**1. Introduction**

Large scale geological storage of CO2 can significantly reduce CO2 emissions and limit global warming [1]. Geological reservoirs are selected for the physical containment of CO2 which guarantees permanent storage in the subsurface. However, CO2 injection wells (and possibly old oil and gas wells) penetrating the reservoirs and the caprocks above can compromise the integrity of the storage complex. Wells have a primary structural seal of casing and annular cement (between the casing and the geological formation) and a cement plug when abandoned. Despite these seals, many oil and gas wells leak during their operational lifetime or after abandonment through leakage pathways formed by cement shrinkage or pressure and temperature fluctuations [1]. If annular cement is placed properly, the most likely leak path for CO2 is along the well through fractures in the cement or microannuli between the cement and the casing or adjacent rock [2–5].

CO2 leakage through microannuli will cause dissolution of CO2 in the pore waters which acidifies the near-wellbore environment and causes cement reactivity. Reactions of cement in contact with CO2-rich water or brine have been extensively studied with experiments and by numerical modelling [6–13]. A typical wellbore cement mineralogy consists of mainly portlandite (Ca(OH)2) and calcium silicate hydrate (CSH), with minor phases such as aluminium-, iron-, magnesium-, or calcium-containing sulphate-, carbonate-, or silicate-hydrates [14]. In general, cement-CO2 interaction is primarily characterized by portlandite dissolution and subsequent precipitation of calcium carbonate (CaCO3) because of the fast reaction kinetics. The dissolving CSH phase forms additional calcium

carbonate and amorphous silica gel (SiO2) [7]. Characteristic successive, inward moving reaction zones are observed consisting of portlandite dissolution, calcite precipitation, and subsequent calcite dissolution from the rim, leaving a porous, silica-rich rim [6,10–12]. However, cement reactivity will most-likely only lead to cement degradation under leaching/flow conditions when reaction products are quickly removed from the reaction site. At no/low flow conditions, calcite precipitation is the dominant process rather than (re-)dissolution [4,15,16]. Depending on the initial flow and chemical conditions, continuous cement leaching occurs, or cement reactivity may actually support natural sealing of the micro annulus [17–19].

In the case of sustained CO2 leakage, a corrective measures plan must be in place and appropriate remediation measures should be taken [20,21]. Intentional clogging of the near-well area with salt has been proposed as a preventative measure against CO2 leakage [22]. This method was based on the capacity of injected CO2 to evaporate water and precipitate pore filling salt. The process of natural sealing of the microannulus by mineral precipitation indicates a potential for chemical clogging of the annulus leak path. Clogging with calcite or silica has already been proposed for a CO2 leak path through the caprock [23–25]. To induce mineralization in a leak path, it was proposed to inject a silicaor calcium-rich suspension or solution into the CO2-containing environment. The injected solution will react in the acid environment to form a solid silica (gel) or carbonate mineral. A modelling study on leakage remediation above a leaking fault through a caprock indicated a final leakage reduction of up to 95% [25]. Experimental [23] and modelling results [24,25] for caprock leakage mitigation support the feasibility of the method for reactive clogging by injecting a CO2-reactive solution into a high permeable leak path to form solid reactants that clog the leak path, reduce permeability and stop leakage. The main objective of this study is to assess the possibility of reactive leakage mitigation for wellbore annulus leakage.

We developed a field scale reactive transport model based on the model reported by Koenen and Wasch [18] to simulate CO2 leakage through a microannulus, resulting in either sustained flow and cement leaching or in natural sealing and reduced leakage. For the leakage cases, we study the potential of induced CO2 mineralization in the leak path, mitigating CO2 leakage. The numerical modelling study includes the following processes:


In this paper, we report on microannulus leakage (versus sealing), the intentional clogging process for leakage remediation, and the post-clogging phase to assess the sustainability of the clogging procedure.
