A synthetic polymer cement retarder has been designed to provide extended pumping times for cement slurries, while having minimal effect on compressive strength development. When compared to traditional lignosulfonate cement retarders, the synthetic polymer provides better response properties, and it is chemically more uniform. Deep wells with bottom-to-top temperature differentials of 150 deg. F have been successfully cemented with slurries containing this retarder.
The function of a cement set retarder is to effectively increase the time the cement slurry remains fluid and pumpable. When mixed at the recommended water to cement pumpable. When mixed at the recommended water to cement ratio, an unretarded slurry containing API Class H or G cement may be safely placed at a depth of 8,000 ft where the bottom hole circulating temperature is less than about 125 deg. F. However, at depths and temperatures in excess of these limits, it is necessary to add chemicals to prevent the slurry from setting prematurely. These additives are especially important in deeper wells where circulating temperatures can exceed 450 deg. F.
The response properties of retarded slurries depend on several factors. For example, the extent of time slurries remain in a fluid state depends on the chemical composition and concentration of the retarder. In addition, retarders must be compatible with other additives present in the slurry. Changing the cement brand or type will change the retarder requirement, i.e., clinker composition and fineness play a major role in retardation. Although increasing temperature decreases thickening time more than increased pressure, pressure does influence the thickening time. Normally, the generalized trend of thickening time dependencies on retarder concentration are similar for many different types of slurries and retarders. That is, a plot of thickening time versus retarder concentration should show an incremental increase in thickening time with each increase in retarder concentration. After some concentration, the thickening time typically increases as an exponential function of retarder concentration. Depending on the chemical and physical properties of the cement, this dependence may be systematically shifted to longer or shorter times for a given retarder level but the basic form of the dependence remains unchanged.
Other parameters influenced by the chemical nature of the retarder include slurry viscosity and gelation. Ideally, cement retarder should not contribute excessive viscosity to the cement slurry. Excessive slurry viscosity will result in high pumping pressure and may, in some cases, result in fracturing pumping pressure and may, in some cases, result in fracturing of the hydrocarbon bearing zone resulting in costly job failure. Another undesirable feature is the induction of slurry gelation during slurry placement. That is, as the temperature of the slurry is increased, the viscosity increases to unacceptably high levels without the development of compressive strength. This may cause the bridging of the annulus resulting in job termination. Optical microscopical examination has revealed the possible premature formation of hydration products in gelled samples. This implies that some retarders either do not adequately prevent the partial hydration of some phase or that they actually induce the process. The viscosity of a slurry containing a suitable retarder will initially be relatively low and remain at that level until the hydration processes commence. At this time the viscosity should increase rapidly and the slurry should soon set and build significant compressive strength.