This article will demonstrate how to perform volume metric well control. There are a total of 5 steps as listed below;
Step 1 – Calculation
Three calculations must be determined before conducting volumetric well control.
1. Safety Factor (SF) – The Safety Factor (SF) in an increase in bottom hole pressure which we allow to happen naturally when gas influx migrates up with the shut in well. SF is important because it will allow the bottom hole pressure to be over formation pressure so the well is not in underbalance condition while conducting later steps. Typically, SF is around 50 – 200 psi. If the initial shut in casing pressure is very close to maximum allowable surface pressure. Personnel must select small safety factor to prevent fracturing formation.
2. Pressure Increment (PI) – It is pressure used as a working pressure while conducting Volume Metric well control. This pressure will be equal amount of hydrostatic pressure of mud bled during each step.
3. Mud Increment (MI) – It is volume of mud bled off from the annulus to reduce hydrostatic pressure by amount of Pressure Increment. It is very important that the rig must have an accurate measurement to measure small amount of mud bled from annulus. Mud Increment is determined by the following equation:
Mud Increment is in bbl.
PI is pressure increment in psi.
ACF is annular capacity factor in bbl/ft
MW is mud weight in ppg.
Step 2 – Allow Casing Pressure To Increase To Safety Factor Plus Pressure Increment
After the first step is completed, the 2nd step is to wait until casing pressure increases by an amount equal to Safety Factor (SF) plus Pressure Increment (PI). At this stage, the bottom hole pressure will increase by surface pressure but hydrostatic pressure is still the same.
For example, if SF is 100 psi and PI is 100 psi, we need to wait until casing pressure increase by 200 psi.
Step 3- Hold Casing Pressure Constant While Mud Increment Is Bled Off
Since we have the overbalance in step-2, in order to keep raising casing pressure due to gas migration, hydrostatic pressure must be taken out by bleeding off mud volume. This step will bleed off amount of mud equal to mud increment. Bleeding mud with constant casing pressure is performed to ensure that the bottom hole pressure is decreased by a loss of hydrostatic pressure only. Failure to keep casing pressure constant while bleeding off mud results in reduction of the bottom hole pressure. This can lead to more severe well control problem.
Every bleed off volume (mud increment) will reduce the bottom hole pressure by the amount of Pressure Increment. Once the bleed off is complete, the bottom hole pressure will be over balance by the safety factor.
Step 4 – Wait For Casing Pressure To Increase By Pressure Increment
At this step, we must wait to gas to migrate up until the surface casing pressure increase by Pressure Increment. When this step finishes, the bottom hole pressure will increase by the amount of Pressure Increment therefore the bottom hole pressure will be over balance by the amount of Safety Factor plug Pressure Increment.
Step 5 – Repeat Step 3 and Step 4 Until The Gas Migrates To Surface
The rest of volumetric well control is to repeat step#3 and step#4 until the gas finally migrates all the way to surface. During each step of bleeding off, the gas bubble expands and its pressure decreases. By the time, the gas reach at surface, the gas pressure will greatly reduce and its volume increases according to Boyles’ Laws.
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