A US Department of Energy (DOE) project is turning stranded natural gas at marginal or low-production oil fields into fuel for distributed electric power.

This breakthrough in using stranded gas is bringing previously idle oil fields back into production and could boost domestic oil production by 28 million bbl per year within 10 years of its inception, according to the DOE.

Typically, associated gas is vented or flared, re-injected, or left in the ground. A project called

A 30 kW flex microturbine is now generating electricity on a California oilfield by burning ultra-low-BTU gas.
Oil Field Flare Gas Electricity Systems (Offgases), which is managed by the Office of Fossil Energy’s National Energy Technology Laboratory (NETL), has recently introduced another way to deal with stranded gas. This new solution is turning waste gas into fuel for distributed generation power units at marginal well sites in California.

Mature oil production sites are often heavy electricity consumers. According to the California Oil Producers Electric Cooperative, electricity accounts for 40% to 60% of the operating cost of oil production and delivery, and it represents one of the highest expenses in producing marginal oil wells. Pump jacks and other oilfield equipment are run by electricity, and in California, power to operate the equipment is purchased from the utility grid. As a result, the cost of energy figures heavily in the decision to produce or abandon a declining field.

By using microturbines to harness the stranded gas and generate low-cost electricity — which according to the DOE is usually 20% to 40% of the cost of utility grid electricity — the Offgases project is increasing oil production in fields that were previously cost prohibitive to produce. In electrical terms, the equivalent of about 45 MW of potential electrical generation has been identified as stranded gas.

The Offgases project demonstrates how associated gas of various heat content values and quality can be used to generate electricity. This research has been applied in four field installations: one using a high-British thermal unit (Btu) stranded gas with a value above 1,600 Btus per standard cubic foot (scf) of gas; one using a medium-Btu gas that does not meet the quality requirements for commercial pipelines in California; one using harshly contaminated fuel gas with high levels of nitrogen, carbon dioxide, and hydrogen sulfide; and one using ultra-low-Btu gas (below 300 Btu/scf).

The high-Btu gas project involves stranded gas containing more than 1,600 Btu/scf. The oilfield had been shut in for eight years because the operator had no means of handling the natural gas associated with the oil being produced from the field. Researchers chose a Capstone 30-kW microturbine for the project, coupled with a horizontal scrubber to remove produced water, and a small compressor to achieve the line pressure needed for the turbine. After the wells were reworked, production increased to 23 b/d. Production was estimated at about 9,000 scf/d of 1,650-Btu/scf gas.

As of June 2007, researchers were monitoring the project and collecting runtime and equipment reliability data as well as air emissions and operating maintenance figures.
A second application of the new technology took place in a well producing medium-Btu gas that did not meet the quality requirements for commercial pipelines in California. Three Capstone 30-kW microturbines were installed on site to generate power, and a large compressor was added to achieve the pressure needed for the microturbines. The solution also included a vertical scrubber to remove associated liquids and a small refrigeration dryer. The 19-well field that had been at risk for abandonment is now producing 150 b/d of oil, according to a report by the DOE.

The third field in the demonstration contained “harsh” gas — gas that contains naturally high levels of nitrogen, carbon dioxide and hydrogen sulfide (H2S). The unusable gas was being flared, but as of Sept. 6, 2006, enough H2S is being scrubbed from the gas to bring air emissions into compliance using a patented sulfur treating system.

Now, an IR 70-kW microturbine generates electricity through an interconnect permit with Pacific Gas & Electric. The system produces two segregated streams of gas, one containing about 6,000 parts per million H2S and the other containing no H2S. The separate streams allow researchers to test various concentrations of H2S. As of late 2007, researchers were in the maintenance and monitoring phase of the project, collecting runtime and equipment reliability data along with operating maintenance figures.

The fourth demonstration in the Offgases project addressed ultra-low-Btu gas, in this case, defined as gas containing as little as 15 Btu/scf. Because gas of this low quality is not flammable, the operators had been spiking the weak gas with purchased commercial natural gas so the gas could be flared.

As part of the NETL-funded project, operators on this field are now using a 30-kW Flex microturbine to generate electricity. This microturbine employs a new technology that uses catalytic combustors and runs on 15-Btu gas. While the microturbine is working, improvements are still needed to turn this field into a success, the DOE said.

Reservations aside, the present situation that has eliminated flaring along with the cost of adding commercial gas to make flaring possible is an obvious step in the right direction.

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