Ch. 8. Wellbore stability

1
Ch. 8. Wellbore stability Ch. 8.1.
Introduction

1. Intro
2. Filtarion control
3. Mechanical
4. Chemical
5. Hole problems / countermeasures
6. Inhibitive muds

Typical stability problems

• Mechanical stress-induced wellbore stability
• Chemical related instability after water
  infiltration
• Time dependent water interaction with chalk,
• limestone, anhydrite, gypsum
  etc
• 1. Ca - ions ? flocculation
• 2. Fines ? LGSC ? rheological problems
• b. Time-dependent drilling fluid-shale
  interaction

2
Ch. 8.2. Filtration control. In sand. Testing
Lab testing of filter cake

1. Spurt loss
2. Initial filter is formed

dpores
dp
Real formation
Typical
Cuttings 5 Barite
55 Clay 10
1. Coarse Particles greater than 2 000 microns
2. Intermediate Particles between 250-2 000microns
3. Mediums Particles between 74-250 microns
4. Fines Particles between 44-74 microns
5. Ultrafine Particles between 2-44 microns
6. Colloidal Particles less than 2 microns
3. Filter cake is formed
3
Ch. 8.2. Filtration control In shale
4
Ch. 8.3. Mechanical stability Failure type
s F/A
Stress related instability
Creep Wellbores that stays open for a long
time (weeks), tend to close in Tensile failure
at high MW Fractures are detected at the
surface as lost circulation. Compression
failure at low MW Cavings, breakouts,
leads eventually to total wellbore
collapse Weakness in the formation
Faults crossing the wellbore. Inter-bedded
formations (between lithology changes, bedding
angle close to wellbore angle) Naturally
weak formations (coal beds, conglomerates, loose
sands, etc)
e Dl/l
Geometry Cause Countermeasure
UBD Abnormal stress regime
Pre-existing fractures/ failures
Increase MW Optimize trajectory
Improve fluid loss Monitor ECD
Increase MW
Reduce hydraulic / mechanical attack
Monitor ECD
5
Ch. 8.4. Chemical stability Shale
interaction
typically 100s of bar
Capillary pressure
typically - 100s of bar
Hydraulic pressure Governed by Darcys law.
Osmotic pressure Activity of pore water,
Aw,pore, gt Aw,mud, controlled through salt c
Shales ability to swell depends on amount of
Montmorilonite Mechanical strength of shale
determines its to resistance to swelling
Diffusion
Salt ions, the opposite direction of water
Symptoms of swelling shale and tight hole

• Pack Off, Overpull, Took Weight on trips

• filter loss
• MBT (CEC)
• Increased PV YP
• (increasing with LGSC
• open hole time) Torque
• Drag

-

• Shaker is blocked off by soft, hydrated clay or
  even clay balls
• Torque fluctuations
• Large irregular rock fragments on shaker
• Fill on bottom
• Shallow depths (lt 2500 m)

6
Ch. 8.5. Hole problems And their symptoms
Bit balling

• Reduced ROP
• Increased SPP - due to reduced dann of
  stabilizers
• Blocked shaker screen (soft clay)
• Overpull on Tripping Out

Mobile fm

• Bit balling and pack off due to reactive shale
• Wellbore erosion when drilling through the salt
  formation and /
• or though shale above or below the salt
  formation
• Excessive torque and pack Off caused by salt
  creep
• Well control issues related to mud losses.
  Rubble zone beneath
• or adjacent to the salt section usually
  consists of series of highly
• reactive shale stringers embedded in
  unconsolidated sands
• Salt-induced casing collapse across mobile salt
  sections is the
• most common type of casing collapse. Several
  wells in
• Southern North Sea and Gulf of Sues have
  suffered SICC

Naturally fractured, faulted and unconsolidated fm

• High CEC
• High clay porosity
• High in-situ Stresses
• Low kshale delayed swelling, f (open hole
  time)
• Soft, hydrated cuttings in flow line, clay balls
• Torque or fluctuation
• Increased filter loss, MBT, PV, VP, LGS,
• Pack offs during drilling
• Bit balling when POOH causing Tight Hole and
  Over pull

Rock Bit Failure
Broken Teeth
7
Ch. 8.6. Inhibitive mud OBM
Na
CH3-CH2-CH2-CH2-CH2
Na
CH3-CH2-CH2-CH2-CH2
Na
CH3-CH2-CH2-CH2-CH2
Na
CH3-CH2-CH2-CH2-CH2
8
Ch. 8.6. Inhibitive mud OBM
1. Base oil the continuous phase 2.
Emulsifier emulsify water in oil 3. Wetting
agent makes the wellbore oil wet mix
123 4. Water forms viscosifying droplets
mix 4 into it 5. Viscosifyer like in WBM 6.
Weighting material like in WBM
Water phase must have same water activity as pore
water of shale

• Advantage of OBM
• Clay / shale do not swell
• No corrosion
• Low filter loss
• T-stable mud

Shale without wetting agent
Shale with wetting agent
9
Ch. 8.6. Inhibitive mud WBM
Acrivity of water Destilled
water 1.000 Ba SO4 0.999 KH2PO4
0.96 NaCl 0.755 CaCl2
0.295 ZnCl2 0.100
pswell -

r
Molecular size (Å)
Not hydrated Hydrated Not hydrated Hydrated
H 1,6 Å 1,6 Å
O 1,3 1,3
H2O 2,9 2,9
K 2,1 7,6
Na 1,8 11,2
Ca 3,0 19,2
Na SiO2 6,1 6,1
Clay pore throat 10-100
days
r
Aw
1.0
0.0
10
Ch. 8.6. Inhibitive mud
WBM
11
Summary
12
Summary
WOB