3.1 Primary Causes of Well Influxes

Principles of Primary Well Control and Influx Initiation

Primary well control is maintained when the hydrostatic pressure ($HP$) of the drilling fluid column exceeds or equals the pore pressure ($P_p$) of any exposed permeable formation:

$HP \ge P_p$

The positive margin between hydrostatic pressure and formation pore pressure is the overbalance margin:

$\Delta P = HP - P_p$

A kick (an unwanted influx of formation fluids into the wellbore) requires two conditions to occur simultaneously:

1. Pressure Underbalance: Downhole pressure falls below formation pore pressure ($BHP < P_p$).
2. Formation Permeability and Porosity: Exposed formations possess connected pore spaces and mobility allowing fluids to enter the wellbore.

Operational causes of kicks fall into four primary categories: insufficient mud density, failure to keep the hole full while tripping, swabbing during pipe withdrawal, and lost circulation.

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1. Insufficient Mud Density

Insufficient mud density occurs when the unit weight of the drilling fluid is inadequate to counterbalance pore pressure, even with a full column of fluid:

$\Delta HP = 0.052 \times (\text{MW}_{\text{required}} - \text{MW}_{\text{actual}}) \times \text{TVD}$

Operational Causes of Insufficient Mud Weight

• Drilling into Unexpected Abnormal Pressure: Penetrating transition zones, geopressured shales, or faulted boundaries can cause pore pressure to rise abruptly. Geopressured zones commonly originate from compaction disequilibrium (undercompaction in rapidly deposited shales where pore water cannot escape) or trapped hydrocarbon generation. When drilling penetrates through an impermeable cap rock into undercompacted sediments, pore pressure gradients can jump from normal (e.g., $0.465 \text{ psi/ft}$ or $8.94 \text{ ppg}$) to severe abnormal levels (e.g., $0.750 - 0.850 \text{ psi/ft}$). Without proactive mud weight increases, an underbalance occurs immediately upon bit penetration.
• Uncontrolled Surface Dilution: Inadvertent addition of base fluid (water or base oil) at surface pits rapidly erodes mud weight. Common triggers include wash water entering the active pits, water added to the system without telling the Driller, solids-control valves left misaligned, centrifuges discarding barite along with fine solids, and heavy rainfall into uncovered pits.
• Barite Sag (Settling): Weighting agents (such as barium sulfate, $\text{BaSO}_4$) can settle out of suspension under static or low-shear conditions. This phenomenon is especially severe in directional wells ($35^\circ$ to $60^\circ$ inclination) due to the Boycott effect, where particles settle vertically over short distances and slide downward along the low side of the hole. When circulation halts for logging or trips, barite beds accumulate; upon resuming circulation, light supernatant mud circulates through the annulus, causing an unexpected, severe drop in bottom-hole hydrostatic pressure.

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2. Failure to Keep the Hole Full While Tripping

When a drill string is pulled from the wellbore, the volume of steel removed must be replaced with drilling fluid. If the hole is not filled continuously, the fluid level drops by vertical height $\Delta h$. With dry pipe the level falls in both the annulus and the open pipe bore, so the falling level sees the casing (or hole) capacity minus the metal displacement of the pipe still in the hole:

$\Delta h = \frac{V_{\text{steel pulled}}}{\text{Casing capacity} - \text{Metal displacement}}$

IWCF's formula sheet expresses the same idea as the pressure drop per foot of dry pipe pulled: $\frac{\text{MW} \times 0.052 \times \text{metal displacement}}{\text{casing capacity} - \text{metal displacement}}$.

As the fluid level falls, hydrostatic pressure decreases proportionally:

$\Delta HP = 0.052 \times \text{MW} \times \Delta h$

Pulling heavy drill collars poses severe danger because large collars displace five to seven times more steel per foot than drill pipe (e.g., $0.040 - 0.055 \text{ bbl/ft}$ for collars vs. $0.0075 \text{ bbl/ft}$ for 5-inch pipe). Unfilled collar pulling rapidly drops the annular column by hundreds of feet, causing swift loss of overbalance.

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3. Swabbing During Pipe Withdrawal

Swabbing is hydrodynamic suction caused by upward drill string movement through viscous drilling fluid. Viscous friction and fluid drag pull mud upward, generating a localized pressure reduction beneath the BHA:

$\text{BHP}_{\text{pulling}} = HP - P_{\text{swab}}$

If the swab pressure reduction ($P_{\text{swab}}$) exceeds the overbalance margin ($HP - P_{\text{swab}} < P_p$), formation fluids enter the wellbore.

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4. Lost Circulation

Lost circulation is the uncontrolled loss of whole mud into formation voids, natural fissures, or induced fractures when wellbore pressure exceeds breakdown pressure ($P_{\text{frac}}$):

$\text{BHP} > P_{\text{frac}} \implies \text{Formation Breakdown and Losses}$

Losses are recognised from falling pit levels and a reduced rate of returns (flow out less than flow in). In total loss scenarios where no returns reach surface, the annular mud level drops until hydrostatic pressure matches the thief zone's pore pressure. This column collapse reduces pressure across the entire wellbore, frequently triggering a severe secondary kick from another permeable zone.

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Gas-Cut Mud Dynamics: Superficial Gas Cutting vs. Active Kick

Gas-cut mud occurs when formation gas enters the fluid stream. The hydrostatic impact of gas cutting is governed by Boyle's law:

$P_1 V_1 = P_2 V_2$

• Bottom-Hole Behavior: Deep in the wellbore (e.g., $10{,}000 \text{ ft TVD}$ under $5{,}200 \text{ psi}$), entrained gas bubbles are compressed into negligible volumes. Consequently, bottom-hole hydrostatic pressure reduction is minor, typically less than $50 \text{ psi}$.
• Surface Behavior: As mud ascends into the top few hundred feet, hydrostatic confining pressure drops toward atmospheric levels. The gas expands rapidly (50- to 100-fold), frothing the mud and dropping surface mud density from $12.0 \text{ ppg}$ to $7.0 - 9.0 \text{ ppg}$.

First action for Level 2 personnel: if you see gas-cut mud at the shakers or a falling mud weight out, alert the Driller immediately. The Driller decides on a flow check; never assume the gas is harmless drilled gas without one.