This report describes work undertaken at the British Geological Survey (BGS) that forms part of the IEA Weyburn CO2 Monitoring and Storage Project. This project aims to monitor and predict the behaviour of injected CO2 into the Midale reservoir at the Weyburn oil field in southern Saskatchewan, Canada, using methods that include time-lapse geophysics, modelling its subsurface distribution and migration, and simulating likely chemical interactions with the host rock. This report describes fluid chemical and mineralogical changes occurring in a series of experiments that have been conducted within the Hydrothermal Laboratory of the British Geological Survey. These experiments were undertaken to identify the changes that would result from the interaction of CO2 and synthetic formation water with borehole cement. The scoping experiments summarised in this report used samples of borehole cement of types currently used at the Weyburn field (��fill�� and ��tail�� cements), together with synthetic porewaters based upon actual measured well fluid compositions. The pressures and temperatures used within the experiments were representative of actual in-situ conditions at Weyburn (60��C, 150 bar [15 MPa]), conditions that will exist even after oil production and CO2 injection have ceased. The experiments were pressurised with either CO2 or N2, and had durations of two weeks. Although this timescale was relatively short, there was enough reaction to provide some insights into the reactions of borehole cements with CO2. No significant changes in the size of the cement monoliths were found after exposure to CO2. However, sample density increased significantly. The fill cement underwent a greater weight increase (approximately 9-12%) compared to the tail cement (approximately 1-4%). However, for both fill and tail cement, weight gain was greater with supercritical CO2 (11% and 4% respectively) compared to dissolved CO2 (10% and 1% respectively). Simple flexture tests showed no significant changes in the tensile strength of the cements after exposure to CO2. However, the fill cement was about twice as strong as the tail cement (both before and after exposure to CO2). There were some tentative indications from the fill cement experiments that leaching by aqueous fluids may have decreased cement strength a little. Both tail and fill cements reacted with the CO2, developing calcite coatings up to 40 ��m thick on most external surfaces. This ��carbonation�� reaction also penetrated into the cement blocks to varying depths up to around 3.5 mm from the block surface (depending on experimental duration and local permeability variations). The carbonation reaction produced a probable calcite-rich front with significantly reduced porosity that varied up to 50-100 ��m in thickness. In contrast to the fill cement, the tail cement reacted extensively with the CO2-rich synthetic ��Marly porewater�� to produce a series of precipitates from probable calcite and CSH gel, to ettringite and Ca-sulphate,chloride. However, more detailed studies are needed to identify definitively and quantify the reaction products developed in these experiments. Changes in fluid chemistry reflected control by cement minerals in both the lower pH CO2 experiments and the hyperalkaline CO2-free experiments. Large decreases in Mg in the CO2-free experiments suggest brucite precipitation, compared to possible brucite dissolution and increases in Mg in the CO2 experiments. Data for Si show similar trends, and suggest control by CSH phases. Very high bicarbonate concentrations were found in the CO2 experiments. Overall, this study found no evidence for significant deleterious reactions �C at least in the short term. However, our relatively simple approach will have not replicated all of the complex spatial and temporal variations around a borehole. Much work remains to be done to develop a comprehensive understanding of the interactions of stored CO2 with borehole cement, and especially those operating over the longer term.
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