Review: Role of chemistry, mechanics, and transport on well integrity in CO2 storage environments

Abstract

a b s t r a c tAmong the various risks associated with CO 2 storage in deep geologic formations, wells are important potential pathways for fluid leaks and groundwater contamination. Injection of CO 2 will perturb the storage reservoir and any wells that penetrate the CO 2 or pressure footprints are potential pathways for leakage of CO 2 and/or reservoir brine. Well leakage is of particular concern for regions with a long history of oil and gas exploration because they are top candidates for geologic CO 2 storage sites. This review explores in detail the ability of wells to retain their integrity against leakage with careful examination of the coupled physical and chemical processes involved. Understanding time-dependent leakage is complicated by the changes in fluid flow, solute transport, chemical reactions, and mechanical stresses over decade or longer time frames for site operations and monitoring.Almost all studies of the potential for well leakage have been laboratory based, as there are limited data on field-scale leakage. Laboratory experiments show that CO 2 and CO 2 -saturated brine still react with cement and casing when leakage occurs by diffusion only. The rate of degradation, however, is transport-limited and alteration of cement and casing properties is low. When a leakage path is already present due to cement shrinkage or fracturing, gaps along interfaces (e.g. casing/cement or cement/rock), or casing failures, chemical and mechanical alteration have the potential to decrease or increase leakage risks. Laboratory experiments and numerical simulations have shown that mineral precipitation or closure of strain-induced fractures can seal a leak pathway over time or conversely open pathways depending on flow-rate, chemistry, and the stress state. Experiments with steel/cement and cement/rock interfaces have indicated that protective mechanisms such as metal passivation, chemical alteration, mechanical deformation, and pore clogging can also help mitigate leakage. The specific rate and nature of alteration depend on the cement, brine, and injected fluid compositions. For example, the presence of co-injected gases (e.g. O 2 , H 2 S, and SO 2 ) and pozzolan amendments (fly ash) to cement influences the rate and the nature of cement reactions. A more complete understanding of the coupled physical-chemical mechanisms involved with sealing and opening of leakage pathways is needed.An important challenge is to take empirically based chemical, mechanical, and transport models reviewed here and assess leakage risk for carbon storage at the field scale. Field observations that accompany laboratory and modeling studies are critical to validating understanding of leakage risk. Long-term risk at the field scale is an area of active research made difficult by the large variability of material types (cement, geologic material, casing), field conditions (pressure, temperature, gradient in potential, residence time), and leaking fluid composition (CO 2 , co-injected gases, brine). Of particular interest are the circum...

Application of a new reduced‐complexity assessment tool to estimate CO2 and brine leakage from reservoir and above‐zone monitoring interval (AZMI) through an abandoned well under geologic carbon storage conditions

“…The goal of any CO 2 storage project is to retain effectively more than 99% of the injected CO 2 in the target reservoir for hundreds to thousands of years, with a maximum cumulative CO 2 mass leakage rate of 0.01–0.1% per year . In some cases, abandoned wells at storage sites can threaten this target.…”

“…10 The goal of any CO 2 storage project is to retain effectively more than 99% of the injected CO 2 in the target reservoir for hundreds to thousands of years, with a maximum cumulative CO 2 mass leakage rate of 0.01-0.1% per year. [11][12][13][14] In some cases, abandoned wells at storage sites can threaten this target. Such wells can potentially compromise the integrity of caprock layers that would otherwise prevent unwanted vertical fluid migration out of the storage system and into receptors of concern (i.e., potable groundwater formations and the atmosphere).…”

Field‐scale well leakage risk assessment using reduced‐order models

“…25,26 Geomechanical and geochemical processes over the life span of the well may significantly affect the well integrity. [27][28][29][30][31][32] Moreover, the interaction of CO 2 -rich brine with well cement may rapidly degrade the integrity of the wellbore cement. 5,18,19,25,27,[33][34][35] Bachu and Bennion 25 noted that the degradation of the cement's ability to act as a barrier is directly related to its initial quality.…”

“…For storage zones in which pressure builds up significantly, accounting for geomechanical effects may be necessary to obtain realistic estimates of leakage rates. [27][28][29][30][31][32][33]41,42 Moreover, other site-specific characteristics will also play a significant role in determining well leakage and conducting the risk assessment process. In heterogeneous and dipping formations, it is possible that some wells may not be exposed to CO 2 .…”

Comparative wellbore integrity evaluation across a complex of oil and gas fields within the Michigan Basin and implications for CO2storage

“…Multiple researchers, including Gasda, Duguid et al ., and Carrol et al ., have characterized leakage pathways. These studies examined leakage occurring through or around the cement matrix that makes up the primary and plug cement in a well (Fig.…”