The working fluid in geothermal wells often contains naturally occurring CO2. In an aqueous environment, CO2 forms carbonic acid and subsequently reacts with the binding components of cement (calcium silicate hydrate and calcium hydroxide) resulting in the formation of carbonates that are brittle and cause cement failure when the cement is exposed to stress fluctuations. Well cement formulations containing hydroxyapatite are considered resistant to the carbonic acid attack. The main goal of this work is to study the effects of hydroxyapatite on cement degradation in high-pressure high-temperature (HPHT) acidic environments. To examine the performance of well cement containing hydroxyapatite, degradation experiments were conducted using cement formulations with and without hydroxyapatite. Cement cores and shear-bond test samples were prepared and aged for 14 days in a high-pressure autoclave at different temperatures (38 to 221°C), pressures (21 to 63 MPa), and gas-phase CO2 concentrations (10 to 100%). After aging, the cores and samples were recovered and tested to assess the level of degradation based on variation in bond and compressive strengths, porosity, and permeability, and mineralogical composition. The results showed that hydroxyapatite-containing (HHO) cement has better resistance to carbonic acid attack as compared to the baseline (HS) cement that does not contain hydroxyapatite. As a result, the baseline cement samples were more carbonated than the HHO samples as indicated by the change in cement properties such as compressive and bond strengths, porosity, permeability, and mineralogical composition. Nevertheless, HHO samples have exhibited unexpectedly high permeability due to the formation of micro-cracks after exposure to carbonic acid at 221°C. The thermal retrogression could be the cause of cement expansion and the formation of microcracks.

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