Effect of Supercritical CO2 and Thermal Loading Cycles on Class G Well Cement Properties
Authors: Agofack Nicolaine , Hoang Hoang Nguyen , Cerasi Pierre , Stroisz Anna , Naumann Marcel 
Published: September 2023

ABSTRACT
In carbon dioxide (CO2) capture and storage (CCS) technology, one of the goals is to prevent the injected CO2 from leaking back into the atmosphere. The leakage paths include fractures on the cement sheath sealing the injection well and/or reactivated faults going through the caprock. During a batch-wise injection of CO2, the wellbore is submitted to thermal loading cycles. After injection, the near-well materials, including cement sheath, are exposed to a CO2-rich environment, especially in the vicinity of the reservoir. These thermal cycles (TC) and exposure environment can affect the properties of the sealing material like cement and create leakage paths. Portland G cement samples were submitted to TC and exposed to a supercritical CO2 environment, respectively. The results show that after submitting to more than 100 TC, the strength of the cement sample can decrease by 40% and its Young's modulus by 20%. On the other hand, after exposure to supercritical CO2 for 52 days, the strength of the cement can increase up to 170% and its Young's modulus up to 25%, depending on the cement slurry formulation.

INTRODUCTION
Cement paste in CCS wells can experience different mechanical and thermal loadings. During hydration, the cement pastes are also exposed to various temperature conditions. After well construction, when cement hydration takes place, the cured cement is submitted to thermal cyclic loadings following a batchwise CO2 injection. Injected from the seabed at a 4°C, the CO2, while going down along the well into the reservoir, cooldown the wellbore and its surroundings to a temperature much lower than the in-situ one. During maintenance or when shifting from one batch to another, the wellbore temperature will raise again towards its initial in-situ value. These thermal loadings can significantly affect the properties of the cement sheath, compromise its integrity by creating leakage paths for the stored CO2 to escape the reservoir. Previous investigations have shown that properties of cement and cement-sand mixture are highly affected when exposed to high temperatures (Cao et al., 2022; Dillenbeck et al., 2002; Milestone et al., 2012; Mindeguia et al., 2010; Peng & Huang, 2008; Rimmelé et al., 2008; Urbonas et al., 2016; Vu, 2013; Zeng et al., 2022). In the case of a leakage path in the cement sheath, remedy solutions include squeeze cementing operations. This consists of performing a perforation to the casing and squeezing cement slurry to seal the leakage paths (Manceau et al., 2014; Todorovic et al., 2016). For remediation of CO2 leakage, part of the squeezed cement slurry may be hydrated while exposed to CO2. A CO2-rich environment is expected to influence the properties of the hydrating cement. Some publications indicate that cement carbonation increases with relative humidity (RH) (Hunt et al., 1958). It is very slow for RH lower than 25% and highly enhanced for RH higher than 90%. The same authors demonstrate a smaller amount of non-evaporable water after hydration accompanied by CO2 exposure for more carbonated cement. According to Fabbri et al. (2009), Johannesson & Utgenannt (2001) and Milestone et al. (2012), the mechanism of carbonation is a chemical reaction of calcium hydroxide and calcium silicate hydrates with CO2. Calcium ions (Ca2+) migrate through a saturated porous medium, leaving a leached zone and then rapidly precipitate as calcium carbonate (CaCO3) when meeting the dissolved CO2 (Fabbri et al., 2009; Johannesson & Utgenannt, 2001; Milestone et al., 2012). After exposing hardened cement and mortar samples to scCO2 for only four hours, significant strength increase and porosity reduction were observed (Urbonas et al., 2016). According to these authors, these changes result from carbonation which leads to microstructure change. Dillenbeck et al. (2002) showed that the carbonation depth is higher in ordinary Portland cement (OPC) with fly-ash (FA/OPC) than in OPC alone. In both cement systems, it increases with the exposure time. Consequently, the compressive strength of OPC cement increased after exposure to CO2 for up to 6 months and then decreased afterwards (Dillenbeck et al., 2002). After exposing Portland cement G to wet CO2 and CO2-saturated water under the pressure of 280 bars and temperature of 90°C, Rimmelé et al. (2008) measured a reduction in the porosity after 3 weeks (resp. 6 weeks) of exposure to CO2-saturated water (resp. wet CO2). Eventually, the porosity increased to a final value, lower than the reference one, after 6 months of exposure. Many of these investigations are rather limited to microstructure analysis in terms of mineral changes or compressive strength. The stiffness of the material, as well as the dynamic properties, are rarely investigated. In this campaign, different cement formulations, hydrated under different conditions and subjected to many thermal loading cycles are tested. Compressive strength, porosity, stiffness and density change after scCO2 exposure and temperature treatment were measured.