The Effect of CO2-Saturated Brine on the Conductivity of Wellbore-Cement Fractures

The efficiency of carbon-capture and -storage (CCS) projects is directly related to the long-term sealing ability of cemented sections in wellbores penetrating CO2-storage reservoirs. The microfractures inside the wellbore cement and/or microannuli are possible pathways for CO2 leakage to the surface and/or freshwater-aquifer leakage and could jeopardize safe and long-term containment of CO2 in the subsurface. This paper presents an experimental study that investigates the changes inside the cement internal structure when exposed to acidic brine through an artificial fracture. A 30-day-long flow-through experiment was conducted using a 1 x 12-in, cement core and CO2-saturated brine as a permeant at a flow rate of 2 mL/min in a coreflooding apparatus with 10 and 600 psi of injection and net overburden pressure, respectively [low-pressure (LP) experiment]. The same experiment was repeated with the same flow rate but with 1,800 and 600 psi of injection and net overburden pressures, respectively, for 10 days in order to account for the effects of pressure on the degradation process of cement [high-pressure (HP) experiment]. High-resolution X-ray computed tomography (CT) was used to image several subvolumes extracted from the flow-through cores. The images were processed and thresholded, followed by calculation of porosity. Total porosity was observed to decrease from 26 to 22% after 30 days of exposure in the LP experiment. The HP experiment did not cause any significant change in total porosity, possibly because of the short duration of the experiment.