Abstract

Fourier transform infrared spectroscopy and synchrotron X-ray powder diffraction methods are described for studying cement hydration chemistry at temperature range of wellbore cementing. The methods provide complementary information on the transformation of silicate, ferrite and sulphate minerals. The thermal decomposition of the cement mineral ettringite is shown to occur at 114 deg C in a sealed system in contact with water. The FTIR spectrum of a well cement slurry hydrating at 150 deg C and 2000 psi is analysed. The anomalous thickening time behaviour of certain cements around 75-100 deg C is discussed in the light of new data on the hydration of a Class G cement at 65 and 95 deg C, with and without retarder.

Introduction

Chemical retardation of thickening and setting lies at the heart of oilwell cement slurry design. However the chemistry of retardation is not yet completely understood. Cements are mineralogically complicated and the hydration chemistry of cements is obscure for that reason. Add to that the lack of scientific techniques for observing directly the chemical changes which occur in cement pastes over the early minutes and hours after mixing. Consider further that the cement slurry is raised rapidly to temperatures as much as 100 deg C above ambient. It is not surprising therefore that until recently the route to new cement additives has been unavoidably empirical.

Today many new scientific tools are becoming available. Cement hydration is now recognised as a challenging topic by materials scientists who can deploy a variety of advanced analytical techniques. This is bringing cement into the mainstream of materials chemistry. We are now beginning to see exciting research making use of such methods as environmental SEM [1,2], NMR [3,4] and electron energy loss spectroscopy [5]. These new experimental approaches are complemented by a variety of modeling methods [6]. For oilwell applications, especially valuable are those methods which can open a window on the chemical changes which occur in the hydrated cement from the first seconds after mixing.

Cement hydration: the chemical clock

It is well known that there is a delay between initial mixing and the start of the main hydration reaction. The induction period is typically several hours in duration and it is this induction period which is lengthened by chemical retarders. Billingham and Coveney [7] were the first to point out that the unusual reaction profile means that cement hydration is an (important) example of the class of inhibited autocatalytic reactions or chemical clock reactions. The chemical clock is started at the moment of first contact between water and anhydrous cement powder, early chemical changes fix certain kinetic parameters and eventually the clock triggers the reawakening of the system, which then passes into an active period of hydration. Billingham and Coveney proposed a sequence of coupled reactions that could reproduce the main features of the kinetic profile of the cement setting reaction. However extensive experimental work is required to identify the specific chemical processes which correspond to the primary inhibitive and autocatalytic steps in cement/water reactions.

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