This example demonstrates the calculations and the steps of the stripping with volumetric control so it will help you understand about what calculations required are and how to perform the stripping with volumetric control.
Gas kick at the bottom but the drillstring is out of bottom. The kick is introduced while pulling out of hole and the following information below is the well information.
- Pit gain = 30 bbl
- Shut in Drill Pipe Pressure = 400 psi
- Shut in Casing Pressure = 400 psi
- Current mud weight = 11.0 ppg
- Casing shoe depth = 6,000’MD/6,000’TVD
- Hole TD = 9,000’MD/9,000’TVD
- Hole size = 12.25”
- Casing ID = 12.5”
- Drill pipe size = 5”, 19 ppf
- BHA consists of 6.5” drill collar
- Length of BHA = 800 ft
- Average pipe per stand = 94 ft
- Rate of increase in casing pressure = 150 psi in 1 hour
- Assume gas density = 2 ppg ( 0.104 psi/ft)
Figure 1 – Well Diagram
The decision is made to strip to the bottom. Safety Factor and Pressure Increment are 100 psi.
Assumption: Gas kick at the bottom
Kick height = kick volume ÷ hole capacity
Hole capacity = 12.252 ÷ 1029.4 = 0.1458 bbl/ft
Kick height = 30 ÷ 0.1458 = 206 ft
The top of gas kick is at 8,974 ft (9,000 – 206) before it migrates.
Figure 2 – Top of Kick
Gas Migration Rate
Gas migration rate, fph = (Rate of increase in casing pressure, psi/hr) ÷ (0.052 x MW)
Gas migration rate, fph= 150 ÷ (0.052 x 11.0)
Gas migration rate = 262 fph
Figure 3 – Gas Migration
Mud Increment (MI) is calculated by the following equation
MI = Mud Increment (MI), bbl
PI = Pressure Increment (PI), psi
ACF = annular capacity factor between casing and drillstring, bbl/ft
MW = mud weight, ppg
ACF = (12.52 – 52) ÷ 1029.4 = 0.1275 bbl/ft
Mud Increment (MI) = 22.3 bbl
Metal Displacement per Stand
Metal Displacement (bbl) = (OD2 x Average Length) ÷ 1029.4
Metal Displacement (bbl) = (52 x 94) ÷ 1029.4
Metal Displacement (bbl) = 2.3 bbl
Stripping with Volumetric Control Procedures
- We determine the Safety Factor (SF), Pressure Increment (PI) and Mud Increment (MI) as per the calculated shown in the above section
- Strip into the well until the casing pressure increase is equal to Safety Factor (SF) + Pressure Increment (PI) which is 200 psi. At this stage, the overbalance is 200 psi (100 + 100) and there is no bleeding back volume. From this example, we strip 2 stands to reach this point.
Figure 4 – Strip to get SF + MI
Figure 5 – Strip to get SF + MI Diagram
- Hold casing pressure constant and bleed off fluid volume while stripping in hole. For this step, we need track the actual volume bled back which is equal to volume bled off minus pipe displacement.
Figure 6 – Bleed off volume while stripping
Let’s take a look at Figure 6, the stand#3 bleeds 2 bbl and the metal displacement is 2.3 bbl. Therefore, you will have only 1.7 bbl of mud bled off. The overbalance reduces to 192 psi because 1.7 bbl of mud is out of the wellbore. You need to strip in until the total bled minus total stripped is equal to 22 a target mud increment which is 22.3 bbl.
Figure 7 – Bleed off while stripping until the mud increment is reached.
According to Figure 7, you need to strip a total of 12 stands in order to reach the target MI at 22.3 bbl and this point will have only 100 pis over balance.
Figure 8 – Stripping until the MI is reached.
- Strip in a shut in well until the safety factor is reach. At this example, we strip in to get additional 100 psi so the casing pressure will be 700 psi and the overbalance is 200 psi.
Figure 9 – Stand#13 stripped in a shut in well
Figure 10 – Strip in a shut in well until 100 psi is reached (700 psi surface pressure)
- We will continue step#3 and step#4 until the drillstring reaches the bottom or the desired depth. Then you can carry on the conventional well control methods as driller’s method, wait and weight etc.
Figure 11 – Continue Stripping by repeating step#3 and step#4
Figure 12 – Strip to desired depth
Reference books: Well Control Books