Phase change-induced internal-external strain of faults during supercritical CO₂ leakage

Highlights

• This study assesses fault deformation morphology under supercritical CO₂ leakage.

• The internal-external strain variation is evaluated in faults under SC-CO₂ leakage.

• The viscosity indicates that T₀ = 45 °C is the critical temperature.

• The distribution of P-T zones may be used as a criterion for fault leakage.

Abstract

For the long-term safe operation of carbon capture and storage (CCS) projects, the leakage of supercritical carbon dioxide (SC-CO₂) along a fault is an important factor that needs to be monitored. To investigate the phase change and fault deformation associated with SC-CO₂ leakage along a fault, this study carried out five groups of SC-CO₂ leakage experiments at different initial CO₂ temperatures. The volume of the CO₂ storage container (100 mL), CO₂ initial pressure (8.07 MPa) and fault confining pressure (10.5 MPa) were controlled at the same values. The CO₂ phase change and fault internal-external strains response were monitored by high-precision fiber Bragg grating sensors throughout the experiment. The results show that (1) Temperature and strains response times were 1 and 2.5 times longer than pressure, respectively. (2) The viscosity greatly influenced the determination of the critical temperature T₀ = 45 °C. (3) The fault internal strains were more than twice that of the external one; except for the entrance, the fault internal and external strains states were opposite at the beginning of leakage; all other positions on the fault surface showed compressive strains except for the tensile strains at the entrance. (4) The distribution of the inflection point of temperature–pressure curves may be used as a basis for distinguishing CO₂ leakage from faults (near the triple point) or pipes (far from the triple point). This study provides technical and theoretical support for laboratory-scale studies on the mechanism of internal and external strains differences caused by phase transition of SC-CO₂ leakage along faults.

Introduction

Carbon capture and storage (CCS) is an effective way to mitigate global warming (Singh, 2013). Researchers have suggested CCS as a new technology that can contribute to mitigating global carbon dioxide (CO₂) emissions from large fossil fuel power stations and to reducing CO₂ release by approximately six billion tons per year in 2050 (Munkejord et al., 2016). The CCS technology mainly includes three types: underground geological storage, deep ocean storage, and mineral carbonation (Metz et al., 2005), in which underground geological storage itself comprises several options including saline aquifers, depleted oil and gas reservoirs, unmineable coal seams, hydrate storage, and CO₂ within enhanced geothermal systems (Yang et al., 2008). The structural trap, confined trap, dissolution trap, and mineralization trap mechanisms are mainly used in oil and gas reservoir storage and saltwater reservoir storage, while the adsorption mechanism is mainly used in coal seam storage (Liu et al., 2021). Reliable CCS monitoring is vital in order to confirm that injected CO₂ stays in the reservoir as intended, and that any occurring leakage is promptly detected allowing corrective actions to be initiated (Waarum et al., 2017). There are various potential modes in which CO₂ can escape from the storage formation. Leakage pathways for CO₂ can correspond to well leakage, diffusive loss, induced migration by capillary pressure, and escape through faults and fracture networks (Aminu et al., 2017). Injection of CO₂, or any other fluid, into subsurface reservoirs might increase the risk of caprock failure and migration of fluid along faults, fracture corridors, and other pre-existing weak zones (Ogata et al., 2014). Furthermore, faults are widespread in geological structures and provide possible channels for CO₂ leakage (Jing et al., 2021), which threaten the seal integrity of CO₂ repositories. In this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions (Zoback and Gorelick, 2012). Many CO₂ leakage events along faults have been found in natural CO₂ reservoirs. For example, in the St. Johns Dome natural CO₂ reservoir in the United States, Miocic et al. (2020) used travertine sedimentary records located above the reservoir and found a long history of CO₂ leakage (>400 years). The distribution of CO₂ springs and seeps along the Little Grand Wash fault and Salt Wash faults in central Utah is controlled by along-fault flow of CO₂-charged groundwater from shallow aquifers (<1 km deep) (Dockrill and Shipton, 2010). A protracted history of fault-related fluid flow is recorded within the footwalls of the Little Grand Wash fault and northern fault of the Salt Wash graben. The study of Dockrill and Shipton (2010) showed that field evidence indicates hydrocarbons have and are migrating along the Little Grand Wash fault with regional studies indicating that migration commenced from the early Tertiary. U-Th–dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances (Burnside et al., 2013). In the Rialdo Plain (Campania Region, southern Italy), the existence of normal faults caused CO₂-saturated fluid to migrate to the critical zone (Cuoco et al., 2020).

Recently, CO₂ leakage along faults has been investigated intensively by natural tests and field observations (Zhang et al., 2018). By integrating structural geology and near-surface gas geochemistry surveys, Annunziatellis et al. (2008) examined that CO₂ (deep, naturally produced) migration to the surface along various buried and exposed faults in the Latilla Crater (central Italy). Their results showed how gas migration was channeled along discrete, high-permeability pathways within the faults, with release typically occurring from spatially restricted gas vents. Do et al. (2020) assessed the risk of CO₂ leakage at a potential geological storage site (a sedimentary basin in Gyeongsang buk-do, South Korea) by analyzing the water chemistry properties. Due to hydrochemistry effects, the levels of Mg, K, F, Cl, SO₄, HCO₃, Li, As Fe, Mn, and U in the water exceeded the WHO guidelines (Do et al., 2020). Tang et al. (2020) deployed monitoring wells and assessed the possibility of gas leakage from the reservoir based on the cumulative sequence. Kim et al. (2020) presented the results of site selection and characterization for onshore 10,000-ton-class CO₂ pilot storage in the Miocene Janggi Basin, SE Korea. It is expected that the stratigraphic and structural traps at the selected site minimize the leakage risk of injected CO₂ due to the scarcity of surface and subsurface fractures, low fault intensity, and limited vertical extents of buried faults. Dockrill and Shipton (2010) investigated a naturally leaking CO₂-rich system at the northern end of the Paradox Basin in central Utah, United States. Their results demonstrated that fault zones can impact fault-parallel leakage from a robust structural trap, and they highlighted the potential risks when assessing the seal integrity of structural traps in the hydrocarbon and emerging geologic CO₂ storage industries. Kampman et al. (2014) examined reactive fluid flow in reservoirs, caprocks and faults during the migration of CO₂ through geological overburden from deep supercritical CO₂ reservoirs. They found that the reservoir fluids were undergoing a complex mix of (i) CO₂-saturated brine inflowing from the fault, (ii) CO₂-undersaturated meteoric groundwater flowing through the reservoir and (iii) reacted CO₂-charged brines flowing through fracture zones in the overlying Carmel Formation caprock into the formations above.

In addition, many researchers have carried out a series of numerical simulations and analytical solutions to investigate CO₂ migration along faults. Biryukov and Kuchuk (2012) presented transient pressure solutions for a vertical well in a reservoir containing arbitrarily distributed finite- and/or infinite-conductivity faults and/or fractures, i.e., in discretely fractured and faulted reservoirs. Studies of two natural analog sites by Keating et al. (2013) provided insights into physical and chemical mechanisms controlling both brackish water and CO₂ intrusion into shallow aquifers along fault zones. Their simulations showed that even at a single site, there can be zones where only CO₂ migrates upward in the fault zone and others where both CO₂ and brackish water upwell. Yang et al. (2018) established a three-dimensional (3D) numerical model to evaluate the effects of inner reservoir faults on the CO₂ migration and storage capacity of an actual CCS demonstration project in the Ordos Basin of China. The results showed that (i) the faults in the layered reservoir system can significantly affect the migration of injected CO₂ and (ii) the cross-layer faults at the bottom of the faulted reservoir can act as preferential passages between the upper and lower geological formations, causing the CO₂ in the reservoir formation to move upward to adjacent layers rather than to laterally migrate. Nakajima et al. (2014) examined hypothetical CO₂ seepage by numerical simulations to assess the potential environmental impacts at geological storage sites. They found that the seepage rate controlled not only the permeability of the fault but also that of the reservoir. Tillner et al. (2013) carried out different fault leakage scenarios by numerical modeling of a prospective storage site in northeastern Germany using a newly developed workflow that includes grid transfer from the geological model generated with the applied preprocessing software Petrel to the reservoir simulator TOUGH2 and the implementation of virtual elements for a discrete description of fault zones. The results showed that closed boundaries generally lead to higher brine migration rates, especially if a number of permeable faults are present, whereas the permeability of fault zones has only a minor impact and does not significantly influence the salinization of shallower aquifers.

However, laboratory experiments on the phase transition mechanism and deformation characteristics of fault structures during CO₂ leakage are rarely reported. Most of the current CO₂ leakage laboratory experiments have focused on the exploration of CO₂ in pipelines (e.g., Cui et al., 2016, Mazzoldi et al., 2009). As a result, the research progress on the laboratory-scale mechanism of fault CO₂ leakage lags behind, which limits the improvement of safety control technology in CO₂ geological storage. Previous research results have shown that the storage of CO₂ in saline aquifers is intended to be at supercritical pressure and temperature conditions, but CO₂ leaking from a geological storage reservoir and migrating toward the earth surface (through faults, fractures, or improperly abandoned wells) would reach subcritical conditions at depths shallower than 500–750 m (Pruess, 2011). Risk assessment must evaluate potential leakage scenarios and develop a rational, mechanistic understanding of CO₂ behavior during leakage. The flow of CO₂ may be subject to positive feedback that could amplify leakage risks and hazards, placing a premium on identifying and avoiding adverse conditions and mechanisms (Pruess, 2008). To develop a rational, mechanistic understanding of CO₂ behavior during leakage, in this study, laboratory-scale experiments of SC-CO₂ leakage along faults at different initial temperatures were carried out. A total of five groups of SC-CO₂ leakage experiments along faults with different initial temperatures were carried out. The initial temperatures were 35, 40, 45, 50 and 55 °C. The initial pressure and volume of CO₂ in the five groups of experiments were 8.07 MPa and 100 mL, respectively. The temperature, strains and pressure of CO₂ were monitored throughout the leakage experiment. In particular, fiber Bragg grating (FBG) sensors were used to monitor the inside and outside strains of faults during CO₂ leakage. Through this experimental study, the characterization and leakage mechanism of SC-CO₂ leakage along faults at the laboratory scale are evaluated.