Initial slurry temperature (IST) and bottomhole static temperature (BHST) are the principal determinants of waiting on cement (WOC) time. The WOC time requirement is driven by
the amount of compressive strength needed, and
governmental regulations applicable to the area of operations.
The following two factors indicate the need for monitoring bulk cement temperature while it is in storage:
Temperature of the dry bulk cement and mix water are principal constituents of IST.
Careful monitoring of the temperature of dry cement in storage can influence the quality of cement slurry.
Bulk cement and mix water temperature information is of special interest in areas of extreme temperature variation, such as the following:
Surface casing applications in which cement slurry is so cold that it takes a long time to reach an acceptable compressive strength, thereby slowing down the drilling operation. This is a common occurrence in the northeastern United States.
Cementing applications in which warm cement slurries do not have sufficient fluid time for pumping operations to be completed, sometimes resulting in job failure. Laboratory charts and case study data from operations in the Michigan Basin document these instances and are presented in this study.
This paper presents methods for
monitoring bulk cement temperatures, and
calculating initial cement slurry temperature at the surface.
When mixing and placing a cement slurry, three important factors to consider are the outside ambient air temperature, the temperature of mix water, and the temperature of the dry cementing materials. When considered at all, the mixed slurry temperature typically has been extrapolated from a nomograph of water temperature and dry cement temperature1 to obtain the mixed slurry temperature (Fig. 1).
In most cases, the nomograph method has not included additives to the mixed slurry, including those that have an exothermic, or heat-releasing reaction, when mixed with water (e.g., calcium sulfate or calcium chloride). In Fig. 2, cement heat-of-hydration curves graphically illustrate the effects of exothermic additives in several blends of Portland cement under laboratory conditions. Cement and mix water with a temperature of 33°F was placed in a simulated well with a BHST of 60°F. The accelerators' effect on the temperatures of the slurries is evident when comparing the two cements with accelerators to the neat cement. The difference in the heat-of-hydration temperatures is the actual heat from the solution of the accelerators in the cement blend.
Temperature-increasing additives are extremely important when cementing shallow oil and gas wells because these additives generate the heat needed to achieve the short-term compressive strengths that help enable drilling operations or completions to resume quickly. Strength data for one common accelerated cement blend (Type 1 with 3% CaCl2 + H2O) are compared in Table 1. These data show slurry temperatures ranging from below wellbore BHST to slurry equal to wellbore temperature and cured at BHST.
Sonic strength testing is the accepted method of testing cement strength as it sets. The sonic strength test is a nondestructive test performed on a slurry to estimate its strength. Correlations have been developed to approximate the compressive strength of a cementing composition based on the time required for the ultrasonic signal to pass through the cement as it sets. Sonic strength tests are performed according to the procedures outlined in the API RP 10B.2
This paper discusses four opportunities for cement slurry excellence and job quality:
Prejob Laboratory Testing - Testing the desired slurry at downhole temperature and pressure conditions.
Quality-Control Monitoring - Monitoring the temperature and quality of physical properties in the cement and cement additives.