Comparing Oilwell Cement Static Gel Strength Development by Ultrasonic, Intermittent and Continuous Rotation Measurement Methods

Abstract

Cement sheath integrity to prevent interzonal communication is closely related to the static gel strength. The API Standard 65-2 puts importance on the critical gel strength period (CGSP) measurement, which begins when the critical static gel strength (CSGS) is developed and ends when 500 lbf/100 ft2 is attained. The recommended duration for this period should be 45 min or less to be effective in isolating flow potentials. The API 10B-6 covers the three methods to measure the static gel strength development accepted in the industry, which are continuous and intermittent rotation followed by ultrasonic. A laboratory-based study is presented in this paper that compares these measurement methods. The slurry frameworks chosen for the comparison ranged between 11.5 to 18 lbm/gal and the temperature extended from 27 to 121°C. The formulation of the fluid system consisted of Class G cement, silica flour, weighting agent, or light weight extender for the blended phase. Liquid phase additives such as antifoam, fluid loss, dispersant, and retarder were used. The formulations were adjusted to simulate two placement times, i.e., one between 3 to 4 hr. and the second between 7 to 8 hr. The testing performed on the selected cement systems provided significant knowledge of the four different types of static gel strength development equipment used during the testing. There are two equipment's from different manufacturers operating using the continuous rotation method followed by one each for the intermittent rotation and the acoustic type. The overall average transit time for each slurry and the respective standard deviation were arranged for ease of comparison. It was found that there are less deviations in certain fluid systems compared with some other systems. As indicated by the API 10B-6, each equipment may well result in generating different static gel profiles due to cement sample size, apparatus configuration, and formulation. Slurry formulations can be modified to improve their transition time depending on conditions as needed.