This section will discuss some practical stand points of the stripping operation with and without volumetric control. Additionally, this will summarize some key points for both methods of stripping for off bottom well control.
General Practical Considerations
Stripping operation requires accurate measurement of fluid bled off therefore it is very critical to have a small trip tank or a stripping tank for the operation. Furthermore, when the drillstring is stripped deeper, you might need to fill the pipe. You need to ensure that the volume filled up will not create any confusion with the bleed of volume. There are some cases when personnel don’t track the volume properly and finally the kick is unintentionally introduced into the wellbore.
Once you identify that you need to strip to the bottom, the stripping operation should be conducted as soon as possible. Gas kick at the deeper depth of the well will have a little expansion therefore you can minimize the expansion effect. In some cases, the drillstring can be successfully stripped back to the bottom before the first mud increment is reached. This will minimize the complication caused by the gas expansion.
Stripping Without Volumetric Control
Stripping to bottom using Non Volumetric Control is applicable for non-migrating influx such as oil and water kick and it has less complexity than the stripping with volumetric control. Since the kick does not migrate, there will be no increase in surface pressure and the concept of stripping is to control bottom hole pressure while a drillstring is stripped into a shut-in well. There are two methods that you can use as follows;
- Volume accounting method – the concept of this method is to bleed of the fluid at the same volume as the drillstring displacement stripped into the well.
- Constant casing pressure method – the concept of the method is to maintain casing pressure constant during the stripping operation but it requires pressure compensation when the drillstring penetrate the influx. Therefore, it is recommended that you add the safety factor that must be more than the pressure increase due to the kick penetration in order to control the bottom hole pressure. If the safety factor is added into the system properly at the beginning of the operation, the well will still be in an overbalanced condition after the string penetrates into the kick.
Stripping With Volumetric Control
Stripping with volumetric control is more complex than the first method because it deals with gas migration. What’s more, there are several small things that you need to consider such as changing in mud increment when the string penetrates the gas bubble, time to penetrate kick, etc. There are also several unknown values associated with the calculation so it makes the operation quite difficult.
In order to be more practical with this stripping method, you need to get rid of the time to penetrate to the bubble and there are two practical ways to do.
Safety Factor – The first way is to add the pressure increase due to kick penetration into the safety factor at the early step of the operation. The safety factor will prevent the additional kick when the gas kick in penetrated. You can estimate the required safety factor here –
Conservative mud increment – Mud Increment (MI) (MI) is calculated by the following equation
MI = Mud Increment (MI), bbl
PI = Pressure Increment (PI), psi
ACF = annular capacity factor, bbl/ft
Before you penetrate the kick (the influx is below the bit), ACF is calculated based on hole capacity but once the drillstring penetrates the kick, the ACF is calculate based on the capacity between around the drillstring.
In order to make the MI more practical, it is recommended to determine the MI based on the ACF around the drillstring and use this one figure for entire operation. The MI will be less than the one that is calculated when the gas kick is below the bit therefore the wellbore will be slightly overbalanced.
Reference books: Well Control Books