Oil and gas fields offer an appealing opportunity for storage, since their existence proves that a seal can keep hydrocarbons contained for millions of years and much is known about subsurface strata in well-explored areas. However, because oil-producing areas in North America have been punctured by many thousands of existing wells, the seal integrity of well cements is a critical factor in determining whether CO2 will stay stored, or leak up through these potential conduits to the surface.
The Scherer-Prevost Group is combining experimental data on cement integrity with small-scale simulations to assess the potential for leakage through aging wells. The prediction of leakage rates requires an analysis that takes account of the flow rate, the corrosion rate of well cement, volume changes in the solid products (which might either enlarge or block the flow path), and phase changes in the fluid (including rapid expansion of supercritical CO2 as the pressure drops). The experimental data that are most important as input for this modeling task are the permeability, diffusion, and mechanical properties of corroded cement and caprock materials.
Analysis of Corroded Cements
During the past year the researchers have focused on methods for preparing and handling cement samples that are uniformly corroded, so that they can be examined by methods such as nuclear magnetic resonance (NMR) that do not have great spatial resolution. They have also acquired major new experimental capabilities, with the arrival of a system for supercritical drying of cements (which allows preparation of delicate corroded samples for microscopic examination) and an environmental SEM for microstructural examination of wet samples (such as cement in acidic brine).
The diffusivity of water within a porous material can be measured by NMR. The group contracted with a company to provide an NMR system specifically designed for analysis of its cement samples, but it was necessary to cancel the contract when it became clear that they could not deliver the system that was promised, costing precious time. They group is now adapting the equipment available on campus (which is intended for a different kind of measurement) to suit their needs. Preliminary results have been obtained on a corroded sample of special iron-free cement (which yields better NMR results, owing to the absence of magnetic impurities). The measured diffusivity (9.2 x 10-9 m2/s) is within a factor of two of the value obtained by Bruno Huet by fitting his simulations to Duguid’s experimental results. Measurements of well cements are underway.
To examine the microstructure and to obtain composition profiles of corroded samples, it is necessary to dry them without alteration by capillary pressure. This can be achieved by supercritical drying, in which the pore solution is replaced by a liquid with a low critical temperature (such as Freon), then raising the sample to a temperature and pressure above the critical point and extracting the fluid. This is conventionally done with CO2, but that would cause carbonation of cement (Figure 8), so the researchers adapted their autoclave to use Freon R23; since this is expensive material, the modifications included a system for purifying and recycling the Freon. The system is now operational.
The group has recently received an environmental scanning electron microscope (ESEM) through an NSF grant (with Scherer as PI). This instrument allows examination of wet samples, allowing observation of cement as it corrodes (but not with the high resolution possible with dried samples). It also has detectors that permit accurate compositional profiling, so the researchers can examine the composition of the sequence of corrosion layers in cement. This will provide data to validate and refine the corrosion model developed by Huet.
Permeability Measurements in Shale
Shale is the most common cap rock material, so estimates of the diffusivity and permeability of this material are important for modeling leakage. A novel beam-bending technique developed in the Scherer lab permits rapid measurement of permeability, and this method has now been demonstrated to work for shale. When a beam of saturated material is bent, the top is compressed and the bottom is stretched, so a pressure gradient is created in the pore liquid; as the liquid flows to relieve the gradient, the force required to sustain the deflection decreases. By measuring the kinetics of relaxation of the force and fitting the data to the theoretical expression, the permeability can be determined. An example of the results is shown in Figure 9. The good quality of the fit indicates that the permeability, as well as the elastic modulus and viscoelastic relaxation rate, can be obtained by this method. This work will be extended to include a variety of types of shale before and after exposure to carbonated brine.