Disclaimer: This paper includes forward-looking statements, Actual future conditions… The publication of the API Standard 65-2 "Isolating Potential Flow Zones During Well Construction" places strong emphasis on measuring the Critical Gel Strength Period (CGSP). The CGSP is the time period starting from when laboratory measurements indicate the slurry has developed Critical Static Gel Strength (CSGS), to when it has developed strength of 500 lbf/100ft2. As per the API standard, CGSP of 45 minutes or less has proven effective in isolating zones with severe flow potential therefore, the CGSP has been identified as a crucial parameter to qualify slurry systems for many oil and gas operators. The API 10B-6 "Recommended Practice on Determining the Static Gel Strength of Cement Formulations" indicates there are three measurement methods; continuous rotation, intermittent rotation, and ultrasonic-type static gel strength. This paper will review a comparative laboratory- based study between two measurement devices commonly used in the industry to measure the CGSP. One device uses the intermittent rotation, and the other uses the ultrasonic method. The slurry systems selected for the study covers a density range of 11.5 to 18 lbm/galUS within the temperature range of 27 to 121 deg C. The selected slurries comprise of Class G cement, silica flour with extenders or weighting agents paired with antifoam, fluid loss additives, dispersant and retarder to represent the typical slurry systems used in cementation of potential flow zones. Additionally, each slurry system is designed for two scenarios; one is to cover a short placement time (3 to 4 hours) and the other for a longer placement time (7 to 8 hours). While some of the slurry systems passed the CGSP criteria based on the ultrasonic method of measurement, they did not pass the criteria when measured by the intermittent rotation method. Possible reasons for this mismatch are further explained in this paper. Results of this study are applicable to all oil and gas service operators involved in cementation of potential flow zones. One would expect that both of these methods would predict the same slurry behavior in either passing or failing the CGSP criteria, however and on the contrary that was not observed on all of the slurry systems selected for this study.
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