Fault sealing and caprock integrity for CO<sub>2</sub> storage: an in situ injection experiment

Abstract

Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

Chemo‐Mechanical Coupling in Fractured Shale With Water and Hydrocarbon Flow

“…The material used in this study was obtained from the shaly facies of the Opalinus claystone at the Mont Terri Underground Laboratory (MTRL), Switzerland (Zappone et al, 2020;Q. C. Wenning et al, 2021).…”

“…The material used in this study was obtained from the shaly facies of the Opalinus claystone at the Mont Terri Underground Laboratory (MTRL), Switzerland (Zappone et al., 2020; Q. C. Wenning et al., 2021). The clay fraction of the sample is very high (70 wt.%) and includes both expandable (illite‐smectite, 12 wt.%) and nonexpandable clays (illite, 30 wt.% and kaolinite, 20 wt.%; Text , Supplementary Methods 1).…”

Transient evolution of permeability and friction in a slowly slipping fault activated by fluid pressurization

“…However, recent in situ experiments of fault activation caused by fluid injection at shallow depths (from 0.3 to 1.5 km) have proved to be well adapted for studying at the decametric scale how a fault responds to a known fluid pressure perturbation 11 – 15 . Such experiments provide quantitative information on the hydromechanical processes 16 – 18 , and significant insights have been gained into the physics of fault slip 11 , 19 . Yet, to date, constraints on the evolution of permeability and friction during fluid pressurization in a natural fault are rare.…”

Physical characterization of fault rocks within the Opalinus Clay formation

“…Previous studies have mostly focused on the microstructural characterization, and the mechanical and transport properties of intact samples cored from the host rock of OPA formation 21 , 22 , 61 , 62 . Conversely, less attention has been paid to its faulted zones and the role of the internal fault core structure 17 , 31 , 43 , 45 , 47 – 49 , 63 – 65 . Thus, in this section, we discuss our results by comparing them with earlier studies of intact samples.…”

“…Finally, recent in-situ tests have revealed contradictory results. Permeability values of the surrounding rock and the Main Fault have been estimated around ~ m 2 to ~ m 2 with no significant difference 48 , 49 . A second study, however, has measured two orders of magnitude higher permeability values around ~ 10 –18 to 10 –19 m 2 for the Main Fault with respect to the surrounding host rock 50 .…”