Summary. This paper describes experiments with a large-scale flow loop (45 ft of 7-in. pipe in nominal 8 1/2-in. open hole) that measure the velocity of the interface between fluids of varying densities and rheologies. The work showed that single-fluid velocity profiles cannot be used to predict interfacial profiles (e.g., between spacer and cement) and confirmed the importance of turbulent flow in minimizing channeling through mechanisms of increased frictional pressure drop a nd mixing across the annulus. Large density differences, both negative and positive, also minimized channeling. Introduction Successful primary cementing is as important today as it has been in the past. A large percentage of wells are being drilled in the difficult offshore environment, where the cost of remedial work is significant, leading to renewed efforts to ensure efficient primary-cementing. Placement is a key part of the primary-cementing primary-cementing. Placement is a key part of the primary-cementing process and has attracted much attention in the past. As a result of process and has attracted much attention in the past. As a result of significant improvements in our understanding, a greater percentage of wells are now cemented efficiently. Nevertheless, a significant number of primary cement jobs still experience problems, particularly in such primary cement jobs still experience problems, particularly in such difficult situations as highly deviated wells or wells prone to severe washouts. The cost of failure in these wells is often extremely high. A quantitative understanding of the placement process, including the ability to predict performance, is required to achieve successful cement jobs over the wide range of conditions currently found in the field. BP Exploration built a large-scale wellbore simulator to improve the qualification of cementing variables. Current practice relies on a number of well-worn guidelines that are not universally applicable. When these guidelines are applied to some of the difficult cases, other important variables can be forgotten, resulting in poor cement placement. Lockyear and Hibber described the wellbore simulator and some preliminary results. This paper addresses the problem of channeling, particularly attempts to quantify the behavior of the cement/spacer and spacer/mud interfaces. Previous Studies Previous Studies Probably the most widely known guideline to achieve good cement placement Probably the most widely known guideline to achieve good cement placement is to pump in turbulent flow. This was first recommended by Howard and Clark, who displaced mud directly by cement without use of a spacer. Current practice is to use a spacer fluid to separate mud and cement, but when a spacer is used, it is often unclear which fluid needs to be pumped in turbulent flow. In contrast, Parker et al. suggested that the viscous reaction between mud and cement could be used to obtain good cement placement if the overall velocity were less than 90 ft/min. This idea led to the plug-flow technique and the recommendation that under such circumstances the cement needed to have a specific gravity (SG) 0.24 greater than the mud. This early work was followed by a number of attempts to build mathematical models of the displacement process, but these apparently did not match experimental behavior. Some of the assumptions were not strictly accurate, however, and some of the experimental data may have been incorrectly interpreted. One important cause of errors was the assumption that interfacial velocity profiles could be modeled by a single-fluid velocity profile. Early models also assumed that no reaction occurs between mud and cement, which generally is not true. With the increasing use of compatible spacers, however, this assumption may now be valid. The problems of deviated-well cementing were not addressed in earlier modeling problems of deviated-well cementing were not addressed in earlier modeling work but clearly have great significance today. After the poor success of the mathematical approach, a more empirical approach was taken to develop a series of field guidelines. The resultant guidelines included 10-minute spacer-contact time, pipe movement, use of spacers, and further evidence of the benefits of turbulent flow. During this period, the first attempts were also made to investigate the problems of cementing highly deviated wells. The empirical approach proved very valuable in increasing the success rate of primary cementing. The usefulness of these guidelines, however, is limited because they are rather rigid and not easily extrapolated to conditions outside the range of the study on which they are based. They do not allow extreme cases, such as horizontal wells, to be cemented reliably. A more detailed understanding of the mechanisms is necessary if reliable predictions are to be made over a wide range of conditions. predictions are to be made over a wide range of conditions. We therefore studied displacement once again to develop a model capable of predicting behavior under all conditions. Theory Cementing Requirements. To place cement around the entire annulus successfully, three conditions must be satisfied.1. Mud Displacement. The mud gel must be broken down so that mud moves on the narrow side of the annulus, This should be done during mud conditioning before cementing. 2. Yield Stress. The yield stress of each fluid (mud, spacer, and cement) must be overcome to allow fluid to flow into or out of the narrow side of the annulus. 3. Channeling. The velocity of the interface between two fluid in the annulus should be the same on the wide and narrow sides. If the interfacial velocity on the wide side is substantially greater than that on the narrow side, severe channeling will result. The following sections address the problems of displacing one fluid by another in an eccentric annulus. These discussions apply equally to the displacement of mud by spacer and spacer by cement. Remember that almost all annuli are eccentric to some extent. even in vertical wells. Therefore, this analysis can be applied to all well types. Mud Displacement. Lockyear and Hibbert discussed the problem of gelled mud in detail. During mud circulation before cementing, problem of gelled mud in detail. During mud circulation before cementing, the mud gel must be broken down so that the mud is flowing in the entire annulus. With no pipe movement, the only force acting on the gelled mud is the frictional pressure drop. The condition necessary to break down mud gel strength, is ................... .................... (1) where = wall shear stress generated by frictional pressure drop. A simple expression for wall shear stress valid for annuli with diameter ratios approaching unity is = ....................................... (2) where b= width of the annular gap on the narrow side of the annulus and = frictional pressure drop. The frictional pressure drop ideally should be estimated with a hydraulics program that accounts for the effect of standoff because computer simulations have shown that pressure drop is very sensitive to standoff. SPEDE P. 201
This paper describes new research into cement placement. The research used a novel large-scale flow loop with 49 ft [15 m] of 8V2-in. [21.6-cm] open hole thatcan be deviated from a to 90 0 . A distinction is made between mud displacement and cement placement and some of the problems associated with each process are described. Examples are given from the largescale flow loop. Other work that used a flow-visualization loop and mathematical modeling is also discussed.Preliminary conclusions confirm the long-held intuitive view that correct casing centralization, circulation rate, and mud properties are critical for mud displacement. The paper also presents a technique to quantify the role of each variable. Efficient cement placement is influenced by spacer properties and centralization.
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