Cement Bond Log Evaluation of Foam- and Synthetic-Cemented Casings

Abstract

Summary Cement bond log (CBL™) studies on foam- and synthetic-cemented wells were initiated to determine the feasibility of, as well as to develop technologies for, evaluating these novel cementing services. Early CBL's on these cementing systems showed little effect on the log amplitude curve. Hence, CBL evaluations were difficult to obtain and interpret. A special sonde with a 1.3-ft [0.4-m] transmitter-to-receiver spacing was developed for this study. Sonic signal amplitudes were determined by using cemented short-casing test sections. Sonic attenuation rates were correlated to compressive strengths for a range of cement densities. Experimental details of the cementing operation and logging studies are discussed. Data relating attenuation rates to compressive strengths and cement densities also are presented. Field results are discussed. Introduction Historically, since the introduction of foamed cement into the oil field in 1978,1 this novel cementing service has been considered primarily for applications to solve lightweight cementing problems.2 The advent of new and improved foamed-cement technology and an understanding of the properties nitrogen imparts to cement slurries have broadened the application range of this innovative service to where it is no longer limited to low-fracture-gradient wells. Although the high compressive strengths and wide density range (8 lbm/gal [958.4 kg/m3] and greater) of foamed cements make this service ideally suited for cementing operations in weak formations, the high 24-hour compressive strengths (500 to 3,000 psi [3448 to 20 685 kPa]) made available by new technologies make this service suitable for many different well applications. Furthermore, the higher performance of the hydrated cement provides perforation capability. Cement bonding and zonal isolation, which are the ultimate criteria of a successful cementing job, are improved by foamed cement. Laboratory results have shown superior bonding when nitrogen is added to cement slurries. Zonal isolation also is aided from the pressurized gas in the slurry. As cement hydrates and loses hydrostatic pressure, cement to casing and formation wall contact is decreased.3 With foamed cements, as the cement hydrates and loses hydrostatic pressure, the gas cells undergo a slight expansion, depending on gel-strength development, thus maintaining cement contact with the wellbore and casing.4 Induced circulation problems caused by high pump pressure used to overcome slurry friction pressure can be a major concern during the cementing operation. Because foamed cements have low frictional pressures, the probability of inducing lost circulation during the pumping operation is reduced. Should lost-circulation zones or cavernous vugs exist, the thixotropic property of slurries containing nitrogen makes these systems ideally suited to solving these types of problems.4 Steamflood, fireflood, and geothermal wells present special challenges with respect to job design. Not only must the hydrated cement provide excellent zonal isolation as well as insulation, but it also must be able to withstand thermal shock and the well's corrosive environment. Thermally stabilized foamed cements, up to 572°F [300°C], are excellent choices for this type of application. The high porosity of these materials offers superior insulating properties compared with other cementing materials, while their low permeability provides good corrosion resistance and zonal isolation.5 Synthetic cements (i.e., epoxy-base systems) are seeing increased usage in the oil field. Their chemical inertness, high strength, and low viscosity are being used to solve several unique application problems. The use of synthetic-base cements is solving many of the problems normally associated with waste-water and acid-disposal wells, which are subject to corrosive environments. The low viscosity and nonparticle nature of the material is seeing wide use in squeeze-cementing applications. More specifically, microannuli, which are prone to cement-particle blockage problems, are squeezed off readily with low-viscosity, epoxy-base cements, such as liner tops having microannular gas flow problems. Injection and production profiles are altered easily by epoxy sealing of the formation. The nonaqueous nature of synthetic cements makes them very applicable to water-sensitive formations.