# slp - wellborestability

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Basic understanding of drilling terms and procedures Stuck Pipe Self Learning Package.

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• Wellbore Stability Self Learning Package Sugar Land Learning Center

raltman@slb.com1

SUGAR LANDLEARNING CENTER

Wellbore StabilitySELF-LEARNING COURSE

USEFUL PRE-REQUISITES

Basic understanding of drilling terms and proceduresStuck Pipe Self Learning Package

• Wellbore Stability Self Learning Package Sugar Land Learning Center

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Table of Contents

OBJECTIVES.. 3

THE STRESS IN THE EARTH BEFORE WE DRILL A BOREHOLE....4THE STRESS IN THE EARTH AFTER WE DRILL A BOREHOLE.......8ROCK FAILURE........10REVIEW QUESTIONS I....14WELLBORE STABILITY PLANNING AND PREVENTATION...15REVIEW QUESTIONS II...29ANSWERS TO REVIEW QUESTIONS ...30

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Objectives

Upon completion of this training module you should be able to:

Describe the stresses in the earth before we drill a borehole Describe the stresses in the earth after we drill a borehole Describe the different types of rock failure Describe the characteristics of a mini-frac Describe the 2 main outputs of wellbore stability planning Understand the differences between Tabular, Angular and Splintered Cavings Describe the common wellbore monitoring techniques and the 4 most common wellbore

instability mechanisms Describe remedial actions that are taken to fix a failed / failing wellbore

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The Stress in the Earth before we Drill a Borehole

Before we drill a borehole the rock in the earth is in a state of equilibrium. This state is called theInitial State.

In the earth, there are 3 stresses that are perpendicular to each other:

v Principal Stress in vertical axish Principal Stress in horizontal axisH Principal Stress in horizontal axis

H is the maximum of the 2 horizontal stresses and h is the minimum.(ie H > h )

In Rock Mechanics we also describe earth stresses in order of magnitude:

1 Maximum Earth Stress2 Intermediate Earth Stress3 Minimum Earth Stress

These can be ordered in any way: for example 1 could be the vertical stress or one of thehorizontal stresses, depending on the sedimentary basin in which we are drilling.

hH

V

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Slip Fault Regime

H = 1

v = 2

h = 3

Thrust (Reverse) FaultRegime

Gentle sloping

H = 1

h = 2

v = 3

Normal FaultRegime

Steep sloping

v = 1

h = 3H = 2

Figure 1: Tectonic dependence on earth stresses

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The earths stresses are related to a number of different variables including:

Tectonic Setting, Pore pressure, Depth, Lithology, Temperature, Structure

The tectonic setting can affect the relationship of the earths stresses. Consider figure 1.1

a) In a Normal Fault Regime, the vertical stress (v) is the maximum principal stress (1): v > H > h

b) In a tectonically stressed regime, horizontal stress (H) is the maximum principal stress (1):H > h > v

c) Slip fault regime, the horizontal stress (H) is the maximum principal stress (1):H > v > h

Pore Pressure supports a portion of the total applied stress in a rock.

In general:

Total stress (in given direction) = Effective Stress of Rock Grains (given direction) + Pore Pressure

If a formation is normally pressured the pore pressure mechanism can be described asfollowing:

Sediment burial full pore fluid escape porosity decreases effective rock stress increases pore pressures are hydrostatic (normal)

If a formation is over-pressured the pressure in the formation is greater than the pressureexerted by a column of water at that same depth.

There are 2 main mechanisms causing overpressure:

a) Loading mechanisms:Sediment burial pore fluid escape fully restricted porosity & effective stress are bothconstant pore pressures increases at the same rate as the overburden (ie overpressure)

b) Unloading mechanisms7:(i) Aquathermal expansion or hydrocarbon generation or mineral dehydration (smectiteillite) or osmosis sealed formation fluid-volume increase can result in rapid pore pressure increases that unload the rock grain matrix.

(ii) Uplift / Erosion unloading rock grain matrix sealed formation formation has same pore pressure as before but due to closed system is abnormally pressured compared with neighbor formations at same depth.

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Figure 2: The 3 Wellbore Stresses

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The Stress in the Earth after we Drill a Borehole

Before a wellbore is drilled the rock is in a state of equilibrium. This state is called the InitialState.

The stresses in the earth under this condition are known as the Far Field Stresses (h , H , v )or in-situ stresses.

When a well is drilled it introduces a perturbation in the initial stress field. The perturbationcauses a new set of stresses known as wellbore stresses that act on the formation at thewellbore wall.

There are 3 wellbore stresses. These are:

Radial Stress Tangential Stress Axial Stress

Figure 2 shows these 3 wellbore stresses.

The wellbore stresses depend on 2 different things:

a) The mud weight usedb) The magnitude of the far field stresses (v , H and h)

If we know what these wellbore stresses are then we will have a better idea of whether aborehole will fail when we drill it.

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Figure 3: The 2 different ways a rock can fail

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Rock Failure

Generally a rock can fail in 2 different ways:

a) Shear Failure:

This is caused by 2 perpendicular stresses that are different in magnitude.

b) Tensile Failure:

This is caused by one stress exceeding the tensile strength of the rock.

Figure 3 shows schematically a shear failure and a tensile failure.

Both of these failures can cause wellbore instability.

When a rock fails by either shear or tensile failure, 2 things can happen depending on the type ofshear/tensile failure:

a) Loss circulation can occur (due to mud losses in the cracks of the rock)b) Stuck pipe can occur (pack off due to the borehole collapsing)

We need to prevent these failures from occurring (if we can) to minimize the amount of NonProductive Time (NPT)

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Leak off Pressure

Formation Breakdown Pressure pbdpw

Time

Pumping stops

Fracture Closure Pressure = h

Tensile Strength To

Fracture opening pressure

Figure 4: A Mini-Frac Test

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Figure 4 shows an example of a mini-frac. The y-axis shows the wellbore pressure (ie the mudweight).

The formation is basically broken down and the pressure trace is examined from this we candetermine certain properties of the rock and this will give us geomechanical information that willultimately help us manage wellbore stability.

It can be seen that there is a linear trend (the elastic region) until The Leak Off Pressure.At this point (the Leak off Pressure) the plot deviates from the straight line; the formation grainsstart to move apart and take mud. The formation is on the threshold of moving from an elasticstate to a plastic state.

The Formation Breakdown Pressure pbd represents the maximum strength of the rock before itbreaks.

This will be equivalent to the pressure exerted by the mud in the borehole. The tensile strengthTo of the rock is the corresponding Tangential Stress at this mud weight. (For simplicity of thisSLP we will neglect Axial and Radial Stress).

Therefore, the condition for tensile failure is when the tangential stress is equal to the tensilestrength of the rock.

Figure 5 shows some examples of borehole failure from RAB images.swbo, ssko and shae are examples of shear failurestver is an example of a tensile failure (a vertical fracture in this case)

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Figure 5: Borehole Failure in RAB images

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Review Questions I

1)What is the relationship between the earth stresses while drilling in a tectonically active region?

2) What are the 2 main mechanisms that cause a formation to be overpressured ?

3) What are Wellbore Stresses and what do they depend on ?

4) Describe the 2 ways that a rock can fail

5) What is the difference between the Leak off Pressure and Formation Breakdown Pressure ?

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Wellbore Stability Planning and Preventation

Wellbore instability / Rock Failure is undesirable because it can lead to Non Productive Time(NPT) such as:

Pack offs (formation failure leading to excess of cuttings) Excessive trip and reaming time Mud losses Stuck Pipe and BHAs Loss of equipment / Fishing / Si