Microstructural Changes and Mechanical Behavior of Cement Paste with Hollow Glass Microspheres under Carbon Dioxide Geological Storage Conditions

MARTÍN, CHRISTIAN MARCELO; TERESA M. PIQUE; SIAVASH GHABEZLOO; JEAN MICHEL PEREIRA; DIEGO GUILLERMO MANZANAL

Aussois

Workshop; Alert Geomaterials - Workshop; 2023

Carbon dioxide capture and geological storage (CCS) is a necessary and well-known climate change mitigation technique that must be implemented to attain the objective of net-zero emissions. It seeks to reduce the CO2 emitted to the atmosphere by injecting it into deep reservoirs in supercritical state (scCO2). There are already existing storage sites all around the world, but the storage capacity needs to exponentially increase soon. This highlights the importance of studying the different components involved in this process. Special attention should be paid to the integrity of the reservoir, the caprock, the casing, and the interphase between the last two, the cement sheath.The cement sheath is generated by injecting the fresh cement paste until it occupies the annular space between the casing and the formation rock. During this procedure, the cement in fluid state exerts some pressure to the formation, which is proportional to its specific weight since it is behaving as a fluid. Depending on the formation rock, this density will need to be controlled so that no fractures and possible fluid migration paths are generated. For this particular purpose, among all the existing alternatives, partial cement replacement with hollow glass microspheres (HGMS) appears to be the most promising one to effectively decrease cement slurry density without severely affecting its properties. HGMS consist of spheres hollow on the inside, of around 50 µm in diameter, and mainly composed of amorphous borosilicate glass. In this research two classes of HGMS were used, HGMS27 and HGMS41, from which the former presents a density of 0.28 g/cc, a mean particle size (D50) of 30 µm, and a strength of 27 MPa, while the latter presents 0.46 g/cc, 40 µm and 41 MPa of density, D50 and strength, respectively. To properly assess the influence of HGMS on cement pastes under CCS conditions two main goals were set to study: change in mechanical and chemical properties due to carbonation of the cement under laboratory CCS conditions, and modelling the behaviour of the compound material, which was calibrated with all the experimental data obtained.Different cement paste formulations were subjected for 60 days to simulated downhole conditions in the laboratory, replicating high-pressure (20 MPa) and high-temperature (90ºC) with a scCO2 saturated atmosphere and the presence of water. Under these conditions, cement pastes undergo carbonation, which involves the chemical reaction between cement hydration products and CO2-derived ions, to form calcium carbonate crystals. This phenomenon has been previously studied, though there still exist some unresolved aspects associated with it. Carbonation does not only cause chemical changes, but also the calcium carbonate crystals that precipitate within the pores of the cement matrix alter the mechanical behaviour of the cement paste. It has been argued that these crystals may precipitate and induce internal stresses that may fracture the cement paste. This would lead to an increase in permeability and a shift in the mechanical behaviour, resulting in a decrease of the cement paste strength.As for the presence of HGMS, it was observed that they contributed to the formation of non-interconnected porosity, which plays a key role in affecting the rate and mechanisms of cement carbonation. This influence is primarily attributed to the limited strength of the HGMS outer shell. While the chemical and mechanical interactions between HGMS and cement are favourable, the increased compacity of the cement paste due to the presence of HGMS is not the predominant factor to consider.HGMS exhibit pozzolanic activity, meaning they partially consumed the calcium hydroxide (CH) present in the cement paste to form secondary calcium silicate hydrate (CSH). CH is known to contribute to the high pH of cement, which acts as the first barrier against carbonation. Moreover, this chemical interaction resulted in a reduction in the thickness of the HGMS outer layer, compromising its strength. This could lead to their rupture when exposed to the high-pressure and high-temperature conditions simulated in this research. The initially non-interconnected porosity shifts to a densely interconnected porosity that ease the way of CO2, which further damages the lightened cement paste.This can be confirmed both by Scanning Electron Microscopy (SEM) images, shown in Figure 1, and Mercury Intrusion Porosimetry (MIP) results, shown in Figure 2. It was also appreciated that calcium carbonate crystals precipitation occurred within the void space inside the HGMS, which could also contribute to the damage suffered by the compound material in terms of microstructural changes.The mechanical properties were obtained from uniaxial compressive strength and drained triaxial tests at 15 and 30 MPa of confining pressure with a back pressure of 5 MPa. An evolution in the yield surface due to carbonation was found and is shown in Figure 3 for cement paste with HGMS27. After carbonation, a reduced cohesion and an increase in the friction angle were obtained for all the cement pastes studied, both with and without cement replacement by HGMS. The microstructural changes shown in Figure 1 are closely related to the evolution of the yield surface. On the non-carbonated cement paste a smooth and seemingly cohesive material can be appreciated, whilst on the carbonated one a disaggregated and almost non-cohesive material is seen.All the laboratory results were considered for adapting an already existing model developed at the École des Ponts ParisTech, which considers the coupled chemo-poro-mechanical problem for traditional cement pastes subjected to CO2 injection well conditions. This model consists of three main parts. Firstly, the homogenization of the compound material for obtaining its elastic properties, with adaptions made to incorporate the presence of HGMS of different classes. Secondly, the mechanical behaviour is addressed by employing an enhanced damage model that properly represents the response of the cement paste as measured from triaxial tests. Lastly, the carbonation of cement hydration products is accounted for based on the thermodynamics of the problem. The model considers the diffusive nature of the carbonation process, yielding an anisotropic material with complex mechanical behaviour.