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Glossary

This glossary defines key terms engineers and developers encounter when working on well integrity, pressure gradients, and CBD visualizations.

General terms​

RKB – Rotary Kelly Bushing​

Description: A physical reference point on the rig floor, often 20–35 meters above mean sea level (MSL). Used as a vertical datum to measure depths (e.g., 1000m RKB).

MSL – Mean Sea Level​

Description: A global reference point for depth and elevation. Used to normalize pressure calculations across wells and locations.

RKB MSL Explanation

TVD – True Vertical Depth​

Description: The vertical depth from RKB or MSL straight down to a point in the well. Used in pressure formulas like:

P = SG × 0.0981 × TVD

MD – Measured Depth​

Description: The actual path length of the wellbore, including any deviation. TVD is used over MD for most pressure-related calculations.

MD TVD Explanation

SG – Specific Gravity​

Description: Ratio of a fluid's density to freshwater.

  • Methane gas ≈ 0.23
  • Condensate ≈ 0.64
  • Seawater ≈ 1.03

Used to calculate pressure gradient:

Pressure (MPa) = SG × 0.0981 × TVD

EMW – Equivalent Mud Weight​

Description: A way to express pressure as an equivalent fluid weight. Commonly shown in ppg (pounds per gallon) or SG.

Formulas:

EMW (SG) = Pressure / (0.0981 × Depth)  
EMW (ppg) = Pressure / (0.052 × Depth)

Casing Shoe / Casing Seat​

Description:

  • Casing Shoe: The bottom end of the casing string
  • Casing Seat: The depth where the casing is set and cemented

Tests like FIT and LOT are performed just below the casing shoe.

FIT – Formation Integrity Test​

Description: A pressure test to determine how much pressure the rock just below a casing shoe can hold. before leaking.
Input is often in SG (e.g., 1.80 SG).

LOT – Leak-Off Test​

Description: A pressure test performed to determine the fracture resistance of the formation just below a casing shoe. The LOT identifies the point at which injected fluid begins to enter the formation — known as the leak-off point.

How it works:

  1. After casing is set and cemented, a small section of open hole is exposed.
  2. Fluid is pumped into the well while pressure is monitored.
  3. The point where pressure stops increasing linearly and begins to deviate is the leak-off point.
  4. In an Extended LOT, pumping is continued until fracture propagation begins.

Why it matters:

  • LOTs help determine the maximum pressure the formation can withstand without fracturing.
  • The resulting Fracture Gradient (FG) is used in safe mud weight planning and to validate wellbore integrity.

Important distinction:

  • LOT ≠ Sh_min. A basic LOT provides fracture initiation pressure, not closure pressure (Sh_min).
  • Sh_min requires post-test analysis with pressure falloff data, typically done via DFIT.

Datum Correction Example​

RKB to MSL conversion:

Depth_MSL = Depth_RKB - RKB_Height  
Pressure = SG × 0.0981 × Depth_MSL

Critical Barrier Depth terms​

CBD – Critical Barrier Depth​

Description: The depth at which the pressure from an SOI equals or exceeds Sh_min, indicating a loss of containment.

Calculated by finding the closest match between:

SOI Gradient ≈ Sh_min Gradient

Critical Barrier Depth vis

Excel representation. The intersection between SH_min and the SOIs BHP (green line) is the Critical Barrier depth.

Excel visualization

Visual representation

CBD Drawing

SOI – Source of Inflow​

Description: A formation layer or zone from which fluids can enter the wellbore. Serves as a reference point to evaluate barrier integrity.

Pressure and curves​

Gradient​

Description: A measure of pressure per unit depth. Common units: MPa/m or psi/ft.

Conversions:

MPa/m = SG × 0.0981  
psi/ft = SG × 0.433

PPFG prognosis chart

Pore Pressure​

Description: The pressure exerted by fluids trapped within the rock's pore spaces. It is a key input in well planning and is essential for understanding formation stability, kick risk, and barrier integrity.

Pore pressure is typically derived from seismic data, well logs, or offset wells and is often expressed as a pressure gradient or equivalent mud weight (EMW).

Formula (hydrostatic assumption):

Pore Pressure = SG × 0.0981 × TVD  // in MPa

Why it matters:
If bottom hole pressure drops below pore pressure, influx of formation fluids (kicks) can occur — a major well control risk. It also defines part of the window for safe mud weights between pore pressure and fracture gradient.

Sh_min – Minimum Horizontal Stress​

Description: The least principal stress acting horizontally within the formation. It represents the pressure required to initiate a fracture in the direction of least resistance. Also referred to as "sigma min"

How it is measured:
Sh_min is commonly determined through a mini-frac or Diagnostic Fracture Injection Test (DFIT). During this test:

  1. Fluid is injected into the formation to initiate a small fracture.
  2. The injection is stopped and the well is shut in.
  3. As the pressure falls off, the fracture begins to close.
  4. The pressure at which the fracture closes is interpreted as Sh_min.

Why it matters:
Understanding Sh_min is critical in evaluating fracture risk, barrier integrity, and determining the Critical Barrier Depth (CBD) when compared to SOI pressure gradients.

Note:
Sh_min typically lies below the fracture gradient (FG), but exceeding Sh_min may still result in temporary or recoverable fracturing.

How to Think About It:

  • The earth squeezes rocks from all directions.

    • Downward = overburden stress Sv
    • Side-to-side = two horizontal stresses:
    • One stronger → Sh_max
    • One weaker → Sh_min

  • SH_min is the weakest sideways stress. If you push harder than this from the inside (with fluid pressure), the rock will start to open a fracture.

📈 On a pressure vs depth plot:

  • The Sh_min curve acts as a “safe limit line”.
  • As long as bottom hole pressure or SOI pressure is below Sh_min, the formation holds.
  • When pressure approaches Sh_min, that’s when the rock can start to break apart — and that's critical when evaluating barrier integrity.

Analogy Think of a sealed cardboard box:

  • If you press gently from the inside, the box holds (within Sh_min).
  • If you press hard enough, the flaps start to separate — that’s you reaching Sh_min.
  • Press even harder, and the box tears open — that’s the fracture gradient.

Fracture Gradient (FG)​

Description: The pressure gradient at which the rock begins to fracture irreversibly. Usually slightly higher than Sh_min.

BHP – Bottom Hole Pressure​

Description: The actual pressure at the bottom of the well, either static (shut-in) or dynamic (while circulating).
Used to assess how close the well is to fracturing or breaching integrity.

Absolute Pressure​

Description: The true pressure at a given depth, referenced to a vacuum (not gauge or relative).
Must correct for RKB height if depth is not referenced from MSL.

Gas Gradient (Excel term)​

Description: A manually entered gradient that represents the hydrostatic pressure behavior of the fluid expected in the formation (e.g., gas, oil, or water). Engineers typically estimate it based on the expected fluid type in the lithology at the Source of Inflow (SOI).

Examples:

  • Gas (methane): ~0.23 SG
  • Condensate/light oil: ~0.64 SG
  • Water: ~1.00 SG

Example:

Pressure = 0.23 × 0.0981 × TVD  // For methane
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