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Introduction to Reservoir Stimulation
Kellyville Training Center
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Well Stimulation
Stimulation is a chemical or mechanical method of increasing flow capacity to a well.
Dowell Schlumberger is mainly concerned with three methods of stimulation:
1. Wellbore Clean-up : “ Fluids not injected into formation”• a. Chemical Treatment• b. Perf Wash
2. Matrix Treatment : “ Injection below frac pressure”• a. Matrix Acidizing• b. Chemical Treatment
3. Fracturing “ Injection above frac pressure”• a. Acid Frac• b. Propped Frac
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Stimulation Techniques
Restores Flow Capacity:• Wellbore Clean-up• Matrix Treatment
These procedures are performed below fracture pressure.
Create New Flow Capacity:• Hydraulic Fracturing (Acid and Sand)
These procedures are performed above fracture pressure.
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Areas Where Reduction in Flow Capacity May Occur
1. Wellbore:• Scale Damage• Sand Fill• Plugged Perforations• Paraffin Plugging• Asphalt Deposits• Etc.
2. Critical Matrix:• Drilling Mud Damage• Cement Damage• Completion Fluids• Production• Native Clays/Fines
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WELLBORE
Primary Purpose :
Restore flow capacity by removing restrictive damage to fluid flow in the wellbore.
Methods :• Mechanical• Chemical Treatment• Acidizing Treatment
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Critical Matrix What is It?
• The area of formation that is 3' to 5' from the wellbore. Why is it critical?
r % Pressure Drop (Drainage Radius) P (psi) P/ft (Pe - P) (Pe - Pwf) * 100
(Pe) 2,000 ft 5,000 0.07 psi/ft 01,000 ft 4,934 2.5100 ft 4,719 10.850 ft 4,654 1.3 psi/ft 13.320 ft 4,568 16.610 ft 4,503 6.5 psi/ft 19.05 ft 4,439 21.53 ft 4,391 23.32 ft 4,000 850 psi/ft 24.81 ft 3,150 27.3
(Pwf) 0 ft 2,000 1,150 psi/f 100
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Major Goals of Matrix Treatment
1. Restore Natural Permeability• By Treating the Critical Matrix
2. Minor Stimulation
3. Leave Zone Barrier Intact
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Matrix Acidizing 1. Sandstone:
• Major Effects: Dissolves/Disperses Damage Restores Permeability
• Minor Effects: Minor Stimulation
2. Limestone:• Major Effects:
Enlarge Flow Channels/Fractures Disperse Damage by Dissolving Surrounding Rock Creation of Highly Conductive Wormholes
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Applications For Matrix Treatment
High Permeability Formation with Damage.
Unproppable Formations.
Treating Limitations.
Thick Zones.
To Supplement Fracturing.
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Low Permeability Reservoir Increase well productivity by creating a highly conductive path
compared to the reservoir permeability.
The fracture will extend through the damaged near wellbore area. The fracture size is limited to two criteria :
• Drainage Radius• Cost
Fracturing is : Pumping fluid into the formation above fracture pressure.
Damage
XL
XL = Fracture half length
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Darcy’s Equation
Oil Well : Oil Well : Gas Well : Gas Well :
q = kh (Pe - Pwf)
141.2 µ (In rerw + S)
q = kh (Pe2 - Pwf2)
1424 µzT (In rerw + S)
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Skin (s) The total Skin (ST) is the combination of mechanical and pseudo-skins. It
is the total skin value that is obtained directly from a well-test analysis.
Mechanical Skin:• Mathematically defined as an infinitely thin zone that creates a steady-
state pressure drop at the sand face.• S > 0 Damaged Formation• S = 0 Neither damaged nor stimulated• S < 0 Stimulated formation
Pseudo Skin:• Includes situations such as fractures, partial penetration, turbulence,
and fissures. The Mechanical Skin is the only type that can be removed by stimulation.
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Skin Example Pseudo Skin:
• Producing at high rates --> turbulence• Collapsed tubing, perforations• Partial penetration / Partial perforation• Low Perforation Density (Shots/ft)• Etc.
Formation Damage:• Scales• Organic/Mixed Deposits• Silts & Clays• Emulsions• Water Block• Wettability Change
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Example An oil well produces 57 B/D under the following reservoir and producing
conditions:
k = 10 md
h = 50 ft
ßo = 1.23 res bbl/stb
µo = 0.6 cp
Pr = 2,000 psi
Pwf = 500 psi
rw = .33 ft
re = 1,320 ft
What is the Skin Factor?
Is there potential for Stimulation?
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INTRODUCTION TO MATRIX TREATMENT
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Formation Damage
Damage Definition :
• Partial or complete plugging of the near wellbore area which reduces the original permeability of the formation.
• Damage is quantified by the skin factor ( S ).
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Types of Formation Damage Emulsions
Wettability Change
Water Block
Scale Formation
Organic Deposits
Mixed Deposits
Silt & Clay
Bacterial Slime
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Areas of Damage
Scales
Organic deposits
Silicates, Aluminosilicates
Emulsion
Water block
Wettability change
Tubing Gravel Pack Perforations Formation
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Emulsions Definition:
• Formed by invasion of filtrates into oil zones or mixing of oil-based filtrates with formation brines.
• Any two immiscible fluids
Keys to Diagnosis:• Sharp decline in production• Water breakthrough• Production of solids• Fluid samples• Injection of inhibitors
Treatment:• Surfactants • Mutual solvents
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Wettability Change Definition:
• Oil wetting of rock from hydrocarbon deposits or adsorption of an oleophilic (attracts oil) surfactant from treating fluid.
Keys to Diagnosis: (Normally difficult to diagnose)• Rapid production decline• Casing leak• Water breakthrough• Water coning• Decrease or disappearance of gas
Treatment:• Mutual solvent followed by water-wetting surfactant.
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Water Block Definition:
• Caused by an increase in water saturation near the wellbore which decreases the relative permeability to hydrocarbons.
Keys to Diagnosis: • Rapid oil or gas production decline• Casing leak• Water breakthrough• Water out• Abnormally high water cut through lower perforations
Treatment:• Mutual solvents or surfactants
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Scale Formation Definition:
• Scales are precipitated mineral deposits. Scale deposition occurs during production because of lower temperatures and pressures encountered in or near the wellbore.
Keys to Diagnosis: • Sharp drop in production• Visible scale on rods/tubing• Water breakthrough
Treatment:• Carbonate (Most Common)
HCl, Aqueous Acetic• Sulfate
EDTA NARS
• Chloride 1 - 3% HCl
IronIron» HCl with various iron control agentsHCl with various iron control agents
SilicaSilica» Mud AcidMud Acid
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Keys to Diagnosis of a SampleFloats in H2O 2
Soluble in H O2
Soluble in HCl
No
Soluble in hot HCl
No
No
Iron Oxide
Magnetic
Magnetite FeCo
Soluble in U42
Soluble in hot HCl/HF
3
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Organic
NaCl (probably)
Odor of rotten eggs
Silica Base (sand/clay)
SrSO (slow) BaSO (very slow)
CO Evolves
FeCO
Fe (CO ) CaCO
MgCO Ca(SO ) slowly soluble (also soluble in U42)
FeS (possible)
3
2 3 3
3
3
4 2
2
4
4
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Scales : Inorganic Mineral Deposits
Types of Scale
Usual Occurrence
Treating Fluids Comments
Carbonates CaCO3 HCl Very Common
Sulfates
CaSO •2H O (gypsum)
BaSO /SrSO
EDTA
EDTA
Common
Rare
Chlorides NaCl H O/HCl Gas Wells
IronFe S
Fe O
HCl + EDTA
HCl + Sequestering Agent
CO /H S Possible Produced
Silica SiO HF Very Fine
Hydroxides Mg/Ca(OH) HCl
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4 4
3
2
2
2
2
22
2
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Organic Deposits Definition:
• Organic deposits are precipitated heavy hydrocarbons (parrafins or asphaltenes). They are typically located in the tubing, perforations and/or the formation.
• The formation of these deposits are usually associated with a change in temperature or pressure in or near the wellbore during production.
Keys to Diagnosis: • Sharp decline in production• Visual parrafin on rods and pump• Operator is "hot oiling"
Treatment:• Aromatic Solvents (Xylene, Toluene)• Mutual Solvents
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Keys to Diagnosis of Actual Organic Deposit
Floats in water Yes Organic Deposit
1. Burns evenly with clean flame Yes Paraffin/wax
No
Black sooty flame Yes Asphaltene
2. Soluble in pentane Yes Paraffin
No
Asphaltene
3. Soluble in Toluene/Xylene Yes Paraffin/ Asphaltene
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Silts & Clays Definition:
• Damage from silts and clays includes the invasion of the reservoir permeability by drilling mud and the swelling and/or migration of reservoir fines.
• Keys to Diagnosis: • Sharp drop in production• Lost circulation during drilling• Production tests• ARC tests
Treatment:• HCl: Carbonate Reservoirs• HF Systems: Sandstone• Quaternary Amine Polymers (L55)• Cationic Surfactant (M38B)• Fusion (Clay Acid)
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Bacterial Slime
Definition:• Anaerobic bacteria grows downhole without oxygen up
to 150°F. Bacteria may chemically reduce sulfate in a reservoir to H2S.
Treatment:• M91 (Bleach+Caustic soda)
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Sources of Formation Damage Drilling
Cementing
Perforating
Completion and Workover
Gravel Packing
Production
Stimulation
Injection Operations
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Successful Matrix Treatment
REQUIREMENTS :
• Enough Treating Fluid Volume
• Correct Reactive Chemicals
• Low Injection Pressure
• Total Zone Coverage
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INTRODUCTION TO FRACTURING
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Applications For Hydraulic Fracturing
If wells natural permeability is low ( Ke < 10 md )
Natural production is below economic potential
Skin By-Pass “ HyperSTIM “ or higher permeability and soft formations.
The injected fluid is pumped at a rate above the fracture pressure of the reservoir to create cracks or fractures within the rock itself.
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Hydraulic Fracturing Treatment
Primary Purpose :• To increase the effective wellbore area by creating a
fracture of length XL whose conductivity is greater than that of the formation.
Dimensionless Conductivity ( Fcd ) = Kf Wf / Ke Xf
Two Methods :• Sand Frac• Acid Frac
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Propped Frac & Acid Frac
1/2"open fractureduring job
fracture tends to closeonce the pressure has been
released
sand used to prop thefrac open
acid etched frac walls
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Propped Fracture Optimization
Optimize the reservoir deliverability by balancing fracture characteristics and reservoir properties
Analyze the effect of production systems :• Perform => Nodal Analysis
Determine the pumping parameters :• DataFRAC
Tailor the fracturing fluid and proppant to the reservoir Determine treatment size (Fluid & proppant amount)
• Calculate XLand FCD
Calculate the benefit of the treatment => $• FracNPV
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Acid Fracture
Bottom hole pressure above fracturing pressure
Acid reacts with the formation
Fracture is etched
Formation must retain integrity without fracture collapse
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Hydraulic Fracturing Accomplishes:
Creates Deep Penetrating Fractures to :
Improve productivity Interconnect formation permeability Improve ultimate recovery Aid in secondary recovery Increase ease of injectivity
• A hydraulic Fracture has to be cost effective to the customer.
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Fracture Penetration is influenced by: FORMATION CHARACTERISTICS :
• Type • Hardness• Permeability• Zone Height “ Presence of Barriers “• Drainage Radius
FRAC FLUID CHARACTERISTICS :• Base Fluid• Viscosity• Volume• Pump Rate• Fluid Loss
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Orientation Of The Fracture
The fracture will extend perpendicular to the axis of the least stress.
• X - Y - Z Coordinate :Overburden Pressure
Least Principal Stress
Favored Fracture Direction
(i.e. Vertical Fracture)
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Vertical Or Horizontal Fracture
Rule-Of-thumb :• Frac Gradient < 0.8 psi / ft --------> Vertical Fracture• Frac Gradient > 1.0 psi / ft --------> Horizontal Fracture
Vertical fracture plane is perpendicular to earth’s surface due to overburden stress being too great to overcome
Horizontal fracture with a pancake likegeometry. Usually associated withshallow wells of less than 3,000 ft. depth
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Fracture Propagation Models
KGD
• XL < h
PKN
• XL > h
Radial
• XL = h/2
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Rock Mechanical Behavior Young’s Modulus :
• E =
Poisson’s Ratio : L1 - L2 / L1
d1 - d2 / d1D1
D2
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Rock Mechanical Behavior
Young’s Modulus :• E =
Poisson’s Ratio : L1 - L2 / L1
d1 - d2 / d1
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Fracture Width
W = ( Q L) 1/4 PKN E
W = ( QL2)1/4 KGD EH
= Viscosity of fluid
• Q = Injection Rate • H = Gross Height
• L = Xf
• E = Young’s Modulus
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Net Present Value FracNPV BENEFITS :
• Design lowest cost job• Realize full production rate potential• Forecast post treatment decline• Study impact of treatment variables
APPLICATION :
• Select optimum XL, W & proppant type
• Aid in determining whether or not to fracture a new well• Determine size of production equipment• Evaluation of the fracture treatment based on well performance
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FracNPV
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0 100 200 300 400 500
Hydraulic Half-Length - ft
-100000
0
100000
200000
300000
400000
500000
600000
Ne
t Pre
se
nt V
alu
e -
$(U
S)
YF120LG
ClearFRAC (3
Production time 1 year
Fluid Type
FracCADE*
*Mark of Schlumberger
Net Present Value
Well XXXX1235.5//1249.508-26-1997
Design
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Conclusion Three Types of Stimulation :
• Wellbore Clean-up• Matrix Treatment• Hydraulic Fracturing
Well Candidate Selection :• What is it ?• How does Dowell Schlumberger use it ?• What are some of the tools associated with it ?
NPV• What is it ?• How can it be used to design a treatment ?• How does the output benefit our customers and us ?