Well design · casing points · field units

Casing seat selection and the mud weight window

Casing setting depths are picked from the mud weight window: the range of mud weights, in ppg equivalent, that sits above the pore pressure (so the well does not flow) and below the fracture gradient (so the formation does not break down). Working from total depth upward, the mud weight needed at the bottom of each section, plus margins for swab, surge and a kick, must be held by the formation at the casing shoe above it. Where the fracture gradient can no longer hold that mud weight, a casing string has to be set.

Reference material only. Casing points are set by the operator's well engineers from offset-well data, pore pressure and fracture gradient predictions, regulations and company policy. The pressure profile in the worked example below is illustrative, not data from a real well.

1. The mud weight window

Everything is compared in equivalent mud weight (EMW), so pressures at different depths can be read off one chart. The lower edge of the window is the pore pressure, or the wellbore stability (collapse) gradient where that is higher. The upper edge is the fracture gradient, confirmed at each casing shoe by a leak-off test (LOT) or formation integrity test (FIT). The window narrows with depth in overpressured sections, because pore pressure climbs faster than fracture gradient.

PP is the pore pressure and FG the fracture gradient, both in ppg EMW. In the kick equation, D int is the depth of the next section's TD, D the trial shoe depth, and the kick increment is a design kick expressed in ppg (0.5 ppg is a widely used value). Pressures come from the hydrostatic pressure formula; units are covered in mud weight conversion.

2. Where the limits come from

A LOT pumps until the pressure curve departs from a straight line, showing where the formation starts to take fluid. A FIT stops at a pre-set pressure: it proves the shoe can hold the equivalent mud weight the next section needs, without finding the actual limit. Either result becomes the upper edge of the window for the next hole section and the basis for MAASP.

3. The bottom-up method, step by step

• Plot the profiles. Pore pressure and fracture gradient in ppg EMW against TVD, from surface to the planned TD.
• Start at TD. Planned mud weight at TD = highest pore pressure in the section + trip margin. The shoe of the casing above must hold that mud weight + surge and safety margins.
• Read up the fracture line. Go up from TD at that required value until it meets the fracture gradient curve. That depth is the shallowest point at which the intermediate casing can be set and still let the next section reach TD.
• Repeat for the next string. At the new shoe, read the pore pressure, add the trip margin to get the mud weight for drilling down to it, add surge and safety margins, and go up again until the fracture curve meets that value.
• Check surface casing for a kick. The surface shoe must also hold the pressure of a design kick taken while drilling the section below (the kick-imposed EMW above).
• Check differential sticking top-down. The overbalance across permeable zones in each open-hole section must stay under the operator's limit; published guidance quotes roughly 2,000 to 2,300 psi in normally pressured zones and 2,500 to 3,000 psi in abnormally pressured zones. If not, a string is set shallower or a liner added.
• Apply the other constraints. Groundwater protection and regulatory minimums, lost-circulation and depleted zones, salt, unstable shale, and where the shoe will sit in competent rock for a good cement job.

4. Worked example: picking casing points for a 12,000 ft well

An illustrative pressure profile for a vertical well to 12,000 ft TVD. Margins: trip 0.3 ppg, surge 0.3 ppg, safety 0.2 ppg, design kick 0.5 ppg. Differential sticking limit: 2,200 psi in normally pressured rock.

• Production section (to TD): MW = 14.0 + 0.3 = 14.3 ppg. Bottom-hole hydrostatic = 0.052 × 14.3 × 12,000 = 8,923 psi against a pore pressure of 8,736 psi, an overbalance of 187 psi. The shoe above must hold 14.3 + 0.3 + 0.2 = 14.8 ppg.
• Intermediate casing point: the fracture gradient reaches 14.8 ppg between 8,000 ft (14.5) and 9,000 ft (15.0). Interpolating: 8,000 + (14.8 − 14.5) ÷ 0.5 × 1,000 = 8,600 ft. Setting the intermediate casing any shallower would leave a shoe that cannot hold the mud needed at TD.
• Mud weight above the intermediate shoe: pore pressure at 8,600 ft ≈ 9.5 + 0.6 × 1.3 = 10.3 ppg, so MW = 10.6 ppg and the shoe above must hold 10.6 + 0.3 + 0.2 = 11.1 ppg. On pressure alone, the fracture curve reaches 11.1 ppg at about 1,100 ft.
• Surface casing, kick check: kick-imposed EMW = (8,600 ÷ D) × 0.5 + 10.6. At 2,000 ft this is 12.75 ppg against a fracture gradient of 12.4 ppg (fails); at 2,300 ft it is 12.47 ppg against about 12.55 ppg (passes); at 2,500 ft, 12.32 against about 12.65 ppg. The kick case governs, so the surface casing goes to at least 2,300 ft, say 2,500 ft in a competent shale.
• Differential sticking: in the production hole, 14.3 ppg mud against 10.3 ppg pore pressure just below the 8,600 ft shoe gives 0.052 × 4.0 × 8,600 = 1,789 psi, under the 2,200 psi limit. In the intermediate hole, 10.6 ppg against 8.8 ppg at 6,000 ft gives 562 psi. Both pass.
• Shoe test: after drilling out the 8,600 ft shoe with 10.6 ppg mud, a FIT to 14.8 ppg EMW needs a surface pressure of (14.8 − 10.6) × 0.052 × 8,600 = about 1,880 psi.

Result: surface casing at about 2,500 ft, intermediate casing at about 8,600 ft and a production string or liner to 12,000 ft. Each string then goes through its own burst, collapse and tension design, and each section's kick tolerance is checked against the shoe strength actually measured.

5. Typical margins and limits

Circulating friction also counts: while drilling, the bottom-hole pressure is the ECD, not the static mud weight, so a narrow window may be limited by ECD at the shoe rather than by surge.

6. When the window is too narrow

In deep overpressured wells the margins can add up to more than the gap between pore pressure and fracture gradient. Options include an extra casing string or a drilling liner, tighter margins backed by better pore pressure data, managed pressure drilling to hold bottom-hole pressure within a smaller range, and lower-ECD fluids and hydraulics. Each extra string costs hole size, so the casing program and bit sizes are planned together from TD upward.

7. Common mistakes

• Picking casing points from mud weight alone and forgetting the surge, safety and kick margins.
• Ignoring the kick case for surface casing, which often sets it deeper than the pressure case does.
• Setting a shoe in weak or permeable rock, then getting a low LOT that the plan did not allow for.
• Not re-checking the window when the measured LOT or the pore pressure seen while drilling differs from the prediction.
• Leaving depleted sands open under a heavy mud, which risks both losses and differential sticking.

8. In DrillX

DrillX brings daily drilling reports with IADC-style time logs, a rig fleet view, NPT insights and engineering calculators into one tool. The calculators cover hydrostatic pressure, ECD, annular pressure loss, triplex pump output, annular capacity and velocity, buoyancy and the kill sheet. This page explains how casing points are chosen from the mud weight window; it does not describe a DrillX casing seat selection tool. DrillX is in final testing; email info@drillx.app for a demo or early access.

Sources and further reading

• SPE PetroWiki, Determining depth to set casing: bottom-up selection, swab/surge/safety margins of about 0.2 to 0.3 lbm/gal each, a 0.5 lbm/gal kick factor and differential sticking limits of 2,000 to 2,300 psi (normal) and 2,500 to 3,000 psi (abnormal pressure).
• Drilling Manual, Casing setting depth selection: the graphical method working up from TD, trip margin for 200 to 500 psi overbalance, kick margin, and non-pressure constraints such as groundwater protection.
• Drilling Formulas, Leak-off test procedure: running a LOT and converting the result to equivalent mud weight.
• A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986), chapters 6 and 7: pore pressure, fracture gradient and casing setting depth.
• N. J. Adams, Drilling Engineering: A Complete Well Planning Approach, PennWell (1985): kick-imposed loads in surface casing selection.

Reference summary only. Worked examples are illustrative arithmetic, not a well design or recommended margins.