Impact of Oil Well Cement Exposed to H2S Saturated Fluid and Gas at High Temperatures and Pressures: Implications for Acid Gas Injection and Co-Sequestration

摘要

Experimental difficulties and safety issues while working with H2S under acid gas exposure conditions (HPHT) have greatly limited the understanding of their impacts on wellbore integrity. To address this, an experimental study was designed to prepare and maintain H2S gas and H2S -containing fluids exposure experiments under relevant subsurface conditions. In this study, several cement formulations (Class H neat and pozzolan amended cements) were cured under subsurface (downhole) conditions and then exposed to H2S for a period of 28 days at a temperature of 50°C and pressure of 15 MPa. The cement was exposed to both an H2S saturated fluid and gaseous H2S to simulate existing wellbores that came into contact with an acid gas injection plume under reservoir conditions. The ensuing cement alteration products were determined by scanning electron microscopy, X-ray diffraction, and ICP-OES. The exposed cement underwent significant physical and chemical alteration via sulfate formation and decalcification. Both the neat and 35:65 pozzolan amended cement were decalcified with a complete loss of structural integrity, while the 65:35 pozzolan cement, although still intact, lost much of its integrity. The integrity of the exposed cement samples increased with an increasing pozzolan addition suggesting that pozzolan amended cements would be beneficial in wellbores expected to come in contact with H2S environments.