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PAB 4323 – WELL STIMULATION

TECHNIQUES

SEMESTER 7

By

Dr. Aliyu Adebayo Sulaimon

([email protected])

(Direct Line: 05-3687051)

mailto:[email protected]

mailto:[email protected]

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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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Common Formation Minerals

Calcite

Dolomite

Siderite

Quartz

Albite (i.e. sodium feldspar)

Orthoclase (potassium feldspar)

Kaolinite

Montmorillonite

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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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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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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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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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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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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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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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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

pab 4323 – l4

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