Experimental assessment of well integrity for CO2 geological storage: Batch experimental results on geochemical interactions between a CO2–brine mixture and a sandstone–cement–steel sample

Abstract

A wellbore constructed in a CO2 geological storage site is considered to be a potential leakage path. Some laboratory experiments showed serious degradation of wellbore cement while a filed experience indicated integrity of the cement over 30 years. To fill the knowledge gap, we prepared a simulated well sample: a carbon steel bar (J-55) was embedded and fixed with Portland cement (Class A) in a hole made in the centre of a sandstone core sample. Batch experiments were conducted at similar to reservoir conditions: 0.5M NaCl brine, 50°C and 10MPa. Simulated well samples were exposed to (1) supercritical CO2, (2) CO2-saturated brine or (3) both CO2 and CO2-saturated brine. Results show that the cement was carbonated to the depth of a few millimetres and the interior cement was not altered after 56 reaction days; also, the alteration of the steel casing was minimal. Apparently, in the sandstone surrounding, the cement seemed to prevent direct interaction between the cement and CO2 and to provide buffer area of pH and/or of carbonate precipitation. The alteration depths for the cement exposed to supercritical CO2 and CO2-saturated brine over 30 years were estimated at 4.6mm and 2.1mm, respectively. Our experimental results were consistent with the previous results of CO2-enhanced oil recovery field samples (1–10mm thick of orange carbonated zone) that were collected from the wells which were exposed to CO2 for over 30 years.