A Circulating and Shut-in Well-Temperature-Profile Simulator

Abstract

Summary This paper describes a comprehensive circulation and shut-inwell-temperature-profile simulator capable of accounting for free fall duringcementing operations. The program can predict temperatures during circulation of drilling mud in a well or during cementing. The system can also simulateperiods of shut-in at any point during a given run. The simulator was verifiedby use of exact analytical solutions to certain problems. Comparison of simulator runs with field-measured well-temperature data indicated that theprogram does a good job of simulating well temperatures during circulation andshut-in. Introduction Temperature is one of the most critical parameters in the design of cementslurry formulations for oil-well cementing. Temperature affects all aspects of a cement slurry design, including thickening time, compressive strength, fluidloss, rheology, and free water. Waiting on cement (WOC) time, an often costlyfactor in drilling, is affected directly by temperature because it dictates thetime required for enough compressive strength to be developed by the cement todrill ahead. Well temperatures affect the rheology not only of cement slurries, but of any fluid pumped into the well. Changes in rheology are related directlyto changes in frictional pressures in the wellbore. Frictional pressures areimportant during drilling and also in the prediction of surface pressures andrates of free fall during cementing operations. In cementing, rheology affectsthe pumping rates needed to achieve turbulent flow, the regime for optimumdisplacement efficiency. Temperature also determines the range of applicability of chemicals used in drilling fluids, cement slurries, and fracturing fluidsbecause of the temperature-degradation limits of these chemicals. As importantas temperature is, only relatively few temperature data generally are collectedduring routine well operations such as logging and cementing. The data that arecollected often consist of one or two measurements, generally at the bottom of the well. Many important decisions affected by temperature made during the life of a well are based on limited and often unreliable data. The economic impact of this situation is difficult to determine, but money lost because of failuresattributed to lack of reliable well-temperature data could be significantthroughout the oil industry. For years, the American Petroleum Inst. (API) hasmade bottomhole circulating temperatures (BHCT's) available to the oil industryfor the approximation of pseudo-steady-state temperatures at the bottom of awell during mud circulation. API also has provided cement-testing schedulesbased on the API BHCT's for casing, liner, and squeeze cementing. The APItemperatures were developed from actual temperature measurements made in wells, and API has improved on these correlations through the years. Although the APIcirculating temperatures are the result of many actual field temperaturemeasurements, the data are still not sufficient to give a complete picture of awell-temperature profile. The API data give an estimate of thepseudo-steady-state circulating temperature at the bottom of an average wellwith a given bottomhole pseudo-temperature gradient. The data do not giveinformation at any other location in the well and do not include the effect of important variables such as mud type, rate, well configuration, mud properties, and rock properties. Temperature tools are available through service companiesto measure circulating and shut-in temperatures in a well. Some measurementsalso have been made with other types of tools during actual cementingoperations. Temperature-measuring tools usually generate very satisfactorydata. For critical wells, running temperature subs in the drillpipe duringcleanup trips before cementing to confirm estimated BHCT's should be a standardpractice, but use of these subs often is rejected by field personnel because of time and cost factors. In any case, the subs can generate data only at a fewlocations in the well. Recently, a technique consisting of dropping probes orpellets in the casing during circulation to measure well temperatures wasintroduced. This method is more economical than the subs but also can generateonly limited data. Perhaps the only way to obtain an approximation to thecomplete transient temperature profile in a circulating well is with a computersimulator. A few of the many papers describing different approaches to thesolution to the wellbore-temperature problem are given in the General References of this paper. Some of the efforts found in the literature usesimplified approaches that do not properly simulate the unsteady-state nature of the problem. JPT P. 1140^