Consider the following about gas:
• Gas is a compressible and expandable fluid. The gas that enters the well does so under formation pressure.
• Gas will expand as pressure and temperature on the gas is reduced provided there is physical room for expansion.
• If gas doesn’t expand, pressure within the gas remains the same—essentially formation pressure.
• Gas is lighter than mud and can migrate up the hole after it enters; however, gas migration is not a given, and there are several things that can affect whether or not gas will migrate, and if it does, how fast.
• If gas migrates up the wellbore after shut-in there exists the possibility of extreme pressures to be exerted on the wellbore and the surface.
• As gas is circulated out of the hole, physical room for controlled expansion is created by bleeding mud through a choke. Since expansion takes place, pressure within the gas is gradually reduced which reduces internal wellbore stress.
The next few pages illustrate what can happen if gas does not expand.
Conclusions:
•Pressure within the gas remains constant as gas migrates up the hole unexpanded.
•Wellbore stress dramatically increases both above and below the gas—this can lead to formation failure in the open hole.
•Excessive surface pressures, can lead to BOP failure and/or casing rupture.
•For these reasons, the gas should be circulated from the hole. By doing this, the gas can expand in a controlled fashion resulting in much lower and manageable wellbore stresses.
•Surface pressures resulting from hydrostatic changes are much lower when gas is circulated from the hole.
•Additionally, the BHP can be maintained equal to or slightly greater than formation pressure.
When we have to shut-in a well we “move” from primary well control to secondary well control. This is the control of formation pressure with the BOP’s and the remaining wellbore hydrostatics.
The importance of promptly shutting in a well can not be overstated.
Shutting in the well means:
We stay safe
- Minimizes wellbore and surface equipment stress which aids in preventing blowouts and keeps us safe
- Minimizes the possibility of underground blowouts which again, minimizes the chance of a surface blowout and helps keep us safe
- Minimizes the release of poisonous gases......by now you know why
- Minimizes the possibility of fire, loss of life, personal injury, damage or loss of expensive equipment
- Minimizes the potential loss of valuable, non-renewable natural resources
- Minimizes the potential for environmental damage
Shutting in at the Christmas Tree during wireline operations with no tools in the hole:
•Close the Upper Master Valve, counting the number of turns to completely close
the valve. It should be the same number of counts as it took to fully open the valve.
•Close the Swab valve
Shutting in at the Christmas Tree during wireline operations with tools in the hole:
•If possible, pull tools into the lubricator and proceed with shut in as above.
•If tools are to remain in the well, close the wireline BOP
•Cut the wireline while closing the Upper Master Valve, if it is capable. If not, cut the wireline through other means. Count the turns going in to ensure that the valve is fully closed.
•Close the Swab valve
Verify shut in by checking for leaks and monitoring pressures and flow meter. Verify line up of flow path and that two barriers are in place above the wellbore pressure. Continue to monitor pressures. Pressures may rise due to thermal expansion or gas migration. If gas is migrating, it must either be circulated out of the well or brought to the surface using the Volumetric Method.
Valves can be damaged when opened or closed under differential pressure. If possible, equalize the pressure across the valve before opening. If a valve must be operated against differential pressure, open the valve slowly. All gate valves should be either fully opened or fully closed under operating conditions.
Pressures must be recorded once the well is shut in to confirm that the well has stabilized. Pressures should be recorded once every minute or every 90 seconds to ensure that the well is closed in and stable. If pressures start to rise, it can be an indication of gas migration, which must be dealt with.