The Effects of Temperature and Cement Admixes on Bond Strength

Abstract

Summary The integrity of a cement job is generally measured by its ability to provide zonal isolation, both at the time of completion and for the life of the well. Historically, hydraulic and shear bond strengths have been considered the laboratory equivalent of Cement Bond Logs (CBL), good indicators of a cement's ability to isolate pressure differentials. However, past bond strength studies have generally overlooked the effect of thermal shock, large fluctuations in downhole temperature. Thermal shocks of +/- 100 deg. F, positive or negative, are common place during stimulation treatments, workover procedures or production of reserves, particularly in Arctic or deep water environments. A cement-to-casing, bond strength study was undertaken to evaluate the relative benefits of various "bond improving" cement designs and admixes and the effects of sudden inductions in downhole temperature. An apparatus was constructed that would cure the cement and permit failure testing while at elevated pressures and downhole temperature. Primary and dehydrated pressures and downhole temperature. Primary and dehydrated or "squeeze" cement slurries were evaluated with and without the presence of thermal cycling. Introduction Laboratory measurements of cement bond strength have long been used to evaluate the zonal isolation potential of a cement design. Shear bond strength tests measure the amount of force necessary to initiate a sliding movement along the bonded casing-cement interface. Hydraulic bond strength tests use either fluid or gas pressure to quantify the force required to breach the cemented interfaces. Past test methods have precluded a complete evaluation of the Past test methods have precluded a complete evaluation of the effects of temperature and pressure and may have introduced some inadvertent errors. Typically, bond test specimens have been cured at either 1) ambient temperature and pressure or 2) elevated temperatures and pressures. While those tests conducted at ambient conditions were not representative of downhole conditions, most of the others were not either. With the exception of Parcevaux and Sault, when elevated temperatures and pressures were used during the curing period, the specimens were pressures were used during the curing period, the specimens were returned to ambient conditions before collecting the bond strength data. This type of procedure exposes the specimens to significant changes in temperature and pressure. Although many of these studies noted that changes in temperature and pressure can impact bond strength, these effects were not extensively investigated. These and other aspects of previous bond testing methods, have lend to their results being characterized as inconsistent, non-reproducible and sometimes contradictory. This study was designed to permit a qualitative evaluation of the zonal isolation potential of various cement designs and their resistance to damage from thermal shocks. Although the possibility of thermally induced damage has been cited by past possibility of thermally induced damage has been cited by past researches, little comparative data has been offered. Past investigations performed by members of this study have noted that sharp temperature reductions could breakdown the hydraulic seal of cement squeeze perforations. Liquid latex or styrene butadiene resin (SBR) cement blends were also cited as "appearing" more resistant to thermal and mechanical These two observations by Blount et al prompted this cement bonding study.