Gas Behavior and Bottom Hole Pressure with Constant Surface Pressure

This example demonstrates how bottom hole pressure will be when maintaining constant surface pressure and letting gas bubble migrating up.

From the previous post, Gas Behavior and Bottom Hole Pressure in a Shut in well, you know that if you take gas kick and you do nothing only letting gas migrating up, you will have an increase in casing pressure (surface pressure) and you may end up breaking shoe or surface equipment. You know that this practice is a bad idea.

What will be happened if the surface pressure is maintained at constant value?

The well will be maintained constant surface pressure by bleeding some mud in order to compensate pressure increment.

Do you think this is a good idea to do that?

Let’s go though all details about it.

This is 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 head on top of gas is 4,000 psi. The well is bleed off mud in order to keep 400 psi constant surface pressure.


  • 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

What happens if we maintain surface pressure?

Surface pressure is kept constant at 400 psi by bleeding 5 bbl of mud. It means that gas is allowed to expand another 5 bbl so the gas influx is 10 bbl in total.

According to the Boyle’s Law,  gas influx pressure can be calculated.

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 the second condition (V2) is 10 bbl.

Bolye’s Law

P1 × V1 = P2 × V2

4,400 × 5 = P2 × 10

P2 = 2,200 psi ->Gas pressure decreases because of expansion.


Figure 3 – Gas Pressure with Constant Surface Pressure

What will happen to Bottom Hole Pressure?

We apply the hydrostatic pressure concept.

Bottom Hole Pressure (BHP) = Hydrostatic Pressure (HP) + Surface Pressure (SP)

Surface pressure: Surface pressure remains constant because it is controlled at a planned value, which is 400 psi in this example.

Hydrostatic pressure: The overall hydrostatic pressure will decrease because gas expansion will displace volume of mud.

Bottom hole pressure: Bottom hole pressure will decrease because of reducing hydrostatic pressure in the wellbore.

Figure 4 – Bottom Hole Pressure Reduction due to Hydrostatic Pressure Reduction

Reduction in bottom hole pressure will result in more influx.

Figure 5 – More Influx to the well


If gas influx is in the well and  casing pressure is maintained at constant value by bleeding off mud of surface, the bottom hole pressure will be decreased. Therefore, additional influx will be allowed into the wellbore. This is not good idea.


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.

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3 Responses to Gas Behavior and Bottom Hole Pressure with Constant Surface Pressure

  1. zayed says:

    In UBD case whene calculate the Mixture desnsity @BHP condition.How to get this

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