ft (1200 and 2860 m) and are generally from 10 to 50 ft (3 to 15 m) thick. There is a normal pressure gradient down to 7,300 ft (2225 m), and the structure is strongly overpressured below this.The thermal gradient is high in the Gulf of Thailand. Temperatures around 325 degrees F (163 degrees C) are reached at 9,025 R (2750 m). and 390 degrees F (200 degrees C) at 11,540 ft (3517 m) is reached at static conditions. This results in high surface wellhead and equipment temperatures under flowing conditions. The CO2 content of the reservoirs also increases with depth from less than 6% at 4,000 ft (1200 m) to more than 38% at 9,400 ft (2860 m). These relatively high CO2 concentrations in the wellstream, combined with the high temperatures, make design of production facilities and equipment a unique challenge. production facilities and equipment a unique challenge. JPT P. 1204