Improving the Flexural Strength of Fibre Reinforced Oil Well Cementsby Addition of a Polymer Latex
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565 Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
incorporation of short chopped fibres into oilwell cements however, does appear to be a feasible technique for improving mechanical properties and this together with other cement toughening methods has been the subject of various studies. It has long been known that improvements in tensile strength and toughness of cements may be achieved by the incorporation of fine polymeric particles [2,3,4,5,6]. These are usually less than 1 jtrm in diameter and are introduced into the slurry during mixing as a water borne suspension known as a latex. Such latexes have been used in oilwell cements for more than a decade and may provide improved bonding to various rocks and steel casing in the wellbore [5,7,8,9]. They also improve the ability to prevent migration of reservoir fluids from one zone to another by decreasing permeability and preventing gas percolation through the setting slurry in the semi-solid state. Furthermore, the incorporation of polymer latexes where the polymer particle size is in the sub-micron range may lead to a significant reduction in the cement viscosity and therefore improve mixability and pumpability. The combined effects of incorporating both fibrous materials and polymer latexes have been reported for various construction materials [10]. In this paper, the effects of combining a polymer latex together with short glass fibres on the mechanical properties of an oilwell cement are reported and discussed. MATERIALS The cement used in this study was a class G, high sulphate resistant oilwell cement supplied by Dykerhoff of Germany. This is a basic well cement intended for use at depths to 2440 metres. The cement is categorised by the American Petroleum Institute as class G and as such has a composition of up to 6% MgO, 3% S03, between 48% and 58% of tricalcium silicate and a maximum of 8% of tricalcium aluminate. The cement particle sizes range from less than 1 gim to approximately 100 jtm with a mean of 30 jim. The polymer latex was a styrene butadiene rubber latex which has a volumetric composition of 50% water and 50% polymer particles with a mean diameter of approximately 200 nm. The latex was stabilised with a low molecular weight non-ionic polglycol surfactant. Glass fibres were 6 mm long Cem-FIL 7(/30 water dispersible strands with a filament diameter of 20 gim. METHODS Mixing Cement slurries were mixed using a 1 litre propeller type mixer capable of rotation speeds up to 12000 rpm. The required quantity of water was placed in the mixer and the rotation speed set to 4000 rpm. The entire volume of cement was then added in not more than 15 seconds and the mixer speed then set to 12000 rpm for a further 35 seconds. In slurries containing glass fibres, they were dry blended with the cement prior to addition to the mix water. In slurries containing polymer, the latex fluid was injected into the mix after all of the cement had been added and prior to mixing at the higher shear rate. The temperature of the mix water and the addit
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