Saline Brine Reaction with Fractured Wellbore Cement and Changes in Hardness and Hydraulic Properties

The integrity of cemented wellbores is fundamental to the containment of subsurface greenhouse gases and hydrocarbons, while minimizing migration of deep subsurface brines. These brines contain salts and hazardous chemicals that could contaminate potable water resources. This study integrated flow measurements, X-ray diffraction, microscopy, and solution chemistry to investigate the physicochemical effects of reacting saline brine waters at circumneutral pH with fractured wellbore cement under varying stress conditions. Chemical reactions of saline brines with cement fracture surfaces can alter the permeability and porosity of the fracture and consequently impact the potential for leakage through the wellbore. Flow measurements on fractured wellbore cement specimens were made with nitrogen, de-ionized water, and finally brine for a range of hydrostatic confining stresses under controlled laboratory conditions. Hydraulic apertures obtained from nitrogen flow measurements decreased in response to the initial increase in confining stress applied to the fracture owing to crushing of asperities. During flow of saline brine, the hydraulic aperture progressively decreased fourfold compared with its unreacted condition. Precipitation of calcite along the flow path likely decreased the hydraulic aperture, limiting the fluid transport through the damaged wellbore system. Our results indicate that saline brine supersaturated with respect to CaCO3(s) promotes precipitation at the wellbore surface showing an initial rapid drop in the fracture aperture followed by a more gradual reduction in the aperture with increasing pore volume. This result suggests injection of brine may reduce leakages of hydrocarbon fluids into the environment.