Evaluation of hydroxyapatite-based cement for geothermal well applications

Abstract

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, resulting in the loss of strength and early failure. Well cement formulations containing hydroxyapatite (HOAP) 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. For evaluating the carbonic acid resistance of HOAP cement, aging experiments were conducted using cement cores and shear-bond test specimens. The aging temperature, pressure, and CO2 content of the gas were varied. After aging, the cores and specimens were recovered and tested to assess the level of degradation based on variation in bond and compressive strengths, porosity, permeability, and mineralogical composition. The results showed that HOAP cement has better resistance to carbonic acid attack than the baseline cement that does not contain hydroxyapatite. Nevertheless, due to the lack of silica flour, HOAP samples were thermally degraded after aging under extremely harsh conditions and exhibited high permeability due to the formation of microcracks.