Understanding Surface Casing Waiting-on-Cement Time

Abstract

AbstractThe waiting-on-cement time in low-temperature surface casing cementing canvery often be reduced to less than what is common practice today through abetter understanding of cementing conditions. Two major factors in the earlydevelopment of the compressive strength of cement are the water-cement ratioand the curing temperature. The latter is partially dependent upon theformer.The strengths of cements required for drill-out are discussed, and thecuring times required to achieve the necessary strengths are presentedgraphically. API Class A, B and C cements are considered, and the advantages ofusing the densified systems are illustrated.Data from field studies concluded over the past several years, includingtemperature measurements on well fluids and temperature surveys followingcement jobs, illustrate the curing temperature behaviours of typical westernCanadian surface casing cement jobs. The effects caused by heat of hydrationand by cement mixing water and displacement fluids which are warmer thanformation temperatures, can be rather significant, producing average curingtemperatures which are higher than the surrounding formation temperatures by asmuch as 5°F to 30°F or more.A laboratory study was made to determine the comparative effects of the heatof hydration of cement slurries under different conditions. The heat evolved isincreased when (1) the slurry is placed at a higher starting temperature, (2)the volume of slurry is increased and (3) the slurry is mixed at a higherdensity.Cementing practices are discussed and recommendations made. With a morethorough knowledge of the basic factors involved, it should be possible in manycases, to more accurately correlate the required waiting-on-cement time withthe conditions as they exist rather than adopt a policy of fixed time whichsatisfies all probable conditions of cementing materials, temperatures andtechniques.IntroductionThe phrase "waiting-on-cement," or "WOC," has long been a misnomer in mostinstances. The nonproductive and expensive time spent waiting has usually notbeen necessary, because, in fact, the cement was firmly set and was actuallywaiting on us. Increasing concern is being expressed by the industry over thiscostly waste, and continual attempts are being made to reduce WOC time to asafe minimum. The area of greatest interest appears to be that of surfacecasing cementing; this is due to the difficulties caused by the lowertemperature curing conditions.