Evaluating Natural Gas Emissions from Pneumatic Controllers from Upstream Oil and Gas Facilities in West Virginia

Abstract

In April of 2018, an optical gas imaging (OGI) and full flow sampler (FFS) emissions measurement study of pneumatic controllers (PCs) was conducted at 15 oil and natural gas production sites in West Virginia. The objective of the study was to identify and characterize PC systems with excessive emissions caused by maintenance issues or nonoptimized process conditions. A total of 391 PC systems were found on the sites and all were classified by the operator as snap-acting (on/off) intermittent venting PCs (IPCs) that should exhibit little gas release while the PC is closed between actuation events. The population was comprised of two groups, 259 infrequently actuating, lower emitting (LE) IPCs and 132 gas processing unit (GPU) liquid level IPCs and associated dump valve actuators that vent more frequently and have larger emission volumes. Using a PC-specific OGI inspection protocol with an assumed whole gas OGI detection threshold of 2.0 scfh, only 2 out of 259 LE-IPCs exhibited OGI detectable emissions indicating good inspection and maintenance practices for this category. Due to combined (ganged) GPU exhaust vents, the OGI inspection of the GPU liquid level IPCs was comparatively less informative and determination of single component IPC emissions by the FFS was more difficult. The time resolved FFS measurements of GPU IPCs defined three categories of operation: one that indicated proper function and two associated with higher emissions that may result from an IPC maintenance or process issues. The overall GPU IPC emission distribution was heavy tailed, with a median value of 12.8 scfh, similar to the 13.5 scfh whole gas IPC emission factor (EF). Total emissions were dominated by non-optimal temporal profile high-emitter IPC cases with the top 20% of IPC systems accounting for between 51.3% and 70.7% of GPU liquid level IPC emissions by volume. The uncertainty in the estimate was due to the ganged nature of the GPU exhaust vents. The highest GPU IPC emission came from a single malfunctioning unit with a measured whole gas value of 157 scfh. Up to six IPCs exceeded 100 scfh. An analysis of FFS emission measurements compared to liquids production per IPC unit employed indicated that production sites operating at a high level of liquids production test the limits of the site engineering, likely resulting in higher IPC emissions. Overall, this study found that the LE-IPCs with OGI-verified low closed bleed rates may emit well below the IPC EF while GPU liquid level IPC systems are likely well represented by the current IPC EF. IPCs that are experiencing a maintenance or process issue or that are operating at sites with a very high product throughput per IPC employed can emit at rates exceeding ten times IPC EF.

Keywords: Greenhouse Gases; Methane Emissions; Oil and Natural Gas Production; Pneumatic Controller Emissions; Volatile Organic Compounds.

Figure 1.

FFS operation schematic where the sample is drawn into the inlet on the left, passes through the system, and exits at the outlet to the right.

Figure 2.

FFS-determined CH₄ MER from liquid level IPC pilots (white/blue circles) and dump valve actuators (white triangles), seven values below 0.2 scfh CH₄ are not shown. Closed black circles represent the normalized MER for CH₄. Emission categories A, B, and C presented in Figure 3 panels A1, A2, B1, B2, and C1, are shown, with C2* the combined result of a single pilot into a split header (RN82A, RN82B). The EF and OGI reference lines are scfh whole gas.

Figure 3.

FFS CH₄ timeseries for three categories GPU IPC pilot emissions: (A1), (A2) category A, (B1), (B2) category B, and (C1), (C2) category C, with (—) indicating valid FFS data, (—) indicating quality assurance-removed FFS data from zero check or other, and (—) showing CH₄ for valid FFS data; histograms adjacent right. Shaded box inserts contain SI RNs, number of IPCs potentially contributing to the measurement, and CH₄ MER.

Figure 4.

(a) Estimate of whole gas emissions from the 132 GPU liquid level IPC systems by category (A, B, or C) found using the equal apportionment (low limit) case assumptions with inset showing emission breakdown by category. (b) same using the high limit case assumptions.

Figure 5.

Comparison of approximate daily liquids production per liquid level IPC installed versus liquid level IPC site whole gas emission ratio. The label indicates the production site number with the RNs for all high emitter IPC devices shown. The dashed line indicates 200% of expected emissions level (whole gas basis).