Unocal solves mercury problem off Thailand

Unocal's deep-well injection scheme [35,863 bytes] The induced-gas flotation unit is one of the key units in the treatment scheme. Photo courtesy of Unocal. [36,837 bytes] The sludge holding tank. Photo courtesy of Unocal. [30,457 bytes] The water disposal well. Photo courtesy of Unocal. [34,391 bytes] Unocal Thailand Ltd. is using water treatment and reinjection to solve a mercury contamination problem in its Gulf of Thailand gas fields. The hydrocyclone is one of the key units in the

Nov. 16, 1998

5 min read

Unocal Thailand Ltd. has solved a difficult problem that threatened to disrupt its extensive natural gas operations in the Gulf of Thailand: the discharge of mercury-contaminated water.

The U.S. concessionaire has completed a full-scale project, costing more than $6.58 million, aimed at resolving the mercury issue, which was of concern to the Thai public. Trace amounts of mercury were found in the produced formation water from all of Unocal Thailand's gas fields. Unocal's solution employs two techniques: water treatment and water reinjection.

The firm installed state-of-the-art water treatment facilities at the central processing platforms in Funan, Platong, and Satun fields. The facilities for these fields collect and process natural gas from nine of the 10 Unocal-operated gas fields in the area.

Unocal has applied direct injection of the treated formation water into deep wells at Erawan field, the oldest gas field in the area. Unocal chose Erawan because some of its depleted wells were available and near the processing facilities.

The injection method is not being used at other gas fields because Unocal's gas platforms are widely scattered in the gulf, and not all depleted wells are within a reasonable distance of the processing facilities, said the firm.

Treatment method

In the first step of the process, the produced mixture of gas, condensate, and formation water is piped to the central processing facilities, where the components are separated. The water then flows into an oil-water separator, which removes the bulk of the hydrocarbons from the water stream.

The water then flows into a hydrocyclone, which removes any remaining traces of condensate from the water. Nontoxic chemicals, approved for use by the U.S. Environmental Protection Agency, are then injected into the water, which is cleaned of the last traces of hydrocarbons in an induced-gas flotation unit (see photos).

The treated water contains, at most, 5-10 ppb of mercury-a level considered safe by Thailand's onshore discharged-water standards.

Of the total quantity of mercury associated with Unocal's production in the area, 65% is contained in the sludge, which is removed and handled by the company. About 28% is found in the condensate, which is taken care of by the condensate buyer, and 3% contained is found in the natural gas, which likewise is removed and handled by Petroleum Authority of Thailand (PTT), the natural gas purchaser.

The remaining 4% of the mercury is contained in produced formation water and is of public concern, because the water would be eventually discharged into the gulf if it were not reinjected.

Unocal Thailand's offshore facilities consist of about 80 platforms, eight of which release pretreated, produced formation water into the gulf. These eight are production platforms that serve all 10 of the Unocal-operated gas fields.

The total volume of water produced by the fields is about 45,000 b/d. By applying water treatment and reinjection, Unocal is able to remove virtually all of the mercury from the water.

A confirmed success

International environmental groups have recently confirmed that fish caught around Unocal's gas platforms in the gulf are safe to eat and do not pose health hazards.

Unocal officials said the mercury content in the vast majority of the fish tissue samples has been found to be well below the United Nations Food and Agriculture Organization standard and is similar to worldwide average mercury concentrations in fish tissue.

Similarly, seawater sampling has indicated that mercury concentrations in the waters around the platforms are normal and consistent with worldwide levels.

Although the water treatment used by Unocal removes nearly all of the mercury present, there is still no technology available that will remove 100% of the mercury from the water, says the firm.

Since implementing the water treatment and reinjection scheme, Unocal has reduced the amount of mercury discharged into the gulf to merely 13-14 kg/year-a far cry from the estimated 300-400 kg/year that would have been released without the treatment and injection.

The concessionaire pledged to continue to use both methods as it looks for even better technologies for mercury removal. Meanwhile, Unocal continues to routinely monitor the quality of seawater and the mercury content in the tissue of fish living in the vicinity of the gas platforms.

Unocal soon will apply what it says is the most thorough water treatment technology employed at any field off Thailand, and possibly in the world. This will take place at Pailin, a recent gas find that is under development and slated for start-up in early 1999.

Meanwhile, France's Total says it expects to achieve a zero discharge of mercury-contaminated water produced in association with gas from its Bongkot field, also in the Gulf of Thailand.

That achievement will follow the full commissioning of water-injection facilities, built and installed at a cost of $8 million. In this scheme, produced water will be reinjected into depleted reservoir via disposal wells.

Total claims that the Bongkot group is the first exploration and production firm in Thailand to achieve zero mercury discharge. The group was previously widely criticized by local environmentalists because of its mercury problems.