drilling engineering notes

8/12/2019 drilling engineering notes

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Kick ToleranceBy

Kanad Kulkarni

29/10/2013


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Gas Behavior in Well

Gas is a highly compressible fluid. Its volumedepends on both pressure and temperature. Tounderstand the behaviour of gas as it is

circulated out of the hole during a well killoperation, we need to use the ideal gas law:

PV/ T = constant


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Consider two cases

Same well size & same amount of Gas injected.

1.Well is open at the top

2. Well is closed at the topWhere do you think you can calculate the gasVolume & what effect will that have on your Well

Pressure?


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The Well is open There is a scope for the gas to expand

The pressure is known at the surface

Reduction in Pressure will produce increase inthe gas volume

Due to Known pressure it will be detectable Effect on the Pore Preesure & fracturegradient???


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The Well is closed Effect on the same amount of gas injected?

Effect on Pressure?

Effect on volume on the gas Effect on the Pore Pressure Fracture gradient

& Well Integrity???


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How to overcome Kick???

Take it in the well Control the pressure using surface choke &

valves

Allow gas expansion in the well to allow wellbore pressure reduction

Keep expanded volume at the surface withinthe manageable levels


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Definition of Kick Tolerance

For practical purposes, kick tolerance may bedefined as the maximum kick size which can betolerated without fracturing the previous casing

shoe. Kick tolerance may also be defined in theterms of the maximum allowable pore pressureat next TD or maximum allowable mud weightwhich can be tolerated without fracturing the

previous casing shoe


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Maximum Kick Tolerance hence depends on thecompany and its policy.

Exploration wells have larger tolerance as compared tothe production wells.

Kick tolerance depends on maximum kick size, maximum formation pressure at next TD maximum mud weight which can be tolerated without

fracturing the weakest point in the open hole,

previous casing shoe. density of the invading fluid circulating temperatures.


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Typical Kick Volume Table


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Kick Tolerance Elements

1. Pore pressure from next TD

2. Maximum mud weight to be used

3. Fracture Gradient at current casing shoe4. Design influx volume that can be safelycirculated out

5. Type of well: exploration or development


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When Should we calculate the tick

tolerance After a leak-off test and prior to drilling ahead, At intervals through the hole section to be drilled at the

expected mud weight.

If a factor such as mud weight or drillstring geometry ischanged, then the kick tolerance must be recalculated. When drilling into areas of overpressure with rapid pore

pressure increase,

Increasing mud weight to compensate, the kicktolerance (limited by formation strength at the previouscasing shoe) will be rapidly reduced.


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How to Calculate the Kick 1. kick volume which can be circulated out without

fracturing the previous casing shoe.

2. Additional mud weight over current mud weight. 3. Drilling Kick Tolerance: This is the maximum pore

pressure which can be tolerated without the need toexceed the maximum allowable mud weight.


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Pressure at the shoe after drillers rotation can becalculated by

Px = Pf - Pg - (TD - H - CSD) xpm

WherePf= Formation Pressure next to TD

Pg= Pressure in gas Bubble= HXG

H = height of gas bubble at casing shoe, ft

G = gradient of gas = 0.05 to 0.15 psi/ft

TD = next hole total depth, ft

CSD = casing setting depth, ftpm = maximum mud weight for next hole section, ppg


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Rearranging in terms of H & replacing Px with FG (fractureGradient)

In vertical and near-vertical holes the FBG is invariably greater thanthe FG. In highly inclined holes the FBG is usually smaller than theFG. For kick tolerance calculations, it is recommended to reduce thevalue recorded during leak-off tests in vertical wells by 100 psi and

to use the resulting value as an approximate value of FG.


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The volume of influx at the casing shoe isV1= H x Ca bbl

where

Ca = capacity between pipe and hole, bbl/ft

At bottom hole conditions the volume of influx (V2) is given by:

P2 V2 = P1 V1

(The effects of T and Z are ignored for the moment)

V2=P1V1/P2

whereP1 =fracture pressure at shoe, psi

P2 =Pf, psi

The value of V2 is the circulation kick tolerance in bbls.


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The maximum allowable drillpipe shut-inpressure (DPSIP) is given by:

DPSIP = (FG - m) x CSD x 0.052

And in terms of additional mud weight,

Kick Tolerance= (FG- pm)


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Calculate the kick tolerance for the following well:9 5/8" casing =14,500 ft

Next TD = 17000 ft

FG at 9 5/8" shoe = 16 ppg

Temperature gradient = 0.02 F/ftMax. mud weight for next hole =14.5 ppg

Max formation pressure at next hole= 14 ppg

Assume next hole 8 " and there is 5" drillpipe fromsurface to TD


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The previous examples clearly show the influence offracture gradient on kick tolerance. If a well is planned fora given kick tolerance say 50 bbls based on an estimatedfracture gradient of say 15 ppg, and if while drilling thewell the actual fracture gradient was found to be different

from the design value, then two scenarios may beconsidered:

1. If the actual FG is greater than the design value, thenthe open hole section below the casing shoe can be drilled

further than planned if desired. In other words, the well isactually stronger than planned.


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2. If the actual FG is less than the planned, then thereverse of the above is true. The open hole section cannot be drilled to it planned depth. The section may thenbe drilled to a shallower depth with less pore pressure ora cement plug is placed at the shoe to artificially

strengthen the shoe


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In case of Exploration well where the fracture gradient is

calculated constantly it is advisable to change the mud weightaccordingly and also the Kick Tolerance should be calculatedrepeatedly.

Revised calculateions for the above problem


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Kick Tolerance graphIn this figure, the kick volume is plotted on the X-axis (point 2), and the SIDP Shut in Pressure isplotted on the Y-axis. Point 1 is the maximum SIDP

as calculated by Equation.Point 2 is the maximum kick volume as obtainedfrom the same equation for zero initial drill pipeshut-in pressure.

The straight line joining points 1 and 2 is called the:Kick Tolerance graph. If the effects of temperatureand gas compressibility are included then a curve isobtained


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All points to the top and right of the line representinternal blowout and lost circulation conditions.Points below the line represent safe conditions andgive kick tolerance for any combination of kick sizeand drillpipe shut-in pressure.

Note that Kick Tolerance is dependent on values ofmud weight and pore pressure and the curve musttherefore be updated each time these valueschange.


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Learning Outcomes

1. List variable affecting kick tolerance

2. List situations when it is required to calculate kick

tolerance.3. Calculate kick tolerance for any well with andwithout temperature corrections

4. List situations when to increase kick tolerance

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