Development of Permeability and Mechanical Properties of Class G Cement from Slurry to Set

Abstract

Abstract As cement changes from liquid slurry to solid, its load-bearing response, strength and permeability characteristics are expected to change with time. Consequently the ability of any cement to withstand changes in wellbore pressure and temperature will be determined, in part, by the changes in elastic properties, failure criteria and permeability that occur over time. An experimental study of time evolution of mechanical and flow properties of Class G neat cement is presented in this study. The objectives of this intra-laboratory test program were to answer the following questions: How rapidly do the mechanical and flow properties evolve over time? Can changes in microtexture be observed and correlated to those properties?What are the optimal times required to observe time-independent responses?What is happening to the water in the cement and does it correlate with the time evolution observed in mechanical properties and permeability? Measurements of liquid permeability, static and dynamic elastic properties, compressive and tensile strengths, pore size distribution and microtexture and fabric photography were recorded over a 10 day interval. The time evolution of mechanical and flow properties for 15.8 pound per gallon Class G cement and their relationships with water content are presented throughout the paper. Permeability is shown to dramatically decrease and equilibrate over the first 24-32 hours, while the mechanical properties continued to increase over a longer time period. The changes in mechanical/flow properties were strongly correlated to the decrease in water content and the shifting of pore size to smaller distribution functions. Complete stabilization of static elastic properties was not observed after 10 days, implying that, for certain cement formulations, these properties may need to cure for longer than industry standard times. From tests results, 96 hour cure times may be insufficient to characterize certain mechanical properties of wellbore cements. This study also sets a base standard for comparison with more complex cement chemistry's that are currently (and in the future) being used in oil-field operations. The authors also concluded that the use of NMR measurements of pore size and, in turn, water content correlate very well with conventionally used methods. Motivation for this study is based on limited data available on the simultaneous response of all critical wellbore cement properties from the very early stages of hydration to long-term set.