## Gas Behavior with Constant Bottom Hole Pressure

You have learned from 2 previous topics, gas behavior in shut in well and gas behavior with constant surface pressure. Both ways are not how you control bottom hole pressure. In this article, we will use the same scenario but we will control bottom hole pressure constant.

You have the same well. The well is shut-in without pipe in hole. 5 bbl of gas kick is taken and initial shut in casing pressure is equal to 400 psi. Hydrostatic pressure on top of gas is 4000 psi. See the diagram in Figure 1.

Assumptions:

• Volume not change
• No temperature change
• Formation not broken
• No must loss
• 5 bbl of mud expansion = 100 psi (equivalent to hydrostatic pressure)

Figure 1 – Well Shut In Diagram

Determine Gas Kick Pressure

Gas kick pressure at the bottom is equal to hydrostatic pressure above gas kick plus shut in pressure

Gas kick pressure = 4,000 + 400

Gas kick pressure = 4,400 psi

At the bottom, gas kick pressure = bottom hole pressure

Figure 2 – Gas Pressure at The Bottom

For this time, bottom hole pressure will be controlled by the following steps;

1. Allow gas to migrate and let pressure increase by hydrostatic pressure of mud that planned to bleed off
2. One pressure increases to the required level, bleed off fluid to planed volume by holding surface pressure constant.

For learning purpose, we assume that 5 bbl of mud equates to 100 psi of hydrostatic pressure. The gas is allowed to migrate and the surface pressure is rose by 100 psi. So surface pressure will gradually increase to 500 psi (see Figure 3). At this point, gas kick is still 5 bbl.

Figure 3 – Allow Surface Pressure to Increase

Once 500 psi surface pressure is reached, mud is bled off while maintaining constant surface pressure of 500 pi until bled back volume reaches 5 bbl (See Figure 4).

Figure 4 – Bleed mud volume by maintaining constant surface pressure

What is the bottom hole pressure after bleeding off mud?

Let’s apply the hydrostatic pressure concept.

Bottom Hole Pressure = Hydrostatic Pressure + Surface Pressure

Surface pressure increases by 100 psi.

Hydrostatic pressure decreases by 100 psi due to bleeding off.

Therefore, the bottom hole pressure will be constant since the increase in surface pressure is compensated by the reduction of hydrostatic pressure (Figure 5).

Figure 5 – Maintain bottom hole pressure constant

For this example, the bottom hole pressure is still at 4,400 psi (Figure 6)

Figure 6 – Bottom Hole Pressure still be maintained at 4,400 psi.

With this method, the well is under control without breaking formation or allowing more influx to come. We use this concept in several well control methods.

Conclusion:

• Well control with bottom hole pressure control is the correct method of well control.
• Surface pressure must rise to account for the loss of hydrostatic pressure due to bleeding off mud.
• This concept is used in several well control methods such as driller’s method, wait and weight, volume metric, and lubricate&bleed.

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.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.

## Gas Behavior and Bottom Hole Pressure in a Shut in well

This is a an example demonstrating how bottom hole pressure will be due to gas migration in a shut in well. This is very important concept in well control.

This example will demonstrate the gas behavior in a shut in well.

The well is shut-in without pipe in hole. 5 bbl of gas kick is taken and initial shut in casing pressure is equal to 400 psi. Hydrostatic head on top of gas is 4,000 psi (see figure 1). The well is shut in and gas migrates up until where hydrostatic pressure underneath gas is 2000 psi. What will happen to bottom hole pressure and shut in pressure?

Assumptions:

• Volume not change
• No temperature change
• Formation not broken
• No must loss

Figure 1 – Well Shut In Diagram

Determine Gas Kick Pressure

Gas kick pressure at the bottom is equal to hydrostatic pressure above gas kick plus shut in pressure

Gas kick pressure = 4,000 + 400

Gas kick pressure = 4,400 psi

Figure 2 – Gas Pressure at The Bottom

Determine Bottom Hole Pressure and Shut In Pressure at The Second Condition

Even though the well is shut in, the gas influx is able to move upwards due to gas migration.

In this case, we will not allow any gas expansion and let the gas gradually migrate.

The well is shut in and gas is allowed to migrate up hole until hydrostatic pressure underneath gas is 2000 psi (see the figure 3).

Figure 3 – Gas Migrate up

### What will happen to bottom hole pressure and shut in pressure?

Determine gas kick pressure –  With Bolye’s Law concept, we will apply it see how much gas bubble should be.

According to this example,

Pressure of gas (P1) is 4,400 which equates to the bottom hole pressure.

Volume of gas at beginning (V1) is 5 bbl.

Volume of gas at this condition (V2) is 5 bbl (volume not change).

Bolye’s Law

P1 × V1 = P2 × V2

4,400 × 5 = P2 × 5

P2 = 4,400 psi ->Gas pressure remains constant.

Determine hydrostatic pressure above kick

Since there is no change in volume of fluid, total hydrostatic pressure remains constant. With this relationship, we can calculate hydrostatic pressure above migrated kick.

Total hydrostatic pressure = Hydrostatic pressure above kick +  Hydrostatic pressure below kick

4,000 = Hydrostatic pressure above kick + 2,000

Hydrostatic pressure above kick = 2,000 psi

You have total of hydrostatic pressure of 4,000 psi at the beginning. Currently, you have 2,000 psi of hydrostatic at the bottom therefore you have 2,000 psi of hydrostatic on top of gas. See the figure 4 below.

Figure 4 – Hydrostatic Pressure above Gas Kick

Determine Shut in Pressure

Let’s see how much shut in pressure will be.

Apply hydrostatic pressure concept to solve this problem.

Gas Kick Pressure = Hydrostatic Pressure above the gas kick+ Shut in Pressure

4400 = 2000 +Shut in pressure

Shut in pressure = 2,400 psi

Figure 5 – Shut In Pressure

Determine Bottom Hole Pressure

Moreover, you can calculate the bottom hole pressure by applying the same concept.

Bottom Hole Pressure = Hydrostatic Pressure  + Shut in Pressure

Bottom hole pressure = 4,000 + 2,400

Bottom hole pressure = 6,400 psi.

Figure 6 – New Bottom Hole Pressure

Conclusions

• If gas migrates in a shut-in well without allowing it to expand, pressures everywhere in the well will go up, except in the gas bubble it self.
• If a well is shut in and the gas influx is allowed to migrate, gas pressure will remain constant; however, bottom hole pressure and casing pressure will be increased.
• If casing pressure (surface pressure) increases too much, you can break formation or damage surface equipment.
• Surface pressure will be increased by the amount of hydrostatic pressure that gas migrates past.
• If there is no change in total hydrostatic pressure in the well, the increase in surface pressure causes a corresponding increase in bottom hole pressure.

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.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.