For any potential job, the data for wellbore configuration, environment, and temperature should be examined to help the designer arrive at the optimum cement job design. This data includes the selection of the basic cement properties and placement scheme. Wellbore conditions indicate whether special materials are required to combat gas migration, salt, and so forth. Cement density is dictated by the mud density. These factors, along with wellbore temperature, determine the selection of slurry properties.
Configurational data include information on vertical and measured depth, casing size and weight, and hole size, hole angle, and azimuth and string type. Hole depth influences application temperature, hydrostatic pressure, and friction pressure. In dealing with the environment in the wellbore, one must consider the potential use of casing hardware (centralizers, scratchers, liner hangers, and so forth), as well as downhole tools such as DV tools, packers, and bridge plugs. Casing Size and Weight - Typically corresponds w/compressive strength for support Deviation - Free water & settling come into play Hole Size & Conditions - washouts (difficult to remove mud); slimhole environment Downhole Tools - Multiple stage cementing tools
The problems posed by the openhole interval traversed by the casing require careful evaluation. Pay zones, fresh water zones, overpressured formations, zones with low fracture gradients, gas zones, and salt sections must be considered during the design phase. The drilling fluid and cement slurry chemical and physical properties should also be considered in the designing of a cement job. Pay zones merit special attention because damage caused from the mud and cement can limit recoverable reserves. Frac Gradients - Densities & flowrates must be controlled to avoid fracturing Formation Pressure - Densities (hydrostatic psi) must remain above F.P. to prevent well from flowing Probably the two biggest design factors OptiCem will help you with are Frac Gradient and Formation Pressure. Want to design to stay above F.P. and below F.G. Basically, have a window to work with.
Both bottomhole circulating and static temperature need to be considered along with the temperature differential between the bottom and top of the cement column. The bottomhole circulating temperature is the temperature to which the cement is exposed during placement. Bottomhole static temperature is important for the assessment of the long-term stability, or the compressive strength development of the cement system. The temperature differential between the top and bottom of the cement can be extremely important to the cement job design. To aid in the accurate prediction of temperatures, simulation programs are available. Cement which has been retarded for an adequate placement at BHCT may remain fluid or have poor strength development when circulated up to the hole to the top of casing.
The two prinicpal functions of primary cementing are to restrict fluid movement between formations and to bond and support the casing. When designing a cement slurry it is most important to remember that wellbore conditions dictate slurry selection. Careful and thorough review of these well characteristics is essential for designing an effective slurry. The overall goal of designing a cement slurry for a specific well application is to select an economical and proper cement mixture that can be placed under existing well conditions. For example, to properly design slurries for gas wells, the mechanism that causes loss of hydrostatic head must be understood. In addition, designers must consider fluid-loss control, cement stability, and setting behavior. All job designs must include proper displacement practices. Mechanical Properties - Cement will retrogress @ 230 o F & above. More recent studies have shown problems with stresses acting on cement during the life of the well.
