In some special well control cases, you will not be able to circulate kick out of the well then the kick is brought up to the surface using special well control procedure like “Volume Metric Method”. At this point, surface pressure is the height because of decreased hydrostatic pressure in the well bore.
How can we remove the gas out of the well bore without allowing more influx coming into the well bore for this scenario?
This is the time that we must perform a special well control procedure called “Lubricate and Bleed”. Lubricate and bleed procedure is the way to remove the gas when the circulation is impossible to conduct. The basic theory is the same as Volumetric Well Control Method but it is just a reverse process. Surface pressure will be replaced with hydrostatic pressure by pumping drilling fluid into the wellbore. The gas and drilling mud are allowed to swap the places and amount of surface pressure will be bled off later.
If you use the current mud weight to perform the lubricate and bleed procedure, the well will not be killed and there is remaining surface casing pressure. Only surface casing pressure will be decreased to where it balances to formation pressure. In many cases, it is sometimes desirable to pump heavier mud in to the wellbore and hopefully it will kill the well too.
You will wonder why I use the phase “hopefully kill the well”. The reason is you may not have enough hydrostatic height to create extra hydrostatic head to just balance the formation pressure. This is based on case by case.
The lubricate and bleed procedure is listed in the following steps:
Step 1 – Determine hydrostatic pressure
Determine hydrostatic pressure of 1 bbl (I use the oil field unit) of mud that will be pumped into the well.
Step 2 – Lubricate
Slowly pump a desired volume into the well. The amount of volume depends on well conditions and it may change during the process. Increasing in surface pressure can be estimated by utilizing Boyle’s Laws (P1V1 = P1V2) and every one bbl of mud pumped into the well, the gas size is reduced by one bbl.
During lubricating, surface casing pressure will be definitely increase. The amount of pressure increase will depend on the volume of gas being compressed. Small pressure increase indicates large volume of gas. Additionally, Maximum Allowable Surface Casing Pressure (MAASCP) will reduce because the increase in hydrostatic pressure during lubrication. Since gas volume also decreases every time that gas is bled off, you may reach the point to stop lubricating operation in order to prevent breaking out the wellbore. At this point you will have gas in the wellbore but the lubricate and bleed procedure cannot be performed any more. In order to know this figure, you may need to play with the kill sheet to find this stopping point. By adjusting parameters in the kill sheet, you can minimize this issue.
Step 3 – Wait
Wait for awhile to allow gas and mud swapping out. Drilling mud properties as mud weight and rheology affects on this step. You need to be patient.
Step 4 – Bleed off pressure
Bleeding gas from the surface until the amount of pressure is equal to hydrostatic pressure of mud pumped in hole. If you know that you lubricate in 50 psi, only 50 psi of gas must be bled off. It is very important to bleed only gas. During this process if you see mud on surface, you must stop and allow gas to swap out. For instant, you plan to bleed a total of 50 psi but you observe mud coming out when you bleed only 30 psi, you stop the bleeding process and shut the well in. Then, you continue bleeding the remaining 20 psi later.
If the mud is accidentally allowed to come out during this bleeding process, the bottom hole pressure will reduce resulting in more influx coming into the wellbore.
Step 5 – Repeat step 2 to 4
Repeat step 2 – 4 until you get the gas out of the well or the desired surface casing pressure is reached. As you know, you may not be able to kill the well with this method because total hydrostatic head is not sufficient to balance the wellbore.
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