Chilingarian G.V. et al. Surface Operations in Petroleum Production, II
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155
SAMPLE QUESTIONS AND PROBLEMS
’
Questions
(1)
What are the uses of hydraulic fracturing? Explain each one in detail.
(2)
Explain why the production increases as a result of a hydraulic fracturing
(3)
What are the factors that should be considered in selecting wells for a
(4)
Define the “fracturing fluid coefficient”. What are the components for the
(5)
What are the properties that a fracturing fluid should possess?
(6)
List and discuss the factors that affect the selection of a fracturing fluid for a
particular well.
(7)
List the types of fracturing fluids used in the industry. Discuss the advantages
and disadvantages of each.
(8)
What is the function of the propping agent? List the types
of
agents used in
the industry. Why are glass beads not used in fracture treatments today?
(9)
List the factors that should be considered in selection of a proppant. What
additives would you consider
if
the fracture fluid was water and the temperature
was
250
OF.
(10)
What are the steps in a hydraulic fracturing treatment? Explain each one in
detail.
treatment.
fracturing treatment?
“composite fracturing coefficient” as defined by Smith
(1965)?
Problems
(1)
The following equation is a mass balance which states that the rate of fluid
injection is equal to the rate of the fluid loss from the fracture to the formation plus
the rate of volume increase of the fracture itself:
dA
Assuming
u(t)
=
C/fi
and
qi
=
constant, show by using Laplace transform that
the fracture area is given by:
A(t)
=
*
[
exp(
G)2
2cr
X
erfc(
7
2cfi
1
+
7
4cfi
-
11
4TC2
(This
equation
is
often referred to as “Carter’s Equation”.)
(2)
Show that the productivity ratio
(PR)
defined as
kavg/k,
where
k
is the
permeability of the unfractured formation and
kavg
is the average permeability of
’
The help extended by M. Parlar
in
preparing this section is greatly appreciated.
156
I-
Fig.
4.1-1.
(a) Sample problem
(2).
a
(b) Sample problem
(3).
TIME
the fractured formation,
is
given by:
Hint: Use Darcy’s law for radial flow in parallel beds to find the average permeabil-
ity of the fractured zone and for the radial flow in beds in series to find the average
permeability of the fractured formation (Reference: Craft et
al.,
1962).
(3)
The following pressure history was recorded during a hydraulic fracturing
treatment of the sandstone formation at 10,000 ft
-
see Fig. 4.1-lb.
(a) What is the fracture initiation pressure?
(b) What is the instantaneous shut-in pressure?
(c) Calculate the frictional pressure gradient.
(d) If the specific gravity (at average well temperature)
of
the fracturing fluid
(e) What is the bottomhole treating pressure (fracture propagation pressure)?
(f)
What would be the pressure on the proppant immediately after the fracture
treatment if the formation pressure is
5000
psi?
(h)
If
the bottomhole flowing pressure is
3500
psi during production, what is
the pressure on the proppant?
(4)
A
7000-ft well
is
to be stimulated by hydraulic fracturing. The following
information is available: Formation permeability to fracturing fluid
=
2 md, forma-
tion porosity fractional
=
0.22, fracturing fluid viscosity at reservoir conditions
=
5
cP, formation fluid compressibility
=
5
X
lo-’
psi-’, viscosity of reservoir fluid
=
10
cP, fracturing gradient
=
0.7
psi/ft, formation pressure
=
3750
psi, injection rate
=
20 bbl/min, total injection volume
=
20,000
gal, and the fracture width
=
0.1 in.
is 1.1, what is the fracture gradient?
157
The filtration data for the fracturing fluid
is
as follows:
vr
t
(cc)
12 1
22
4
30
9
51 25
55
30
Calculate
:
(a) Fracture area using Carter’s Equation.
(b) Fracture penetration assuming a single vertical fracture extending from the
wellbore out equally in both directions, with a vertical extent of
50
ft.
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Acme Resin,
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1982.
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2,
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1964.
Subsurface fracturing from shoal nuclear deronation.
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3001,
USAEC-
USMB,
18
pp.
Atkinson, C.H. and Lekas,
M.A.,
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Atomic-age fracturing may soon open up stubborn reservoirs.
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Gas
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Baker, H.R., Bolster, R.N., Leach, P.B. and Little, R.C.,
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Advantageous use
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Economics of contained nuclear explosions
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1980.
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1957.
Derivation of the general equation for estimating the extent of the fractured area.
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1978.
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Christianovich, S.A. and Zheltov, Y.P., 1955. Formation
of
vertical fractures by means of highly viscous
Clark, J.B., Fast, C.R. and Howard, G.C., 1952. A multiple-fracturing process for increasing productivity
Clark, R.C., Freeman, H.G., Bolstead, J.H. and Coffer, H.F., 1953. Application of hydraulic fracturing to
Coffer, H.F., Bray, B.G. and Knutson, C.F., 1964. Application of nuclear explosives to increase effective
Cooke,
C.E.,
1973. Conductivity of fracture proppants in multiple layers.
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Pet. Technol.,
9: 1101-1107
Cooke, D., 1976.
Hydraulic fracturing with a high strength proppant. SPE Paper 6213
presented at 1976
Craft, B.C., Holden, W.R. and Graves Jr., E.D.,' 1962.
Well Design, Drilling and Production.
Prentice-Hall,
Crawford, D.L., Earl, R.B. and Monroe, R.F., 1973. Friction reducing and gelling agent for organic
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Pet. Technol.,
25(1): 83-97.
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Pet. Eng.,
Oct.: 17-24.
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Oil Wkly,
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Dowell Div. of Dow Chemical Co., 1965.
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Dow Chemical Company, Tulsa, Okla.
Farris, R.F., 1946. Unpublished report, Pan American Petroleum Corporation, Tulsa, Okla. In: G.C.
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Hydraulic Fracturing
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Bitumens Asphalts and Tar Sands.
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Petroleum Production Handbook.
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Grant, B.F., Duvall, W.I., Obert, L., Rough, R.L. and Atchison,
T.C.,
1950. Research on shooting oil and
gas
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Drill. Prod. Pract. API,
p. 303.
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Chem. Eng. News,
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Halliburton Co., 1976.
Hydraulic Fracturing.
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210: 153-166.
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Knox, J.A., Lasater, R.M. and Dill, W.P., 1964.
A New Concept in Acidizing Utilizing Chemical
Langnes, G.L., Robertson Jr., J.O. and Chilingar, G.V., 1972.
Secondary Recovery and Carbonate
Maly, J.W. and Morton, T.E., 1951. Selection and evaluation of wells for hydrafrac treatment.
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Gas
J.,
Matthews, C.S., 1961. Analysis of pressure build-up and
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Pet. Technol.,
13(9): 862-870.
McGuire, W.J. and Sikora, V.J., 1960. The effect
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vertical fractures on well productivity.
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AIME,
219: 401-403.
McLeod,
H.O.
and Coulter, A.W., 1966.
The Use of Alcohol in Gas Well Stimulation. Paper SPE 1633,
presented at SPE Third Annu. East. Reg. Meet., Columbus,
Ohio,
Nov.
10-11.
Minich, A,, 1945. Bodying agent for liquid hydrocarbons.
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Patent No.
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M.,
1949.
Physical Principles of Oil Production.
McGraw-Hill, New York,
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Proc. Fourth World Pet. Congr.,
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Drilling Prod. Pract., API,
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U.S.
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Patent
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3,046,222 (July 24).
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This Page Intentionally Left Blank
161
Chapter
5
ACIDIZING
OILWELLS
JOHN
0.
ROBERTSON Jr. and GEORGE V. CHILINGARIAN
HISTORICAL
USE
OF
ACIDS
IN
OILWELLS
Williams et
al.
(1979) noted that the first acid treatments for oilwells were likely
performed in 1895. Herman Frasch, a chemist at Standard Oil Co.’s Solar Refinery
of Lima, Ohio, is credited with developing the concept for acidizing oilwells.
Herman Frasch and
his
close friend John Van Dyke, manager of the Solar Refinery,
obtained many of the early patents for acidizing oilwells and worked closely in
developing the concepts still used today (see Putman, 1933, and Frasch, 1896).
The Frasch patent proposed the use of a reagent (hydrochloric acid) that reacts
with limestone to produce carbon dioxide (gas) and calcium chloride (salt that is
soluble in water), which are later removed from the formation as the well fluids are
produced. The Van Dyke process, using sulfuric acid, produced insoluble salt
(calcium sulfate) which is capable of plugging the pores and pore throats of the
formation. Williams et al. (1979) noted that these pioneers conceived the acidizing
process as a means to increase fluid production in oilwells for the Lima, Ohio, area,
which at that time accounted for one-thrd of the domestic oil production of the
United States (see Hidy and Hidy, 1955). Their first recorded acid treatment was
made on August 10, 1895. The experiment had reasonably good success and the oil
production for the well they treated increased by 300%, whereas gas production
increased by over
400%.
Although the process was recorded in literature several
times over the next few years, the use of acid as a production stimulation tool
decreased until little evidence of acidizing could be found in literature over the next
30
years (Williams et al., 1979, p. 2).
The next significant use of acid in oilwells did not take place until 1928 in
Oklahoma by the Gypsy Oil
Co.,
which was a subsidiary of Gulf Oil Co. (Williams
et
al., 1979). The problem successfully solved was the removal of oilfield scale from
pipe and well equipment (Chapman, 1933). An inhibitor was used with the acid
similar to that used in steel mills to protect the iron from corrosion. Decline in oil
prices and high cost of acid made this type of treatment less economic and once
again acidizing fell into disuse (Chapman, 1933).
The “modern era” of acidizing is directly traced to the experiences of Pure Oil
Co.
and Dow Chemical Co. (Williams et al., 1979). On February
11,
1932, a Pure
Oil Co. well, located in Chippewa Township, Isabella County, Michigan, was
treated with
500
gal of hydrochloric acid. The acid was brought to the wellsite by a
wooden tank (36 in. in diameter and 12
ft
long) mounted on a wagon. Two gallons
162
of arsenic inkbitor were added to the acid to reduce the corrosion of the tubing and
other well equipment. About half of the acid was siphoned into the tubing, followed
by 6 bbl of oil, and allowed to stand over-night. The well was swabbed the following
morning and the remainder of acid was siphoned into the tubing, being diluted by
the oil flush. The limestone formation responded as the oil production increased
from
0
to 16 bbl/D (Newcombe, 1933). Interest in acidizing spread quickly
resulting in the growth of several companies to provide acidizing services. Compa-
nies originating during this era include Dowell Inc. (1932), Oil Maker’s
Co.
(1932),
Chemical Process Co. (1932), and Williams Brothers Treating Corp. (1932). Halli-
burton Oil Well Cementing Co. entered the acidizing market in 1935 (Williams et
al., 1979, p. 2).
Jesse Russell Wilson, who was workmg for the Standard Oil
Co.
of Indiana, filed
a patent
on
March 16, 1933, for treating a sandstone formation with a hydrofluoric
acid (Wilson, 1935). During the same time period, A.M. McPherson, unaware of
Wilson’s work, proposed ths method to the Halliburton Co. and started working for
the company. A well belonging to the King Royalty Co., Archer County, Texas, was
treated by the Halliburton Co. on May 3, 1933, with a mixture of hydrochloric and
hydrofluoric acids. The results were disappointing because the acid dissolved the
calcareous matrix and cementing material of the sandstone and left the wellbore
filled with a large quantity of unconsolidated sand. Consequently, the Halliburton
Co. discontinued work along this line and did not commercially offer
hydrofluoric-hydrochloric
acid mixtures until the middle 1950’s (Williams et al.,
1979, p. 3). Dowell offered a product known as “mud acid” in the 1940’s which was
composed of hydrofluoric and hydrochloric acids. The intent of ths product was to
dissolve the filter cake formed from the drilling fluid in the wellbore during the
rotary drilling process.
Since the mid 1930’s, advancements in the science of oilwell acidizing have been
mainly in the use of additives to improve acid performance and handle a wide range
of problems. Great strides have been made in the development of (1) surfactants to
reduce emulsion formation, improve wettability of rock by acid, to speed cleanup,
and prevent sludge formation, (2) inhibitors to reduce corrosion, (3) buffering
agents to control pH, (4) retarders to retard reaction rates,
(5)
friction reducers, (6)
fluid loss additives to reduce fluid loss from hydraulically induced fracture to the
formation, and
(7)
complexing agents to prevent the precipitation of iron hydrox-
ides.
PURPOSE
OF
OILWELL
ACIDIZING
The primary purpose of acidizing an oilwell is to increase fluid production by
improving the drainage efficiency of the reservoir rock around the wellbore. Acidiz-
ing is particularly applicable to those oilwells producing from carbonate formations
(dolomites and limestones) or clastics in which the cementing material is composed
of carbonates. When forced out into the flow channels of the formation through
163
natural or induced fractures, acid dissolves a portion of the channel. This increases
the cross-sectional area of the channel and, thus, increases the permeability and
porosity of the reservoir rock in the vicinity of the wellbore. Inasmuch as approxi-
mately
50%
of
the reservoir energy to move fluids is expended within a radius of a
few feet from the wellbore (Muskat,
1949),
the increase in permeability about the
well, created by acidization, increases the rate of fluid flow into the well. The cost of
acidization is thus offset by the increase in oil and/or gas production.
ACID TYPES
An acid is a chemical compound containing hydrogen which
is
capable of being
replaced by positive elements or radicals to form salts. Many types of acids, which
are used in acid stimulation of oilwells, fall into two major categories-organic and
inorganic (Harp and Dobbs,
1967,
p.
1).
TABLE
5-LA
Acid properties
Acid type Formula Acid properties
Formula Molarity Specific Ionization Concentrated
weight gravity for acids aqueous
in water solutions
(25
C)
(%
wt)
Mineral type acids
Carbonic O=C(OH)
,
Hydrochloric HCl
Hydrofluoric HF
Nitric HNO,
Nitrous HNO,
Phosphoric
H
,
PO,
Sulfurous
H,SO,
Sulfuric
*,SO,
Organic type acids
Acetic CH, (COOH)
Citric (CO0H)CH ,C(OH)
(COOH)
CH,COOH-H,O
Formic H( COOH)
Oxalic HO,C(CO, )H
46.03
36.47
20.01
63.02
41.02
98.00
82.08
98.08
60.05
192.12
46.03
90.04
12.0 1.18
28.9
1.17
18.0 1.84
17.5 1.05
- -
4.45x10-’(KIA)
-
5 x10-”
(K2A)
strong acid
36.5-38
strong acid
48-51
strong acid
-
7.516
X
lor3
(KIA)
85
4.5 x~o-~(K,)
-
6.34
xlO-*
(K2A)
1.7 x10-,
(KIA)
-
5
X10-6(K2A)
strong acid
95-98
1.754X10-5
(KA)
99-100
7.45 x~o-~(K,,)
-
1.73
X~O-~
(K,~)
4.02
xio-’
(K~~)
5.18
X~O-~
(K,~)
1.772~10-~
(KA)
-
3.8
xlO-,
(KIA)
-
164
TABLE
5-1.B
Conversion formulae for solutions
having
concentrations expressed in various
ways
(After
CRC
Handbook
of
Chemistry
and
Physics,
p.
D-154)
Concentration
of
8ol”te-
SOUGHT
A
N
c:
M
F
A
=
Weight
per
cent of salute
B
=
Molecular weight
of
solvent
E
=
Moleeular weipht
of
solute
F
=
Grams of solute
per
liter of solution
G
=
Molslity
M
=
Molarity
N
=
Mole
fraction
R
=
Density
of
solution
grams
per
cc
A
-
A
E
A
100
-A
~
E+B
IOOOA
E(100
-
A)
10R
X A
E
104R
__
Concentration
of
solute-GIVEN
N
I
G
IOON
X
E
IOOG
X
E
N
X
E
+
(1
-N)E
1
1000
tG
XE
lOOON
B-NXB
I
-
lOOOR
X
N
IOOOR
X
G
N
X
E
i-
(1
-
N)E
1
-EX
IOOOR
X
N
?(
E
IOOOR
X
G
X
E
N
XF.+II -N)B
1
1000+G
XE
M
EE
10R
BXM
hl(B
-
E)
+
IOOOR
______
ioooni
IOOOR
-
(M
X
E)
-
M
XE
F
F
10R
-
B XF
F(B
-
E)
+
IOOOR
X
E
F
E
-
Organic acids are derivatives
of
hydrocarbons containing one or more carboxyl
groups
(-COOH).
These compounds are acidic due to the ionization potential
of
hydrogen of the carboxyl group:
CH,COOH
+
H20
a
CH,COO-
+
H+
+
H20
(5-1)
Inasmuch as organic acids ionize slowly, they are considered to have a lower
“strength”. An acid’s “strength”, or ability to react as an acid, is measured by an
ionization constant. The ionization constant is higher as the acid is more highly
ionized (Table 5-LA). Several organic acids commonly used in oilwell treatment are
formic
(HCOOH),
acetic
(CH,COOH),
oxalic, and citric.
Inorganic acids are derived from mineral sources. In general, they have a greater
“strength” (Table 5-1) and are less expensive to prepare than similar organic acids.
Most
mineral acids are capable
of
being rapidly and highly ionized, and by
definition are considered strong acids. Inorganic acids commonly used in treating
oilwells are hydrochloric, sulfuric, nitric, phosphoric, and sulfamic
(HS0,NH2)
(Williams et al., 1979). Powdered acids (sulfamic and chloroacetic) have only limited
use in well stimulation. Usually they are mixed with water at or near the wellsite.
The main advantage of these expensive acids is their portability to remote locations.
Acid mixtures (acetic
+
hydrochloric, formic
+
hydrochloric, formic
+
hydrofluoric) have been designed mainly to exploit the dissolving power economics
of
hydrochloric acid and also to obtain the lower corrosivity
of
organic acids
(Williams et al., 1979, p. 11). The latter is critical particularly at high temperatures
when corrosion inhibition costs are great.