Mid-Continent Methane Emission Estimates ľA Collaborative Study for the Department of Energy
In this first-of-its-kind Mid-Continent Methane Emissions Study, researchers from across the region, universities, government agencies, and industries joined forces determined to reconcile longstanding discrepancies between measurement methods in methane loss rates from onshore oil and gas developments in multiple basins. Seeking to bring public and private sectors better understanding of this issue, the combined resources of these groups resulted in weeks of field study and conclusive data. This team discovered that in previous studies, top-down measurements reported much higher methane leak rates than bottom-up methods. Equipped with that knowledge, this team used paired measurements from the same natural gas sources to determine the inconsistencies in measurement methods. The result of a field campaign, which happened over a five-week period from late September to early October 2015 is described in the study overview below.
Research Outcomes Reconciling Top-down and Bottom-up Methane Emission Estimates from Onshore Oil and Gas Development in Multiple Basins
(Funding for this work was initially provided by RPSEA/NETL contract no 12122-95/DE-AC26-07NT42677 to the Colorado School of Mines with full acknowledgement of this funding that attracted additional industry and Govt. cost share is noted in Sponsors section).
This study was designed to develop independent estimates of methane emissions using top-down and bottom-up measurement approaches and then to compare the estimates, factoring in necessary consideration of uncertainty. These approaches were applied at two stages: (1) basin and (2) facility. On the facility scale, multiple methods were used to measure methane emissions of the whole facility – such as controlled dual tracer and single tracer releases, aircraft-based mass balance and Gaussian back-trajectory – which are considered top-down approaches. The bottom-up approach consisted of emissions from identified point sources – measured using appropriate source-level measurement techniques (e.g., high-flow meters). On the basin scale, the top-down estimate came from boundary layer airborne measurements upwind and downwind of the basin, using a regional mass balance model plus approaches to separate atmospheric methane emissions attributed to the oil and gas sector. The bottom-up estimate resulted from statistical modeling – also known as scaling up – of measurements made at selected facilities, with gaps filled through measurements and other estimates based on other related studies. The relative comparison of the bottom-up and top-down estimates made at both scales were used to help improve understanding of the accuracy of the tested measurement and modeling approaches.
This study addressed several additional scientific questions:
- What portion of total basin methane emissions can be attributed to oil and gas industry sources?
- What portion of total basin methane emissions from oil and gas operations is contributed by large emission sources?
- What portion of total basin oil and gas emissions are contributed by episodic emission sources as opposed to routine ones?
- What is the distribution of facility-scale emissions from well sites and gathering facilities? Does it exhibit a similar behavior to that observed in recent studies on wellsite components and other supply chain facilities, with a skewed, “fat tail” distribution of emissions?
- How large are the uncertainties for both bottom-up and top down estimates?