Kick tolerance worksheet, release 9, issued 19 August 2013. JIT version.

(updated and simplified)

Kick tolerance definition; “A description of the volume and pressure of a gas influx that can be safely circulated out of the well“.

This worksheet calculates the volume of an influx taken from drilling into an overpressured formation that can be circulated out of the well without exceeding the assumed or tested strengths of the formations exposed just below the casing shoe. It does not account for gas migration or dispersion in the mud column and it also assumes that no heavy mud exits the bit before the top of the influx reaches the casing shoe. These calculations should not be used for diverter drilling, slimhole drilling (<6″), HPHT conditions or with oil muds where BHP exceeds 6000 psi. The default influx gradient is 1131 Pa/m or 0.05 psi/ft, which is a reasonable gas gradient to use at the pressures encountered at normal well depths.

A swabbed kick may also be calculated for but in this case, please review the JIT version first of all and note the common error that can result in a large overestimate of the kick volume that can be handled. Understanding this, feel free to use the work version afterwards for swabbed kicks.

All inputs can be made in metric or oilfield units. Outputs are given in both unit sets.

User input

Casing shoe TVD. L_Vshoe =

Equivalent mud density for the casing shoe strength, estimated or FIT. ρemw =

Optional; safety margin for the casing shoe (reduces the maximum shoe pressure). Pshoe_s =

Drilling fluid density anticipated or in use for the section at casing point. ρmud =

Estimated influx gradient. ρinf =

Planned hole section TVD. L_Vtd =

Planned hole section MD. L_Mtd =

Enter the required kick intensity (kick formation pressure gradient – mud gradient). 0 for swabbed influx. ρΔfm_mud =

Enter the temperature at the casing shoe. Tfmg =

Enter the geothermal gradient below the shoe (zero if not accounting for temperature). ZTm =

Enter the hole diameter. L_dhole =

Enter the drillpipe OD. L_Ddp =

Enter the inclination at the casing shoe. ΘΔ_Vcs =

Click here when any values are modified to update the result.

Calculations

Calculate the shoe strength

Calculate the current MAASP

Calculate the bottom hole pressure after closing the BOP. This should not be less than mud hydrostatic, if it is a negative kick intensity was used.

Bottom hole pressure

Mud hydrostatic pressure

Methodology

Think of your MAASP as a budget for kick tolerance. You have £500 to spend on food and beer. The more you spend on food, the less you can spend on beer. While you can spend less than you have, you cannot spend more as Nature doesn’t allow you to borrow. Food = kick intensity, beer = loss of hydrostatic due to gas replacing mud. The higher the kick intensity, the less gas you can circulate out of the well without breaking the shoe.

You can also spend a bit on chocolate (choke margin) but this reduces how much you can spend on food and beer combined. Personally I don’t apply a choke margin because there is a hidden safety factor by virtue of the fact that while we assume a single bubble, it is a strung out column of gassified mud which means that the average expansion of the gas is less than if it was a single bubble. The kick tolerance calculations are very sensitive to this, you’re effectively reducing your MAASP by the choke margin applied and the volume you can safely circulate out becomes less.

MAASP =   

If a choke margin is applied,subtract this from the MAASP to give the Total Kick Tolerance Budget (combination of kick intensity and gas volume).

From that Total Kick Tolerance Budget, subtract the extra pressure imposed on the casing shoe as a result of the kick intensity to give the Gas Budget. This is the rise in well pressure when the well is closed in and equals kick intensity x true vertical depth. (It will equal Pdp after closing in the well.)

Gas Budget = Total Kick Tolerance Budget – Pdp 

The Gas Budget represents the loss in hydrostatic pressure by gas replacing mud. 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). From this we can determine the gas volume at the critical point; with the top of the gas bubble at the casing shoe.

The gas volume must be calculated from the length along hole, accounting for hole inclination.

The volume of gas can now be calculated from the annular capacity between hole and drillstring.

The combined gas law is now used to calculate the volume of influx at the bottom of the well. First, calculate the absolute shoe and bottom hole temperatures and the closed in bottom hole pressure.

Initial shut in influx volume, but test to see if influx height is greater than open hole height, if it is then the influx volume is unlimited.

Results

Kick situation assumed; 

Current Maximum Allowable Annular Surface Pressure

Leakoff shoe pressure

Bottom hole pressure after closing in the well

Maximum influx vertical length

Maximum influx volume (gas on bottom) after closing the BOP’s

Recommended Baryte reserve

NOTE – a negative value for the maximum influx volume shows that any kick taken would exceed the maximum shoe pressure.

For the data entered, the 

Worksheet references

API RP59. Recommended Practices for Well Control Operations.

Drillers.com Incorporated. Practical Well Planning and Drilling Manual.

Horning, MR. Kick prevention, detection and control; planning and training guidelines for drilling deep, high pressure wells. IADC/SPE 19990.

Sedco Forex Schlumberger. Well Control Manual. Edition of March 1991.

Version 1 of this worksheet was released on 12 February 2008.

Version 6 was released after correcting the conversion factor for kPa/m.

Version 6a was changed for the JIT version, to display intermediate results in both metric and imperial units.

Version 6b was released after revising the variable names in line with the new DDC convention.

Version 6e was released after adding a specific warning if a negative kick intensity is used.

Version 6f was released after adding some more information in the Results.

Version 7 was released after correcting a small error, to account for any safety margin pressure in calculating the influx volume on bottom.

Version 8 was released after realising that there was a much easier way to calculate kick tolerance!

Version 9 corrected an error when calculating for hole inclination at the casing shoe.