CO2, Methane, and Brine Leakage Through Subsurface Pathways: Exploring Modeling, Measurement, and Policy Options

Abstract

Subsurface pathways, such as abandoned oil and gas wells and faults, can serve as leakage pathways for CO2, methane, brine, and other fluids. These pathways allow fluids from deep subsurface formations to migrate into shallow groundwater aquifers or to the atmosphere.

To estimate leakage rates and the associated pressure effects on adjacent aquifers, analytical models representing fluid flow in the vicinity of leaky faults are developed in Chapter 2. The incorporation of this kind of fault model in larger basin-wide multi-scale models allows sub-grid-scale effects due to leakage through faults to be captured with improved efficiency. The corresponding multi-scale framework that accounts for vertical leakage to the overlying aquifer, and horizontal flows perpendicular and parallel to a fault within a grid block, is presented in Chapter 3.

In Chapter 4, first-of-a-kind direct measurements of methane fluxes from abandoned oil and gas (AOG) wells in Pennsylvania are reported. These measurements may bridge the current gap between top-down and bottom-up methane emission estimates. The mean methane flux at the 19 wells for which fluxes were measured is 0.27 kg/day/well, while the mean methane flux at the control locations near the measured wells is 4.5x10<super>-6</super> kg/day/location. All measured wells showed positive methane leakage. The presence of ethane, propane, and n-butane, along with the methane isotopic composition, indicate that the methane emitted from the measured wells is predominantly of thermogenic origin.

In Chapter 5, the number of AOG wells in Pennsylvania is estimated, based on historical records, and found to be in the range of 280,000 to 970,000. When the mean flux rate from the measured wells is applied to these estimated total number of wells in Pennsylvania, methane emissions are 4 to 13% of currently estimated annual statewide anthropogenic methane emissions. To reduce the uncertainty in methane emission estimates, policies that promote reporting and monitoring of these wells need to be developed and implemented. Finally, to reduce impacts of these emissions, strategies for mitigation in addition to well plugging should be considered.