There is no oil industry guidance on how to calculate kick tolerance volumes or what kick tolerance actually is and why it is important to well design.
Here are some of the “definitions” that can be found on the web which are wrong adds to the confusion over what Kick Tolerance is.
1. Kick Tolerance is the maximum increase in mud weight with zero influx without breaking down the formation at the Casing Shoe. Why this definition is wrong: Kick tolerance is a calculation of a volume not a mud weight.
2. Kick Tolerance is the maximum formation pressure that a kick of a specific volume can be taken and circulated out without breaking down the formation at the Casing Shoe. Why this definition is wrong: Kick tolerance is a calculation of a volume not a maximum formation pressure.
3. Petropedia says “Kick Tolerance is the highest volume of a gas kick in barrels which can help in successfully shutting the well without causing any formation damage.” Why this definition is wrong: Kick Tolerance is solely related to well barrier integrity at the casing shoe and nothing to do about formation damage.
The UKTC uses the following simplified definition as the guiding principle for the Kick Tolerance Volume Calculations: –
“Kick Tolerance is defined as the maximum volume of gas kick influx that can be safely taken into the well bore and circulated out of the well without breaking down the cement/formation at the casing shoe, is a measurement of casing shoe well barrier integrity and is used in well design to plan the position of the casing shoes.”
This position is supported by the Norwegian Petroleum Directorate who confirm in their Norsok Standard D-010 Rev 4 June 2013 section 3.1.25 that “Kick Tolerance – maximum influx volume that can be circulated out of well without breaking down the weakest zone in well.”
The UKTC calculates 1. the maximum volume of gas that can be taken at initial conditions without breaking down the formation at the casing shoe and 2. the maximum volume of gas at initial conditions that can be circulated to the casing shoe without breaking down the formation at the casing shoe.
The calculations are dependent on the mud weight in use, the formation strength at the casing shoe, the volume and type of influx, the kick pressure, the depth of hole and the wellbore / drill string geometry / lengths.
Some companies also calculate the kick tolerance for any weak zones in the open hole to understand the underground blowout risk should a kick be taken. Weak zone kick tolerance calculations should not be confused with calculating kick tolerance volumes for casing design.
The philosophy used to develop the UKTC well control risk based method of calculating kick tolerance is given below: –
The UKTC philosophy
Kick tolerance is a safety critical measurement of well barrier design at the previous casing shoe based on the pressure integrity test performed just after drilling out shoe.
Kick tolerance calculations are used in well design to plan the position of the casing shoes.
The kick tolerance volume is the maximum volume of gas influx in a single bubble (worst case) that can be taken and circulated out of the well without exceeding the maximum pressure that the formations and cement at the casing shoe were previously tested too during the pressure integrity test.
The well barrier integrity calculations should only be performed using the single bubble model as it is worst case and ensures that well barrier integrity calculations are a recognised industry standard.
The industry standard measure of well barrier integrity at the casing shoe is that the kick tolerance volume should be greater than 25 bbls for a gas influx in a single bubble (worst case).
The two conditions that need to be checked are 1. at initial conditions when the gas influx was taken (Vi) and 2. when the top of the gas is at the casing shoe (Vs). The KT Vs volume should be taken back to section TD using Boyles law (PsVs = PtdVtd) to calculate the maximum gas influx volume Vtd at initial conditions that the shoe can take. The kick tolerance volume for the section is the lowest value of either Vi or Vtd.
The planned mud weight in kick tolerance calculations should provide a minimum 200 psi overbalance, for conventional drilling operations, over the formation pressures in the planned section.
The “swabbed” kick tolerance volume is the maximum volume of gas that can be circulated out where the influx pressure at initial conditions equals the planned mud weight pressure.
The overbalance pressure for the planned mud weight should be calculated to determine the actual risk of taking a “swabbed” kick. The overbalance pressure can then be used to determine if it is acceptable to drill the section with kick tolerance volumes less than 25 bbls.
The “drilled” kick tolerance volume is the maximum volume of gas that can be circulated out where the influx pressure at initial conditions is greater than the planned mud weight pressure. The “drilled” kick influx pressure should be based on the risk of drilling into a high-pressure formation just below the planned section TD or by applying a kick intensity factor (EQMW) to the planned mud weight.
Both the “swabbed” and “drilled” kick scenarios should be considered when calculating kick tolerance volumes.
Kick tolerance calculations should not be applied to weak zones in the hole section unless the risk of an underground blowout into the weak zone could compromise the well barrier integrity.
Temperatures should not be used in the KT calculations as it is the circulating temperatures during the well kill operation that would be required, and these are generally not known when performing kick tolerance calculations. Note: If static temperatures are used in KT calculations, then this results in invalid and unrealistically high kick tolerance volumes.
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