A casing design is a critical aspect of any drilling project. It is the outermost layer of a wellbore and serves several important functions, including:
Providing structural support to the wellbore: The casing helps to maintain the integrity of the wellbore and prevent it from collapsing. This is especially important in cases where the wellbore passes through unstable or weak formations.
Protecting the wellbore from external pressures: The casing acts as a barrier, preventing the influx of water, oil, or gas from surrounding formations. This helps to maintain the stability of the wellbore and prevent blowouts or other accidents.
Controlling the flow of fluids: The casing can be used to direct the flow of fluids, such as oil or gas, to the surface. This is achieved through the use of perforations, which allow the fluid to flow through the casing and up to the surface.
There are several factors to consider when designing a casing for a drilling project, including:
The geology of the area: The type and strength of the formations that the wellbore will pass through will influence the size and strength of the casing required.
The depth of the wellbore: The deeper the wellbore, the more pressure the casing will be subjected to. As a result, deeper wells will require stronger casing.
The fluids being produced: The type of fluids being produced, such as oil or gas, will also influence the design of the casing. For example, gas may be more corrosive and require the use of corrosion-resistant materials.
Environmental considerations: The location of the wellbore may also impact the design of the casing. For example, offshore wells will require casing that is resistant to the corrosive effects of saltwater.
In conclusion, casing design is a crucial aspect of any drilling project. It serves several important functions, including providing structural support, protecting the wellbore, and controlling the flow of fluids. The design of the casing must be carefully considered, taking into account the geology of the area, the depth of the wellbore, the fluids being produced, and any environmental considerations.
Casing Design
This establishes the setting point of the 11¾-in. When depths agree, accept answer typically 2-4 iterations agreement to within 30 ft will be satisfactory 3. J Pet Technol 29 2 : 195—208; Trans. Thread shape is square to reduce unzipping tendency. Increase in temperature and pressure can impose tension loadings in casing Casing and casing design casing design: safety factors Because of uncertainties in determining actual loadings and as well as casing properties a factor is used to allow for such uncertainties and to ensure that casing properties always remain greater than loadings. Choose a coupling whose joint strength is greater than the design loading.
For Collapse design, assume no buoyancy effectThe casing string must be designed to stand up to the expected conditions in burst, collapse and tension. Two of the four weights are unacceptable to us everywhere in the string because they do not satisfy the burst requirements. Additional cost savings sometimes can be achieved by the use of liner combination strings instead of full strings running from the surface to the bottom of the hole. However, the potential savings must be weighted against the additional risks and costs of a successful, leak-free tieback operation as well as the additional casing wear that results from a longer exposure of the upper casing to rotation and translation of the drill string. Bottom-up design This is the standard method for casing seat selection. In three modes out of four modes of collapse resistances equations, except elastic collapse, collapse strength is directly proportional to the yield strength of material. Perhaps this string will run all the way to the surface check tension , or perhaps an even more economical string would include some 43.
Above conditions are quite conservative. The procedure is repeated for other casing strings, usually until a specified surface casing depth is reached. Gravity of mud during lowering : 1. This should include installation, drilling and production as appropriate operations. Yield strength :tensile stress required to produce a total elongation per unit length of 0. Determine the depth capability without axial stress 2. Oil industry has no uniform policy on safety factors of casing design Safety factors are normally decided by the individual company in accordance of their company policy.
In practice only a few weights and grades will be kept in stock. Type to threads and couplings e. The plastic flowing behavior of salt zones must also be considered. Pressure loads are produced by fluids within the casing, cement and fluids outside the casing, pressures imposed at the surface by drilling and workover operations, and pressures imposed by the formation during drilling and production. Detailed design method Load cases In order to select appropriate weights, grades, and connections during the detailed design phase using sound engineering judgment, design criteria must be established.
Impact on Casing Design of Thermal Expansion of Fluids in Confined Annuli. Second, they do not necessarily give the same number of strings. Pore and fracture gradient lines must be drawn on a well-depth vs. Typical casing and rock bit sizes are given in Table 1. This factor is called design factor or safety factor Safety factor is defined as the ratio of rating of casing and resultant loadings Casing And Casing Design : Safety factors Contd.
Tan Nguyen Casing Design Process 1. Design method The design process can be divided into two distinct phases. The top-down method is more like actually drilling a well, in which the casing is set when necessary to protect the previous casing shoe. In addition, other considerations can also be included in the design. If you are designing a casing string for a particular company, this load information must be obtained from them. The weight of this pipe will reduce the collapse resistance of the 47.
Determine depth where this pressure exists 5. Install director system on it. Casing Design Prepared by: Dr. Thread compound must be used to fill the voids and obtain a seal. These are the solid lines in Fig.
Detailed design The detailed design phase includes selection of pipe weights and grades for each casing string. Commission of the European Communities. London: Construction Industry Research and Information Association. Gravity of mud in next phase : 1. Structural Reliability Under Combined Random Load Sequences. The problem of choosing the casing setting depths is more complicated in exploratory wells because of shortage of information on geology, pore pressures, and fracture pressures. In such a situation, a number of assumptions must be made.
Apply the design factor for the casing and load conditions considered and check that the pipe body yield strength of the selected casing exceeds the axial design loading. Movement, Forces, and Stresses Associated With Combination Tubing Strings Sealed in Packers. Jeopardize drilling progress with possible pipe sticking, excessive hole enlargement. Top-down design From the setting depth of the 16-in. Recommendations for Loading and Safety Regulations for Structural Design. No analytic expression has been derived that accurately models collapse behavior in this regime.
Uncertainty in predicted formation properties Exploration wells often require additional strings to compensate for the uncertainty in the pore pressure and fracture gradient predictions. This slight error can be fixed by resetting the surface casing depth. Other tension loadings can arise due to bending, drag, shock loading and during pressure testing. Such a casing string is called a combination string. For the casing chosen, calculate the axial loading on the casing. Directional drilling concerns A casing string is often run after an angle building section has been drilled. This implies that either field makeup practices are not adequate, or the connection design basis is not consistent with the pipe-body design basis.
