Numerical Modeling of Methane Leakage from a Faulty Natural Gas Well into Fractured Tight Formations

Abstract

Horizontal drilling and hydraulic fracturing have enabled hydrocarbon recovery from unconventional reservoirs, but led to natural gas contamination of shallow groundwaters. We describe and apply numerical models of gas-phase migration associated with leaking natural gas wells. Three leakage scenarios are simulated: (1) high-pressure natural gas pulse released into a fractured aquifer; (2) continuous slow leakage into a tilted fractured formation; and (3) continuous slow leakage into an unfractured aquifer with fluvial channels, to facilitate a generalized evaluation of natural gas transport from faulty natural gas wells. High-pressure pulses of gas leakage into sparsely fractured media are needed to produce the extensive and rapid lateral spreading of free gas previously observed in field studies. Transport in fractures explains how methane can travel vastly different distances and directions laterally away from a leaking well, which leads to variable levels of methane contamination in nearby groundwater wells. Lower rates of methane leakage (≤1 Mcf/day) produce shorter length scales of gas transport than determined by the high-pressure scenario or field studies, unless aquifers have low vertical permeabilities (≤1 millidarcy) and fractures and bedding planes have sufficient tilt (∼10°) to allow a lateral buoyancy component. Similarly, in fractured rock aquifers or where permeability is controlled by channelized fluvial deposits, lateral flow is not sufficiently developed to explain fast-developing gas contamination (0-3 months) or large length scales (∼1 km) documented in field studies. Thus, current efforts to evaluate the frequency, mechanism, and impacts of natural gas leakage from faulty natural gas wells likely underestimate contributions from small-volume, low-pressure leakage events.

Impacts of Methane on Carbon Dioxide Storage in Brine Formations

“…Conversely, it is also possible that CH 4 ‐rich gas bypasses such pathways to shallower depths, depending on the locations of fluvial channels relative to failures of the cap rock integrity. This is similar to the migration of stray gas contamination from compromised natural‐gas wells through fractured tight formations discussed elsewhere in this issue (Moortgat et al ).…”

“…This makes Osures capable of capturing the small‐scale onset of viscous and gravitational flow instabilities, if such fingering occurs (Moortgat ). The complex phase behavior of brine‐CO 2 and CH 4 is modeled using unique EOS‐based phase stability and phase split algorithms developed in prior work (see Li and Firoozabadi , ; Moortgat and Firoozabadi ; Moortgat et al , ). The governing equations for species transport in the two‐phase brine‐CO 2 and CH 4 system are provided in Soltanian et al (a).…”

Conventional Oil—The Forgotten Part of the Water‐Energy Nexus

“…Previous studies have indicated high permeability pathways, such as faults, leaky wells, or other breaches in aquitards, are probably required to allow for transport of contaminants NGWA.org J.C. McIntosh and G. Ferguson Groundwater 57, no. 5: 669-677 associated with HVHF to shallow aquifers Flewelling and Sharma 2014;Birdsell et al 2015;Moortgat et al 2018). The larger areal extent of the pressure anomaly also suggests that encountering a high permeability pathway is more likely with EOR than HVHF.…”

“…Transport of formation waters and injected fluids over longer distances is more likely to occur with EOR than HVHF due to the longer time periods, despite the lower hydraulic head increases. Previous studies have indicated high permeability pathways, such as faults, leaky wells, or other breaches in aquitards, are probably required to allow for transport of contaminants associated with HVHF to shallow aquifers (Jackson et al ; Flewelling and Sharma ; Birdsell et al ; Moortgat et al ). The larger areal extent of the pressure anomaly also suggests that encountering a high permeability pathway is more likely with EOR than HVHF.…”

Moving beyond forensic monitoring to understand and manage impacts of hydraulic fracturing for oil and gas development

“…Similar studies have begun to appear for identifying methane leakage from shale gas development, using either conceptual or numerical models to assess transport pathways and geochemical outcomes (17,18). Models and monitoring programs are often synergistic: Monitoring is necessary to calibrate, verify, and refine the conceptual and parametric representation of the subsurface embodied by the model, while the modeling helps to identify the sampling needs that are most important for reducing the uncertainties that matter most for both ongoing model improvements and management decisions.…”