Mercury has been known to be a trace contaminant in natural gas and condensate produced from Unocal Thailand's operations in the Gulf of Thailand since 1985. It has been detected in well streams, produced formation water, and also in solid waste. Mercury is recovered from process vessels, wellhead chokes, piping, and storage tanks. Almost all of the mercury found in natural gas and produced formation water is in elemental form, but in condensate, mercury is mostly in the colloidal form. Unocal Thailand has taken various proactive steps to minimize and limit both people's exposure to mercury and any impact on the environment, in full compliance with applicable laws and regulations. An example of environmentally sound treatment to dispose of mercury-contaminated solid waste is the deep-well injection technique. Another successful in-house developed technology, currently applied to the treatment of discharge water containing trace mercury, is a system using both gravitational separation and chemical flocculation techniques.
Unocal Thailand, a subsidiary of Unocal Corporation, is currently producing offshore natural gas and condensate from nine fields in the Gulf of Thailand. Unocal Thailand's offshore facilities include four central processing platforms, four production platforms, and sixty well platforms. During the first six months of 1996, Unocal Thailand produced a dally average of 757 million cubic feet of natural gas and 27,090 barrels of condensate. Following gas and condensate processing, production platforms and central processing platforms discharge produced formation water into the sea at a daily average of 23,800 barrels/day.
Liquid hydrocarbons in the subsurface reservoirs in the Gulf of Thailand contain trace mercury in organic complex form. Due to high temperature in the reservoirs, this mercury is thermally decomposed to elemental form. Unocal Thailand has detected mercury in gas, condensate, and produced formation water. However, most of the mercury has been recovered from the process with the solid sludge.
Mercury in the Gulf of Thailand
The Gulf of Thailand is approximately 720 km. in length, with a coastline of 2,900 km. and a maximum depth of 84 m. The central portion of the Gulf is over 60 m. deep and is separated from the South China Sea by two ridges at depths of 25 and 50 m., respectively. Unocal Thailand's facilities are located near the center of the Gulf; approximately 100 km. from the coast.
Sources of mercury which migrates in aquatic environments as illustrated in Figure 1 can be man-made discharges through industrial and domestic wastes and natural discharges through leaching and volatilization of the geological formations containing significant quantities of mercury such as the volcanic fumaroles.
There is some evidence indicating natural mercury leaching in the Gulf. The data collected from 12 plugs from conventional cores in Unocal's new exploration area, Pailin field, indicated high mercury content, ranging from 0.02 ppm to 0.23 ppm, associated with coal stringers. Other sources and pathways responsible for mercury contamination in sea water in the Gulf also include atmospheric fallout and direct discharge of industrial and domestic wastes into the five major rivers (the Chao Phraya, the Mae Klong, the Ta Chin, the Bang Pakong, and the Tapi) which flow into the Gulf. These rivers pass through the industrial areas that contribute a significant amount of heavy metals to the river estuary. The estimated amount of mercury which enters the Gulf from atmospheric deposition is 5,000 kg/yr., and estimates from terrestrial runoff range from 5,000 to 10,000 kg/yr. There are various forms of mercury found in river runoff such as inorganic divalent-mercury, metallic-mercury, phenyl-mercury, methyl-mercury, and alkoxyalkyl-mercury.