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Remedial cementing requires as much technical, engineering, and operational experience, as primary cementing but is often done when wellbore conditions are unknown or out of control, and when wasted rig time and escalating costs force poor decisions and high risk. Squeeze cementing is a "correction" process that is usually only necessary to correct a problem in the wellbore. Before using a squeeze application, a series of decisions must be made to determine (1) if a problem exists, (2) the magnitude of the problem, (3) if squeeze cementing will correct it, (4) the risk factors present, and (5) if economics will support it. Most squeeze applications are unnecessary because they result from poor primary-cement-job evaluations or job diagnostics. Squeeze cementing is a dehydration process.
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Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper.
Field results indicate that displacing cement in turbulent flow during the primary cementing operation has materially reduced remedial cementing. Lower displacement rates with subsequently reduced frictional pressures have been achieved by using dispersants or thinners in the cement slurry. Failures caused by lost circulation and formation breakdown during primary cementing, sometimes encountered when high displacement rates are necessary for turbulent flow, have been substantially reduced.
The paper presents laboratory analyses of the effects of some dispersants on the rheological properties of cement slurries. These dispersants include (1) lignosulfonates; (2) phosphates; (3) alkyl-aryl sulfonates; (4) synthetic polymers. Such dispersants probably function somewhat like viscosity-reducing agents in muds, although the viscosity resulting from the gel structure of the hydration products apparently is reduced also.
Results on actual jobs using slurries containing thinners investigated are discussed. These results indicate that laboratory rheological measurements on slurries can be used successfully to predict pump rates necessary to achieve turbulency. Cementing operations using both low- and high-velocity displacement rates are compared.
High displacement rates required during casing cementing operations to improve mud displacement impose certain restrictions limiting use of this technique. Specifically, rheological properties of many commonly used cement systems preclude approaching turbulency in the annulus because the frictional pressures induced at the critical velocity approach or exceed the formation breakdown pressure.
Conversion of drilling mud to oilwell cement has advanced from an unpredictable laboratory curiosity to a practical reality. Recent field introduction of polymer dispersants, organic accelerators, and an alternative cementitious material have provided two refined and practical conversion methods. Each method claims universal applicability plus performance superior to that of conventionally mixed and pumped Portland cement. Both blast-furnace-slag (BFS) and Portland cement are used for drilling-mud conversion. Portland and BFS mud conversions can use the same recently developed polymer dispersants, filtration-control materials, defoamers, and other additives that are typically used to treat high-temperature, highly-salt-contaminated drilling muds. Experience in the field and laboratory has demonstrated that conversion with BFS or Portland cement is essentially one technology from a pilot-test and application standpoint. While use of these two materials reflects essentially one technology, distinct performance and cost differences exist. These differences define the specific economic application advantages and must be considered when a decision to use BFS or Portland cement is made. Rational selection of mud-to-cement conversion depends on a detailed economic comparison of basic materials, logistics, and equipment availability.
BFS, introduced by Emil Langens in Germany in 1862, has long been used as sulfate- and seawater-resistant construction cement. It was first proposed as a cementitious material for conversion of drilling mud in a 1972 patent. Portland cement has a longer history than BFS cement for construction and for conversion of drilling muds. However, as a rule, BFS cement can be introduced to drilling muds with less effect on rheological properties and filtrate volume than Portland cement. The additional flocculative effect of Portland cement is the result of the calcium oxide (quicklime) content of the material. If calcium oxide or calcium hydroxide (lime) is used as a BFS activator, flocculation of the drilling mud is nearly identical to that of Portland cement.
Conversions of dispersed mud systems may be accomplished more easily and allow higher compressive strength by use of BFS cement with sodium hydroxide and sodium carbonate as activators. Laboratory and field experience have demonstrated the superiority of BFS conversion of dispersed fresh- and seawater-based mud systems at temperatures up to 250 F. While this advantage is evident for other common mud types, drilling muds are available that can be converted more economically with conventional Portland cement. Experience in the laboratory has shown that as drilling-mud flocculation increases, the rheological effect of subsequent addition of Portland cement decreases. Therefore, and especially, in salt-mud systems, conversion performance and economics favor use of less-expensive Portland cement for drilling-mud conversion.
A 1-year strength-retrogression study revealed that BFS cement, while more temperature-stable than Portland cement, does benefit from the addition of silica flour to retain strength at elevated temperatures. No measurable shrinkage or physical degradation of either neat BFS cements or BFS-converted drilling mud was detected in the study. Preliminary strength-retrogression studies show a need for expanded long-term aging studies on BFS cements and drilling-mud conversions at >250 F.
Early laboratory work indicated that application of Portland cement to several types of drilling-mud conversions consistently resulted in excess free water and shrinkage. Laboratory-developed batch-mixing techniques that established minimum slurry residence times and mixing energy were applied to field applications. Shrinkage and free water of Portland cement applications have been elimated in most cases or controlled to acceptable levels.
Shrinkage has also been noted in BFS formulas but is thought to be the result of a phenomenon unrelated to shrinkage of Portland cement. Shrinkage and instability of slag cements is primarily a laboratory-handling problem that is not manifested in the relatively static temperature, pressure, and humidity downhole.
Previously published reports on performance of BFS- and Portland-cement conversions focused on specialty applications. This paper will look at more-typical, simple land jobs. Three well histories suggest that significant savings are available to operators willing to implement this new technology. A single-stage 11.0-lbm/gal BFS conversion can provide 18% to 36% savings compared with the typical cost of a two-stage cement job applied to a New Mexico well. Use of a Portland-cement conversion on a closed-loop brine-mud system near Odessa, TX, resulted in a 20% savings compared with a traditional cement job. Ongoing BFS conversions in Colorado and Canada typically provide 10% savings on a per-barrel basis, with potential for up to 21% savings to the operator.
The cementation of tophole sections in deepwater operations is very challenging due to the conditions in which those jobs take place. Tophole sections in deepwater operations often have a narrow margin between the pore and fracture gradients and shallow hazards, such as gas or water flow, associated with them. The low temperature found at seabed results in high fluid viscosities, slow gel strength development, and delayed early compressive strength development of the slurry. To optimize the low-temperature slurries currently used, a novel low-temperature dispersant was developed that enables designing the slurry to meet the required properties. The new dispersant has several main benefits in slurry design and properties. The dispersant enables the slurry to develop faster gel strength and has no retarding effect compared to conventional dispersants. The new dispersant will provide a flat rheology over time, allowing better control of the circulating pressures during placement. Laboratory tests show less gelation and an improvement in static gel strength development as compared to current dispersants. The dispersant also provides better rheological properties, often at lower concentrations, thereby facilitating a simplified slurry design. Following laboratory testing, the low-temperature dispersant was introduced in Gulf of Mexico operations with very good results. Operationally, the dispersant is easy to handle. It is compatible with all typical cementing additives and can be used with conventional cementing equipment and liquid additive systems. The low-temperature dispersant has been used in various types of deepwater slurries such as lightweight, conventional, and foamed slurry system, using drill water or seawater as the base fluid, often requiring less concentration and improving key slurry properties to achieve successful cementing operations in challenging low-temperature conditions.
A dependable method for converting fluid into a cementitiuos slurry has been sought for many years. Recent advancements in copolymer technology have made it possible to introduce Portland cement directly into drilling fluid while maintaining control over the rheology of the resultant slurry. For purposes of brevity, converted drilling fluid as described herein will be referred to as mud-to-cement, or MTC.
Conversion of drilling mud into a cement slurry has several unique advantages. As examples, conversion of spent drilling mud reduces disposal costs. Secondly, the superior compatibility into MTC significantly improves annular displacement efficiency by controlling the flocculation that normally results when drilling mud and cement contact. Thirdly, MTC makes the most benefit of manpower, equipment, time and materials by consolidating services at the rig site.
Consolidation of drilling mud and cementing technologies to cut costs while improving the quality of completions is the underlying topic of discourse in this paper. In addition to presentation of the slurry design paper. In addition to presentation of the slurry design process, laboratory data, mixing and handling concepts process, laboratory data, mixing and handling concepts and case histories are discussed. Also, several important avenues of potential savings are suggested. Applications in a variety of locations using different types of completions have all been successful both operationally and from the standpoint of results.
The basic purpose of cementing a wellbore is to return the annulus to as near the original state as possible. In general, this means removal of all drilling mud from the annulus and replacement with a contiguous cement sheath. Cement should provide at least four basic functions: (1) zone isolation, (2) casing support, (3) formation support, and (4) casing corrosion protection. The more complete the removal and replacement process, the more complete will be the provision of process, the more complete will be the provision of these four required functions. It is in this critical removal replacement process, however, that convention all cementing practice has often failed. The reason for that failure can be best explained as follows. Drilling mud and cement slurry are incompatible and formed a gelled mass when intermixed. Therefore, conventional cementing practice usually calls for a preflush or spacer fluid to separate mud and cement. preflush or spacer fluid to separate mud and cement. However, some spacer fluids available on today's market are incompatible with either mud or cement. Many spacer fluids are overwhelmed when all three slurries are commingled. A good spacer should have the dispersion power necessary to reduce the viscosity of such mixtures back to a level near that of the original fluids involved. Otherwise, large quantities of clabbered or severely gelled mixture will remain in the annulus. As a result, expensive remedial cementing is often required to correct inferior primary cementing applications.
To reduce the problems associated with mud and cement incompatibility into a cementitiuus slurry has been developed. Converting drilling fluid into a cement slurry has been a topic of research for many years. IN 1970, a Portland cement based formulation was introduced by Wyant et al., but it had a complex formulation, was very sensitive to temperature, and did not incorporate an adequate dispersant. Even so some field applications were done. Another formulation employing a magnesium based drilling fluid was introduced by Miller et al. in 1975.