Influence of pre-carbonation on hydro-mechanical properties of cement paste subjected to leaching

Depleted oil reservoirs have been one of the most practical options for CO2 sequestration. To ensure the overall sealing performance of oilwell for CO2 injection, it is necessary to understand the long-term hydro-mechanical behaviour of cement paste used as oilwell casing. In the presence of high CO2 concentration and water, cement paste is firstly carbonated and then may be degraded by carbonic acid in CO2 storage. The present work aims to experimentally investigate the pre-carbonation effects on the leaching of oilwell cement paste, with special attention to hydro-mechanical properties. By using a series of uniaxial compression tests and triaxial compression tests with permeability measurements, permeability and mechanical properties of sound and pre-carbonated materials, before and after leaching treatment, are measured and analysed. Moreover, the time-dependent behaviour of pre-carbonated material subjected to leaching is studied via a coupled creep test. The obtained results exhibit that after leaching treatment, comparing with the initially sound samples, the pre-carbonated samples exhibit higher mechanical properties, lower permeability, and smaller creep deformation. Therefore, one can conclude that the degradation of mechanical and hydraulic properties induced by leaching is reduced by the pre-carbonation of cement paste and the carbonation has a positive effect against leaching. To verify and understand the macroscopic experimental observations, some micro-indentation tests are performed at a mesoscopic scale and a mineralogy and chemistry analysis are also conducted via SEM-EDS and Raman spectroscopy tests. Experimental investigation, micromechanical analysis and microstructure will advance the understanding of long-term durability of CO2 geological storage.