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Fluid-Loss-Control Additives (FLAs) are used to maintain a consistent fluid volume within a cement slurry to ensure that the slurry performance properties remain within an acceptable range. The variability of each of these parameters (slurry performance properties) is dependent upon the water content of the slurry. If the water content is less than intended, the opposite will normally occur. The magnitude of change is directly related to the amount of fluid lost from the slurry. Because predictability of performance is typically the most important parameter in a cementing operation, considerable attention has been paid to mechanical control of slurry density during the mixing of the slurry to assure reproducibility.
To evaluate a given casing design, a set of loads is necessary. Casing loads result from running the casing, cementing the casing, subsequent drilling operations, production and well workover operations. Temperature changes and resulting thermal expansion loads are induced in casing by drilling, production, and workovers, and these loads might cause buckling (bending stress) loads in uncemented intervals. In shallow normal-pressured wells, temperature will typically have a secondary effect on tubular design. In other situations, loads induced by temperature can be the governing criteria in the design.
Remedial cementing is performed to correct problems associated with the primary cementing job. This article describes some common techniques for placing the cement in the correct location to correct the problem. These techniques are described in basic terms; they can be custom-designed for specific situations, and depend upon the type of remedial cementing operation being undertaken. Dump bailers are used for placing very small volumes of plugging material precisely and economically. Different types of dump bailers, including gravity and positive-displacement bailers, are shown in Figure 1.
The American Petroleum Institute (API) has numerous manufacturing requirements for tubing. Many API standards have also been adopted by the International Standards Organization (ISO). The tubing purchaser and designer should be aware of API requirements and testing procedures (see API Spec. All tubing should meet API minimum requirements. In critical wells, the purchaser may want to receive and review the manufacturer's test results.
To evaluate a given casing design, a set of loads is necessary. Casing loads result from running the casing, cementing the casing, subsequent drilling operations, production and well workover operations. Internal pressure loads result from fluids within the casing and are modeled with pressure distributions. Pressure distributions are typically used to model the internal pressures. These pressure distributions are discussed next.
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.
Equipment for monitoring and control are important elements of any coiled tubing unit. The control-console design for the coiled-tubing (CT) unit may vary with manufacturers, but normally, all controls are positioned on one remote console panel. A diagram of a typical well-intervention unit control panel is seen in Figure 1. The console assembly is complete with all controls and gauges required to operate and monitor all of the components in use and may be skid-mounted for offshore use or permanently mounted as with the land units. The skid-mounted console may be placed where needed at the wellsite as desired by the operator.
Over the years, attempts have been made to track the working history of coiled tubing (CT) strings in service to maximize the service utility of the tube while minimizing fatigue failures. As a result, three commonly used methodologies for predicting the fatigue condition of the CT were developed. A relatively simplistic approach used to predict the working life of coil tubing is commonly described as the "running-feet" method, in which the footage of tubing deployed into a wellbore is recorded for each job performed. This deployed footage is then added to the existing record of footage deployed in service for any given string. Depending upon the service environment, type of commonly performed services, and local field history, the CT string is retired when the total number of running feet reaches a predetermined amount.
The service reel serves as the coiled tubing(CT) storage apparatus during transport, and as the spooling device during CT well-intervention and drilling operations. The inboard end of the CT may be connected either to the hollow segment of the reel shaft (spoke and axle design), or to a high-pressure piping segment (concave flange plates), both of which are then connected to a high-pressure rotating swivel. This high-pressure fluid swivel is secured to a stationary piping manifold, which provides connection to the treatment-fluid pumping system. As a result, continuous pumping and circulation can be maintained throughout the job. A high-pressure shutoff valve should be installed between the CT and reel shaft swivel for emergency use in isolating the tubing from the surface pump lines.
The pressures, temperatures, and potentially corrosive conditions that may be found in a wellbore create the need for metals and materials that can resist such conditions. Metallurgical requirements are dictated both by the downhole well environment and the design and performance requirements of the packer. Consideration must be given to both when selecting and specifying materials for corrosive environments. Many types of materials that are applicable for tubing and casing in corrosive environments are not always suitable (or practical) for packer manufacture. The NACE International (formerly National Association of Corrosion Engineers) Standard MR-01-75 establishes guidelines and acceptance criteria for material selection for sour service in H2S environments.