Abstract

With the search for hydrocarbons moving to more extreme environments, includingdeepwater, one of the challenges associated with cementing is ensuring thelong-term integrity and mechanical properties of the cement at hightemperatures (HT). To avoid strength retrogression at temperatures above about 110°C, silicais added to the cement. This makes the hydration process (setting and curing ofthe cement) more complex, as initially formed hydration products are replacedby more stable phases over time. To study the nanostructure and mechanicalproperties of HT-cured cement, we applied a variety of techniques includingsmall-angle neutron scattering (SANS), transmission electron microscopy (TEM), nanoindentation, and micro-scratch testing on small cement samples andinvestigated their structure and properties under a variety of conditions. Weobserved that, at HT, there is a general coarsening of the nanometer-scalestructure of the set cement paste over time, with associated degradation of the properties. We showthat the rate of coarsening depends strongly on the initial curing conditions, providing possible strategies for improving the properties and performance ofHT-cured cement. The findings may have particular application to geothermal wells andsteam injection wells.

1. Introduction

Oilwell cementing, in which the annular space between the drilled formationrock and the steel casing pipe is filled with a cement slurry, presentssignificant challenges due to the wide range of environmental conditionsencountered. In particular, the high temperatures and pressures experienced in deep wells generatesignificant changes in the chemical nature and morphology of the hydrationproducts (Taylor, 1997). At near-ambient conditions, the main hydration productof Portland cement is calcium-silicate-hydrate (C-S-H) gel with a Ca/Si molar ratio (C/S)of about 1.7 (Richardson and Groves, 1992, Taylor, 1997). The C-S-H phase hasan amorphous or poorly crystalline nature and a high specific surface area, andis responsible for the generally excellent engineering properties of cement andconcrete cured under normal conditions. As the curing temperature increases toabout 110!C, the C-S-H phase becomes increasingly crystalline but retains itsgeneral structure and properties. Above 110!C, a different phase, alpha-dicalcium silicate hydrate (Ca2SiO2(OH)2) becomes stable and replacesC-S-H. As the C-S-H phase converts to Ca2SiO2(OH)2, the permeability increasesand the strength decreases.

To avoid this, oilwell cements for use above 110!C contain a significant amountof silica (approximately 35% by weight of cement) to lower the C/S of thehydration products to about 1. Under these conditions, a range of crystallinecalcium silicate hydrate minerals with generally acceptable engineering properties can form(Taylor, 1964, Kalousek, 1968, Eilers and Root, 1976).

While the equilibrium phases over a wide range of conditions are wellestablished (e.g. Shaw et al., 2000, Meller et al., 2009), metastable phasesoften form and linger for extended periods of time. In addition, the morphologyof the products are also highly variable. Thus, it is quite difficult to predict the nature of thehydration products that will develop under particular conditions of time, temperature and pressure. Here we apply novel techniques to investigate theproperties of cement/silica blends cured at high temperature and pressure. Small-angle neutron scattering (SANS) and transmission electron microscopy(TEM) are used to obtain information about the nanometer-scale structure, andnanoindentation and microscratch testing are used to determine the mechanicalproperties.

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