Characterization of the Initial, Transitional, and Set Properties of Oilwell Cement
Authors: Mueller Dan T. , Lacy Lewis L. , Boncan Virgilio Go 
Published: December 2007

Upon hydration, Portland cement-based materials vary in physical and mechanical properties as a function of time, temperature, and pressure. A new laboratory instrument, which determines the dynamic modulus of a substance, has been developed to measure the properties of a Portland cement as it solidifies. This information serves to improve the understanding of the various physical and mechanical phenomenon that occur within a setting cement. Measurable properties include:the duration of the cement's hydraulic phase;beginning and end of a static gel period;time of transition into initial compressive strength;current compressive strength; andcement expansion/shrinkage as a function of time.

As such, this laboratory device can be used to characterize the conversion of a cement from a fully hydraulic fluid, through its static gel stage, into a solid mass. With this information, a more complete understanding of the interrelationship of fluid-state, gel-state, and solid-state phenomenon, as they relate to cement shrinkage and compressive strength development, can be obtained.

Introduction

The use of ultrasound to determine the integrity of structural concrete has been practiced in civil engineering applications for over 40 years. In the early 1980's this technology was used as the basis for a non-destructive compressive strength test method for oilwell cements. The major advantage of this device, an ultrasonic cement analyzer (UCA), over the more conventional American Society of Testing Materials (ASTM) C-109 test method is the ability to continuously monitor the compressive strength history of the sample under high temperature and pressure (HTHP) conditions. This feature allowed for the determination of the time and rate of initial compressive strength development (50 and 500 psi) which is of greater practical significance than the more traditional 8-, 12-, 24-, or 72- hour strength values.

A UCA monitors and records the acoustic transit time of a compressional wave traveling through the cement sample, converts the transit time to an apparent sonic strength, and stores this data for printout or display. The UCA also provides a realistic simulation of the actual temperature history profile found during and after placement by incorporating ramp-and-soak type temperature controllers. Curing pressures of up to 20000 psi (137.8 MPa) could also be placed on the sample. The development of the UCA was a significant advancement over the conventional ASTM method for determining compressive strength. This advancement was accomplished while reducing the labor demands associated with the conventional technique. The widespread use and acceptance of the UCA was recently demonstrated as the device was accepted as a recommended practice for determining sonic strength in API RP10B.

The use of ultrasound to determine compressive strength of a cement is based upon the fundamental interrelationship of ultrasonic compressional wave transit time, the density of the substance that its traveling through, and the compressive strength of the material. As described in the literature, a predetermined, empirically derived relation is used to establish the sonic strength of the sample as a function of ultrasonic transit time. These correlations are specific to a range of densities, and are believed to be based upon the ultrasonic transit time of samples whose unconfined compressive strength was determined by destructive methods.

The force per unit area required to mechanically fail the molecular structure of a material is related to the modulus of elasticity (Young's modulus) of the substance. Test data indicates the Young's modulus of cement changes from 0.6 × 106 psi to 2.5 × 106 psi as the compressive strength changes from 50 (344 KPa) to 3500 psi (24.1 MPa). The dynamic modulus of a material is closely correlated to the static Young's modulus. It is the determination of a cement's dynamic Young's modulus that serves as the basis for the dynamic modulus test apparatus (DMTA) used to generate the data contained in this paper.

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