Methane Emissions from Natural Gas Compression Operations in Gaslift and Gathering and Boosting Service
Summary:
Modern advancements in methane detection have led to the increased utilization of remote sensing platforms to augment traditional emission surveys. Recent aerial surveys have frequently identified emissions originating from gas compressors, often at emission rates greater than expected. To confirm these findings, better understand the cause(s), and provide “ground truth” estimates for comparison to aerial estimates, a three-week-long field study was performed. Methane emission rates from gas compression operations in upstream (gaslift) and midstream (gathering and boosting) services were measured directly at 24 facilities in both the Permian and Bakken/Williston basins. Measurements were limited to compressor packages (gas compressors driven by natural gas-fired engines or electric motors); other onsite equipment was not within the scope of the current study. Several steps were taken to completely quantify methane emissions originating from compressor packages. Units were screened using optical gas imaging to identify vented and fugitive sources. Identified sources were quantified using a high-flow sampler. Stack testing was performed to quantify the methane portion of “combustion slip”, the fraction of fuel gas that escapes the engines unburned. Engine crankcase vents were also measured.
Results:
Measurements were performed on 79 units, 48 of which were quantified completely, without issue. Among gas-fired units quantified completely, combustion slip from lean-burn engines contributed the majority of emissions encountered (76 %), while fugitives contributed very little (3 %). A roughly even split between rod packing vents (10 %) and engine crankcase vents (11 %) made up the remainder. The majority of units surveyed were four-stroke, lean-burn; only six four-stroke, rich-burn units were encountered in this study. A handful of small, electric compressors were encountered that had high emissions relative to their throughput. Excess emissions due to maintenance issues were observed on a few units, though the majority were operating within normal specifications. Preventing these issues would reduce overall emissions, but would not likely change the observed emission fraction by source in a significant way. Fundamental engine combustion efficiency improvements would be required. Several implementations for incremental emission reductions were observed in this study including crankcase vent capture and recycle, and the routing of rod packing (and other) vents to flares, or low pressure (e.g., suction) lines. Observed combustion slip emissions were compared to proposed default factors in Table W-9 to Subpart W of Part 98 for both lean-burn and rich-burn units. Rich-burn units, as observed, exceed the factor by an order of magnitude and are roughly in line with the proposed factor when excluding study outliers. Lean-burn units, as observed, and with study outliers removed are similar to, but less than the proposed factor by 6 % and 23 %, respectively.
Funding Provided by:
