Explanation of Pressure Terminologies in Drilling Operations

In drilling engineering, understanding various pressure-related terms is essential to ensure safe and efficient operations. This document provides detailed explanations of common pressure concepts encountered during drilling.

1. Hydrostatic Pressure

Hydrostatic pressure refers to the pressure exerted by the drilling fluid due to its own weight in the wellbore. It is calculated using the formula:

$$P=\rho \cdot g\cdot h$$

Where:

• $P$: Hydrostatic pressure (MPa)

• $\rho$: Drilling fluid density (g/cm³)

• $g$: Gravitational acceleration (~9.8 m/s²)

• $h$: Vertical depth of the well (m)

Functions:

• Prevents formation fluids from entering the wellbore

• Maintains wellbore stability

• Controls bottom hole pressure (BHP)

2. Equivalent Circulating Density (ECD)

During circulation, frictional pressure loss increases the bottom hole pressure above the static level. Equivalent Circulating Density (ECD) is used to represent the effective density accounting for this additional pressure.

The formula is:

ECD=ρ+ΔPfriction0.00981⋅DECD = \rho + \frac{\Delta P_{friction}}{0.00981 \cdot D}ECD=ρ+0.00981⋅DΔPfriction

Where:

• ΔPfriction\Delta P_{friction}ΔPfriction: Frictional pressure loss during circulation (MPa)

• $D$: Measured well depth (m)

• 0.00981: Unit conversion constant

Significance:

• ECD is critical for assessing real-time bottom hole pressure

• Essential for well design, especially in high-pressure or low-fracture-gradient formations

3. Surge Pressure

Surge pressure is a transient pressure spike generated when running tools (e.g., casing, drill string) into the hole too quickly, displacing drilling fluid downward.

Characteristics:

• Occurs during rapid tool insertion

• Can cause formation breakdown or lost circulation

• In extreme cases, may lead to well control incidents

Prevention:

• Run tools slowly

• Optimize tool and wellbore clearance

• Use drilling fluids with suitable rheology

4. Swab Pressure

Swab pressure occurs when pulling the drill string or tools out of the hole, creating upward fluid movement and negative pressure at the bottom of the well. This may lead to formation fluid influx (kick).

Influencing factors:

• Pulling speed

• Tool diameter

• Drilling fluid viscosity and flow characteristics

• Wellbore geometry

Control measures:

• Pull out slowly

• Maintain wellbore pressure with mud pumps if needed

• Monitor for signs of influx during trips

5. Bottom Hole Pressure (BHP)

Bottom Hole Pressure is the total pressure acting at the bottom of the wellbore, consisting of:

$$BHP=HydrostaticPressure+FrictionalPressureLoss+Surge/SwabPressure$$

Alternatively expressed as:

$$BHP=ECD\cdot 0.00981\cdot D$$

Importance:

• Maintains well control and safety

• Prevents kicks and blowouts

• Ensures proper hole cleaning and mud circulation

6. Importance of Managing Drilling Fluid Density

Drilling fluid density directly influences both hydrostatic pressure and ECD, and thus plays a key role in controlling bottom hole pressure.

Optimal condition:

$$Formationporepressure<Hydrostaticpressure<Formationfracturepressure$$

If the density is too low:

• Insufficient hydrostatic pressure

• Risk of formation fluid influx (kick or blowout)

If the density is too high:

• Exceeds fracture pressure of the formation

• Causes lost circulation, wellbore instability, and operational hazards