Experimental study of the diffusion-controlled acid degradation of Class H Portland cement

The rate of acid corrosion of Class H Portland cement was measured using time-lapse video over a range of temperature (T=30-80°C) and pH (0-3.7), both for hydrochloric acid and carbonic acid. The process was found to be diffusion-controlled, and the dependence of the slope, S, of corrosion depth vs square root of time was obtained as a function of T and pH. The slope decreases by about a factor of 3, if the leachate is allowed to accumulate on the corroded surface; however, a flow rate as low as 4cm/h is sufficient to flush the surface and establish an equilibrium rate. With or without accumulation of leachate, the cumulative cation mass loss is proportional to the depth of leaching, indicating that the phases extracted are very similar in both cases. The corrosion rate between 30 and 80°C is characterized by an activation energy of 39.6kJ/mol and a power-law dependence on acid concentration, S~[H⁺]^0.35. The resulting equation describes the present results, and agrees within a factor of 2 with rates reported in the literature by other workers. On the basis of this equation, estimates are provided of the leakage rates from a reservoir that is sealed with a sound plug of cement (where escape of the acid would take millions of years) or where an annular gap extends through the caprock. In the latter case, corrosion in small annuli (≤10μm diameter) is predicted to be concentrated near the bottom (i.e., near the reservoir-caprock boundary), so that penetration of acid into the overlying formation will take centuries.