Stripping well control becomes more complex when encountering an influx, simply called influx penetration.. This article explores how influx penetration, the drill string entering the influx zone, affects well control procedures.
As the drill string enters the influx, the height of the influx increases. This larger influx volume translates to a decrease in hydrostatic pressure within the wellbore. To maintain well control and prevent additional influx due to underbalance condition, the casing pressure at the surface needs to compensate for this pressure reduction. Figure 1 demonstrates influx height change when BHA penetrates into influx. This penetration will elongate the influx casing reduction in hydrostatic pressure.
Figure 1: Influx Penetration
The impact of influx penetration is particularly significant for gas kicks. Due to the lower density of gas compared to wellbore fluids, a gas influx causes a much larger decrease in hydrostatic pressure, requiring a more substantial increase in casing pressure.
The well control method employed during stripping also plays a role. When using the volume accounting method, casing pressure automatically adjusts as the drill string penetrates the influx. This eliminates the need for precise calculations regarding penetration timing.
However, the constant surface pressure method requires manual adjustments. Here, the operator must calculate the necessary increase in casing pressure based on the influx volume using the equation:
ΔCP = ΔH × (PGM – PGI)
In this formula,
ΔCP represents the required casing pressure increase (psi).
ΔH is the change of the height of the influx zone (ft).
PGM is the Pressure gradient of drilling mud (psi/ft).
PGI is the Pressure gradient of influx (psi/ft).
** This calculation is suitable for non-migrate influx such as oil or water kick.
Influx Penetration Calculation Example
The information given shows in the figure 2.
Figure 2 – Information given for influx penetration
Solution
Hole capacity = 8.5²÷1029.4 = 0.0702 bbl/ft
Capacity between hole and BHA = (8.5² – 6.5²) ÷ 1029.4 = 0.02914 bbl/ft
Capacity between hole and DP= (8.5² – 5²) ÷ 1029.4 = 0.0459 bbl/ft
Initial length of influx = 30 bbl ÷ 0.0702 bbl/ft = 427 ft
Volume of influx between hole and BHA = 90 ft × 0.02914 bbl/ft = 2.62 bbl
Therefore, volume of influx between hole and drill pipe is equal to initial volume (30 bbl) minus volume of influx between hole and BHA(2.62 bbl).
Volume of influx between hole and drill pipe = 30 – 2.62 = 27.38 bbl.
Height of influx between hole and drill pipe = 27.38 bbl ÷ 0.0459 bbl/ft = 597 ft
Total influx height once the BHA penetrates into the influx = 90 + 597 = 687 ft
Mud Gradient (PGM) = 9 × 0.052 = 0.468 psi/ft
ΔH = 687 – 427 = 260 ft
ΔCP = 260× (0.468 – 0.3) psi
ΔCP = 44 psi (round up figure)
New casing pressure = 150 + 44 = 194 psi
Figure 3 – Casing pressure once the influx is penetrated.
For non-migrating influxes, such as oil or saltwater kicks, estimating the penetration time is relatively straightforward. We simply calculate the length of pipe needed to reach the influx depth and subtract it from the initial distance between the drill string and the bottom of the wellbore.
In practice, a safety factor can be added to the calculated casing pressure increase. This ensures the well remains overbalanced even after encountering the influx, providing an additional layer of security. However, it’s crucial to ensure this safety factor doesn’t exceed the maximum pressure the well can withstand.
The situation becomes more complex with migrating gas kicks. Relying solely on surface pressure or the volume of fluid bled off (volume accounting) is insufficient for maintaining well control. In such cases, Volumetric Control techniques become essential. This method involves simultaneously measuring the volume of fluid bled off and monitoring the volume of drill pipe stripped into the well. The next section will delve deeper into combining Volumetric Control with stripping operations.
By understanding the impact of influx penetration on well control procedures, operators can ensure safe and efficient stripping operations even when encountering unexpected situation.
References
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
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