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Borehole instability is the undesirable condition of an openhole interval that does not maintain its gauge size and shape and/or its structural integrity. This articles discusses the causes, types, effects, and possible prevention of borehole instability.
Borehole instability is the undesirable condition of an openhole interval that does not maintain its gauge size and shape and/or its structural integrity. This articles discusses the causes, types, effects, and possible prevention of borehole instability.


==Causes==
== Causes ==
 
The causes can be grouped into the following categories:
The causes can be grouped into the following categories:
* Mechanical failure caused by in-situ stresses
* Erosion caused by fluid circulation
* Chemical caused by interaction of borehole fluid with the formation


==Types and associated problems==
*Mechanical failure caused by in-situ stresses
*Erosion caused by fluid circulation
*Chemical caused by interaction of borehole fluid with the formation


There are four different types of borehole instabilities:
== Types and associated problems ==
* Hole closure or narrowing
* Hole enlargement or washouts
* Fracturing
* Collapse


'''Fig. 1''' illustrates hole-instability problems.
There are four different types of borehole instabilities:


<gallery widths=300px heights=200px>
*Hole closure or narrowing
*Hole enlargement or washouts
*Fracturing
*Collapse
 
'''Fig. 1''' illustrates hole-instability problems.
 
<gallery widths="300px" heights="200px">
File:Devol2 1102final Page 442 Image 0001.png|'''Fig. 1—Types of hole instability problems.'''
File:Devol2 1102final Page 442 Image 0001.png|'''Fig. 1—Types of hole instability problems.'''
</gallery>
</gallery>


===Hole closure===  
=== Hole closure ===
 
Hole closure is a narrowing time-dependent process of borehole instability. It sometimes is referred to as creep under the overburden pressure, and it generally occurs in plastic-flowing shale and salt sections. Problems associated with hole closure are:
Hole closure is a narrowing time-dependent process of borehole instability. It sometimes is referred to as creep under the overburden pressure, and it generally occurs in plastic-flowing shale and salt sections. Problems associated with hole closure are:
* Increase in torque and drag
* Increase in potential [[Stuck pipe|pipe sticking]]
* Increase in the difficulty of casings landing


===Hole enlargement===  
*Increase in torque and drag
*Increase in potential [[Stuck_pipe|pipe sticking]]
*Increase in the difficulty of casings landing
 
=== Hole enlargement ===
 
Hole enlargements are commonly called washouts because the hole becomes undesirably larger than intended. Hole enlargements are generally caused by:
Hole enlargements are commonly called washouts because the hole becomes undesirably larger than intended. Hole enlargements are generally caused by:
* Hydraulic erosion
 
* Mechanical abrasion caused by drillstring
*Hydraulic erosion
* Inherently sloughing shale
*Mechanical abrasion caused by drillstring
*Inherently sloughing shale


The problems associated with hole enlargement are:
The problems associated with hole enlargement are:
* Increase in cementing difficulty
* Increase in potential hole deviation
* Increase in hydraulic requirements for effective [[Hole cleaning|hole cleaning]]
* Increase in potential problems during logging operations


===Fracturing===
*Increase in cementing difficulty
Fracturing occurs when the wellbore drilling-fluid pressure exceeds the formation-fracture pressure. The associated problems are [[Lost circulation|lost circulation]] and possible [[Kicks|kick]] occurrence.
*Increase in potential hole deviation
*Increase in hydraulic requirements for effective [[Hole_cleaning|hole cleaning]]
*Increase in potential problems during logging operations
 
=== Fracturing ===
 
Fracturing occurs when the wellbore drilling-fluid pressure exceeds the formation-fracture pressure. The associated problems are [[Lost_circulation|lost circulation]] and possible [[Kicks|kick]] occurrence.
 
=== Collapse ===


===Collapse===
Borehole collapse occurs when the drilling-fluid pressure is too low to maintain the structural integrity of the drilled hole. The associated problems are pipe sticking and possible loss of well.
Borehole collapse occurs when the drilling-fluid pressure is too low to maintain the structural integrity of the drilled hole. The associated problems are pipe sticking and possible loss of well.  
 
== Principles of borehole instability ==


==Principles of borehole instability==
Before drilling, the rock strength at some depth is in equilibrium with the in-situ rock stresses (effective overburden stress, effective horizontal confining stresses). While a hole is being drilled, however, the balance between the rock strength and the in-situ stresses is disturbed. In addition, foreign fluids are introduced, and an interaction process begins between the formation and borehole fluids. The result is a potential hole-instability problem. Although a vast amount of research has resulted in many borehole-stability simulation models, all share the same shortcoming of uncertainty in the input data needed to run the analysis. Such data include:
Before drilling, the rock strength at some depth is in equilibrium with the in-situ rock stresses (effective overburden stress, effective horizontal confining stresses). While a hole is being drilled, however, the balance between the rock strength and the in-situ stresses is disturbed. In addition, foreign fluids are introduced, and an interaction process begins between the formation and borehole fluids. The result is a potential hole-instability problem. Although a vast amount of research has resulted in many borehole-stability simulation models, all share the same shortcoming of uncertainty in the input data needed to run the analysis. Such data include:
* In-situ stresses
* [[Subsurface stress and pore pressure#Pore pressure|Pore pressure]]
* Rock mechanical properties
* Formation and drilling-fluids chemistry


==Mechanical rock-failure mechanisms==
*In-situ stresses
Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the rock. Compressive failure is caused by shear stresses as a result of low mud weight, while tensile failure is caused by normal stresses as a result of excessive mud weight.
*[[Subsurface_stress_and_pore_pressure#Pore_pressure|Pore pressure]]
*Rock mechanical properties
*Formation and drilling-fluids chemistry


The failure criteria that are used to [[Predicting wellbore stability|predict hole-instability]] problems are the maximum-normal-stress criterion for tensile failure and the maximum strain energy of distortion criterion for compressive failure. In the maximum-normal-stress criterion, failure is said to occur when, under the action of combined stresses, one of the acting principal stresses reaches the failure value of the rock tensile strength. In the maximum of energy of distortion criterion, failure is said to occur when, under the action of combined stresses, the energy of distortion reaches the same energy of failure of the rock under pure tension.
== Mechanical rock-failure mechanisms ==


==Shale instability==
Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the rock. Compressive failure is caused by shear stresses as a result of low mud weight, while tensile failure is caused by normal stresses as a result of excessive mud weight.
Shales make up the majority of drilled formations, and cause most wellbore-instability problems, ranging from washout to complete collapse of the hole. Shales are fine-grained sedimentary rocks composed of clay, silt, and, in some cases, fine sand. Shale types range from clay-rich gumbo (relatively weak) to shaly siltstone (highly cemented), and have in common the characteristics of extremely low permeability and a high proportion of clay minerals. More than 75% of drilled formations worldwide are shale formations. The drilling cost attributed to shale-instability problems is reported to be in excess of one-half billion U.S dollars per year. The cause of shale instability is two-fold: mechanical (stress change vs. shale strength environment) and chemical (shale/fluid interaction—capillary pressure, osmotic pressure, pressure diffusion, borehole-fluid invasion into shale).  


===Mechanical instability===
The failure criteria that are used to [[Predicting_wellbore_stability|predict hole-instability]] problems are the maximum-normal-stress criterion for tensile failure and the maximum strain energy of distortion criterion for compressive failure. In the maximum-normal-stress criterion, failure is said to occur when, under the action of combined stresses, one of the acting principal stresses reaches the failure value of the rock tensile strength. In the maximum of energy of distortion criterion, failure is said to occur when, under the action of combined stresses, the energy of distortion reaches the same energy of failure of the rock under pure tension.
As stated previously, mechanical rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. The mud in use with a certain density may not bring the altered stresses to the original state, therefore, shale may become mechanically unstable.  
 
== Shale instability ==
 
Shales make up the majority of drilled formations, and cause most wellbore-instability problems, ranging from washout to complete collapse of the hole. Shales are fine-grained sedimentary rocks composed of clay, silt, and, in some cases, fine sand. Shale types range from clay-rich gumbo (relatively weak) to shaly siltstone (highly cemented), and have in common the characteristics of extremely low permeability and a high proportion of clay minerals. More than 75% of drilled formations worldwide are shale formations. The drilling cost attributed to shale-instability problems is reported to be in excess of one-half billion U.S dollars per year. The cause of shale instability is two-fold: mechanical (stress change vs. shale strength environment) and chemical (shale/fluid interaction—capillary pressure, osmotic pressure, pressure diffusion, borehole-fluid invasion into shale).
 
=== Mechanical instability ===
 
As stated previously, mechanical rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. The mud in use with a certain density may not bring the altered stresses to the original state, therefore, shale may become mechanically unstable.
 
=== Chemical instability ===


===Chemical instability===
Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The mechanisms that contribute to this problem include:
Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The mechanisms that contribute to this problem include:
* Capillary pressure
* Osmotic pressure
* Pressure diffusion in the vicinity of the borehole walls
* Borehole-fluid invasion into the shale when drilling overbalanced


===Capillary pressure===
*Capillary pressure
*Osmotic pressure
*Pressure diffusion in the vicinity of the borehole walls
*Borehole-fluid invasion into the shale when drilling overbalanced
 
=== Capillary pressure ===
 
During drilling, the mud in the borehole contacts the native pore fluid in the shale through the pore-throat interface. This results in the development of capillary pressure, p<sub>cap</sub> , which is expressed as
During drilling, the mud in the borehole contacts the native pore fluid in the shale through the pore-throat interface. This results in the development of capillary pressure, p<sub>cap</sub> , which is expressed as


[[File:Vol2 page 0443 eq 001.png]]....................(1)
[[File:Vol2 page 0443 eq 001.png|RTENOTITLE]]....................(1)


where ''σ'' is the interfacial tension, ''ϴ'' is the contact angle between the two fluids, and ''r'' is the pore-throat radius. To prevent borehole fluids from entering the shale and stabilizing it, an increase in capillary pressure is required, which can be achieved with oil-based or other organic low-polar mud systems.  
where ''σ'' is the interfacial tension, ''ϴ'' is the contact angle between the two fluids, and ''r'' is the pore-throat radius. To prevent borehole fluids from entering the shale and stabilizing it, an increase in capillary pressure is required, which can be achieved with oil-based or other organic low-polar mud systems.
 
=== Osmotic pressure ===


===Osmotic pressure===
When the energy level or activity in shale pore fluid, ''a''<sub>''s''</sub>, is different from the activity in drilling mud, ''a''<sub>''m''</sub> , water movement can occur in either direction across a semipermeable membrane as a result of the development of osmotic pressure, ''p''<sub>''os''</sub> , or chemical potential, ''μ''<sub>''c''</sub> . To prevent or reduce water movement across this semipermeable membrane that has certain efficiency, ''E''<sub>''m''</sub>, the activities need to be equalized or, at least, their differentials minimized. If ''a''<sub>''m''</sub> is lower than ''a''<sub>''s''</sub>, it is suggested to increase ''E''<sub>''m''</sub> and vice versa. The mud activity can be reduced by adding electrolytes that can be brought about through the use of mud systems such as:
When the energy level or activity in shale pore fluid, ''a''<sub>''s''</sub>, is different from the activity in drilling mud, ''a''<sub>''m''</sub> , water movement can occur in either direction across a semipermeable membrane as a result of the development of osmotic pressure, ''p''<sub>''os''</sub> , or chemical potential, ''μ''<sub>''c''</sub> . To prevent or reduce water movement across this semipermeable membrane that has certain efficiency, ''E''<sub>''m''</sub>, the activities need to be equalized or, at least, their differentials minimized. If ''a''<sub>''m''</sub> is lower than ''a''<sub>''s''</sub>, it is suggested to increase ''E''<sub>''m''</sub> and vice versa. The mud activity can be reduced by adding electrolytes that can be brought about through the use of mud systems such as:
* Seawater
* Saturated-salt/polymer
* KCl/NaCl/polymer
* Lime/gypsum


===Pressure diffusion===
*Seawater
Pressure diffusion is a phenomenon of pressure change near the borehole walls that occurs over time. This pressure change is caused by the compression of the native pore fluid by the borehole-fluid pressure, ''p''<sub>''wfl''</sub>, and the osmotic pressure, ''p''<sub>''os''</sub>.  
*Saturated-salt/polymer
*KCl/NaCl/polymer
*Lime/gypsum
 
=== Pressure diffusion ===
 
Pressure diffusion is a phenomenon of pressure change near the borehole walls that occurs over time. This pressure change is caused by the compression of the native pore fluid by the borehole-fluid pressure, ''p''<sub>''wfl''</sub>, and the osmotic pressure, ''p''<sub>''os''</sub>.
 
=== Borehole fluid invasion into shale ===


===Borehole fluid invasion into shale===
In conventional drilling, a positive differential pressure (the difference between the borehole-fluid pressure and the pore-fluid pressure) is always maintained. As a result, borehole fluid is forced to flow into the formation (fluid-loss phenomenon), which may cause chemical interaction that can lead to shale instabilities. To mitigate this problem, an increase of mud viscosity or, in extreme cases, gilsonite is used to seal off microfractures.
In conventional drilling, a positive differential pressure (the difference between the borehole-fluid pressure and the pore-fluid pressure) is always maintained. As a result, borehole fluid is forced to flow into the formation (fluid-loss phenomenon), which may cause chemical interaction that can lead to shale instabilities. To mitigate this problem, an increase of mud viscosity or, in extreme cases, gilsonite is used to seal off microfractures.


===Use of drilling fluid===  
=== Use of drilling fluid ===
[[Underbalanced drilling (UBD)|Drilling overbalanced]] through a shale formation with a [[Drilling fluid types#Water-based fluids|water-based fluid (WBF)]] allows drilling-fluid pressure to penetrate the formation. Because of the saturation and low permeability of the formation, the penetration of a small volume of mud filtrate into the formation causes a considerable increase in pore-fluid pressure near the wellbore wall. The increase in pore-fluid pressure reduces the effective mud support, which can cause instability. Several polymer WBF systems have made shale-inhibition gains on [[Drilling fluid types#Oil-based fluids|oil-based fluids (OBFs)]] and [[Drilling fluid types#Synthetic-based drilling fluids|synthetic-based fluids (SBFs)]] through the use of powerful inhibitors and encapsulators that help prevent shale hydration and dispersion.


==Wellbore-stability analysis==
[[Underbalanced_drilling_(UBD)|Drilling overbalanced]] through a shale formation with a [[Drilling_fluid_types#Water-based_fluids|water-based fluid (WBF)]] allows drilling-fluid pressure to penetrate the formation. Because of the saturation and low permeability of the formation, the penetration of a small volume of mud filtrate into the formation causes a considerable increase in pore-fluid pressure near the wellbore wall. The increase in pore-fluid pressure reduces the effective mud support, which can cause instability. Several polymer WBF systems have made shale-inhibition gains on [[Drilling_fluid_types#Oil-based_fluids|oil-based fluids (OBFs)]] and [[Drilling_fluid_types#Synthetic-based_drilling_fluids|synthetic-based fluids (SBFs)]] through the use of powerful inhibitors and encapsulators that help prevent shale hydration and dispersion.
Several models in the literature address wellbore-stability analysis.<ref name="r1"/> These include very-simple to very-complex models such as:
 
* Linear elastic
== Wellbore-stability analysis ==
* Nonlinear
 
* Elastoplastic
Several models in the literature address wellbore-stability analysis.<ref name="r1">McLean, M.R. and Addis, M.A. 1990. Wellbore Stability Analysis: A Review of Current Methods of Analysis and Their Field Application. Presented at the SPE/IADC Drilling Conference, Houston, Texas, 27 February-2 March. SPE-19941-MS. http://dx.doi.org/10.2118/19941-MS.</ref> These include very-simple to very-complex models such as:
* Purely mechanical
 
* Physicochemical
*Linear elastic
*Nonlinear
*Elastoplastic
*Purely mechanical
*Physicochemical


Regardless of the model, the data needed includes:
Regardless of the model, the data needed includes:
* Rock properties (Poisson ratio, strength, modulus of elasticity)
 
* In-situ stresses (overburden, horizontal)
*Rock properties (Poisson ratio, strength, modulus of elasticity)
* Pore-fluid pressure and chemistry
*In-situ stresses (overburden, horizontal)
* Mud properties and chemistry
*Pore-fluid pressure and chemistry
*Mud properties and chemistry


Other than the mud data, the data are often compounded with problems of availability and/or uncertainties. However, sensitivity analysis can be conducted by assuming data for the many variables to establish safety windows for mud selection and design.
Other than the mud data, the data are often compounded with problems of availability and/or uncertainties. However, sensitivity analysis can be conducted by assuming data for the many variables to establish safety windows for mud selection and design.


==Borehole-instability prevention==
== Borehole-instability prevention ==
 
Total prevention of borehole instability is unrealistic, because restoring the physical and chemical in-situ conditions of the rock is impossible. However, the drilling engineer can mitigate the problems of borehole instabilities by adhering to good field practices. These practices include:
Total prevention of borehole instability is unrealistic, because restoring the physical and chemical in-situ conditions of the rock is impossible. However, the drilling engineer can mitigate the problems of borehole instabilities by adhering to good field practices. These practices include:
* Proper mud-weight selection and maintenance
 
* Use of proper hydraulics to control the equivalent circulating density (ECD)
*Proper mud-weight selection and maintenance
* Proper hole-trajectory selection
*Use of proper hydraulics to control the equivalent circulating density (ECD)
* Use of borehole fluid compatible with the formation being drilled
*Proper hole-trajectory selection
*Use of borehole fluid compatible with the formation being drilled


Additional field practices that should be followed are:
Additional field practices that should be followed are:
* Minimizing time spent in open hole
* Using offset-well data (use of the learning curve)
* Monitoring trend changes (torque, circulating pressure, drag, fill-in during tripping)
* Collaborating and sharing information


==Nomenclature==
*Minimizing time spent in open hole
{|border="0" cellspacing="4" width="70%"
*Using offset-well data (use of the learning curve)
|''α''<sub>''m''</sub>
*Monitoring trend changes (torque, circulating pressure, drag, fill-in during tripping)
|=
*Collaborating and sharing information
|activity in drilling mud, dimensionless  
 
== Nomenclature ==
 
{| border="0" cellspacing="4" width="70%"
|-
| ''α''<sub>''m''</sub>
| =
| activity in drilling mud, dimensionless
|-
|-
|''α''<sub>''s''</sub>
| ''α''<sub>''s''</sub>
|=
| =
|activity in shale pore fluid, dimensionless  
| activity in shale pore fluid, dimensionless
|-
|-
|''A''<sub>''c''</sub>
| ''A''<sub>''c''</sub>
|=
| =
|area of contact, L<sup>2</sup> , in.<sup>2</sup>  
| area of contact, L<sup>2</sup> , in.<sup>2</sup>
|-
|-
|''D''<sub>''h''</sub>
| ''D''<sub>''h''</sub>
|=
| =
|diameter of the hole, L, in.  
| diameter of the hole, L, in.
|-
|-
|''D''<sub>''op''</sub>
| ''D''<sub>''op''</sub>
|=
| =
|outside diameter of the pipe, L, in.  
| outside diameter of the pipe, L, in.
|-
|-
|''E''<sub>''m''</sub>
| ''E''<sub>''m''</sub>
|=
| =
|efficiency, dimensionless  
| efficiency, dimensionless
|-
|-
|''f''
| ''f''
|=
| =
|coefficient of friction, dimensionless  
| coefficient of friction, dimensionless
|-
|-
|''F''<sub>''l''</sub>
| ''F''<sub>''l''</sub>
|=
| =
|lateral force, F, lbf  
| lateral force, F, lbf
|-
|-
|''F''<sub>''p''</sub>
| ''F''<sub>''p''</sub>
|=
| =
|pull force, F, lbf  
| pull force, F, lbf
|-
|-
|''h''<sub>''mc''</sub>
| ''h''<sub>''mc''</sub>
|=
| =
|mudcake thickness, L, in.  
| mudcake thickness, L, in.
|-
|-
|''L''<sub>''ep''</sub>
| ''L''<sub>''ep''</sub>
|=
| =
|length of the permeable zone, L, in.  
| length of the permeable zone, L, in.
|-
|-
|''p''<sub>''cap''</sub>
| ''p''<sub>''cap''</sub>
|=
| =
|capillary pressure, F/L<sup>2</sup>, psi  
| capillary pressure, F/L<sup>2</sup>, psi
|-
|-
|''p''<sub>''ff''</sub>
| ''p''<sub>''ff''</sub>
|=
| =
|formation-fluid pressure, F/L<sup>2</sup>, psi  
| formation-fluid pressure, F/L<sup>2</sup>, psi
|-
|-
|''p''<sub>''m''</sub>
| ''p''<sub>''m''</sub>
|=
| =
|mud pressure, F/L<sup>2</sup>, psi  
| mud pressure, F/L<sup>2</sup>, psi
|-
|-
|''p''<sub>''os''</sub>
| ''p''<sub>''os''</sub>
|=
| =
|osmotic pressure, F/L<sup>2</sup>, psi  
| osmotic pressure, F/L<sup>2</sup>, psi
|-
|-
|''r''
| ''r''
|=
| =
|pore-throat radius, L, in.  
| pore-throat radius, L, in.
|-
|-
|''T''
| ''T''
|=
| =
|tension in the drillstring just above the key-seat area, F, lbf  
| tension in the drillstring just above the key-seat area, F, lbf
|-
|-
|Δ''p''
| Δ''p''
|=
| =
|differential pressure, F/L<sup>2</sup> , psi  
| differential pressure, F/L<sup>2</sup> , psi
|-
|-
|Δλ<sub>''αf''</sub>
| Δλ<sub>''αf''</sub>
|=
| =
|additional mud weight caused by friction pressure loss in annulus, F/L<sup>3</sup>, lbm/gal  
| additional mud weight caused by friction pressure loss in annulus, F/L<sup>3</sup>, lbm/gal
|-
|-
|Δλ<sub>''s''</sub>
| Δλ<sub>''s''</sub>
|=
| =
|additional mud weight caused by surge pressure, F/L<sup>3</sup>, lbm/gal
| additional mud weight caused by surge pressure, F/L<sup>3</sup>, lbm/gal
|-
|-
|''ϴ''
| ''ϴ''
|=
| =
|contact angle between the two fluids, degrees  
| contact angle between the two fluids, degrees
|-
|-
|''ϴ''<sub>''dl''</sub>
| ''ϴ''<sub>''dl''</sub>
|=
| =
|abrupt change in hole angle, degrees  
| abrupt change in hole angle, degrees
|-
|-
|λ<sub>''eq''</sub>
| λ<sub>''eq''</sub>
|=
| =
|equivalent mud circulating density, F/L<sup>3</sup>, lbm/gal  
| equivalent mud circulating density, F/L<sup>3</sup>, lbm/gal
|-
|-
|λ<sub>frac</sub>
| λ<sub>frac</sub>
|=
| =
|formation-pressure fracture gradient in equivalent mud weight, F/L<sup>3</sup>, lbm/gal  
| formation-pressure fracture gradient in equivalent mud weight, F/L<sup>3</sup>, lbm/gal
|-
|-
|λ<sub>''mh''</sub>
| λ<sub>''mh''</sub>
|=
| =
|static mud weight, F/L<sup>3</sup>, lbm/gal  
| static mud weight, F/L<sup>3</sup>, lbm/gal
|-
|-
|''μ''<sub>''c''</sub>
| ''μ''<sub>''c''</sub>
|=
| =
|chemical potential, dimensionless  
| chemical potential, dimensionless
|-
|-
|''σ''
| ''σ''
|=
| =
|interfacial tension, F/L, lbf/in.  
| interfacial tension, F/L, lbf/in.
|}
|}


==References==
== References ==
<references>
 
<ref name="r1">McLean, M.R. and Addis, M.A. 1990. Wellbore Stability Analysis: A Review of Current Methods of Analysis and Their Field Application. Presented at the SPE/IADC Drilling Conference, Houston, Texas, 27 February-2 March. SPE-19941-MS. http://dx.doi.org/10.2118/19941-MS.</ref>
<references />
</references>
 
== See also ==
 
[[Drilling_problems|Drilling problems]]
 
[[PEH:Drilling_Fluids]]


==See also==
[[PEH:Drilling_Problems_and_Solutions|PEH:Drilling Problems and Solutions]]
[[Drilling problems]]


[[PEH:Drilling Fluids]]
== Noteworthy papers in OnePetro ==


[[PEH:Drilling Problems and Solutions|PEH:Drilling Problems and Solutions]]
1. Aadnoy, B.S. 1988. Modeling of the Stability of Highly Inclined Boreholes in Anisotropic Rock Formations (includes associated papers 19213 and 19886 ). SPE Drill Eng 3 (3): 259-268. SPE-16526-PA. [http://dx.doi.org/10.2118/16526-PA http://dx.doi.org/10.2118/16526-PA].


==Noteworthy papers in OnePetro==
2. Hale, A.H., Mody, F.K., and Salisbury, D.P. 1993. The Influence of Chemical Potential on Wellbore Stability. SPE Drill & Compl 8 (3): 207-216. SPE-23885-PA. [http://dx.doi.org/10.2118/23885-PA http://dx.doi.org/10.2118/23885-PA].
1. Aadnoy, B.S. 1988. Modeling of the Stability of Highly Inclined Boreholes in Anisotropic Rock Formations (includes associated papers 19213 and 19886 ). SPE Drill Eng 3 (3): 259-268. SPE-16526-PA. http://dx.doi.org/10.2118/16526-PA.  


2. Hale, A.H., Mody, F.K., and Salisbury, D.P. 1993. The Influence of Chemical Potential on Wellbore Stability. SPE Drill & Compl 8 (3): 207-216. SPE-23885-PA. http://dx.doi.org/10.2118/23885-PA.
== External links ==


==External links==
==Category==
[[Category: 1.6 Drilling Operations]]
[[Category:1.6 Drilling operations]]  [[Category:YR]]

Latest revision as of 13:24, 26 June 2015

Borehole instability is the undesirable condition of an openhole interval that does not maintain its gauge size and shape and/or its structural integrity. This articles discusses the causes, types, effects, and possible prevention of borehole instability.

Causes

The causes can be grouped into the following categories:

  • Mechanical failure caused by in-situ stresses
  • Erosion caused by fluid circulation
  • Chemical caused by interaction of borehole fluid with the formation

Types and associated problems

There are four different types of borehole instabilities:

  • Hole closure or narrowing
  • Hole enlargement or washouts
  • Fracturing
  • Collapse

Fig. 1 illustrates hole-instability problems.

  • Fig. 1—Types of hole instability problems.

    Fig. 1—Types of hole instability problems.

Hole closure

Hole closure is a narrowing time-dependent process of borehole instability. It sometimes is referred to as creep under the overburden pressure, and it generally occurs in plastic-flowing shale and salt sections. Problems associated with hole closure are:

  • Increase in torque and drag
  • Increase in potential pipe sticking
  • Increase in the difficulty of casings landing

Hole enlargement

Hole enlargements are commonly called washouts because the hole becomes undesirably larger than intended. Hole enlargements are generally caused by:

  • Hydraulic erosion
  • Mechanical abrasion caused by drillstring
  • Inherently sloughing shale

The problems associated with hole enlargement are:

  • Increase in cementing difficulty
  • Increase in potential hole deviation
  • Increase in hydraulic requirements for effective hole cleaning
  • Increase in potential problems during logging operations

Fracturing

Fracturing occurs when the wellbore drilling-fluid pressure exceeds the formation-fracture pressure. The associated problems are lost circulation and possible kick occurrence.

Collapse

Borehole collapse occurs when the drilling-fluid pressure is too low to maintain the structural integrity of the drilled hole. The associated problems are pipe sticking and possible loss of well.

Principles of borehole instability

Before drilling, the rock strength at some depth is in equilibrium with the in-situ rock stresses (effective overburden stress, effective horizontal confining stresses). While a hole is being drilled, however, the balance between the rock strength and the in-situ stresses is disturbed. In addition, foreign fluids are introduced, and an interaction process begins between the formation and borehole fluids. The result is a potential hole-instability problem. Although a vast amount of research has resulted in many borehole-stability simulation models, all share the same shortcoming of uncertainty in the input data needed to run the analysis. Such data include:

  • In-situ stresses
  • Pore pressure
  • Rock mechanical properties
  • Formation and drilling-fluids chemistry

Mechanical rock-failure mechanisms

Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive or the tensile strength of the rock. Compressive failure is caused by shear stresses as a result of low mud weight, while tensile failure is caused by normal stresses as a result of excessive mud weight.

The failure criteria that are used to predict hole-instability problems are the maximum-normal-stress criterion for tensile failure and the maximum strain energy of distortion criterion for compressive failure. In the maximum-normal-stress criterion, failure is said to occur when, under the action of combined stresses, one of the acting principal stresses reaches the failure value of the rock tensile strength. In the maximum of energy of distortion criterion, failure is said to occur when, under the action of combined stresses, the energy of distortion reaches the same energy of failure of the rock under pure tension.

Shale instability

Shales make up the majority of drilled formations, and cause most wellbore-instability problems, ranging from washout to complete collapse of the hole. Shales are fine-grained sedimentary rocks composed of clay, silt, and, in some cases, fine sand. Shale types range from clay-rich gumbo (relatively weak) to shaly siltstone (highly cemented), and have in common the characteristics of extremely low permeability and a high proportion of clay minerals. More than 75% of drilled formations worldwide are shale formations. The drilling cost attributed to shale-instability problems is reported to be in excess of one-half billion U.S dollars per year. The cause of shale instability is two-fold: mechanical (stress change vs. shale strength environment) and chemical (shale/fluid interaction—capillary pressure, osmotic pressure, pressure diffusion, borehole-fluid invasion into shale).

Mechanical instability

As stated previously, mechanical rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. The mud in use with a certain density may not bring the altered stresses to the original state, therefore, shale may become mechanically unstable.

Chemical instability

Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The mechanisms that contribute to this problem include:

  • Capillary pressure
  • Osmotic pressure
  • Pressure diffusion in the vicinity of the borehole walls
  • Borehole-fluid invasion into the shale when drilling overbalanced

Capillary pressure

During drilling, the mud in the borehole contacts the native pore fluid in the shale through the pore-throat interface. This results in the development of capillary pressure, pcap , which is expressed as

RTENOTITLE....................(1)

where σ is the interfacial tension, ϴ is the contact angle between the two fluids, and r is the pore-throat radius. To prevent borehole fluids from entering the shale and stabilizing it, an increase in capillary pressure is required, which can be achieved with oil-based or other organic low-polar mud systems.

Osmotic pressure

When the energy level or activity in shale pore fluid, as, is different from the activity in drilling mud, am , water movement can occur in either direction across a semipermeable membrane as a result of the development of osmotic pressure, pos , or chemical potential, μc . To prevent or reduce water movement across this semipermeable membrane that has certain efficiency, Em, the activities need to be equalized or, at least, their differentials minimized. If am is lower than as, it is suggested to increase Em and vice versa. The mud activity can be reduced by adding electrolytes that can be brought about through the use of mud systems such as:

  • Seawater
  • Saturated-salt/polymer
  • KCl/NaCl/polymer
  • Lime/gypsum

Pressure diffusion

Pressure diffusion is a phenomenon of pressure change near the borehole walls that occurs over time. This pressure change is caused by the compression of the native pore fluid by the borehole-fluid pressure, pwfl, and the osmotic pressure, pos.

Borehole fluid invasion into shale

In conventional drilling, a positive differential pressure (the difference between the borehole-fluid pressure and the pore-fluid pressure) is always maintained. As a result, borehole fluid is forced to flow into the formation (fluid-loss phenomenon), which may cause chemical interaction that can lead to shale instabilities. To mitigate this problem, an increase of mud viscosity or, in extreme cases, gilsonite is used to seal off microfractures.

Use of drilling fluid

Drilling overbalanced through a shale formation with a water-based fluid (WBF) allows drilling-fluid pressure to penetrate the formation. Because of the saturation and low permeability of the formation, the penetration of a small volume of mud filtrate into the formation causes a considerable increase in pore-fluid pressure near the wellbore wall. The increase in pore-fluid pressure reduces the effective mud support, which can cause instability. Several polymer WBF systems have made shale-inhibition gains on oil-based fluids (OBFs) and synthetic-based fluids (SBFs) through the use of powerful inhibitors and encapsulators that help prevent shale hydration and dispersion.

Wellbore-stability analysis

Several models in the literature address wellbore-stability analysis.[1] These include very-simple to very-complex models such as:

  • Linear elastic
  • Nonlinear
  • Elastoplastic
  • Purely mechanical
  • Physicochemical

Regardless of the model, the data needed includes:

  • Rock properties (Poisson ratio, strength, modulus of elasticity)
  • In-situ stresses (overburden, horizontal)
  • Pore-fluid pressure and chemistry
  • Mud properties and chemistry

Other than the mud data, the data are often compounded with problems of availability and/or uncertainties. However, sensitivity analysis can be conducted by assuming data for the many variables to establish safety windows for mud selection and design.

Borehole-instability prevention

Total prevention of borehole instability is unrealistic, because restoring the physical and chemical in-situ conditions of the rock is impossible. However, the drilling engineer can mitigate the problems of borehole instabilities by adhering to good field practices. These practices include:

  • Proper mud-weight selection and maintenance
  • Use of proper hydraulics to control the equivalent circulating density (ECD)
  • Proper hole-trajectory selection
  • Use of borehole fluid compatible with the formation being drilled

Additional field practices that should be followed are:

  • Minimizing time spent in open hole
  • Using offset-well data (use of the learning curve)
  • Monitoring trend changes (torque, circulating pressure, drag, fill-in during tripping)
  • Collaborating and sharing information

Nomenclature

αm = activity in drilling mud, dimensionless
αs = activity in shale pore fluid, dimensionless
Ac = area of contact, L2 , in.2
Dh = diameter of the hole, L, in.
Dop = outside diameter of the pipe, L, in.
Em = efficiency, dimensionless
f = coefficient of friction, dimensionless
Fl = lateral force, F, lbf
Fp = pull force, F, lbf
hmc = mudcake thickness, L, in.
Lep = length of the permeable zone, L, in.
pcap = capillary pressure, F/L2, psi
pff = formation-fluid pressure, F/L2, psi
pm = mud pressure, F/L2, psi
pos = osmotic pressure, F/L2, psi
r = pore-throat radius, L, in.
T = tension in the drillstring just above the key-seat area, F, lbf
Δp = differential pressure, F/L2 , psi
Δλαf = additional mud weight caused by friction pressure loss in annulus, F/L3, lbm/gal
Δλs = additional mud weight caused by surge pressure, F/L3, lbm/gal
ϴ = contact angle between the two fluids, degrees
ϴdl = abrupt change in hole angle, degrees
λeq = equivalent mud circulating density, F/L3, lbm/gal
λfrac = formation-pressure fracture gradient in equivalent mud weight, F/L3, lbm/gal
λmh = static mud weight, F/L3, lbm/gal
μc = chemical potential, dimensionless
σ = interfacial tension, F/L, lbf/in.

References

  1. ↑ McLean, M.R. and Addis, M.A. 1990. Wellbore Stability Analysis: A Review of Current Methods of Analysis and Their Field Application. Presented at the SPE/IADC Drilling Conference, Houston, Texas, 27 February-2 March. SPE-19941-MS. http://dx.doi.org/10.2118/19941-MS.

See also

Drilling problems

PEH:Drilling_Fluids

PEH:Drilling Problems and Solutions

Noteworthy papers in OnePetro

1. Aadnoy, B.S. 1988. Modeling of the Stability of Highly Inclined Boreholes in Anisotropic Rock Formations (includes associated papers 19213 and 19886 ). SPE Drill Eng 3 (3): 259-268. SPE-16526-PA. http://dx.doi.org/10.2118/16526-PA.

2. Hale, A.H., Mody, F.K., and Salisbury, D.P. 1993. The Influence of Chemical Potential on Wellbore Stability. SPE Drill & Compl 8 (3): 207-216. SPE-23885-PA. http://dx.doi.org/10.2118/23885-PA.

External links

Category

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