Cement Analysis Under Simulated Pumping Conditions (1993)

Abstract A recently developed portable cement laboratory instrument has been used to test dynamic cement performance under actual downhole conditions. This instrument allows the user to change such control variables as pressure, temperature, and shear to follow conditions actually experienced by a cement slurry during a job. During a simulated temporary suspension of pumping, the testing device can perform static gel strength evaluations to help predict the pumping capability of the cement. Static gel strength evaluations can also be performed after the job to allow additional cement quality evaluations. By using this new microcomputer-controlled method, the user can provide values for simulated job variables if necessary. Different test environments can be specified for the lead, intermediate, or tail cements to improve accuracy of the test. The user can still use the conventional, standard API schedules to control the test. This paper will demonstrate the importance of using values obtained from true job simulations for controlling the tests. This information will be supported by test results from various cement slurries tested with the portable cement laboratory. As a result of using this new simulation method, a closer prediction of cement on-the-job performance can be provided. Introduction A recently introduced cement testing method allows users to measure the consistency and static gel strength of a cement slurry under actual downhole conditions. When the compact cement laboratory instrument used in this method is connected to a microcomputer the device can effectively monitor and control pressure, temperature, and shear rate to simulate pumping conditions (Figure 1). The conventional high-pressure, high-temperature consitometer used in thickening time tests uses a slurry cup that rotates at 150 rpm. This speed does not properly simulate actual pumping conditions. In fact, shear produced in the slurry cup is often greater than the actual shear generated by pumping downhole in field operations(1). Shear rates also change as the slurry travels downhole inside the calling and back up through the annulus. The ecently designed cement testing instrument allows various shear rates to be simulated by modifying the speed or the stirring paddle. As a result, test results can better represent actual slurry conditionswhen these conditions are better defined. FIGURE 1: Cement laboratory system. Left to right: micro-computer, 14HPHT consistometer, control system. Centre: plotter. Illustrations available in full paper. Standard API schedules often do not address pressure changes that a cement slurry will experience during a job. For example, the lead cement will encounter completely different pressures than the tail cement because of the different friction and hydrostatic pressure acting on these cement portions. This new instrument can model different pressure conditions that the slurry will encounter as it is pumped downhole, providing more accurate simulations and test results. Cement Slurry Mechanics A successful cement test required accurate environmental control. Different temperatures, pressures, and shear rates can affect the results of thickening time tests so much that the resulting errors can cause slurry design flaws downhole. Inaccurate temperature control can cause negligible to drastic effects, depending on the slurry type tested(2, 3, 4).