A New Low Density Slag Cementing Slurry

Floating beads and hollow glass beads are commonly used lightweight additives in cement slurries. These lightweight additives are expensive, and generally large amount of them are needed in formulating cement slurries of required density. Cement slurries treated with floating beads and hollow glass beads have poor compatibility with drilling fluids. A clue borrowed from MTC technology can be used to solve these problems, that is, using slag as a gelling material to replace oil well cement in formulating cementing slurries. Activators and retarders as assorted agents in this new technology have been studied. An alkaline metal hydroxide JHQ and an alkaline metal salt of silicate JGY were preliminarily screened out as the activators through large amount of laboratory experiments, and the concentration of the activators were determined to be 3% and 2%, respectively. At these concentrations of activators, the set cementing slurry had compressive strength of 12.5 MPa. The retarder selected, HNJ, has hydroxycarboxylic acid groups on the α and β carbon atoms that have strong chelating capacity to calcium ions. The result of the chelating was the formation of highly stable pentacyclic or hexacyclic structures. HNJ was adsorbed on the surface of slag particles to retard the hydration process and prolong the thickening time. The thickening time was almost in a positive linear relationship with the concentration of HNJ. A bentonitic suspending agent, GYW-201, was selected to enhance the stability of the cementing slurry and to control flter loss. GYW-301, a high polymer suspending agent, was used in combination with GYW-201. The slag cementing slurry is suitable for use at 50-90℃, and has density adjustable between 1.30 g/cm³ and 1.50 g/cm³. Low cost, low flter loss, good sedimentation stability, good compatibility with drilling fluids, linearly adjustable thickening time, fast-developing low temperature strength were the advantages of the cementing slurry. Good cementing job quality has been obtained in the Jiangsu oilfeld. As a low cost technology, this new slag cementing slurry has the potential to replace low density cement slurry in cementing low pressure wells liable to lost circulation, wells with long cementing section, and under-balanced wells.