PEFTEC - Abstract

Abstract Information :

The environmental community is broadly aware of the many recent methane emission studies of oil and gas (O&G) facilities. However, many participants are unaware of several studies comparing common measurement methods in practical, real-world, conditions. Three recent studies[1]-[6] compared downwind, facility-scale, measurement methods with on-site, device-scale, measurement methods. While methods generally agree, studies indicate significant differences between methods. We review study results and likely causes of agreement and disagreement between methods.

The "dual tracer flux" method ("tracer") compares downwind concentrations of tracer gases released at a known rate to concentrations of methane downwind of a facility to estimate emissions concentration [7]. Properly implemented, the method is considered both well developed and robust. However, questions have been raised about the ability of the method to capture exhaust emissions [3], [6] and efficacy for very small emissions [5].

Onsite surveys, commonly referred to as "leak detection and measurement" or LDAM, utilize a several methods (e.g. optical gas imaging (OGI) or laser gas detectors) to identify emission locations and a variety of measurement methods capture and quantify measurements. LDAM is well-recognized and codified in emission reporting regulations, including the US Environmental Protection Agency's (EPA) Greenhouse Gas Reporting Program (GHGRP). However, significant variations exist in implementation of the method. Omissions in measurement requirements has been identified as a potential cause of under-reporting [2]. In addition, issues have been identified with some measurement equipment, including high-flow instruments [8]-[10] and portable acoustic meters.

A second downwind method, commonly known as EPA Other Test Method 33A (OTM33A), has been utilized to assess emissions from well pads [11], [12] and distribution systems. OTM33A estimates emissions from point sources by measuring methane concentration downwind for an extended period. The method relies on variations in wind direction to reconstruct the Gaussian profile of the emission plume and uses plume modeling methods to estimate emission rates. Concerns have been raised about the method's accuracy, particularly for diffuse sources or lofted plumes.

Two studies of compressor stations [1], [6] compare tracer methods to on-site measurement techniques augmented with emissions modeling. Compressor station emissions include methane entrained in engine exhaust as well as raw gas emissions from facility equipment. Studies indicate good agreement between methods, but illustrate that both LDAM and tracer techniques must be applied with caution. In addition, engine exhaust emissions must be analyzed concurrently, or carefully modeled using correct emission factors, to accurately estimate total emissions.

A study comparing tracer, OTM33A, and onsite methods at O&G well pads indicates significant variation between methods. The study highlights significant variation - and likely inaccuracies - in OTM33A under a range of conditions, including large or lofted emissions. LDAM methods encounter problems assessing intermittent pneumatic device and liquid unloading emissions. Tracer methods exhibit difficulties distinguishing low emissions from method failure.

A thorough review of these studies provides insight into efficacy and limitations of common measurement and reporting methods. Results can be utilized to advise the structure of reporting programs, requirements for surveys and reporting, and proper protocols for applications of downwind methods.