Wellbore Instability Types

1
Wellbore Instability - Types Causes

• Lecture 28
• Rock Mechanics for Petroleum Engineers
• Dr. A. Abdulraheem

2
Wellbore Instability

• Stable and Unstable Borehole
• Types of Instabilities
• Mechanical
• Compressive
• Tensile
• Chemical
• History

3
Wellbore Instabilies
Stable Wellbore a wellbore that stays the same
diameter as the drill bit, in all
directions Enlarged Restricted Breakout Fractur
e
Some examples of Unstable conditions
4
Wellbore Instability

• Wellbore Instability can occur as a result of
• Mechanical effects
• Chemical effects or
• A combination of both
• Chemical effects are mud-type related and are due
  to
• Inappropriate mud type
• Inadequate inhibition

5
Types of borehole failures

• Basically two types
• Compressive failure, caused by an insufficient
  mud weight compared with the rock's strength and
  the stresses around the borehole and

6
Types of borehole failures

• Tensile failure, caused by an excessive mud
  weight compared with the minimum in situ stress

7
Compressive failure

• Two main types
• an increased borehole diameter due to brittle
  failure and the subsequent caving of the wellbore
  wall. (in brittle rocks), and
• a reduced borehole diameter which occurs in weak
  (plastic) shales, sandstones, and salts.

8
History

• The problem was not severe because the wells
  were
• Shallow and
• Vertical
• But with deep and inclined/hzl. Wells, the
  problem has assumed significant importance

9
For production engineers?

• In case these wells give problems during
  production, it is nice to know the history of
  problems, if any, during their drilling
  operations

10
Types of Problems(Operational Problems as
reported by drillers)
1. Stuck Pipe 2. Tight Hole 3. Overpull 4.
Obstruction 5. Jarring 6. Reaming 7.
Circulation Loss
11
1. Stuck Pipe
The portion of the Drillstring that cannot be
rotated or moved vertically. Most expensive
One may lose the drill bit May require
sidetrack
12
2. Tight Hole
A section of a wellbore, usually openhole, where
larger diameter components of the drillstring,
such as drillpipe tool joints, drill collars,
stabilizers, and the bit, may experience
resistance when the driller attempts to pull them
through these sections.
13
3. Overpull
The force which is applied to the drillstring to
free the stuck pipe
14
4. Obstruction
Inability to move the bit
15
5. Jarring
A process of delivering an impact load to another
downhole component, especially when that
component is stuck
16
6. Reaming

• To enlarge a wellbore. Reaming is required when
• a bit has been worn down from its original size,
  and is therefore not able to make a true size
  hole. The hole is a little smaller in diameter
  than planned. Reaming corrects it.
• Some plastic formations may slowly flow into the
  wellbore over time and therefore make the hole
  smaller than its original size.

17
7. Circulation Loss

• Lack of mud returning to the surface after
  being pumped down a well.
• Occurs when the drill bit encounters natural
  fissures, fractures or caverns, and mud flows
  into the newly available space.
• May also be caused by applying more mud pressure
  (that is, drilling overbalanced) on the formation
  than it is strong enough to withstand, thereby
  opening up a fracture into which mud flows

18
Causes of Problems (Instability Mechanisms Why
different types of problems occur? What causes
those problems
19
Outline

• Instability can be due to
• Rock type
• Drilling operations
• Drill string
• Geometry
• Other
• The above cases may overlap each other.
• Controllable Factors
• Uncontrollable Factors
• Solution

20
A. Instability due to Rock Type

• 1. Unconsolidated Zone
• 2. Erosion
• 3. Fractured or Faulted Zone
• - Prevention
• 4. Mobile Formation
• 5. Reactive Formation
• - The Case of Shales

21
1. Unconsolidated Zone

• Unconsolidated and weak zone may lead to huge
  washouts
• Generate debris which may lead to difficulty in
  pipe movement during tripping
• Can cause stuck pipe
• Hole cleaning become laborious
• May damage downhole equipment

22
2. Erosion

• High horizontal forces cause wall failure and bit
  restrictions.
• Lower formation compressive strength
• Higher hydraulic HP expended at the bit
• Longer nozzles (mini-extended or fully extended)
• Nozzles pointing towards the wall of the hole
• Lower rates of penetration (the wall gets more
  time exposed to the bit hydraulic flow), e.g.
  during controlled ROP drilling

23
3. Fractured or Faulted Zone

• Create excessive debris which may results in
  other stability problems such as tight hole and
  stuck pipe
• Results in Loss circulation
• Hole cleaning becomes difficult due to generation
  of debris

24
Fractured or Faulted Zone - Prevention

• Avoid sudden reductions in wellbore pressure
• Use WBM with high spurt loss and plugging
  additives
• Silicate muds can stabilize the fractures
• Avoid using OBM (it lubricates the fracture
  surface)
• Keep the mud density as low as possible

25
4. Mobile Formation

• Formation like salt deform under constant stress
  (Creep). Results in
• Stuck pipe
• Tight hole
• Casing failure
• Requires excessive reaming and backreaming
• Stuck pipe in salt layer may be freed by jarring
  and circulating fresh-water pill.

26
5. Reactive Formation

• Some shales which are sensitive to water
  (contained in the drilling fluid) swell. The
  swelling creates a number of problems.
• The instability is caused by chemistry but it
  is then changed to mechanical instability.
• We will study it later.

27
Reactive Formation

• Drilling in Reactive Formation may leads to
• Stuck pipe
• Tight hole
• Sloughing
• Bit balling

28
B. Instability Drilling Operations Related

• 1. Differential Sticking
• 2. Pore Pressure Penetration
• 3. Undergauge hole

29
1. Differential Sticking

• A condition whereby the drillstring cannot be
  moved (rotated or reciprocated) along the axis of
  the wellbore.
• Typically occurs when high-contact forces caused
  by low reservoir pressures, high wellbore
  pressures, or both, are exerted over a
  sufficiently large area of the drillstring

With time
30
2. Pore Pressure Penetration

• To prevent it
• Avoid sudden reductions in wellbore pressure by
  shutting off pumps slowly while making connection
• Use a mud which can plug off the pore throats
• Use a mud which has oil as the continuous phase
• Silicate muds and Cloud Point Glycol Muds are
  also an alternative

31
3. Undergauge hole

• Caused by
• worn bit
• Mobile formation
• Results in
• tight hole
• restriction

32
C. Instability Drill String Related

• 1. Drillstring Vibration
• 2. Cantilever Effect
• 3. Spiralling

33
1. Drillstring Vibration

• It can cause
• wall failure
• Generate debris
• Increase hole diameter
• May result in keyseating

34
2. Cantilever Effect

• Caused by Huge washouts in horizontal sections
• Leads to problems while making re-entry

35
3. Spiralling
Caused by Improper BHA (BottomHole Assembly)
36
D. Instability Geometry Related

• 1. Wellbore Geometry
• 2. Keyseating
• 3. Poor Hole Cleaning

37
1. Wellbore Geometry

• Especially in stringers (multiple layers of sand
  and shale)

38
2. Keyseating

• A small-diameter channel worn into the side of a
  larger diameter wellbore.
• This can be the result of a sharp change in
  direction of the wellbore (a dogleg), or if a
  hard formation ledge is left between softer
  formations that enlarge over time.

39
3. Poor Hole Cleaning

• Results in
• Tight hole due
• Stuck pipe
• Hole Pack off

40
E. Instability Geometry Related

• 1. Casing Collapse
• 2. Junk
• 3. Cement Related

41
1. Casing Collapse

• Can occur due to
• High Horizontal stresses
• Movement along the discontinuity
• Leads to
• Stuck pipe
• Tight hole

42
2. Junk

• Anything in the wellbore that is not supposed to
  be there.
• The term is usually reserved for small pieces of
  steel such as hand tools, small parts, bit
  nozzles, pieces of bits or other downhole tools,
  and remnants of milling operations

43
3. Cement Related

• Poor cementing job Can lead to tight hole due to
  cement debris

44
Controllable Factors

• Drilling parameters (WOB, )
• Geometry (angle of deviation, azimuth)
• Open-hole time (ROP, stuck pipe, casing, )
• Mud weight (type of mud, )
• Mud flow rate (bit cleaning, )
• Downhole temperature (Thermal stresses )

45
Controllable Factors (contd.)

• Completion choices (open-hole, cased, )
• Treatment choices (acidizing, )
• Stimulation choices (hydrofrac, )
• Production choices (drawdown rate, )
• Production management (reservoir pressure,
  injection .)

46
Uncontrollable Factors
Mechanical instability is controlled by the
following two factors 1. Rock mechanical behavior
(stress-strain behavior, strength, creep,
swelling, ) 2. In-situ stresses Chemical
instability In reactive shales, the rock's
strength is related directly to the water content
which can be altered by drilling mud.
47
Solution?

• Wellbore stability requires a proper balance
  between
• the uncontrollable factors of earth stresses,
  rock strength, anisotropy, and pore pressure,
• and the controllable factors (those listed
  earlier, especially mudweight)

48
End result

• During Drilling Keep the mudweight such that it
  is
• Above wellbore collapse pressure
• Above pore pressure (to avoid in-flow) during
  drilling
• Below fracture pressure
• For reactive shales Use oil-based-mud to avoid
  swelling of shales