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Kick tolerance calculations for well design and drilling operations

July 5, 2011 - Correctly calculating kick tolerance is essential to safe well design and drilling. Interestingly, neither the

API publications nor the IADC Drilling Manual seem to provide a method.

The length of hole that can be planned between each casing shoe is determined by Kick Tolerance, among other

factors. While drilling, the actual kick tolerance should be recalculated after a leakoff test or whenever the mud density

changes to ensure that sufficient strength remains in the well to handle a gas influx.

Although there are spreadsheets and programs to calculate kick tolerance, some allow unrealistic inputs to be made

without warning the user. This can lead to over-optimistic results. Moreover, some may have bugs, but can’t be

audited by the user. Luckily, kick tolerance can be easily calculated by hand to verify program results, or if the driller

only has a calculator handy.

There are two values necessary to define a kick tolerance:

Kick Intensity: This is the amount of overpressure that is penetrated when the well flows. It is normally expressed in

the same units as the mud density. If the mud density is 10 lbs/gal and the kick intensity is 0.5 lbs./gal, then the

equivalent pore pressure of the kicking formation is the addition of these two values; 10.5 lb/gal in this example. In the

case of a swabbed kick, the kick intensity is zero. It can never be negative as the BHP cannot be less than mud

hydrostatic when circulating an influx out.

Influx Volume: This is the quantity of gas to enter the well from the kicking formation. Gas is normally assumed due to

the expansion effect; it’s the ‘worst case.’

A swabbed kick scenario (zero kick intensity) should only be assumed for a development well in a known area, where

an overpressured zone is definitely known not to exist. It is very dangerous to assume in an exploration well that only a

swabbed kick can possibly be taken. This practice is becoming common.

The required influx volume that the well can circulate out, at the required kick intensity, should be a realistic amount

that the drill crew can detect and close in the well on.

A useful guide to the required influx volume is 300 feet of open hole capacity. A larger hole will allow in a larger influx

volume, all being equal, than a smaller hole as more hole is exposed to the kicking formation. A bigger hole can also

handle a higher volume as the annular capacity is higher.

First, the strength of the rock at the top of the hole section (under the casing shoe) is assumed (at the well planning

stage) or measured (Formation Integrity Test while drilling). The well is assumed to kick at the depth of interest,

usually the bottom of that hole section, with the given kick intensity. The calculation then leads to the volume of gas

that can be circulated out of the well using the Drillers method without exceeding the strength of the rock at the top of

the hole section. This volume must meet or exceed that required by company policy or legal regulation.

The strength of the formation at the top of the hole section (the ‘shoe strength’ of the previous casing) exceeds the

hydrostatic pressure of the drilling fluid at the shoe depth by an amount which is called the Maximum Allowable

Annular Surface Pressure (MAASP). If the BOP is closed and mud is pumped in to the well, damage to the rock below

the shoe would be expected if MAASP is exceeded.

Think of your MAASP as a budget for kick tolerance. You have $50 to spend on food and drinks. The more you spend

on food, the less you can spend on drinks. While you can spend less than you have, you cannot spend more as

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nature doesn’t allow you to borrow. Food = kick intensity, drinks = loss of hydrostatic due to gas replacing mud. You

can also spend some of this budget on chocolate (choke safety margin) but if you do, you might regret it afterwards.

Example calculation

The following example calculates the volume of a bubble of gas at the shoe. Maximum shoe pressure occurs when the

top gas reaches the shoe. A single gas bubble is assumed.

Using easy figures:

Shoe at 5000 feet; shoe strength gradient 0.6 psi/ft.

Hole drilled to 8000 feet; assume a kick intensity of 0.05 psi/ft and a gas density gradient of 0.05 psi/ft.

Mud in hole has a density gradient of 0.5 psi/ft.

Calculations:

MAASP = (0.6 psi/ft shoe strength – 0.5 psi/ft mud gradient) x 5000 ft = 500 psi [total kick tolerance budget].

From the total kick tolerance budget of 500 psi, subtract the extra pressure imposed on the shoe as a result of the kick

intensity at section Total Depth. This gives the gas budget, which can be spent on the length of gas. (Note: if there

was any Choke Safety Pressure, this must also be subtracted to give the gas budget, not used in this example).

Gas budget = 500 psi MAASP – (8000 ft depth x 0.05 psi/ft kick intensity) = 100 psi.

The gas budget represents the loss in hydrostatic pressure caused by gas replacing mud in the well, when the top of

the gas is at the shoe. Assuming a gas gradient of 0.05 psi/ft, the loss of hydrostatic is the length of the gas column,

multiplied by (mud gradient – gas gradient).

Therefore, the length of the gas column is the gas budget divided by (mud gradient – gas gradient).

Length of gas = 100 ÷ (0.5 – 0.05) = 100 ÷ 0.45 = 222 feet.

If we have a 12 1/4-inch hole with 5-inch pipe, the annular capacity is 0.1214 bbl/ft., and therefore, the volume of gas

at the shoe is 222 x 0.1214 = 27 bbls. The Combined Gas Law can now be used to calculate the Influx Volume when it

enters the bottom of the well.

Things to remember:

Higher mud weight = lower MAASP = reduced kick tolerance gas volume for a particular kick intensity. If the

mudweight increase swallows 100 psi of MAASP, then the gas budget goes down by that 100 psi.

Some operators mandate a Choke Safety Pressure be used, so that MAASP is not exceeded even if the choke

operator doesn’t accurately control the choke opening while killing the well. However, the kick tolerance calculation is

very sensitive to this and a 100 psi CSP will significantly reduce the volume of gas that can be handled. It’s also not

necessary because there are built in safety factors in the assumptions made.

Kick tolerance calculations can be done online at www.drillers.com/pages/view/oilfield-tools.

– Steve Devereux CEng is the CEO of Drillers.com and the author of two drilling books.