Leak Off Test Calculator

About This Calculator

This engineering calculator estimates casing shoe integrity from leak off test data. It converts the observed pressure into fracture gradient, equivalent mud weight, integrity window, planned pressure limits, and exportable results for field review.

Calculator Input Form

Test Type

Mud Weight (ppg)

Casing Shoe TVD (ft)

Casing Shoe MD (ft)

Observed Leak Off Pressure (psi)

Planned Next Section TVD (ft)

Safety Margin (ppg)

External Gradient (ppg)

Mud Temperature (°F)

Preferred Pressure Display

Use field-measured values. Review final limits against your drilling program and well-specific policies.

Example Data Table

Scenario	Mud Weight (ppg)	Shoe TVD (ft)	LOT Pressure (psi)	Equivalent MW (ppg)	Fracture Gradient (psi/ft)
Well A	10.20	8,500	1,260	13.05	0.6786
Well B	9.80	7,900	980	12.18	0.6334
Well C	11.30	10,200	1,540	14.20	0.7384

Formula Used

1) Hydrostatic pressure at casing shoe
Hydrostatic Pressure = 0.052 × Mud Weight (ppg) × Shoe TVD (ft)

2) Total shoe pressure during test
Total Shoe Pressure = Hydrostatic Pressure + Leak Off Surface Pressure

3) Fracture gradient
Fracture Gradient = Total Shoe Pressure ÷ Shoe TVD

4) Equivalent mud weight
Equivalent Mud Weight = Total Shoe Pressure ÷ (0.052 × Shoe TVD)

5) Integrity window
Integrity Window = Equivalent Mud Weight − Current Mud Weight

6) Recommended maximum mud weight
Recommended Maximum MW = Equivalent Mud Weight − Safety Margin

The constant 0.052 converts mud weight in ppg and depth in feet into psi. These formulas are commonly used in drilling operations for casing shoe strength assessment, fracture limit interpretation, and safe pressure planning.

How To Use This Calculator

Enter the current mud weight in ppg.
Provide casing shoe TVD and measured depth.
Enter the observed leak off pressure at surface.
Add the next planned section TVD for planning checks.
Set your safety margin in ppg.
Optionally enter an outside gradient for differential checks.
Click the calculate button.
Review equivalent mud weight, fracture gradient, and recommended limit.
Download the result as CSV or PDF.

Frequently Asked Questions

1. What does a leak off test measure?

A leak off test estimates the pressure strength near the casing shoe. It helps define fracture resistance, safe mud weight limits, and pressure margins for the next drilling section.

2. What is equivalent mud weight?

Equivalent mud weight converts total shoe pressure into an equivalent drilling fluid density. It is useful because engineers often compare formation strength directly against mud weight in ppg.

3. Why is shoe TVD used instead of measured depth?

Hydrostatic pressure depends on true vertical depth, not the full well path length. Measured depth is still useful for documentation and trajectory context, but TVD drives pressure calculations.

4. How should I choose the safety margin?

Use your company standard, well program, and local risk profile. Higher uncertainty, narrow windows, or poor data quality usually justify a larger margin.

5. Can this calculator replace field procedures?

No. It is a planning and interpretation tool. Final decisions should follow the drilling program, pressure test procedure, offset data, and real-time engineering judgment.

6. What is the difference between LOT and FIT?

A FIT confirms the formation can hold a target pressure without deliberately breaking down the formation. A LOT continues until leak off behavior is observed.

7. Why does external gradient matter?

External fluid pressure can affect net loading across the casing shoe or exposed formation. Including it helps compare internal and outside pressures more realistically.

8. Why export CSV and PDF results?

Exports help with drilling reports, engineering reviews, shift handovers, and audit trails. They also make scenario comparisons easier during planning meetings.

Engineering Notes

Use clean and validated field data whenever possible. Unexpected gauge drift, trapped pressure, poor calibration, or weak cement can distort interpretation. Always review the pressure response shape, pumping history, and any operational anomalies before finalizing casing shoe limits.