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PAB 4323 – WELL STIMULATION
TECHNIQUES
SEMESTER 7
By
Dr. Aliyu Adebayo Sulaimon
(Direct Line: 05-3687051)
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Learning Outcomes
At the end of this lecture, students should be able to:
Describe factors that must be considered in the design of matrix
stimulation
Identify and mention at least five formation minerals
Mention and explain the three basic mechanisms of acid-mineralinteractions
Calculate the amount and volume of acid required to dissolve a given
amount and volume of a formation mineral
Mention the typical acid types and describe how they are used in
sandstone reservoir or carbonate formation
Calculate both the acid injection rate and surface injection pressure
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Design of Matrix Acidizing
Design Considerations
Type and concentration of acid required
Amount of acid needed to dissolve sufficient mineralaround the wellbore
Optimal injection rate
Placement of the acid solution
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Design of Matrix Acidizing
Common Formation Minerals
Calcite
Dolomite
Siderite
Quartz
Albite (i.e. sodium feldspar)
Orthoclase (potassium feldspar)
Kaolinite
Montmorillonite
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Design of Matrix Acidizing
Mechanisms of Interaction
There are three basic mechanisms:
Stoichiometry
• Determines the amount of acid needed to dissolve a given amount of mineral, e.g.
→ • That is, 2 moles of are required to dissolve 1 mole of
• The numbers 2 and 1 are the stoichiometric coefficients; &
Reaction kinetics• This deals with the rates at which acids react with various minerals
Diffusion rates
• These control how rapidly acid is transported to the rock surfaces
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Design of Matrix Acidizing
Stoichiometry Acid‟s “dissolving power” is a more convenient means (apart
from the no. of moles) of expressing reaction stoichiometry.
There are two types: Gravimetric dissolving power
• This is the mass of mineral consumed by a given mass of acid
Volumetric dissolving power• This is the volume of mineral dissolved by a given volume of acid
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Design of Matrix Acidizing
Gravimetric dissolving power
% =
where n and M represent the number of moles and molecular mass;
x % is the weight fraction of acid in the acid solution
Volumetric dissolving power
% = %
where ρ is the density in /
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Example CalculationCalculate the gravimetric and volumetric dissolving powers of 15wt% HCl reacting with
Dolomite
Siderite
Solution Dolomite:
→
= . ; = . ; = . . ∗ = . ; = . = .
% =
=
. ∗ . . ∗ .
= . /
= . ; = .
% = %
= . . ∗ .
. ∗ .
= .
%
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Solution (Cont‟d)
Siderite: →
= . ; = . ; = . ∗ = . ; = . = .
% =
=
. ∗ . . ∗ .
= . /
= . ; = .
% = %
= . . ∗ .
. ∗ . = .
%
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Design of Matrix Acidizing
Selection of Acids Sandstone Formation
• The purpose of sandstone acidizing is to remove the damage nearthe wellbore
• Typical treatments usually consist of a mixture of 3.0wt%HF and
12.0wt%HCl, preceded by a 15.0wt%HCl preflush
Carbonate Formation
• HCl is the most widely used acid for carbonate matrix acidizing
• Weak acids are recommended for perforating fluid and perforationcleanup
• Strong acids are recommended for other treatments
•
Concentrated (high strength) acids provide deeper penetration
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Design of Matrix Acidizing
Acid Volume Requirement
Sandstone Formation
=
where = preflush volume or the required acid volume, cu ft
=volume of minerals to be removed, cu ft
= ∅
= initial pore volume, cu ft= ∅
= radius of acid treatment, ft
= wellbore radius, ft
= mineral content, volume fraction
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Design of Matrix Acidizing
Acid Volume Requirement
Carbonate Formation
= ∅
where = required acid volume per unit thickness of formation, cuft/ft
= desired radius of wormhole penetration, ft
= wellbore radius, ft
= no. of pore volumes of acid injected at the time of wormhole
breakthrough at the end of the core
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Design of Matrix Acidizing
Acid Injection Rate
Assuming pseudo-steady state flow, the maximum injection rate limited by
the fracture pressure is expressed as
, =.× −−∆
−
+
where, = maximum injection rate, bbl/min = permeability of undamaged formation, mD = pay zone thickness, ft = formation fracture pressure, psia
= reservoir pressure, psia∆ = safety margin, (200 to 500psi) = viscosity of acid solution, cp = drainage radius, ft = wellbore radius, ft = skin factor
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Design of Matrix Acidizing
Acid Injection Pressure
In most acid treatment operations, only the surface tubing (injection)pressure is monitored. It is important to predict the surface injection pressureat the design stage for pump selection. It is expressed as
= ∆ ∆
where
= surface injection pressure, psia
= flowing bottom-hole pressure, psia
∆ = hydrostatic pressure drop, psia
∆ = frictional pressure drop, psia
=...
.; < /
where ρ = density (g/cc), q = injection rate (bb/min),µ = viscosity (cp), D = tubing diameter (inches), andL = tubing length (ft)
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Design of Matrix Acidizing - (Short Review)
Selection of Acids In sandstone formations
In carbonate formation
Acid Volume Requirement In sandstone formations
In carbonate formation
Acid Injection Rate
Acid Injection Pressure
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Example 1
A sandstone with a porosity of 0.2 containing 10%(volume) calcite () is to be acidized with HF/HCl
mixture solution. A preflush of 15wt% HCl solution is to be
injected ahead of the mixture to dissolve the carbonateminerals and establish a low PH environment. If the HCl
preflush is to remove all carbonates in a region within 1-ft
beyond a 0.328-ft radius wellbore before the HF/HCL
stage enters the formation, what minimum preflush
volume is required in gal/ft of formation thickness?
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Solution 1
Given:∅ = . ; = . ; = ; = . ; = . ;
=
= ∅
= ∗ . ∗ . . ∗ .
= .
= ∅
= ∗ . ∗ . . = .
To calculate X, we need .
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Solution 1
→
= . ; = . ; = . ∗ = . ; = . = .
% =
=
. ∗ .
. ∗ . = . /
= . ; = .
% = %
= . ∗. ∗ .
. ∗ . = .
%
Therefore,
=
= . ∗
. .
= .
%/
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Example 2
A 28wt% HCl is required to propagate wormholes 3-ftfrom a 0.328-ft radius wellbore in a limestone formation(ρ = 2.71) with a porosity of 0.15. The design injection rate
is 0.1 bbl/min-ft, the diffusion coefficient is −
/, and the density of the 28wt% HCl is 1.14g/cc. In
linear core floods, 1.5PV (pore volume) is needed for
wormhole breakthrough at the end of the core. Calculate
the acid volume requirement.
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Solution 2
Given: ∅ = . ; = . ; = . ; ()= .
= ∅
= ∗ . ∗ . . ∗ .
= .
= /
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Example 3
A 60-ft thick, 50mD sandstone pay zone at a depth of9,500-ft is to be acidized with an acid solution having aspecific gravity of 1.07 and a viscosity of 1.5cp down a 2-inch internal diameter coil tubing. The formation fracture
gradient is 0.7psi/ft. The wellbore radius is 0.328ft.Assuming a reservoir pressure of 4,000psia, drainagearea radius of 1,000ft, and a skin factor of 15, calculate
a) The max. acid injection rate using safety margin300psi
b) The max. expected surface injection pressure at themax. injection rate.
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Solution 3
Given: = ; = ; = ; = . ; = . ;
= .
;
= ; . = ; = ; =
a) The max. acid injection rate is
, = . × −
∆ ln
34
=. × − ∗ ∗ ∗ . ∗
. ∗ .
= . /
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Solution 3 (Cont‟d)
a) The max. expected surface injection pressure is
= ∆ ∆
= ∆ = . ∗ = ,
∆ = . = . ∗ . ∗ = ,
∆ =
.
.
.
.=
∗ ∗ . .
∗..
. .
.=
Therefore,
= ∆ ∆ = , ,
= ,
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Homework 1
1) Calculate the gravimetric dissolving power ofa) 15wt% HCl reacting with
i. Calcite
ii. Siderite
b) 3wt% HF reacting with
i. Orthoclase feldspar
ii. Kaolinite
iii. Montmorillonite
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Homework 1 (Cont‟d)
1) Calculate the volumetric dissolving power ofa) 15wt% HCl reacting with
i. Calcite
ii. Siderite
b) 3wt% HF reacting with
i. Orthoclase feldspar
ii. Kaolinite
iii. Montmorillonite
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Homework 2
Solve questions 16.1, 16.3, 16.5 and 16.7 in Chapter 16 of
Guo, B.; Lyons, W.C.; and Ghalambor, A. (2007): „Petroleum Production
Engineering‟, Elseviers‟ Gulf Professional Publishing, Oxford, U.K
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Questions?
Thank you