Short-term effects of brine-cement interaction
Graduate student Andrew Duguid completed his study of the attack of cements by carbonated brines and defended his thesis in October, 2006. (He is now working for Schlumberger in Pittsburgh, PA.) His data on cement decomposition are being used by Prof. Jean Prévost and Dr. Bruno Huet to test the reactive transport module that they have implement in Dynaflow.
The team will assess the importance of pressure on the corrosion rate of cement as results from the high-pressure studies at NETL become available. The NETL results will be compared to predictions from the geochemical module of Dynaflow if necessary, improved pressure dependence will be introduced into the model.
Long-term effects of injection on cement properties
A new graduate student will also join the team in September, 2007, to study the transport and mechanical properties of cement subjected to attack by carbonated brine. Duguid’s results indicate a catastrophic loss of integrity of the cements when the corrosion is well advanced. However, there may be a more subtle deterioration that occurs in limestone formations, where our short-term (i.e., 1-year) experiments indicate little or no attack, that might have significant impact on leakage over the course of a century. We will use NMR to measure diffusion coefficients and a variety of techniques to measure permeability changes during leaching. We will also study the change in strength and stiffness as corrosion proceeds. This information will be needed to refine the leakage model and expand our confidence regarding the risk of leakage over the long term. Andrew Duguid’s experiments reveal that a layer of calcium carbonate precipitates in the reaction zone, resulting in a decrease in porosity and a temporary slowing of attack. It is possible that this precipitate would heal small cracks, while larger ones would be opened further. This possibility will be explored by forcing flow through cracks of controlled width to see whether there is a threshold size below which the flow is arrested.
Characterization of field samples
Following a visit to CMI by Tony Hayward, BP mobilized a team to acquaint us with BP’s field experience regarding durability of cement exposed to carbon dioxide, and to obtain samples of cement from wells. Prof. Scherer has offered advice on the handling and analysis of those samples. If they are provided to Princeton, we will study their microstructure and composition, and subject them directly to corrosion testing to see how they compare to our home-made samples. This will provide the first systematic data regarding features of the structure and/or chemistry of samples subjected to long term aging at elevated temperature and pressure, and thereby enable us to refine our predictions of risk.
The direction of the experimental program beyond Year 7 will depend on availability of cement samples and field data for leakage rates from wells. Field samples, if available, will be used for direct tests of corrosion rate, as well as transport and mechanical properties. If we obtain field data regarding methane leakage rates, those results will be used to infer the size of annuli that are likely to be present in sealed wells and provide initial conditions for the simulations of corrosion of cement.