Fracture opening or self-sealing: Critical residence time as a unifying parameter for cement–CO2–brine interactions

Abstract

Understanding long-term property evolution of cement fractures is essential for assessing well integrity during geological carbon sequestration (GCS). Cement fractures represent preferential leakage pathways in abandoned wells upon exposure to CO 2-rich fluid. Contrasting self-sealing and fracture opening behavior have been observed while a unifying framework is still missing. Here we developed a processbased reactive transport model that explicitly simulates flow and multi-component reactive transport in fractured cement by reproducing experimental observation of sharp flow rate reduction during exposure to carbonated water. The simulation shows similar reaction network as in diffusion-controlled systems without flow. That is, the CO 2-rich water accelerates the portlandite dissolution, releasing calcium that further reacted with carbonate to form calcite. The calibrated model was used for CO 2-flooding numerical experiments in 250 cement fractures with varying initial hydraulic aperture (b) and residence time (τ) defined as the ratio of fracture volume over flow rate. A long τ leads to slow replenishment of carbonated water, calcite precipitation, and self-sealing. The opposite occurs when τ is small with short fracture and fast flow rates. Simulation results indicate a critical residence time  cthe minimum τ required for selfsealing-divides the conditions that trigger the diverging opening and self-sealing behavior. The  c value depends on the initial aperture size through 42 9.8 10 0.254 c bb       . Among the 250 simulated fracture cases, significant changes in effective permeabilityself-healing or openingtypically occur within hours to a day, thus providing supporting argument for the extrapolation of short-term laboratory observation (hours to months) to long-term prediction at relevant GCS time scales (years to hundreds of years).

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

“…Bachu and Bennion (2009) In order to investigate the relationship of permeability, aperture and flow rate observed in experiments, Brunet et al (2016) developed a reactive transport model calibrated against the experiments of Huerta et al (2015). The model calculates change in fracture permeability as a consequence of diffusion and dispersion when CO 2 -saturated water reacts with cement.…”

“…Illustration of the critical threshold between fluid residence time and initial fracture aperture on the ability of CO2-H2O-Ca reactions to seal or open fracture pathways within cement-cement interfaces based on numerical simulations. The green and blue symbols represent the initial aperture and residence time of experiments from Huerta et al (2015) and Luquot et al (2013), respectively (Brunet et al, 2016). (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)…”

Reaction-driven casing expansion: potential for wellbore leakage mitigation

“…However, even this limited expansion could still be effective, particularly in remedying leakage of CO 2 -bearing fluids. For CO 2 -rich fluids, it may be sufficient to reduce the aperture of fractures and debonding defects only to the point where reactive transport leads to self-sealing behaviour [17,20,53,130], rather than completely closing interfacial flaws by purely mechanical means. Yet, it would not be trivial to expand conventional casing strings by even 1-2% using the pull-through mandrel methods that are now being applied to expandable wellbore casing tubes.…”

Factors affecting self‐sealing of geological faults due to CO2‐leakage

“…11,12 Having a complete understanding of such effects is crucial for successful deployment of geologic CO 2 sequestration. 11,12 Having a complete understanding of such effects is crucial for successful deployment of geologic CO 2 sequestration.…”

“…If CO 2 -laden water migrates along a fault or fracture, there may be CO 2 -induced water-rock reactions that will affect the fluid-flow properties of the fault. 11,12 Having a complete understanding of such effects is crucial for successful deployment of geologic CO 2 sequestration. 13 We investigated possible reactive transport processes involved in long-term leakage of CO 2 -rich fluids along a vertical fault zone in the shallow stratum.…”