Modeling methane emissions in oil and gas industry: pathway for measurement-informed inventories

Abstract

Methane emissions from oil & gas (O&G) operations are under increasing scrutiny due to their climate impact and the growing demand for transparent emissions reporting. This work presents the results from two studies that are under the same scope. 1) The development of a mechanistic emissions model for natural gas dehydration units using a state-machine model integrated within the Mechanistic Air Emissions Simulator (MAES) tool. The model is parameterized by species-specific absorption and emission curves derived from ProMax simulations, with second-order polynomial regressions fitted to key operational variables, namely, pressure, temperature, and glycol circulation ratio. This approach enables site-specific simulation of dehydrator components, including the contactor, flash tank, and still vent, while accounting for emissions from gas-assisted glycol pumps and stripping gas. 2) A novel application of the MAES to simulate methane emissions that incorporate field measurements to generate a more representative emission inventory, hereby referred to as measurement-informed inventory (MII). The study included 168 midstream facilities (gas plants, compressor stations, and dehydration stations) operated by members of the Appalachian Methane Initiative (AMI) across the Appalachian Basin as part of the AMI 2024 Project. The dehydrator model results demonstrated high predictive accuracy, with root mean square error (RMSE) values below 0.01 across all key natural gas (NG) species and outlet stages. Field deployment indicated that gas-assisted glycol pumps accounted for up to 90% of dehydrator methane emissions if gas-assisted pump installed, highlighting substantial mitigation potential through conversion to electric or air-driven pumps. The MII simulated results estimated 37.8% higher than reported Greenhouse Gas Reporting Program (GHGRP) total methane emissions. Notably, tank-related emissions contributed 11.8% under the MAES MII, compared to only 0.6% in the reported GHGRP inventory. This substantial discrepancy suggests a possible underrepresentation of tank emissions in conventional inventories, likely due to the exclusion of water tank emissions from GHGRP requirements for the 2023 reporting year. The average methane leak rate estimated for AMI facilities was 0.046%, remaining well below the leakage thresholds reported in recent literature at which natural gas ceases to maintain a climate benefit relative to coal. These results demonstrate that as the industry continues to seek effective ways to integrate measurements into emissions inventories, MAES offers a robust framework capable of capturing rare but significant emission events, enabling reconciliation between site-level measurements and equipment-level emission estimates.