Improved Cementing Success Through Real-Time Job Monitoring

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Definition The major areas of consideration in any cementing operation are jobplanning, slurry design, blending of bulk materials. slurry mixing, wellpreparation, and slurry pumping. These areas require special attention andoffer many challenges. They reach fruition in job execution on location. Job Execution and Monitoring Proper execution of a cementing job includes Proper execution of a cementingjob includes monitoring, controlling, and analyzing several factors. Certaindata must be collected and analyzed simultaneously to ensure that timelydecisions affecting the success of the overall operation are made. The minimumamount of real-time data that should be collected include pump rate in, annulusrate out, density of fluids pumped in and of those returned, wellhead pressure(at the cementing head), cumulative volume pumped, cumulative volume returned, and hook load during pipe reciprocation. Data Collection To aid data collection during the job, recorder vans, treatment-monitoringvans, and portable automatic data recorders are available. These devices recorddata on tapes or disks for later analysis. They also display many of thevariables on a CRT and/or plotter during the job, which allows the job plotterduring the job, which allows the job supervisor to observe the entire operationfrom one central location and to make timely decisions. The decisions that can be made when sufficient performance data aremonitored and recorded include performance data are monitored and recordedinclude changing pump rate to maintain free-fall rate (mud return rate) at thedesired level for good mud displacement efficiency in the annulus. reducing thepumping rate if partial or complete mud returns are pumping rate if partial orcomplete mud returns are lost, correcting density of the cement slurry if it isout of range, and terminating the job because of high surface pressure or toprevent over displacement. To make correct decisions during the job, it isnecessary to compare predicted ' job performance (obtained with a cementingsimulator) with actual performance. If field performance does not agree withpredicted performance, then changes must be made. Timely performance, thenchanges must be made. Timely changes cannot be made unless sufficient data arecollected and compared with predictions. Fig. 1 (from an x-y plotter) shows some of the variables monitored during afield cementing job. Often the data are displayed simultaneously on a CRT. Thecumulative volumes are calculated from the instantaneous rates. Of special importance is the measurement of the mud return rate during thejob. As a result of free fall of the cement slurry in the casing, the mudreturn rate can exceed the pumping rate while a well is on a vacuum. This isshown in the first 55 minutes in Fig. However, later in the job (from 60 to95 minutes in Fig. 1), the return rate was significantly less than the pumpingrate as the free-fall rate slows down. This lower rate is often mistaken forpartial loss of returns. In this case, full returns are confirmed later in thejob when displacement volume catches up with the top plug. At this time, thereturn rate is equal to the pump rate. Full returns can be confirmed as the jobprogresses by making a comparison of the cumulative volumes pumped in andreturned. In addition, the measured surface pressures for the example in Fig. 1followed the predicted surface pressures exactly as shown in Fig 2. Thisindicates pressures exactly as shown in Fig 2. This indicates that the job wasperformed as planned. Note, however, that the surface pressure was zero for 70%of the job, which renders it of little value earlier in the job. Monitoring themud return rate is so important in order to know what is happening downholeduring the entire job. Predicted Rates Predicted Rates An example of lost circulation duringcementing is shown in Fig. 3 for a different well. Notice that at a cumulativevolume of 200 bbl [32 m ], the return rate dropped to zero (complete lostcirculation) while pumping at a rate of 6.5 bbl/min [1.03 m /min]. The pumpingat a rate of 6.5 bbl/min [1.03 m /min]. The pumping rate at this time wasimmediately reduced to pumping rate at this time was immediately reduced to 3bbl/min [0.48 m /min], which restored complete returns. Full circulation forthe last 30 bbl [4.8 m ] of displacement was important because this brought thecement top 700 ft [215 m] higher in the well and covered critical zones. If mudreturns had not been monitored with flowmeters, complete lost circulation oreven partial returns might not have been detected in time. This reiterates theimportance of monitoring the mud return rate during cementing operations. Full-opening magnetic flowmeters are available to measure the return rate forwater-based systems. It is also important to predict and to measure the mud return rate tocontrol the downhole displacement rate for improved mud displacementefficiency. This is necessary to ensure proper flow regime during displacement. If turbulent flow displacement is required throughout the job, then the annulusreturn rate should not be allowed to drop below this minimum critical rate. P. 619