Summary. Highly flexible cement systems can he designed to accommodate a broad range of well conditions with a relatively small number of additives that are readily available from any cementing service company. Fluid-loss additives can be used with dispersants and small amounts of KCI (when applicable) to design slurries for most primary cementing applications-e.g., cementing in CO, and salt-zone environments and prevention of annular flow after cementing. This paper illustrates how materials and slurry properties that are easily specified can be used to improve wellsite success. Introduction The vast array of cementing additives available to the operator today can be mind-boggling, especially if the user does not have direct access to a cementing laboratory for the design and testing of slurries. What at first may appear as a confusing jumble of cementing systems with different applications can be simplified greatly by use of a few well-chosen additives and good cementing practices. Industry has developed several practices that aid in cement-slurry placement. These includepipe movement (either rotation or reciprocation);centralization;wipers across pay zonesuse of both float shoe and collar;minimum two-joint casing between the float shoe and collar;use of both top and bottom cementing plugs;use of appropriate spacers, washes, or scavenger cement slurries;minimization of surge pressures while pipe is run;good displacement practices (to avoid cement channeling); andoptimum use of mixing equipment for density control. We do not intend to expand upon any of these practices. They were mentioned to emphasize that slurry design cannot substitute for well-engineered placement techniques, nor can good mechanical practices substitute for poor slurry design. We advocate the use of fluid-loss additives, coupled with dispersants and small amounts of KCl, as the basis of sound cement-slurry design, a concept that has applications over a wide range of cementing conditions. This paper discussesthe prevention of annular flow after cementing,cementing in plastic salt environments, andcementing in CO, environments. Prevention of Annular Flow After Cementing Annular flow after cementing occurs when the cement column placed in the annulus fails to contain formation pressures and allows formation fluids (gas, oil, or water) to flow into the wellbore. This is, in fact, a kick that occurs after cement has been pumped into place, a phenomenon that has been studied extensively by a number of investigators. Several mechanisms have been postulated to explain this occurrence, including the following.Dehydration of the cement slurry. Dehydration is caused by insufficient fluid-loss control. If it is severe enough, cement solids will bridge the annulus and prevent transmission of the hydrostatic pressure from the still-fluid cement column above the bridge. If the hydrostatic pressure is reduced to a level less than existing formation pressure below the bridge, formation fluids can enter the wellbore.Free-water pockets and channels. These are caused by a separation of cement solids and mix water and can lead to annular flow after cementing because a continuous water channel can exert only the hydrostatic pressure of the mix water itself (usually 8.33 Ibm/gal [998 kg/m 3 1). If this equivalent pressure is less than the formation pressure, annular flow can occur.Slurry; gelation. This can cause flow after cementing because the attraction between the hydrating cement particles may prevent the hydrostatic pressure of the fluid cement column from being fully transmitted to the exposed formations. In effect, the hydrating cement column "supports some of its own weight." Again, if the effective hydrostatic head of the fluid column is reduced to a level below formation pressure, formation fluids can enter the wellbore. Each of these three mechanisms explains why a cement column with a density greater than that of the drilling-fluid column used to drill the well could fail to contain formation pressures. In reality, these mechanisms are not as discrete and as tightly defined as a simple listing may lead the reader to conclude. All three may coexist during a given cement jobs which mechanism will predominate depends on both the well conditions and the cement-slurry design. Some techniques for predicting the potential for annular flow after cementing have been presented in the literature, but sound slurry design using good fluid-loss additives can alleviate all these potential problems. The Cement Unit of Chevron Services' Drilling Technology Center (DTC) initiated a test program to address the problem areas of flow after cementing. Table 1 summarizes the test results for the majority of fluid-loss additives currently, on the market. These tests are intended to show typical field formulations covering a wide range of bottomhole circulating temperatures (BHCT's). The slurries were designed to have thickening times in the 4- to 5-hour range and slurry densities were designed at 16.4 lbm/gal 1965 kg/m, 3, with the exception of Additive F, which was intend by the particular service to be mixed at a density of 15.6 lbm/gal 1869 kg/m] 3. API Class H cement was used for all slurry designs in Table 1, with all tests performed according to API Spec. guidelines. The data in Table 1 illustrate the concept of using a simple slurry design to combat each of the three mechanisms of annular flow alter cementing. First, as the name implies, the function of a fluid-loss additive is to prevent the uncontrolled dehydration of the cement slurry. While there is general agreement that fluid-loss control is beneficial to the cement slurry, there is some contention as to optimum fluid- loss values. Although such factors as permeability and differential pressure affect dehydration of the cement slurry, good field results have been obtained with a maximum value of 100 ml in 30 minutes for casing jobs and 50 mL in 30 minutes for liner cementations. These values are easily obtained because of the multiple role of the fluid-loss additive. Because these materials are organic, they retard the set of the cement slurry. When sufficient fluid-loss additive is used to obtain the required thickening time, the desired level of fluid-loss control is also obtained. Second, fluid-loss control decreases the volume reduction during slurry hydration. Compare the volume-reduction values in Table 1. with the same values for slurries lacking fluid-loss control in Table 2. The fluid-loss-controlled values are generally an order of magnitude smaller than the noncontrolled values. Third, the fluid-loss additive controls free water. The concentration of fluid-loss additive that provides thickening time and hydration control will also control water separation, provided that reasonable cement/water ratios are used. Zero free water under downhole conditions of temperature and pressure is the goal when the cement slurry is designed. Fourth, and finally, cement slurries designed with fluid-loss additives control slurry gelation. Slurries that rely on fluid-loss ad ditives and dispersants exhibit a "right-angle set" as opposed to a "gel set." These two concepts are illustrated by the strip charts in Fig. 1 . The gel set is characterized by a slow gain in consistency until the final pumping time is reached. By contrast, the right- angleset slurry remains thin until close to the end of the thickening time period and then thickens rapidly to final consistency. Also important is the static gel-strength development.
Summary A newly built mobile cement-testing laboratory assists in monitoring blended cement quality and design. The unit contains the equipment required to perform tests for oilwell cements described in API's Spec 10. Case histories are presented describing how use of the mobile cement laboratory improved cementing success on critical cement jobs. Introduction Cementing practices, equipment, and materials have changed as wells have become deeper and technology has advanced. Many cementing systems have been devised. The number of different systems that can be designed by varying the components is almost endless. The pumping time of a cementing system is controlled by the class of cement, the well temperature and pressure, and the type and amounts of additives. In 1939, the first pressure/temperature thickening-time tester was developed, enabling, the industry to forecast slurry performance accurately. Proper "aboveground" density is essential to the successful accomplishment of any well-cementing operation. Only a small amount of water (about 25%) is necessary for cement to set satisfactorily. More water must be added, however, for the cement system to be pumpable. Mixing and pumping equipment has evolved continuously over the past few years. Unfortunately, maintaining slurry density during, cement jobs is still a problem. Cement is usually handled in bulk form, In most cases, additives can be blended with bulk cement to suit any well condition. To achieve success. critical cement-jobs require accurate testing in the laboratory, proper blending of the cement and additives, and correct mixing of the slurry to designed density. A gamut of pilot tests must be run to ensure a good slurry design. After the system has been blended, samples also must be tested. Many oil companies depend solely on the cement service companies to meet these criteria. Chevron, however, has developed a cooperative testing program that works closely with the service company field and laboratory personnel. This program relies heavily on monitoring the quality and performance of cement and additives. Using this program, Chevron and the service company can verify the slurry performance. Over the past few years, a number of major oil companies have experienced situations in which pilot test and blend sample test results did not correlate. Some companies are willing, to accept a 40% variation in thickening time between the pilot and blend tests. In 1982, we implemented a field study to evaluate service companies' blending equipment and procedures. The following recommendations resulted from this study:layer the cement and additives,weigh additives on a close tolerance scale,move the cement a minimum of six times before samples are taken, andobtain accurate (representative) samples while the blend is going to the tank in which it will be transported to the rig. Very close correlation between pilot and blend sample tests was achieved with the implementation of these recommended procedures. While developing these recommendations. we observed a long period between blending, of the cement with additives and testing of the samples. Although the samples were "hot shotted" to both the Chevron and service company laboratories, in most cases, the cement was on location before testing of the blend samples began. In the past, other methods of analyzing cement blend samples. such as a chemical analysis, have been attempted, but none provided the accuracy of a thickening-time test on a high-pressure, high-temperature consistometer. Because of the time required to ship samples and inadequate alternative testing methods, a full-scale testing laboratory with the ability to operate at remote wellsites or service company blending facilities was needed. Capabilities We designed the mobile cement-testing laboratory with the following objectives:equip a self-contained vehicle with equipment that would reliably perform thickening-time. fluid-loss, free-water, and rheology tests on cement slurries for critical casing strings on any well:to provide living accommodations for two people on location:to provide reserve capabilities;to be within the weight limitation of the vehicle:to meet safety requirements;to provide access to laboratory equipment for maintenance and service:to provide for the calibration of the testing equipment; andto be cost-effective. JPT P. 1032^
Under Federal Court precedents, mentally ill patients have a qualified right to refuse treatment. The amount of due process that may be required to override treatment refusals by active duty military patients is discussed. Due process for these individuals need not be judicial, since medical review satisfies federal requirements. Involuntary administration of medication to active duty military personnel is justified in some circumstances. Specific criteria for overriding treatment refusals are suggested. A sample protocol for overriding the treatment refusals of active duty personnel is offered.
Summary This paper shows the results of a 2-year study on the quality of cement-grade bentonite now available commercially. A comparison of laboratory-designed slurries with field-blend samples shows drastic alteration of cement-slurry performance from poor blending procedures and/or poor-quality bentonite, which could cause catastrophic consequences. Evidence from microscopic examination, chemical-quality procedures, and cement-slurry performance data shows a failure rate approaching 50% among more than 150 samples tested from field stocks. Laboratory data on cement-slurry performance with bentonite that passed and failed the current API specification test are presented. Introduction Problems with bentonite quality and difficulties in determining Problems with bentonite quality and difficulties in determining quality variations among manufacturers' plants and service-company stock points continue to occur. The important considerations in the use of a premium bentonite are the properties that good bentonite gives a lightweight cementing system: zero free water, good fluidloss control, good rheological properties, adequate compressive strengths for filler cement, and cost-effectiveness. The API Specification for Materials and Testing for Well Cements defines bentonite as "a natural material consisting principally of the clay mineral, montmorillonite. It is dried and ground, principally of the clay mineral, montmorillonite. It is dried and ground, but otherwise untreated in processing. No beneficiating agents or other material shall be added to bentonite used in well cements." Additives have been used widely to improve the performance of the bentonite temporarily in drilling fluids to meet API specifications. It has become apparent that treated bentonite is also being used in cementing applications as API cement-grade bentonite. Past API specifications on cement-grade bentonite were violated by the addition of polyacrylamides to increase the yield of lowgrade bentonites. The polyacrylamide concentrations were so small that detection by sophisticated analytical techniques was impossible, but the yield of bentonite was beneficiated so that the bentonite appeared to pass the specification test. The current API specification test fails this type of beneficiated bentonite. API Specifications on Bentonite Beneficiating agents have long been forbidden by the description of cement-grade bentonite. In the past, enforcement of this specification has been difficult because of the shortcomings of classic analytical techniques mentioned earlier. Because of viscosity and filtration problems resulting from the use of beneficiated bentonite, the API Committee on Drilling Fluids (Committee 13) developed a test procedure that uses sodium hex-ametaphosphate to screen bentonite for beneficiation. This screening test has been accepted by the Committee on Well Cements. The test does not detect beneficiating agents directly but involves measuring properties of a bentonite slurry (such as plastic viscosity, yield point, and fluid-loss control) both before and after treatment with a solution of sodium hexametaphosphate to ensure that they fall within specified ranges. The sodium hexametaphosphate negates the beneficiating effects of added polymers and allows the detection of treated bentonite.
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