The US natural gas industry received a preview of a new regulatory push on Apr. 15 when the Environmental Protection Agency published white papers on emissions of methane and volatile organic compounds (VOCs) in five operational categories.
The agency released the papers for external peer review as part of a White House strategy to reduce methane emissions (OGJ Online, Mar. 29, 2014).
The white papers cover emissions from compressors, completions and production of hydraulically fractured oil wells, leaks, liquids unloading, and pneumatic devices.
EPA will accept public comments on its emissions estimates and technical assessments until June 16. Methane is a greenhouse gas (GHG), and VOCs are precursors to ground-level ozone.
Here are summary comments the white papers:
• Compressors. Net methane emissions in 2012 in EPA's current inventory of GHG emissions were 86,259 tonnes from production, 724,295 tonnes from gas processing, and 1.261 million tonnes from transmission and storage. Possible controls for reciprocating compressor emissions include period replacement of rod-packing systems. New technologies in use with compressors capture emissions and route them back to the process. For centrifugal compressors, emissions may be controlled by dry-seal designs. Dry-seal centrifugal compressors have lower emissions, require less maintenance, and are more energy-efficient than wet-seal units, and costs are comparable, EPA said. With wet-seal compressors, it might be feasible to capture emissions from seal oil and reroute recovered gas back to the compressor or another process or to burn the gas.
• Completions and production from hydraulically fractured oil wells. Information is limited on uncontrolled emissions from fraced oil-well recompletions and controlled emission factors for fraced oil-well completions and recompletions. Estimates of uncontrolled methane emissions from hydraulically fractured oil-well completions range from 44,306 tons/year to 247,000 tons/year. Reduced-emission completions (RECs) can be effective in some oil wells producing associated gas. National data aren't available for numbers of RECs or for shares of wells for which emissions were vented or flared during completion. Other technologies under investigation for controlling emissions during completion include gas reinjection, NGL recovery, and use of gas for power generation for local use.
• Leaks. EPA's inventory estimates annual potential methane emissions from leaks at 332,662 tonnes from gas production, 33,681 tonnes from gas processing, and 114,348 tonnes from transmission. Studies indicate most methane and VOC leaks occur from a minority of components, and one indicated the leaks come from a minority of sites. Portable analyzers can effectively locate and measure leaks from production sites. The use of optical gas imaging and ambient/mobile monitoring is increasing and can be used in tandem. EPA's studies indicate the cost of leak detection is "generally far larger" than that of leak repair. Directed inspection and maintenance programs can decrease leak emissions.
• Liquids unloading. Liquids unloading of gas wells, according to EPA's GHG inventory, accounted for 14% of all emissions from gas production. Most emissions from this source come from "a small percentage of wells." Among several technologies the industry has developed to remove liquids from wells and lower emissions during blowdowns, the use of plunger lifts is the most common. Efficiency of plunger lifts in emission reduction varies greatly depending on how the system is operated.
• Pneumatic devices. According to the EPA inventory, pneumatic-controller emissions are 13% of overall methane emissions from the oil and gas industry. Emissions from natural gas-driven pneumatic controllers vary depending on the industry sector—production, transmission, or storage—and the controller type. Low-bleed, gas-driven controllers can replace high-bleed, gas-driven controllers in many but not all applications. Where electric power is available, instrument air systems can replace natural gas-driven pneumatic controllers and eliminate methane and VOC emissions. Zero-bleed, mechanical, and solar-powered controllers can replace continuous-bleed controllers in some applications but aren't broadly applicable. Techniques for mitigating emissions from pneumatic pumps, which result in venting, include use of instrument air pumps and electric pumps. EPA's inventory estimates pneumatic pumps account for 16% of overall methane emissions from gas production and processing.