SLP - WellboreStability

Wellbore Stability

SELF-LEARNING COURSE

USEFUL PRE-REQUISITES

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

Table of Contents

OBJECTIVES……….. 3

THE STRESS IN THE EARTH BEFORE WE DRILL A BOREHOLE………...……….……

4

THE STRESS IN THE EARTH AFTER WE DRILL A BOREHOLE….……….…...……..

8

ROCK FAILURE………..………..……...….

10

REVIEW QUESTIONS I………..……..

14

WELLBORE STABILITY PLANNING AND PREVENTATION……….………..

15

REVIEW QUESTIONS II……….…..

29

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

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 the “Initial State”.

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

σ

v

Principal Stress in vertical axis

σ

h

Principal Stress in horizontal axis

σ

H

Principal Stress in horizontal axis

σ

H is the maximum of the 2 horizontal stresses and

σ

h is the minimum.

(ie

_σ

H

>

σ

h )

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

σσσσ

1 Maximum Earth Stress

σσσσ

2 Intermediate Earth Stress

σσσσ

3 Minimum Earth Stress

σ

σσσσh

σσσσH σσσσV

Slip Fault Regime

σ

v

= σ

2 Thrust (Reverse) Fault

Regime Gentle sloping

σ

H

= σ

1

σ

h

= σ

2

σ

v

= σ

3

σ

h

= σ

3

σ

H

= σ

2

The earth’s 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 earth’s stresses. Consider figure 1.1 a) In a Normal Fault Regime, the vertical stress (

σ

v) is the maximum principal stress (

σ

1):

σ

v

> σ

H

> σ

h

b) In a tectonically stressed regime, horizontal stress (

σ

H) is the maximum principal stress (

σ

1):

σ

H

> σ

h

> σ

v

c) Slip fault regime, the horizontal stress (

σ

H) is the maximum principal stress (

σ

1):

σ

H

> σ

v

> σ

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 as following:

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 pressure exerted by a column of water at that same depth.

There are 2 main mechanisms causing overpressure: a) Loading mechanisms:

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 “Initial State”.

The stresses in the earth under this condition are known as the Far Field Stresses (

σσσσ

h

,

σσσσ

H

,

σσσσ

v )

or in-situ stresses.

When a well is drilled it introduces a perturbation in the initial stress field. The perturbation causes a ‘new’ set of stresses known as wellbore stresses that act on the formation at the wellbore 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 used

b)

The magnitude of the far field stresses (

σ

v

, σ

H

and σ

h)

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

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 of shear/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 Non Productive Time (NPT)

Leak off Pressure

Formation Breakdown Pressure

p

bd

p

w

Time

Pumping stops

Fracture Closure Pressure =

σ

h

Tensile Strength

T

o

Fracture opening pressure

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

The formation is basically broken down and the pressure trace is examined – from this we can determine certain properties of the rock and this will give us geomechanical information that will ultimately 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 grains start to move apart and take mud. The formation is on the threshold of moving from an elastic state to a plastic state.

The Formation Breakdown Pressure

p

bd represents the “maximum strength” of the rock before it breaks.

This will be equivalent to the pressure exerted by the mud in the borehole. The tensile strength To of the rock is the corresponding Tangential Stress at this mud weight. (For simplicity of this

SLP we will neglect Axial and Radial Stress).

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

Figure 5 shows some examples of borehole failure from RAB images. swbo, ssko and shae are examples of shear failures

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

•

Stuck Pipe and BHAs _{→ Loss of equipment / Fishing / Sidetracks}

•

Inability to land casing, casing collapse

•

Poor logging and cementing conditions These can be caused by the following:

breakouts, sloughing,

natural fractures/weak planes, drilling induced fractures, faulting, undergauge hole, interbedded sequence, overpressured formation, unconsolidated formation, mobile formation, permeable formation, chemical activity.

Even relatively minor wellbore stability problems in tectonically passive settings can be extremely expensive ($100,000 to $250,000 per day offshore).

The key to effective reduction of NPT is planning for wellbore stability. One process used to reduce the NPT is the Mechanical Earth Model.

This integrates all geomechanical data available from a field/basin into one “database” which is then used to predict wellbore stability problems that are likely to occur in an upcoming well.

Safe Mud Weight Window

σσσσ

h

Pore

Pressure

snbo

swbo

shae

sdko

slae

ssko

tcyl

tver

Increasing Mud Weight

Mud

Weight

(g/cc)

Increase the mud

weight or increase

the risk of shear

failure

Vertical well

Figure 8: Trajectory Analysis for Anisotropic Stress Field, Relaxed Basin (

σ

v

is max)

Two of the most important outputs that emerge from wellbore stability planning are the

determination of a safe mud weight window and the safest direction to drill, especially for highly deviated wells.

Figure 6 shows that it is often desirable to drill with a mud weight between swbo (a shear failure condition) and

σ

h (the minimum horizontal stress).

Figure 7 shows an example from the North Sea where the safe mud weight window should between the black dashed line (Minimum Borehole Stability or shear failure) and the formation propagation pressure (or the minimum horizontal stress).

Figure 8 shows that in a relaxed basin it is often safer to drill the well in the direction of the minimum horizontal stress

(

σ

h). Also it can be seen that the safe mud weight window narrows as well deviation increases (ie you need to increase the mud weight to keep the wellbore stable but be careful because the maximum mud weight before borehole instability occurs will now be lower).

The open hole section of a wellbore must be maintained in a condition that is good enough to allow drilling and casing to be run. This does not mean that it is necessary to eliminate all formation failure.

Indeed the wellbore can remain stable even after a period of prolonged formation failure.

An example of this is the Cuisiana field, Colombia where the wellbore has remained stable because the cavings from borehole failures can be cleaned out of the hole.

In this example the wellbore instability was managed (or contained) rather than prevented. In these cases it becomes difficult to find a solution that will completely prevent the instability from occurring in the first place and wellbore stability management is required: for example, loss circulation might be avoided at all costs, and techniques to manage the shear failure are implemented such as good hole cleaning practices.

Pit volumes – Gains (overpressured zone), losses, Surface Drilling Parameters

MWD data:

Downhole Drilling Parameters DWOB, DTORQ – Friction / Drag ECD behaviour – Hole Cleaning, pack off LWD data:

Gamma Ray, Resistivity – Identify zones of potential instability from MEM Sonic – Pore pressure prediction while drilling

Caliper measurements – if pattern is forming in some intervals, can identify unstable formations

A reliable diagnosis of the instability mechanism requires use of all available data. If tabular cavings due to natural fracturing are observed then the resistivity log should be checked for evidence of mud invasion into fractures and the mud records require examining for losses.

Similarly, if splintered cavings due to over-pressured formations are seen then high gas levels, kicks or mud gains may also be present.

The observation of angular cavings due to breakouts requires the debris levels in the hole to be discerned. In all cases, the cavings volume should be compared to the ECD and the degrees of tight hole and restricted circulation to discern the effectiveness of the hole cleaning and the severity of instability.

Tabular, Angular, Splintered

Those which cannot be characterized.

Tabular cavings are the result of natural fractures or weak planes.

In the case of natural fractures, the fluid pressure in the annulus exceeds the minimum horizontal stress, resulting in mud invasion of fracture networks surrounding the wellbore.

This can result in severe destabilization of the near wellbore region, due to the movement of blocks of rock, leading rapidly to high cavings rates, lost returns and stuck pipe.

The blocks of rock are bounded by natural fractures planes and, therefore, have flat, parallel, faces.

Figure 9 shows examples of tabular cavings due to natural fractures.

The other characteristic is that bedding, if any, will not be parallel to the faces of the caving. In the case of weak planes, the combination of low mud weight and a borehole axis that is within approximately 15 degrees of the bedding direction can induce massive failure along the planes of weakness, leading to the symptoms described above.

Cavings that are the result of weak planes are characterized by having flat, parallel, faces. The bedding direction is also parallel to the faces.

Angular cavings are a consequence of breakouts. These cavings are characterized by curved

faces with a rough surface structure. The surfaces intersect at acute angles (much less than 90 degrees). Figure 10 shows Angular Cavings.

Splintered cavings have two nearly-parallel faces with plume structures. This type of caving is

due to tensile failure occurring parallel to the borehole wall and commonly occurs in overpressured zones drilled with a small overbalance. Figure 11 shows Splintered Cavings.

Figure 10:

Angular Cavings

Figure 11: Splintered

Cavings

In wellbore stability monitoring, it is important to determine whether a particular drilling problem is mechanical or chemical in origin.

Figure 12 describes how to diagnose the 4 most important wellbore stability mechanisms. 3 of these are mechanical and 1 of these is chemical in origin. The 3 tables that follow show examples of wellbore stability from surface, downhole and miscellaneous signatures.

Mechanism

Lost Time

Wellbore

Trajectory

In-situ

stresses

Formation

Stength

Pore

Pressure

Geology

Permeable

formation

Stuck pipe

Low

compared to mud pressure

Interbedded

soft/strong rocks

Stuck pipe Tortuous

Frequent

changes Thick sections collapse more

Fault slip/

activation

Stuck pipe, excessive reaming High stress deviation Faults present

Sloughing

Hole fill after trips

Weak Proximity to salt dome or faults,

tectonically active

Overpressured

formation

Hole fill after trips High Recently crossed fault

Undergauge hole

Excessive slack

off while tripping

High mean stress Low yield strength

Unconsolidated

formation

Restricted pipe movement

Large sand or fractured section

Mobile formation

Hole fill after trips

High overburden

Proximity to salt dome, evaporate sequence

Chemical activity

Problems worsen with time, slight flow

Fault slip/

activation

Decrease Local borehole elongation Detected. Rotation of breakouts

Sloughing

Decreases Low Borehole

enlargement GAPI > 60 High dip (> 60_°) Borehole enlargement

Overpressured

formation

High, given rock strength Low Borehole enlargement Borehole enlargement

Undergauge hole

Low

Low Diameter less

than gauge

Diameter less than gauge

Unconsolidated

formation

High Decreases

Borehole enlargement GAPI < 60 Borehole enlargement

Mobile formation

Decreases

Chemical activity

Decreases Decreases

Hole tightens with time, or dissolves GAPI > 60 Swelling detected

Breakouts

Decreases Low

Borehole enlargement Orientation & span detected

Drilling induced

fractures

Low GAPI > 60 Diametrically opposed &long Possible detection

Close spaced

fracs/weak planes

Decreases

Low Borehole enlargement GAPI > 60 Fracture & bedding plane orientation Borehole geometry

Mechanism

Pump pressure

Circulation

Mud

Cuttings and

cavings

Hookload

Surface

Torque

Drillstring

Permeable

formation

Gradual decrease Flow

decreases

Water loss, high solids

Increases Higher

Interbedded

soft/strong rocks

Spikes Flow erratic

Volume rate changes frequently

High Erratic Packed off

Fault slip/

activation

Spikes Flow erratic Loss High Increase Diametrical wear

Sloughing

Increase Flow

decreases

Large & flat High when pumps off

High Packed off

Overpressured

formation

Increase

Pit level

increase Backgroundgas high Large, brittle,fissile, concave Large overpullat connections Increase

Undergauge hole

Spikes

Flow erratic Abrasive &

hard

High High &

erratic Undergauge BHA

Unconsolidated

formation

Increase Flow decreases Unconsolidated & uncemented Large overpull at connections Erratic

Mobile formation

Increase Flow decreases Salt present, rise in Cl

Salt grains Large overpull

at connections

Erratic Packed off

Chemical activity

Increase

Flow decreases MW & solids increase Soft,water soluble.Gumbo Large overpull at connections Increases

Induced Fractures Fault Activation 0 0 0 0 0 0 0 0 1 0 1 0 1 Undergauge Hole 1 0 0 0 0 1 1 0 0 0 0 0 1 Interbedded Sequence 1 0 0 1 1 0 1 0 0 0 0 0 1 Overpressured Formation 0 0 0 0 0 0 0 0 0 1 0 0 1 Unconsolidated Formation 0 1 0 1 1 0 1 0 0 0 0 0 1 Mobile Formation 0 1 0 0 1 0 1 0 0 0 0 0 1 Permeable Formation 0 1 0 0 0 0 0 0 1 0 1 0 1 Chemical Activity 0 1 0 0 0 0 0 0 0 0 0 1 0

Figure 16: Actions inhibiting the instability mechanisms. A "1" indicates that the action suppresses the instability.

A "0" indicates that the action has no influence on instability or

makes it worse.

b) Remedial Actions

If wellbore instability becomes severe as detected from a) continuous monitoring, and hole cleaning cannot remove cavings from the wellbore then the wellbore would be unstable. The ability to deal effectively with the consequences of the unstable wellbore depend on the instability mechanism and its severity.

Remedial action generally involves the control of surface parameters (e.g. ROP, RPM, flow rate, mud weight/rheology).

For example, if mud losses are currently occurring, but a mud weight decrease is not possible due to conditions that will be encountered while drilling through formations below the current hole bottom (cavings generation), then decreasing the ROP will reduce cuttings loading and therefore the ECD. This may be sufficient to eliminate mud losses and also reduce cuttings loading in deeper intervals

The emphasis when considering remedial actions, which either suppress instabilities or minimize their consequences, should be the entire open hole interval, rather than focusing on problem fixing at the bit.

The ROP and hole cleaning efficiency form the key links between wellbore instability and operations. Rock debris in the annulus, resulting from drilling and/or wall failure, will increase if hole cleaning is inadequate, raising the risk of pack-offs and stuck pipe. The ability to clean the hole is also related to the ROP.

Figure 16 outlines the various actions that are recommended for various given wellbore stability mechanisms. It can be seen that minimizing swabbing and surging affects helps to suppress more instability mechanisms than any other drilling practice.

Also it can be seen that drilling practices such as wiper trips that are often considered as routine are sometimes detrimental to wellbore stability. Minimising wiper trips can help suppress actions that are sensitive to mechanical agitation of the formation such as mobile formations / sloughing shales, weak planes.

Increasing mud weight is not necessarily the answer to wellbore stability problems. Whilst this practice can help suppress breakouts, it can cause drilling induced fractures or activate natural

9) For the following wellbore instability problems, what drilling practices would you use to surpress or control the problem ?

a) Borehole Breakouts

b) Natural Fractures / Weak Planes c) Unconsolidated formations

Answers to Review Questions

1)

σ

H

> σ

h

> σ

v

2) Loading mechanisms – where the pore fluid cannot escape as quickly as the rock compaction rate, and the pore fluid gets squeezed and pressured because it can’t escape. Unloading mechanisms – where a formation rises to a shallower depth, and the pore fluids cannot escape, then the formation is overpressured compared to surrounding (shallower) formations (because the pore fluids still have the same pressure as before the formation rose). Hydrocarbon generation where the pore fluids are trapped is another example. 3) When we drill a hole in the rock, we replace the rock with a cylinder of mud and a set of

stresses are created in the region of the wellbore wall. These stresses are known as “Wellbore Stresses”.

They depend on the mud weight used, and the far field stresses

σ

H

, σ

h

and σ

v 4) Tensile failure – occurs when the rock grains are held in tension and are pulled apart.

Shear failure – occurs when the rock grains are under a state of compression by 2 stresses that are acting perpendicular to each other and their magnitudes are very different. 5) Leak off Pressure – the wellbore pressure at which the rock begins to yield and the formation grains begin to move apart and take mud.

Formation Breakdown Pressure – the wellbore pressure at which the rock physically breaks down.

6) Often (but not always) between the condition for shear failure and the minimum horizontal stress

σ

h

7) It generally becomes more narrow (ie you have to less of a margin in which to drill safely ) 8) Tabular – from natural fractures (where the cavings will have flat, parallel faces with bedding not parallel to the parallel faces of the caving).

or from weak planes (the same as natural fractures but the bedding is parallel to the faces of the cavings).

Angular – from borehole breakouts (they have curved faces with rough surface structure) Splintered – from overpressured zones (concave flat, thin, planar structures)