Rheological Evaluation Of Cement Slurries: Methods And Models

Abstract

Abstract The determination of the rheological behavior of cement slurries is essential for the proper evaluation of displacement pressures and flow rates for optimum cement placement. Several cement slurries have been examined, using pipe flow and concentric cylinder viscometers, in an effort to determine which method is better suited for determining such flow characteristics. Comparative analysis of the data indicates that the concentric cylinder viscometer may be inadequate for measurement of the rheological properties of cement slurries. Studies using a pipe-flow rheometer indicate that an apparent "slip" at the pipe surface occurs during rheological evaluation of cement slurries. This wall "slip" is attributed to particle migration when cement slurries are sheared. Field evaluation of the rheological properties of cement slurries flowing in large diameter pipes confirm the results of the pipe-flow rheometer. pipe-flow rheometer. Based on data obtained with a pipe-flow rheometer, a recommendation is offered as to which mathematical model most accurately describes the flow characteristics of cement slurries. Introduction Knowing what displacement pressure and flow rate will maintain a cement slurry in turbulent or plug flow in the wellbore annulus is essential in the design of primary cement jobs. Cement in plug or turbulent primary cement jobs. Cement in plug or turbulent flow exerts a uniform displacement force against the mud in the wellbore annulus. In laminar flow, cement has a parabolic or "bullet-shaped" velocity profile across the area of flow. This results in cement "jetting" through the drilling fluid. Incomplete mud removal can result in poor cement bonding, zone communication and ineffective stimulation treatments. Characterization of the flow properties of fluids is determined by the relationship between the flow rate (shear rate) and pressure (shear stress) required for fluid movement. Extensive studies have resulted in the development of several mathematical models which describe the relationship between shear stress and shear rate. The three most commonly used models are the Newtonian. Bingham Plastic and Power Law models. Most drilling fluids and cement slurries are non-Newtonian fluids and have been treated using a Bingham Plastic or Power Law type model. Other models such a the Herschel-Bulkley and Robertson-Stiff models are not widely used at this time. Mathematical modeling of the flow behavior of cement slurries requires the accurate measurement of shear stress and shear rate. At the present time concentric rotational viscometers are extensively used for cement slurries. This type of viscometer permits the fluid placed in the annular space between permits the fluid placed in the annular space between a stationary and a rotating cylinder to be subjected to shear. The rate of shear is determined from the geometry and speed of the rotating cylinder. The shear stress is obtained by the measured torque induced by the fluid on the stationary cylinder (bob). With a small annular gap the shear rate is nearly constant through the fluid. The use of concentric cylinder viscometers is hindered by particle settling during measurement and the high particle settling during measurement and the high shear rates the cement slurries experienced within the small annular gap. An investigation will compare the results of concentric cylinder viscometers with pipe-flow rheometers. The pipe-flow rheometer is pipe-flow rheometers. The pipe-flow rheometer is similar to capillary-type viscometers. The rheological properties of a cement slurry are determined by measurement of the pressure drop across a length of pipe at a given slurry flow rate. The rheometer has been designed to permit rapid testing with minimal particle settling. The shear rates obtained by the pipe-flow rheometer resemble downhole conditions by being zero at the pipe axis and at a maximum at the pipe wail. In addition, the rheological data can be easily compared with fluids flowing in large diameter pipe. Experimental Procedure Rheological measurements were obtained for a series of cement slurries using both a 12-speed Fann 35/SR12 viscometer and a pipe-flow rheometer. Each cement slurry was prepared according to API procedures using either Oklahoma or Longhorn Class H cement. The amount of water needed to prepare 600-ml cement slurries was determined by weight, not volume.