Kick tolerance is the maximum gas volume for a given degree of underbalance which the circulation can be performed without exceeding the weakest formation in the wellbore. This article is the extended version of Kick Tolerance Calculation which will explain more on this topic. It is very critical that drilling personnel understand its importance to well design and drilling operation.
There are two important factors used for determining the kick tolerance
• Kick Intensity – It is the different between the maximum anticipated formation pressure and planned mud weight. For example, the planned mud weight is 13.0 ppg and the possible kick pressure is 13.5 ppg. Therefore, the kick intensity is 0.5 ppg (13.5 – 13.0).
A zero kick intensity (swabbed kick scenario) should be used for a know area where you have less uncertainty about an overpressure zone.
• Kick Volume – It is a gas influx entering into the wellbore from the formation. Gas kick is always used for well control calculation because it is the worst case scenario. The kick volume should be realistic figure which personal can detect the influx on the rig. In a larger hole, it allows bigger influx volume than a small hole.
Maximum Allowable Annular Surface Pressure (MAASP) and Kick Tolerance
Weakest formation point in the open hole is assumed to be at the shoe depth of the previous casing. The well bore will be fractured if a summation of hydrostatic and surface pressure exceeds the weakest pressure (Leak Off Test pressure). The maximum surface pressure before breaking the formation is called “Maximum Allowable Shut In Casing Pressure” (MASICP).
Make it simpler for your understanding. MASICP is the total of kick tolerance budget. It consists of pressure from kick intensity and hydrostatic pressure loss due to gas.
Kick Tolerance Example Calculation
Previous casing shoe (9-5/8” casing) at 6,000’ MD/ 6,000’ TVD
Predicted formation pressure at TD (10,000’MD/10,000’TVD) = 14.0 ppg
Pore pressure uncertainty = 1.0 ppg
Planned mud weight = 14.5 ppg (0.754 psi/ft)
Gas gradient = 0.1 psi/ft
LOT = 16.0 ppg
Hole size = 8-1/2”
Drill Pipe = 5”
BHA + Drill Collar = 7”
Length of BHA+Drill Collar = 400 ft
Annular capacity between open hole and BHA = 0.0226 bbl/ft
Annular capacity between open hole and 5” DP = 0.0459 bbl/ft
Calculation Steps
Maximum anticipated pressure = 14.0 + 1 = 15.0 ppg
Maximum Allowable Shut In Casing Pressure (MASICP) = (LOT – MW) x 0.052 x Shoe TVD
Maximum Allowable Shut In Casing Pressure (MASICP) = (16 – 14.5) x 0.052 x 6,000 = 468 psi
Kick Intensity = 15.0 – 14.5 = 0.5 ppg
Underbalance due to kick intensity = 0.5 x 0.052 x 10,000 = 260 psi
As you can see, when the well is in underbalance condition (260 psi), the shoe will not be broken because the MASICP is more than underbalance pressure (468 > 260).
We know that 0.5 ppg kick intensity we will have 208 psi (468 – 260 = 208 psi) before shoe broken.
It means that gas bubble can replace mud in equivalent to 208 psi before fracturing the shoe. With this relationship, we can determine height of gas kick by the following equation.
Height of gas kick = remaining pressure, psi ÷ (mud gradient, psi/ft – gas gradient, psi/ft)
Height of gas kick = 208 ÷ (0.754 – 0.1) = 318 ft.
Determine gas kick volume base on height of gas kick
We need to separate into two cases and compare the smallest volume.
1st case – Gas at the bottom
Volume of gas kick = Annular capacity between open hole and BHA x Height of gas kick
Volume of gas kick (bbl) = 0.0226 bbl/ft x 318 ft = 7.2 bbl
2nd case – Gas right below casing shoe
For this case, we need to convert gas at the shoe to the bottom condition by applying Boyle’s Laws.
Volume of gas kick = Annular capacity between open hole and 5” DP x Height of gas kick
Volume of gas kick (bbl) = 0.0459 bbl/ft x 318 ft = 14.6 bbl
Convert to the bottom hole condition
Volume at the bottom = (volume of gas kick at shoe x Leak off test) ÷ formation pressure
Leak off test = 0.052 x 16 x 6,000 = 4,992 psi
Formation pressure (gas kick condition) = 0.052 x 15 x 10,000 = 7,800 psi
Volume at the bottom = (14.6 x 4992) ÷ 7800 = 9.3 bbl
We can compare the kick volume from two cases like this.
1 st case : kick volume = 3.9 bbl
2nd case : kick volume = 7.2 bbl
The smallest number must be selected to represent maximum kick volume therefore kick volume is 7.2 bbl.
We wish this article could help you get more understanding about Kick Tolerance.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition