Investigating temperature effect on degradation of well cement in HPHT carbonic acid environment

Exposure of well cement to carbonated brine results in severe degradation. Under high-pressure high-temperature (HPHT) condition, the rate of attack increases which leads to loss of structural integrity. In geothermal wells, for instance, temperature and pressure could be as high as 260 degrees C (500 degrees F) and 69 MPa (10,000 psi) respectively. These down-hole conditions have serious impact on well integrity. This paper presents results of an experimental study that was conducted to investigate the effect of temperature on compressive strength of degraded oil well cements. Cement cores were prepared and aged for 14 days in autoclave filled with 2% NaCI solution saturated with methane gas containing carbon dioxide. Temperature was varied up to 221 degrees C under different conditions of pressure (up to 62 MPa) and CO2 gas concentration (up to 100%). Compressive strengths of unaged and aged samples were measured to assess the level of degradation. After exposure, compressive strength of the cements showed mixed trend with increase in temperature due to interrelated mechanisms of carbonation, leaching, and structural transformation of calcium silicate hydrate. Furthermore, this study shows that temperature is the most critical environmental factor that affects the dynamics of cement degradation in acidic environment. (C) 2015 Elsevier B.V. All rights reserved.