Stranded gas

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 systems.

The project is bringing previously idle oil fields back into production and could boost US oil production by some 28 million barrels per year within the next 10 years.

Stranded gas is natural gas that is uneconomic to produce for one or more reasons: the energy, or Btu content, may be too low; the gas may be too impure to use, or, the volume may be too small to warrant a pipeline connection to the gas infrastructure.

Non-commercial gas is sometimes produced along with oil, becoming an environmental liability. This unwanted by product of oil production has become a major problem in California oil fields where producers have been forced to abandon sites early, leaving valuable reserves of domestic oil untapped.

Typically, there are three ways to deal with stranded gas: venting or flaring the gas contributes to air pollution without any beneficial offsets from the gas, using electrical energy to re-inject the gas incurs significant extra costs and, shutting down oil production leaves valuable oil in the ground.

But now researchers have recently found another, useful, way to solve the stranded gas problem.

A project managed by the Office of Fossil Energy's National Energy Technology Laboratory (NETL) called the Oil Field Flare Gas Electricity Systems (OFFGASES) project is turning the waste gas into a valuable fuel for distributed generation power units at marginal well sites in California.

Oil production sites are heavy electricity consumers. According to the California Oil Producers Electric Cooperative, electricity accounts for 40 to 60 percent of the operating cost of oil production and delivery, and it represents one of the highest expenses in producing marginal oil wells.

In California, equipment such as pump jacks are all run by electricity, and this power must be purchased from the utility grid. This figures heavily in deciding which sites remain economical to produce as oil production declines and which ones must be abandoned.

By using microturbines to harness the stranded gas and generate low-cost electricity - usually at 20 to 40 percent of the costs of utility grid electricity - the Distributed Generation/OFFGASES project is increasing oil production in previously hopeless fields, making use of a fuel that was previously considered unusable and uneconomic to produce.

The project is conducting four field demonstrations with fuels of varying energy contents and quality. Three of the demonstrations have shown success so far.

In the first, a demonstration using high-Btu gas, which contains more than 1,600 Btu per standard cubic foot of gas, boosted oil production in its three-well marginal oil field from 10 barrels per day to 23 barrels per day.

In the second, a demonstration with medium-Btu gas, which does not meet the quality requirements for commercial pipelines in California, is now producing 150 barrels of oil per day in a 19-well field that had been at risk for abandonment.

In the third, a field containing "harsh" gas, which contains naturally high levels of nitrogen, carbon dioxide, and hydrogen sulphide, has been brought into compliance with air emissions regulations by scrubbing hydrogen sulphide from the gas using a patented sulphide-treating system.

The fourth demonstration deals with ultralow-Btu gas, which has as little as 15 Btu per standard cubic foot of gas. This gas is of such low quality that it's not immediately flammable and therefore cannot even be flared - operators have been spiking the weak gas with purchased commercial natural gas just to flare it.

As part of the NETL-funded project, operators are now using FlexEnergy's Flex-Microturbine, a new technology that uses catalytic combustors and actually runs on 15 Btu gas. While the microturbine is working, improvements are still needed, and researchers are testing the equipment needed to turn this field into another success.

NETL demonstration partners include FlexEnergy, the Interstate Oil and Gas Compact Commission, California Oil Producers Electrical Cooperative, California Energy Commission, and California South Coast Air Quality Management District.