Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
To design a casing string, one must have knowledge of:
- Purpose of the well
- Geological cross section
- Available casing and bit sizes
- Cementing and drilling practices
- Rig performance
- Safety and environmental regulations
To arrive at the optimal solution, the design engineer must consider casing as a part of a whole drilling system. A brief description of the elements involved in the design process is presented next.
The engineer responsible for developing the well plan and casing design is faced with a number of tasks that can be briefly characterized.
- Ensure the well’s mechanical integrity by providing a design basis that accounts for all the anticipated loads that can be encountered during the life of the well.
- Design strings to minimize well costs over the life of the well.
- Provide clear documentation of the design basis to operational personnel at the well site. This will help prevent exceeding the design envelope by application of loads not considered in the original design.
While the intention is to provide reliable well construction at a minimum cost, at times failures occur. Most documented failures occur because the pipe was exposed to loads for which it was not designed. These failures are called “off-design” failures. “On-design” failures are rather rare. This implies that casing-design practices are mostly conservative. Many failures occur at connections. This implies that either field makeup practices are not adequate, or the connection design basis is not consistent with the pipe-body design basis.
The design process can be divided into two distinct phases.
Typically the largest opportunities for saving money are present while performing this task. This design phase includes:
- Data gathering and interpretation
- Determination of casing shoe depths and number of strings
- Selection of hole and casing sizes
- Mud-weight design
- Directional design
The quality of the gathered data will have a large impact on the appropriate choice of casing sizes and shoe depths and whether the casing design objective is successfully met.
The detailed design phase includes selection of pipe weights and grades for each casing string. The selection process consists of comparing pipe ratings with design loads and applying minimum acceptable safety standards (i.e., design factors). A cost-effective design meets all the design criteria with the least expensive available pipe.
The items listed next are a checklist, which is provided to aid the well planners/casing designers in both the preliminary and detailed design.
- Formation properties: pore pressure; formation fracture pressure; formation strength (borehole failure); temperature profile; location of squeezing salt and shale zones; location of permeable zones; chemical stability/sensitive shales (mud type and exposure time); lost-circulation zones, shallow gas; location of freshwater sands; and presence of H2S and/or CO2.
- Directional data: surface location; geologic target(s); and well interference data.
- Minimum diameter requirements: minimum hole size required to meet drilling and production objectives; logging tool outside diameter (OD); tubing size(s); packer and related equipment requirements; subsurface safety valve OD (offshore well); and completion requirements.
- Production data: packer-fluid density; produced-fluid composition; and worst-case loads that might occur during completion, production, and workover operations.
- Other: available inventory; regulatory requirements; and rig equipment limitations.
Preliminary design method
- The purpose of preliminary design is to establish:
- Casing and corresponding drill-bit sizes
- Casing setting depths
- The number of casing strings
Casing program (well plan) is obtained as a result of preliminary design. Casing program design is accomplished in three major steps:
- Mud program is prepared
- The casing sizes and corresponding drill-bit sizes are determined
- The setting depths of individual casing strings are found
The most important mud program parameter used in casing design is the “mud weight.” The complete mud program is determined from:
- Pore pressure
- Formation strength (fracture and borehole stability)
- Hole cleaning and cuttings transport capability
- Potential formation damage, stability problems, and drilling rate
- Formation evaluation requirement
- Environmental and regulatory requirements
Hole and pipe diameters
Hole and casing diameters are based on the requirements discussed next.
The production equipment requirements include:
- Subsurface safety valve
- Submersible pump and gas lift mandrel size
- Completion requirements (e.g., gravel packing)
- Weighing the benefits of increased tubing performance of larger tubing against the higher cost of larger casing over the life of the well
Evaluation requirements include logging interpretation and tool diameters.
Drilling requirements include:
- A minimum bit diameter for adequate directional control and drilling performance
- Available downhole equipment
- Rig specifications
- Available blowout prevention (BOP) equipment
These requirements normally impact the final hole or casing diameter. Because of this, casing sizes should be determined from the inside outward starting from the bottom of the hole. The design sequence is, usually, as follows:
- Proper tubing size is selected, based upon reservoir inflow and tubing intake performance
- The required production casing size is determined, considering completion requirements
- The diameter of the drill bit is selected for drilling the production section of the hole, considering drilling and cementing stipulations
- The smallest casing through which the drill bit will pass is determined
- The process is repeated
Large cost savings are possible by becoming more aggressive (using smaller clearances) during this portion of the preliminary design phase. This has been one of the principal motivations in the increased popularity of slimhole drilling. Typical casing and rock bit sizes are given in Table 1.
Casing shoe depths and the number of strings
Following the selection of drillbit and casing sizes, the setting depth of individual casing strings must be determined. In conventional rotary drilling operations, the setting depths are determined principally by the mud weight and the fracture gradient, as schematically depicted in Fig. 1, which is sometimes called a well plan. Equivalent mud weight (EMW) is pressure divided by true vertical depth and converted to units of lbm/gal. EMW equals actual mud weight when the fluid column is uniform and static. Pore and fracture gradient lines must be drawn on a well-depth vs. EMW chart. These are the solid lines in Fig. 1. Safety margins are introduced, and broken lines are drawn, which establish the design ranges. The offset from the predicted pore pressure and fracture gradient nominally accounts for kick tolerance and the increased equivalent circulating density (ECD) during drilling. There are two possible ways to estimate setting depths from this figure.
This is the standard method for casing seat selection. From Point A in Fig. 1 (the highest mud weight required at the total depth), draw a vertical line upward to Point B. A protective 7 5/8-in. casing string must be set at 12,000 ft, corresponding to Point B, to enable safe drilling on the section AB. To determine the setting depth of the next casing, draw a horizontal line BC and then a vertical line CD. In such a manner, Point D is determined for setting the 9 5/8-in. casing at 9,500 ft. The procedure is repeated for other casing strings, usually until a specified surface casing depth is reached.
From the setting depth of the 16-in. surface casing (here assumed to be at 2,000 ft), draw a vertical line from the fracture gradient dotted line, Point A, to the pore pressure dashed line, Point B. This establishes the setting point of the 11¾-in. casing at about 9,800 ft. Draw a horizontal line from Point B to the intersection with the dotted frac gradient line at Point C; then, draw a vertical line to Point D at the pore pressure curve intersection. This establishes the 9 5/8-in. casing setting depth. This process is repeated until bottom hole is reached.
There are several things to observe about these two methods. First, they do not necessarily give the same setting depths. Second, they do not necessarily give the same number of strings. In the top-down design, the bottomhole pressure is missed by a slight amount that requires a short 7-in. liner section. This slight error can be fixed by resetting the surface casing depth. 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. This analysis can help anticipate the need for additional strings, given that the pore pressure and fracture gradient curves have some uncertainty associated with them.
In practice, a number of regulatory requirements can affect shoe depth design. These factors are discussed next.
This can be a function of mud weight, deviation and stress at the wellbore wall, or can be chemical in nature. Often, hole stability problems exhibit time-dependent behavior (making shoe selection a function of penetration rate). The plastic flowing behavior of salt zones must also be considered.
The probability of becoming differentially stuck increases along with:
- An increase in differential pressure between the wellbore and formation
- An increase in permeability of the formation
- An Increase in fluid loss of the drilling fluid (i.e., thicker mudcake)
Zonal Isolation. Shallow freshwater sands must be isolated to prevent contamination. Lost-circulation zones must be isolated before a higher-pressure formation is penetrated.
Directional drilling concerns
A casing string is often run after an angle building section has been drilled. This avoids keyseating problems in the curved portion of the wellbore because of the increased normal force between the wall and the drillpipe.
Uncertainty in predicted formation properties
Exploration wells often require additional strings to compensate for the uncertainty in the pore pressure and fracture gradient predictions.
Another approach that could be used for determining casing setting depths relies on plotting formation and fracturing pressures vs. hole depth, rather than gradients, as shown in Fig. 2 and Fig. 1. This procedure, however, typically yields many strings, and is considered to be very conservative.
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. Commonly, the formation pressure gradient is taken as 0.54 psi/ft for hole depths less then 8,000 ft and taken as 0.65 psi/ft for depths greater than 8,000 ft. Overburden gradients are generally taken as 0.8 psi/ft at shallow depth and as 1.0 psi/ft for greater depths.
Top-of-cement (TOC) depths for each casing string should be selected in the preliminary design phase, because this selection will influence axial load distributions and external pressure profiles used during the detailed design phase. TOC depths are typically based on:
- Zonal isolation
- Regulatory requirements
- Prior shoe depths
- Formation strength
- Annular pressure buildup(in subsea wells)
Buckling calculations are not performed until the detailed design phase. Hence, the TOC depth may be adjusted, as a result of the buckling analysis, to help reduce buckling in some cases.
For casing design purposes, establishing a directional plan consists of determining the wellpath from the surface to the geological targets. The directional plan influences all aspects of casing design including:
- Mud weight and mud chemistry selection for hole stability
- Shoe seat selection
- Casing axial load profiles
- Casing wear
- Bending stresses
It is based on factors that include:
- Geological targets
- Surface location
- Interference from other wellbores
- Torque and drag considerations
- Casing wear considerations
- Bottomhole assembly [(BHA) an assembly of drill collars, stabilizers, and bits]
- Drill-bit performance in the local geological setting
To account for the variance from the planned build, drop, and turn rates, which occur because of the BHAs used and operational practices employed, higher doglegs are often superimposed over the wellbore. This increases the calculated bending stress in the detailed design phase.
Detailed design method
In order to select appropriate weights, grades, and connections during the detailed design phase using sound engineering judgment, design criteria must be established. These criteria normally consist of load cases and their corresponding design factors that are compared to pipe ratings. Load cases are typically placed into categories that include:
- Burst loads
- Drilling loads
- Production loads
- Collapse loads
- Axial loads
- Running and cementing loads
- Service loads
Design factors (DF)
DF = design factor (the minimum acceptable safety factor), and
SF = safety factor.
It follows that
Hence, by multiplying the load by the DF, a direct comparison can be made with the pipe rating. As long as the rating is greater than or equal to the modified load (which we will call the design load), the design criteria have been satisfied.
After performing a design based on burst, collapse and axial considerations, an initial design is achieved. Before a final design is reached, design issues (connection selection, wear, and corrosion) must be addressed. In addition, other considerations can also be included in the design. These considerations are triaxial stresses because of combined loading (e.g., ballooning and thermal effects)—this is often called “service life analysis”; other temperature effects; and buckling.
Loads on casing and tubing strings
In order 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
Casing loads are principally pressure loads, mechanical loads, and thermal loads. 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.
Mechanical loads are associated with:
- Casing hanging weight
- Shock loads during running
- Packer loads during production and workovers
- Hanger loads
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.
The casing loads that are typically used in preliminary casing design are:
- External Pressure Loads
- Internal Pressure Loads
- Mechanical Loads
- Thermal Loads and Temperature Effects
However, each operating company usually has its own special set of design loads for casing, based on their experience. If you are designing a casing string for a particular company, this load information must be obtained from them. Because there are so many possible loads that must be evaluated, most casing design today is done with computer programs that generate the appropriate load sets (often custom tailored for a particular operator), evaluate the results, and can sometimes determine a minimum-cost design automatically.
Noteworthy papers in OnePetro
Adams, A.J. and Hodgson, T. 1999. Calibration of Casing/Tubing Design Criteria by Use of Structural Reliability Techniques. SPE Drill & Compl 14 (1): 21-27. SPE-55041-PA. http://dx.doi.org/10.2118/55041-PA.
Adams, A.J. and MacEachran, A. 1994. Impact on Casing Design of Thermal Expansion of Fluids in Confined Annuli. SPE Drill & Compl 9 (3): 210-216. SPE-21911-PA. http://dx.doi.org/10.2118/21911-PA.
Halal, A.S. and Mitchell, R.F. 1994. Casing Design for Trapped Annular Pressure Buildup. SPE Drill & Compl 9 (2): 107-114. SPE-25694-PA. http://dx.doi.org/10.2118/25694-PA.
Halal, A.S., Mitchell, R.F., and Wagner, R.R. 1997. Multi-String Casing Design with Wellhead Movement. Presented at the SPE Production Operations Symposium, Oklahoma City, Oklahoma, USA, 9-11 March. SPE-37443-MS. http://dx.doi.org/10.2118/37443-MS.
Hammerlindl, D.J. 1977. Movement, Forces, and Stresses Associated With Combination Tubing Strings Sealed in Packers. J Pet Technol 29 (2): 195–208; Trans., AIME, 263. SPE-5143-PA. http://dx.doi.org/10.2118/5143-PA.
Klementich, E.F. and Jellison, M.J. 1986. A Service-Life Model for Casing Strings. SPE Drill Eng 1 (2): 141-152. SPE-12361-PA. http://dx.doi.org/10.2118/12361-PA.
Prentice, C.M. 1970. "Maximum Load" Casing Design. J. Pet Tech 22 (7): 805-811. SPE-2560-PA. http://dx.doi.org/10.2118/2560-PA.
CIRIA Report 63, Rationalisation of Safety and Serviceability Factors in Structural Codes. 1977. London: Construction Industry Research and Information Association. WorldCat
Det Norske Veritas. 1981. Rules for the Design, Construction and Inspection of Offshore Structures. Hovik, Norway: DNV. WorldCat
Economides, M.J., Waters, L.T., and Dunn-Norman S. 1998. Petroleum Well Construction. New York City: John Wiley & Sons. WorldCat
EUROCODE 3, Common Unified Rules for Steel Structures. 1984. Commission of the European Communities. WorldCat
Mitchell, R.F.: “Casing Design,” in Drilling Engineering, ed. R. F. Mitchell, vol. 2 of Petroleum Engineering Handbook, ed. L. W. Lake. (USA: Society of Petroleum Engineers, 2006). 287-342. SPEBookstore and WorldCat
Rabia, H. 1987. Fundamentals of Casing Design. London: Graham & Trotman. WorldCat
Recommendations for Loading and Safety Regulations for Structural Design. 1978. Report No. 36, Nordic Committee on Building Regulations, NKB, Copenhagen. WorldCat
Other noteworthy papers
Bull. D7, Casing Landing Recommendations, first edition. 1955. Dallas: API. Standard: API - BULL D7
Rackvitz, R. and Fiessler, B. 1978. Structural Reliability Under Combined Random Load Sequences. Computers and Structures 9: 489. Abstract