Modeling Gas Migration, Sustained Casing Pressure, and Surface Casing Vent Flow in Onshore Oil and Gas Wells

Abstract

Faulty oil and gas wellbores are a primary pathway of concern for gas migration from the deep subsurface into shallow freshwater aquifers. Leaked gases migrating vertically along wellbores either collect and build a pressure at the wellhead known as sustained casing pressure (SCP) or escape into the atmosphere as surface casing vent flow (SCVF). SCP and SCVF are valuable indicators of integrity loss that provide insight into the potential for groundwater contamination through gas migration. Previous models of SCP and SCVF have focused on offshore wells and have not considered the relationship between SCP/SCVF and gas migration away from onshore wells. We present the first modeling framework for SCP and SCVF that is applicable to onshore oil and gas wells constructed with outermost annuli that are hydraulically connected or “open” to the surrounding formation. Our results show that SCP behavior is unique in onshore wells with open annuli, because steady state SCP is not achieved in an open annulus unless gas escapes the wellbore via gas migration (Darcy Flow), a wellhead leak, or a controlled bleed‐off test. We show that appropriately modeling gas leakage along the wellbore and SCP/SCVF helps to constrain methane leakage fluxes from faulty wells and could be integrated with subsurface flow and transport models of stray gas contamination. By relating gas fluxes to SCP/SCVF, our model can provide information used to support regulatory actions informed by SCP/SCVF, which are easy to measure and monitor.

Fugitive methane gas migration around Alberta's petroleum wells

“…1), 5,6 this is referred to as surface casing vent flow (SCVF). Gas pressures may also accumulate in the outermost, or surface, casing, contributing to a sustained casing pressure, particularly where surface casing vent assemblies are not present in a well assembly 7 . Conversely, methane migration in sediments outside the casing string (i.e., via cement perforations, fracture, and/or micro‐annuli) through soils is known as gas migration (GM) 8 .…”

Numerical Investigation of Wellbore Methane Leakage From a Dual‐Porosity Reservoir and Subsequent Transport in Groundwater

“…The implication is that either leakage at legacy wells has begun recently or that methane migration has been delayed. Processes with the potential to retard or attenuate methane migration include multiphase migration (Lackey & Rajaram, 2019;Rice, McCray, & Singha, 2018) which can lower effective permeability for a given phase by orders of magnitude (e.g., Mualem, 1976), methane oxidation (Forde et al, 2018;Roy et al, 2016;Van Stempvoort et al, 2005), and diffusive transfer and storage of solutes into less-mobile (also called "immobile") pore space by dual-domain mass transfer (DDMT) (e.g., Becker & Shapiro, 2000;Coats et al, 1964;Haggerty et al, 2000Haggerty et al, , 2001van Genuchten & Wierenga, 1976).…”

Remediation of Annular Gas Migration along Cemented Wellbores Using Reactive Mineral Fluids: Experimental Assessment of Sodium Bicarbonate and Sodium Silicate-Based Solutions

“…Such defects can have various causes, including (i) ineffective cement placement [9][10][11], (ii) autogenous shrinkage and debonding upon setting of the cement [12][13][14][15][16][17][18], or (iii) mechanical damage sustained by the set cement [19][20][21][22][23][24][25][26]. The defects offer possible routes for annular gas migration, potentially leading to sustained casing pressure (SCP), surface-casing vent flow (SCVF), or other sealing integrity issues [27,28]. Both SCP and SCVF are relatively prevalent [29][30][31], though statistics vary widely with factors such as wellbore age, design and status, as well as the exploitation and regulatory history of the basin [32].…”