Analysis of Cementing Turbulators

Abstract

Summary This paper presents the results of a study to determine the ability of commercially available mechanical aids called "turbulators" to alter the flow profile and thus enhance the displacement efficiency of typical cementing operations. Results are presented for both the predictions and the measurements showing the effects of pump rate, predictions and the measurements showing the effects of pump rate, viscosity, density, and turbulator blade geometry on the induced flow angle along the pipe axis (thus effective swirl length). The results indicate that mud-displacement efficiency can be improved dramatically with mechanical aids that alter the flow profile in the annulus between the casing and the hole. Simple guidelines are provided for spacing turbulators, and two case histories are presented that substantiate the laboratory findings. Introduction In recent years, an influx of mechanical devices has surfaced to improve the quality of cementing operations by centralizing the casing string and/or diverting the flow circumferentially around the annulus. Casing turbulators are mechanical devices that do both. Turbulators are solid cylindrical devices with blades welded to the outer surface at angles 30 to 50 from on axis (Fig. 1). Cement flowing past the turbulator is then redirected circumferentially around the casing by the blades, thereby flushing the narrow side of the annulus and enhancing drilling-fluids displacement. Although casing turbulators increase turbulence levels in the fluid immediately following the turbulator, this increase is minimal and quickly dissipates. The positive effect from the turbulator is the induced swirl flow. To date, response to the use of turbulators in field applications has been favorable. Cement-bond logs (CBL's) generally provide higher displacement efficiencies. This study examines the effect of the various turbulator geometric parameters, flow rate, and cement rheology on the "effective swirl length," Ls, established in the flow passing the turbulator. The effective swirl length is the axial distance along the casing annulus where the flow angle remains greater than 3 from on axis. Background Casing turbulators were first evaluated in 1986. On the first well, 19 turbulators were run in three separate intervals; two are discussed here. These two intervals were widely spaced in two different wellbore diameters. Fig. 2 shows the wellbore arrangement and the cement-bond, sonic, and gamma ray logs for the first interval. Thirteen turbulators with 6-in.-OD blades were run on 4 1/2 in. casing. One turbulator was run per 40-ft joint across 500 ft of open hole. The bottom turbulator was at 18,762 ft, and casing was set at 19,602 ft. The pipe amplitude curve on the CBL in Fig. 2 shows much better bonding opposite the turbulators compared with the interval below 18,762 ft. The hole size throughout the interval, as indicated by the caliper log, varied from 6 1/5 in. (bit size) to greater than 16 in. The best bonding is achieved in the near-gauge hole for the 6-in.-OD turbulator blades, demonstrating that the effect of the turbulator diminishes as the ratio of hole size to blade diameter increases. Three 6 1/4 in.-OD turbulators were also run farther up the hole in the same well. In this case, the turbulators were inside a 7 5/8-in. drilling liner with an approximate ID of 6 1/5 in. The CBL pipe amplitude curve, shown in Fig. 3, indicates good bonding in the 15,525- to 15,615-ft interval opposite the turbulators. Bonding was poor above and below this interval where turbulators were not run. poor above and below this interval where turbulators were not run. Prompted by the good results obtained on the first well, turbulators were tested a second time on another well (Fig. 4). On this second well, 3,500 ft of 7-in. liner was run to 17,988 ft in 8 1/2 -in. hole and equipped with 81 turbulators with 8 1/4-in.-OD blades. Turbulators were run one per 40-ft joint from 14,454 to 17,956 ft. The CBL pipe amplitude curve indicates excellent bonding throughout the interval, which is representative of the entire section where turbulators were used. In this case, good bonding was realized even though the hole size ranged from 9 to 10 in. across the interval. The field evidence clearly indicates that use of casing turbulators on liners and casing strings improves mud displacement and thus cement bonding. The following sections outline the degree of influence the turbulator has on the cement flow and provide techniques for proper turbulator placement and pump rates for maximum fluid proper turbulator placement and pump rates for maximum fluid displacement.