Analyzing Cements and Completion Gels Using Dynamic Modulus
Authors: Lacy Lewis L. , Rickards Allan 
Published: December 2007

The measurement and control of the physical properties of completion fluids are important problems to the oil and gas industry. A new laboratory instrument, a dynamic modulus analyzer (DMA), has been developed that analyses the physical and mechanical properties of fluids and cement slurries under downhole conditions by using high resolution ultrasonics.

A dynamic modulus analyzer can measure compressive strength, dynamic Young's modulus, and the shrinkage or expansion of cements. The DMA can also be used to determine viscosity changes and changes in the density of fracturing and completion gels under static (10-4 s-1) or zero shear conditions. Test data indicate the DMA is 20 to 100 times more sensitive than current laboratory instruments in evaluating changes in cements or gel properties. Cement shrinkage was measured simultaneously with compressive strength and dynamic modulus. The times required to achieve maximum gel strength and gel breaking were also determined for fracturing gels and a temporary blocking gel.

Introduction

Ultrasonic testing of metals, rocks, cements and other materials is very widely used in various industries. Ultrasonic devices can be used to measure the physical properties of materials or detect hidden flaws in finished products such as welded joints, pipes or small devices. Ultrasonic testing is also widely used in medical applications to detect tumors, heart defects, kidney stones and fetal development. In rock mechanics, ultrasonic testing of cores is often used to determine the dynamic Young's modulus and Poisson ratio of rock samples as a function of temperature and pressure. Ultrasonic velocity is also used to determine in situ stress directions and predict hydraulic fracture azimuth. Ultrasonic velocity information is sometimes used to correlate laboratory data with open hole sonic log data.

An ultrasonic cement analyzer (UCA) measures the ultrasonic transit time (tt) through cement samples at a constant temperature and pressure. By comparing ultrasonic transit time data on cement cubes to unconfined compressive strength (CS) data, Rao and Sutton et al. developed an empirical correlation, and a successful high-temperature and high-pressure (ht-hp) apparatus that predicts CS as a function of time. This nondestructive and labor-saving apparatus is now widely used to test cement compositions before pumping cements in oil and gas wells.

The current paper describes the theory, design and operation of an ultrasonic apparatus that measures the dynamic modulus of cements and gels. The modulus of elasticity is fundamentally related to other material properties such as compressive strength, density and the amount of cementation in solids, and is related to viscosity or gel strength in fluids. The purpose of this study was to determine the potential usefulness of a high-resolution dynamic modulus analyzer to characterize the ht-hp properties of cements and completions gels.

A DMA apparatus was designed, developed, calibrated and used to investigate cements and gel properties. Initial room temperature and pressure tests were conduced on various cement cubes to determine Young's modulus, Poisson's ratio and compressive strength of a variety of cements. In addition to ht-hp compressive strength this laboratory instrument provides additional data on Young's modulus, gel strength and cement shrinkage. Applications of the DMA to measure changes in gel viscosity for hydraulic fracturing fluids and gel blockers for completions operations have also been investigated.

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