A common occurrence in mature oil fields around the world is that many wells are no longer producing. This means these are either idle or producing next to no oil, not only necessarily due to depleting reserves but also because of declining reservoir pressure. Continuous efforts are being made in the E&P sector to ensure incremental production from these fields is optimized and the maximum amount of recoverable oil extracted.

One innovation is being employed by Chevron on its mature assets in the northern Pattani basin in the Gulf of Thailand. The Benchamas field has 20 platforms and hundreds of wells, with production processed on separate processing platforms and off-loaded to the FSO Benchamas Explorer. Benchamas and its satellites have produced more than 360 MMboe since first oil flowed in 1997.

An inevitable result on oil fields such as these is that a high number of wells eventually lay idle or shut in because their economics are not good enough to justify the installation of a conventional gas lift compressor or electrical submersible pump (ESP) to bring the well back on production. Space and weight restrictions on many of the existing wellhead platforms are other limiting factors.

Two artificial lift techniques combined

Chevron and its partners have achieved a dramatic improvement in production levels in the field area by combining two production enhancement methods for oil wells into one system – mobile well unloading units with a gas lift compressor system. It is the combination of these two forms of artificial lift techniques acting simultaneously that makes the system effective in mitigating the inherent problem in many mature oil wells, which is the rapid decline in reservoir pressure and increase in water production leading to short well life. Both of these production enhancement techniques are widely used in the industry, but each has its own inherent challenges offshore, where cost and space are such limiting factors.

For gas lift compressors these factors are a big hurdle: a typical gas lift compressor usually requires more than US $2 million of investment and has a large topsides footprint, making it unattractive for low-cost operations such as those in the Gulf of Thailand.

The well unloading unit and compressor (WUUC) system, however, has a smaller footprint offshore than previous portable compressors and is therefore much more mobile. For example, each piece of equipment weighs less than 8 metric tons and can be lifted with standard platform cranes.

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The FSO Benchamas Explorer has been receiving production from multiple platforms on the Benchamas field in the Gulf of Thailand since 1999, with oil production levels still strong because of Chevron’s efforts to enhance recovery rates from its existing reservoirs. (Photo courtesy of Tanker Pacific)

Pilot test

In 2009, a small portable gas compressor was built after six months of studies and design work. The system began a pilot test in July that year on the remote REX wellhead platform on the Benchamas field in the 2,460-sq-km (950-sqmile) B8/32 block, where many wells had been shut in as these no longer flowed hydrocarbons to surface naturally.

The system underwent a successful trial on the field, allowing wells to be subject to very low backpressure at surface while also being gas-lifted using captured gas from the unloading system that would otherwise be vented.

A three-phase separator and surge tank are incorporated to separate fluids from the production stream, with an inlet pressure as low as 30 psi. Liquids are pumped back into the production pipeline at system pressure while the associated gas (around 2 MMcf/d) is captured from the production

stream in a three-stage reciprocating compressor, compressed to approximately 1,100 psi, and directed to a gas lift manifold that distributes the gas to four oil wells for injection into the annulus for gas lifting. Using the pilot system, this added 2,100 b/d of incremental oil production from the wells, which can be both flowing to the WUU separator (20 psi to 50 psi suction pressure) and gas-lifted at the same time, creating a closed-loop gas lift system.

Outstanding results

The production results and system reliability have been outstanding, according to Chevron. After two and a half years of operation, the unit has had 97% reliability and added 903,000 bbl of incremental oil in total from the four wells. Those wells continue to produce at a combined rate of more than 1,000 b/d of oil more than two years after the WUUC pilot system began, far exceeding expectations.

Chevron said the development of the WUUC system has taken this drilling project from a sub-economic program to a major success, more than tripling the predrill recovery estimates.

The system also now underpins future development drilling in this area of the field and the accessing of reserves that otherwise would have remained untapped because of poor fluid properties, low well recoveries, long distances from the central processing platform, and the economic constraints of installing a conventional gas lift compressor and/or ESP on platforms.

An additional and environmental benefit of the system is that it has so far prevented the release of more than 1.5

Bcf of greenhouse gas that would otherwise have been vented to the atmosphere.

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The Benchamas field is located in Blocks B8/32 and B9A in the northern Pattani Basin offshore Thailand. (Map courtesy of Kris Energy)

System mobility a key aspect

The system's mobility also is a unique aspect, according to the company, because it is modular and can be rigged up or down in a single 12-hour shift offshore. This enables the unit to service multiple wellhead platforms for maximum usage, supporting a low-cost "pay-as-you-go" rental model for the equipment.

This is a vital aspect in the Gulf of Thailand where the subsurface environment is highly complex and difficult to image on seismic. Reservoir volumes and future production rates are hard to predict, and designing artificial lift systems can be challenging. These subsurface factors make the above business model more attractive by minimizing financial risk and enabling flexibility to mobilize the unit when required.

Its small footprint offshore (80 sq m or 861 sq ft) also allows the flexibility to perform work activities such as slickline and electric line simultaneously while operating the unit. Again, this is said to be critical by Chevron in offshore environments like the Gulf of Thailand where frequent well interventions are required.

At a total operating cost during the pilot trial of $7,000/day, and based on the production gains achieved, this equates to a total cost of $7/bbl over the pilot period, making it a highly economic venture.

There are now three WUUC units operating on Chevron's Gulf of Thailand oil fields, and numerous additional candidate platforms will benefit from the installation of such systems.

The WUUC system has the potential to be applied in multiple offshore locations globally, adding significant oil production and unlocking reserves while reducing greenhouse gas emissions.

Editor's Note: Excerpts for this article have been taken from OTC 2012 paper 23079 presented by Jarrad Rexilius, Tipparat Wamanon, and Akshay Sahni of Chevron.

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