Volumetric Well Control Method: A Step-by-Step Guideline

Step 1 – Perform Essential Calculations

Prior to executing the Volumetric Control procedure, three key calculations are necessary:

1. Safety Factor (SF)
2. Pressure Increment (PI)
3. Mud Increment (MI)

Safety Factor (SF): This is the additional bottomhole pressure permitted to occur naturally as gas migrates up the annulus with the well shut in. It ensures the bottomhole pressure remains sufficiently above the formation pressure to prevent underbalance. A typical Safety Factor ranges from 50 to 200 psi. The appropriate value depends on factors such as depth, angle, hole size, and well fluid. Migration time for the gas bubble to increase the casing pressure by this amount can vary from minutes to several hours.

Pressure Increment (PI): This is the working pressure range for the Volumetric Control Method. It represents both the surface pressure increase tolerated per step and the reduction in hydrostatic pressure during each step. The Pressure Increment is generally set equal to the Safety Factor (rounded to the nearest 10 psi). For example, with a Safety Factor of 100 psi, the recommended PI would also be 100 psi.

Mud Increment (MI): This is the volume of mud that must be bled from the annulus to decrease the annular hydrostatic pressure by the Pressure Increment. The Mud Increment is calculated using the formula:

Mud Increment (MI) = (PI×ACF) ÷ (MW×0.052)

where:

• PI = Pressure Increment (psi)
• ACF = Annulus Capacity Factor (bbl/ft)
• MW = Mud Weight (ppg)

For instance, with a PI of 100 psi, an ACF of 0.0802 bbl/ft, and a MW of 12.0 ppg, the Mud Increment (MI) is approximately 12.85 bbl.

Step 2 – Allow Casing Pressure to Increase

With calculations complete, allow the gas bubble to migrate up the annulus, increasing the shut-in casing pressure by the Safety Factor (SF) plus the Pressure Increment (PI). Initially, no mud is bled from the annulus, so the hydrostatic pressure remains unchanged. The bottomhole pressure increases by the combined Safety Factor (SF) and Pressure Increment (PI), resulting in a controlled overbalance to a desired casing pressure.

While allowing casing pressure:

$\text{Bottomhole Pressure ↑}=\text{Hydrostatic Pressure (constant) }+\; Surface\text{Pressure ↑}\text{(Initial Casing Pressure + Safety Factor + Pressure Increment )}$

Step 3 – Maintain Casing Pressure Constant While Bleeding Mud

To prevent further pressure increase, bleed the first Mud Increment (MI) from the annulus while keeping the casing pressure constant. This ensures that the reduction in bottomhole pressure results solely from the hydrostatic pressure decrease. Multiple small choke adjustments may be required to maintain constant surface pressure. During this process, the bottomhole pressure decreases by the Pressure Increment.

While bleeding mud:

$\text{Bottomhole Pressure}\downarrow =\text{Hydrostatic Pressure}\downarrow +\text{Surface Pressure}\text{ (constant)}$

As mud is bled, the gas bubble expands to occupy the vacated volume, decreasing the bubble’s pressure according to Boyle’s Law. Note that improper control of surface pressure may allow additional influx from the formation, exacerbating well control issues.

Step 4 – Wait for Casing Pressure to Increase

After bleeding the Mud Increment (MI), wait for the gas bubble to migrate upward, causing the surface pressures to increase by the Pressure Increment (PI). This restores the overbalance to the Safety Factor plus Pressure Increment (PI).

Step 5 – Repeat Mud Bleeding to Maintain Constant Casing Pressure

Once the maximum overbalance is reached, hold the casing pressure constant by bleeding another Mud Increment. This decreases the bottomhole pressure by the Pressure Increment (PI) and allows further gas expansion.

Step 6 – Alternate Between Pressure Holding and Gas Bubble Migration

Continue alternating between maintaining constant casing pressure and allowing it to rise as the gas bubble migrates, repeating steps 4 and 5. Each cycle involves bleeding mud to reduce bottomhole pressure and waiting for the casing pressure to rise as the gas migrates. By the time the gas reaches the surface, it has expanded significantly, reducing its pressure substantially.

Figure 1  demonstrates casing pressure and overbalance while performing volumetric well control. If you want to see how the example of the volumetric well control, please check this article. Volumetric Well Control Example Calculations 

Critical Notes:

The Volumetric Well Control Method allows controlled wellbore pressure management during gas influx migration. By following the outlined steps and maintaining constant casing pressure during mud bleeding, bottom hole pressure will be maintained almost constant until conventional well control procedures can be implemented or gas is safely controlled migration to surface.

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.