AMOCO PLANS TEST OF NITROGEN INJECTION FOR COALBED METHANE

Amoco Production Co. hopes by late November to begin a 2 year nitrogen enhanced, coalbed methane recovery pilot in the San Juan basin. Amoco wants to quantify methane recovery gains achieved by injecting nitrogen gas into methane bearing coal seams. The company plans to inject 1 MMcfd of nitrogen for 1 year into a 20 ft interval of Cretaceous Fruitland coal on a lease in LaPlata County, Colo., 9 miles southeast of Durango. It expects to spend about $2 million on the 2 year project.

Oct. 28, 1991

5 min read

Amoco Production Co. hopes by late November to begin a 2 year nitrogen enhanced, coalbed methane recovery pilot in the San Juan basin.

Amoco wants to quantify methane recovery gains achieved by injecting nitrogen gas into methane bearing coal seams.

The company plans to inject 1 MMcfd of nitrogen for 1 year into a 20 ft interval of Cretaceous Fruitland coal on a lease in LaPlata County, Colo., 9 miles southeast of Durango. It expects to spend about $2 million on the 2 year project.

Field work can begin after Amoco receives an underground injection permit from the Environmental Protection Agency. EPA will issue the permit after a 30 day public appeal period that is expected to begin this month.

After a Sept. 25 hearing on Amoco's injection permit application, EPA received written comments until Oct. 11. EPA was expected to respond to all written comments within 3-4 weeks, after which the appeal period will begin. Amoco applied for the permit more than 1 year ago.

Amoco's laboratory tests have shown that more methane can be recovered at faster rates by flooding coal seams with a less adsorbing, inert gas like nitrogen. Methane desorption is achieved by reducing partial pressure of methane rather than total reservoir pressure.

Nitrogen can strip methane from coal without reducing total system pressure.

PILOT FIELD WORK

When it has the EPA permit, Amoco will begin pilot field work by drilling two injection wells-15-3 Simon and 15-4 Simon.

The lease where the pilot will take place currently holds three wells.

The 15-2 Simon, drilled in June 1986 and producing 230 Mcfd of methane, will be the pilot production well. The 15-2R Simon, drilled in December 1989, and 22-1 Simon, drilled in April 1989, will be converted to injection wells for the project.

If Amoco is allowed to start drilling the two new wells by late November, it expects to be ready to start injecting nitrogen early in 1992.

Nitrogen will be generated on site with a membrane filter system. Amoco plans to use a 350 hp lean burn compressor to increase nitrogen pressure for injection to 2,000 psi at 200 F. from 135 psi at 126 F.

In a full scale field application, some nitrogen would be recovered from the production stream and recycled through injection wells into the reservoir. For the pilot, Amoco intends to blend the full production stream with other methane for sales.

The nitrogen injection rate will remain the same throughout the first year of the test. But Amoco officials expect to be forced to increase injection pressures as time passes to offset a net increase in reservoir pressure from nitrogen input.

Higher reservoir pressure is expected to increase permeability by opening fractures in the cleat system. That is expected to increase production rates and help offset the need for higher pressure to inject the specified daily volume of nitrogen.

During most of the test, Amoco will divide nitrogen equally among the four injection wells. But the company plans to measure changes in the coal seam's response when the injection pattern is changed.

Amoco will not inject nitrogen during the second year. The reservoir is expected to continue responding to the nitrogen flood for some time after injection ceases.

To focus the pilot solely on testing and analyzing performance of the targeted reservoir, Amoco plans to isolate the pilot coal seam with packers. The company will collect core samples to test relative permeability, porosity, and other variables.

Amoco will inject nonradioactive tracers-sulfur hexafluoride gas and several freon gases-into the Fruitland coal seam to help track reservoir performance. The company also plans to collect p-wave seismic data to help image the reservoir's physical parameters.

MORE EFFICIENT PRODUCTION

In most instances, operators produce methane from coal seams by depleting reservoir pressure. While pressure depletion is a simple, effective production method, Amoco has confirmed in laboratory tests that methane can be recovered more efficiently from coal by reducing only the partial pressure of methane.

Coalbed methane production rates usually are low, and ultimate recovery is limited generally to less than 50% of the gas in place, even after decades of production.

Amoco said desorbing methane from coal essentially is the same process as regenerating a saturated adsorbent bed such as silica gels, charcoals, zeolites, catalysts, or molecular sieves. So methods of regenerating adsorption beds are equally applicable to recovering methane from a coal seam.

Helium is another inert gas that effectively strips methane from coal seams. But Amoco chose nitrogen for its San Juan pilot because nitrogen is cheap and abundant. That makes nitrogen a practical choice for accelerating methane recovery in field applications.

Nitrogen sorbs on coal but less than methane. And it still can effectively strip methane from coal. Because it releases methane both by sorption displacement and partial reservoir reduction, nitrogen helps maintain reservoir pressure.