4 Advice to Choose a Casing Drilling System

In this step of the well life cycle load simulation, the devil lies in the details.

For drilling engineers, it is crucial to understand each input data and how it can "pass" or "fail" a casing string.

For a base case, stick to your company's well engineering manual (WEMS) for relevant inputs on each casing string from the conductor all the way to production casing.

For example, ensure that you consciously understand why a level drop load is used for collapse design and how much the level would drop and its reason (fractured formation below shoe, depleted one, faults, etc.).

Without going into detail in casing design loads input, casing string can easily pass or fail in early stages, which might misguide the project team on critical lead items.

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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 faces 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 the application of loads not considered in the original design.

While the intention is to provide reliable well construction at a minimum cost, failures sometimes occur. Most documented failures happen because the pipe was exposed to loads for which it wasn’t designed, known as “off-design” failures. “On-design” failures are rather rare, implying that casing-design practices are mostly conservative. Many failures occur at connections, suggesting 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.

Preliminary design

The largest opportunities for saving money typically arise during this phase. It includes:

• Data gathering and interpretation
• Determination of casing shoe depths and the number of strings
• Selection of hole and casing sizes
• Mud-weight design
• Directional design

The quality of the gathered data will greatly impact the appropriate choice of casing sizes and shoe depths and whether the casing design objectives are successfully met.

Detailed design

The detailed design phase includes the selection of pipe weights and grades for each casing string. This involves 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 checklist provided below aids well planners/casing designers in both the preliminary and detailed design phases.

• 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.

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The purpose of preliminary design is to establish:

• Casing and corresponding drill-bit sizes
• Casing setting depths
• The number of casing strings

The casing program (well plan) is obtained as a result of preliminary design. This is achieved 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)
• Lithology
• Hole cleaning and cuttings transport capability
• Potential formation damage, stability problems, and drilling rate
• Formation evaluation requirement
• Environmental and regulatory requirements

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Hole and casing diameters are based on the requirements discussed next.

Production

The production equipment requirements include:

• Tubing
• 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

Evaluation requirements include logging interpretation and tool diameters.

Drilling

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.

• Table 1- Commonly Used Bit Sizes That Will Pass Through API Casing

• Table 1 Continued- Commonly Used Bit Sizes That Will Pass Through API Casing

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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.

• Fig. 1—Casing setting depths—bottom-up design.

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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 note 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.

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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.

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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.

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.

• Fig. 2—Casing setting depths—top-down design.

The problem of choosing the casing setting depths is more complicated in exploratory wells because of a shortage of information on geology, pore pressures, and fracture pressures. In such situations, a number of assumptions must be made. Commonly, the formation pressure gradient is taken as 0.54 psi/ft for hole depths less than 8,000 ft and 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 1.0 psi/ft for greater depths.

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Top-of-cement (TOC) depths for each casing string should be selected in the preliminary design phase, as 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
• Buckling
• 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.

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