Malin Torsæter

In-situ X-ray tomography study of cement exposed to CO2 saturated brine

For successful CO2 storage in underground reservoirs, the potential problem of CO2 leakage needs to be addressed. A profoundly improved understanding of the behavior of fractured cement under realistic subsurface conditions including elevated temperature, high pressure and the presence of CO2 saturated brine is required. Here, we report in situ X-ray micro computed tomography (μ-CT) studies visualizing the microstructural changes upon exposure of cured Portland cement with an artificially engineered leakage path (cavity) to CO2 saturated brine at high pressure. Carbonation of the bulk cement, self-healing of the leakage path in the cement specimen, and leaching of CaCO3 were thus directly observed. The precipitation of CaCO3, which is of key importance as a possible healing mechanism of fractured cement, was found to be enhanced in confined regions having limited access to CO2. For the first time, the growth kinetics of CaCO3 under more realistic well conditions have thus been estimated quantitatively. Combining the μ-CT observations with scanning electron microscopy resulted in a detailed understanding of the processes involved in the carbonation of cement.

Thermal Stresses in Annular Cement

Heating of casing, e.g. by the drilling fluid returning to surface, expands the casing string. This results in the hoop stress in the cement sheath becoming less compressive (more tensile). Similarly, cooling of casing, e.g. by injecting cold water down the well, makes the casing contract. This results in the radial stress in cement sheath becoming less compressive (more tensile). These stress changes may induce radial cracks or debonding at cement-casing and cement-rock interfaces. Finite-element simulations are performed in order to estimate the magnitude of the stress variations in cement sheath caused by the temperature variation at the inner side of the casing. Simulations are performed for different combinations of thermal expansion coefficients of cement, steel, and rock. It is shown that, at least in some cases, it is beneficial to have cement formulations that result in lower Young’s modulus and higher tensile strength of cement upon hardening. The role of initial stresses in cement sheath for practical evaluation of cement sheath stability during wellbore heating/cooling is discussed.

Properties of Well Cement

Well cementing involves pumping a sequence of fluids into the well. Often these fluids, such as spacers and cement slurries, have non-Newtonian yield-stress rheology. After the cement slurry has been placed in the annulus, it hardens into a low-permeability annular seal. The complexity of these processes and the multitude of materials involved (drilling fluid, spacer, chemical wash, cement, casing, rocks) call for a sufficiently detailed material characterization in order to design and optimize cement jobs. A review of properties describing cements and other materials used in primary cementing is presented in this chapter. Rheological properties of washes, spacers, and cement slurries that control their flow down the well and up the annulus are discussed. Basics of non-Newtonian fluid rheology required to understand the subsequent chapters are laid out. Transition properties of cement slurry related to its solidification are reviewed. Mechanical, interfacial, hydraulic, and thermal properties of hardened cement that control e.g. response of cement to thermal stresses, vibrations, etc. are introduced, along with laboratory techniques used for their measurement (Brazilian test, uniaxial test, triaxial test, push-out test).

Heterogeneities in Cement

It is not possible to create a perfectly homogeneous annular cement sheath in a well. Defects will always be present at different scales, ranging from intergranular microcracks to bubbles and gas channels. Defects can be produced at all stages of primary cementing, i.e. during cement slurry mixing, placement, hardening, and subsequent loading. Defects have a strong impact on well integrity since they may provide leakage paths for formation fluids along the cemented annulus. In this chapter we discuss some important heterogeneities in cement (channels, pockets, cement porosity, slurry segregation issues, and interface defects). It is outlined how they can be studied by state-of-the-art imaging techniques, such as X-ray computed tomography. Special emphasis is given to the unavoidable interface transition zone (ITZ) forming along cement interfaces, and the debonding of cement from steel/rock. The chapter forms the basis for the further discussion of operation-induced defects in Chaps. 5 and 6.

Fluid Flow and Displacement in the Annulus

During a primary cementing job, a sequence of fluids is pumped into the annulus in order to displace the mud and prepare the annulus for cement placement. The factors affecting the mud displacement efficiency are discussed in this chapter. The effects of pipe eccentricity, breakouts, and irregular wellbore cross-section on the displacement efficiency are demonstrated using a simple kinematic model of annular cementing. In particular, it is shown that breakouts may have a substantial detrimental effect on the displacement efficiency since the displacing fluids might be flowing only in the breakouts. Channelization is also shown to occur when the wellbore has neither breakouts nor washouts, but rather a slightly irregular cross-section, like real wells normally do in sedimentary formations. In this case, viscous instabilities occur for unfavorable mobility ratios. Channelization may in this case be prevented most effectively by increasing the yield stress of the displacing fluid. The effects of well inclination, pipe movement and flow regime are discussed. A brief overview of numerical models of well cementing is provided. Unresolved issues in modelling are summarized.

Knowledge Gaps and Outstanding Issues

Despite substantial progress made in research and development of cement formulations, preflush engineering, cementing technologies, and numerical modelling over the past decades, several knowledge gaps and unresolved problems still exist. These problems are likely to persist in the future as more complicated cementing conditions are encountered, e.g. in deepwater wells, HPHT wells, geothermal wells, and during underground CO2 storage. New challenges are due either to harsh downhole conditions (e.g. HPHT) or more stringent environmental and safety regulations (e.g. geothermal and CO2 wells). Challenges are found in the design, where perfect mud displacement and cement placement are still rare. Numerical models used to design cementing jobs are still often either too complicated (i.e. too slow) or inaccurate. Uncertainties about the formation properties (permeability, temperature, etc) and about the behavior of cement at downhole conditions reduce the practical value of even the most promising models. Despite these knowledge gaps, continuous progress over the past decades suggests that, in the years to come, the technology of primary cementing will continue to improve, based on our steadily improving knowledge of its physics and mechanics.