Postanalysis of Abnormal Cementing Jobs With a Cementing Simulator

Abstract

Summary. Postanalysis often cannot be applied because of lack of prediction and real-time performance data. Proper postevaluation includes prediction and real-time performance data. Proper postevaluation includes collection of actual job performance data and comparison with predicted cementing performance. A comprehensive cementing simulator is required that accounts for fluid properties, well geometry, changing displacement and mud return rates, and the free-fall effect. Such a simulator has been used successfully in the design of many cementing operations and quite frequently in actual jobs to monitor the progress of the operation. In this paper, case histories are presented to show how this model and on-sit measurements can be used for postevaluation of cementing jobs that did not perform as predicted. Two examples illustrate job problems caused by a perform as predicted. Two examples illustrate job problems caused by a restriction either in the annulus or in the casing, a third case points out problems associated with use of improperly calibrated pressure gauges problems associated with use of improperly calibrated pressure gauges during a cementing job. Introduction The productivity of a well is directly related to the quality of the primary cementing job performed on all casing strings in the well. A successful cement job is one that has eliminated all mud and gas channels in the cement sheath and has obtained a complete peripheral hydraulic seal with the casing and the formation face throughout the zones of interest. One key factor in obtaining high-quality cementing success is job postevaluation. This permits adjustments to be made for improvement on subsequent jobs. Phenomenon of Free Fall. Beirute gave a detailed Phenomenon of Free Fall. Beirute gave a detailed explanation of the phenomenon of free fall. Subsequently, others also have developed free-fall models. In the great majority of primary cementing jobs, the densities of the spacer fluid and cement slurries are greater than the density of the mud in the well. Because of this density difference, the well often goes on a vacuum or U-tubes while the heavier fluids are pumped down the casing. During the time the well is on a vacuum, the column of heavier fluids in the casing is free-falling at rates different from the pump rates at the surface. During the early stages of free fall, the free-falling column of fluids accelerates to rates that are higher than the surface pump rate, as shown in Fig. 1. The free-fall rate is shown as the annulus return rate in Fig. 1. Later, toward the end of the free-fall period, the free-falling column of fluids decelerates and the period, the free-falling column of fluids decelerates and the free-fall rate can drop below the surface pump rate. During free fall, the surface pressure at the cementing head is essentially zero. Free fall causes a discontinuous zone to form between the wellhead and the top of the continuous free-falling column of fluid inside the casing, as shown in Fig. 2. Toward the last part of free fall (Fig. 1), the rate of returns decreases, and this gap decreases in length while the displacement fluid is pumped. When the gap has been completely filled, free fall ends and the annulus return rate is equal to the surface pump rate. Also, surface pressure again becomes positive at this time, as shown in Fig. 3. Cementing Simulator The cementing simulator Beirute described has been verified with measured field data. It accurately predicts flow behavior for a given set of conditions. One case is repeated here. Fig. 4 presents the job measured pumping rate and rate of mud returns for the field example. Note that free fall, which accounts for 70% of the job, begins soon after the lead slurry is started and ends about 85 minutes later. The simulator-predicted rate of returns is superimposed in Fig. 5 over the field data from Fig. 4. Note that the predicted rate of returns matches the field measured data very well throughout the entire job. The predicted surface pressure and the measured field pressure for this example are shown in Fig. 6. Note the pressure for this example are shown in Fig. 6. Note the excellent match of the pressure data. The close agreement between predicted performance and actual performance in the example well and other wells has shown that the simulator is accurate and can be used with confidence. In this paper, the simulator is used in the postanalysis of three wells that did not follow the predicted behavior during the jobs. Well A Casing and Cementing. A 7 5/8-in., 52.57-lbf/ft [19.4-cm, 767-N/m], plain-end, NT-95-SS, flush-joint production string was run and cemented at 18,000 ft [5490 m]. The casing contained a high-temperature float shoe and float collar with three joints (120 ft [37 m]) between floats. SPEPE P. 157