Gas Behavior and Bottom Hole Pressure in a Shut in well

This is a classic 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.

gas-Behavior-and-Bottom-Hole-Pressure-in-a-Shut-in-well-cover

Assumption: For this example, since the volume of the well does not change, and assuming that no mud or fluid is lost to the formation.

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 4000 psi (see figure below).

 Gas Behavior and Bottom Hole Pressure in a Shut in well 1

Even though the well is shut in, the gas influx is able to move upward 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 below).

Gas Behavior and Bottom Hole Pressure in a Shut in well 2

What will happen to bottom hole pressure and casing 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 4400 which equates to the bottom hole pressure.

Volume of gas at beginning (V1) is 5 bbl

P1 x V1 = P2 x V2

4400 x 5 = P2 x 5

P2 = 4400 psi ->Gas pressure remains constant.

You have total of hydrostatic pressure of 4,000 psi at the beginning. Currently, you have 2000 psi of hydrostatic at the bottom therefore you have 2000 psi of hydrostatic on top of gas. See the figure below.

Gas Behavior and Bottom Hole Pressure in a Shut in well 3

Let’s see how much casing pressure will be.

Apply hydrostatic pressure concept to solve this problem.

Gas influx pressure = hydrostatic pressure above the gas influx + casing pressure

4400 = 2000 + casing pressure

Casing pressure = 2400 psi

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

Bottom hole pressure = hydrostatic pressure of mud + casing pressure

Bottom hole pressure = 4,000 + 2,400

Bottom hole pressure = 6,400 psi.

Gas Behavior and Bottom Hole Pressure in a Shut in well 4

Conclusion:

If the 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 increase. If you let casing pressure (surface pressure) increase too much, you can break formation or damage surface equipment.

Reference book: Well Control Books

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9 Responses to Gas Behavior and Bottom Hole Pressure in a Shut in well

  1. rick says:

    Want to understand fully if a gas bubble comes in from the bottom of the well & the wells is shut-in. If the gas bubble migrates to the surface does the volume of the gas expands if the well is shut-in?

  2. Alfredo says:

    I think it would be important to clarify a small but important concept.
    “Since the volume of the well does not change, and assuming that no mud or fluid is lost to the formation, the volume of gas does not change, either.”

  3. In Gas Behavior and Bottom Hole Pressure in a Shut in well, the second operation is no good (bottom hole pressure = 4,000 + 2,200 = 6,400 psi.), the correct answer is: 4.400 plus the hydrostatic below gas pressure 2000 psi = 6.400 psi

    Regards..Jairo C. Molero

  4. Wolfgang Romero says:

    Jairo comments are totally right and the result (despite it’s the same) is easier to understand.
    Thanks for this topic
    Wolfgang

  5. Gholam Reza Soori says:

    Dear DrillingFormulas team,

    Deeply appreciated.

    Cheers,

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