Assessing wellbore integrity during carbon sequestration in depleted oil reservoirs

Storing carbon dioxide in subsurface formations is a potential strategy for stabilizing and/or reducing the concentration of atmospheric carbon dioxide. Petroleum reservoirs are an attractive option as storage sites since both a regulatory framework and financial incentive are readily available. However, assessing the risk of leakage from the formation is critical not only to proving the technical feasibility of carbon sequestration, but also for ensuring that neither human health nor the environment are endangered by migrating carbon dioxide. Once injected, CO2 mixes with subsurface brine to produce carbonic acid, which in turn reacts with and degrades wellbore cement through a progressive decalcification reaction that turns the cement hydrates into an amorphous silica gel. It is conceivable that degradation of the well cement could allow a pre-existing annular gap in the wellbore to grow, increasing both the rate and risk of leakage. In light of this, wellbore integrity is central to assessing the leakage potential, since an open pathway within the wellbore could allow CO2 or CO2-saturated brine to flow into a neighboring subsurface formation or escape to the surface.;Bench-top experiments have been performed to: (1) investigate the parameter space of relevant brine compositions, e.g. acid flow rate, pH, CO2 concentration, and calcium ion concentration; (2) produce a uniaxial degradation front such that samples can be profiled to quantify the transport and mechanical properties of acid corroded Class H well cement; and (3) compare the properties of samples exposed to accelerated leaching tests with those prepared under the milder conditions expected for a wellbore in the subsurface. Accelerated batch experiments quantify the effect of leachate accumulation on the rate of attack. Flow-through experiments yield an empirical correlation that accurately predicts the rate of attack across a range of both pH and temperature. This correlation can be used to predict the behavior of an annular gap within the wellbore during the progressive degradation of well cement. Corrosion rate data, compositional analysis, and diffusion measurements are used to quantify the extent to which brine composition influences the material properties of acid damaged well cement. The results enable us to identify the dominant reaction mechanisms and to predict whether plugging or enlargement of an annular leak will occur.