[FREDD, C N ; FOGLER, H S ] [1996] Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing

Prévia do material em texto

Society of Petroleum Engineers 
SPE 31074 
Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing 
C.N. Fredd, SPE, and H.S. Fogler, SPE, University of Michigan 
Copyright 1996, Society of Petroleum Engineers, Inc. 
This paper was prepared for presentation at the SPE Formation Damage Control Symposium held 
in Lafayette, Louisiana, U.S.A., 14·15 February, 1996. 
This paper was selected for presentation by an SPE Program Committee following review of 
information contained in an abstract submitted by the author(s). C~>ntents of the pap~r. as 
presented, have not been reviewed by the Society of Petroleum Eng1n~ers and are sub,1~ct to 
correction by the author(s). The material as presented, does not necessanly reflect any pos1t1~n of 
the Society of Petroleum Engineers, its officers, or members. Paper~ presented at SPE m~etmgs 
are subject to publication review by Editorial Committees of the SoCiety of Petroleur:n Eng1neers. 
Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not 
be copied. The abstract sh?uld .cont~in conspicuous acknowledgme~t of where and by whom the 
paper was presented. Wnte L1branan, SPE, P.O. Box 833836, Richardson, TX 75083-3836, 
U.S.A., fax 01-214-952-9435. 
Abstract 
Conventione1l matrix acidizing treatments rely on hydrochloric 
acid to stimulate carbonate formations. However, the success 
of these treatments is often limited because of rapid acid 
spending at low injection rates and asphaltic sludge 
precipitation. This study investigated ethylenediamine-
tetraacetic acid (EDTA) as an alternative stimulation fluid. 
Results show that EDTA can effectively wormhole in 
limestone, even when injected at moderate or non-acidic pH 
values (4 to 13) and at low flow rates where only face 
dissolution would occur with HCl. Stimulation with EDTA at 
low injection rates is consistent with the dependence of the 
wormhole structure on the Damkohler number for flow and 
reaction. Sludge tests show that EDTA does not induce the 
precipitation of asphaltic sludge from crude oil, even in the 
presence of 3000 ppm of ferric iron. This result is attributed to 
EDT A being able to form stable chelates with ferric and 
ferrous iron. 
Introduction 
Acidizing treatments are commonly used to remove near-
wellbore damage and create artificial flow channels in carbonate 
formations. Matrix acidizing treatments are most useful when 
fracture acidizing is undesirable, such as when a shale break or 
other natural boundary must be maintained to prevent water or 
gas production, 1 or where fracture acidizing is ineffective, such 
as in soft chalk formations. 2 Unfortunately, matrix treatments 
often require low injection rates to prevent fracturing the 
formation rock or are required in heterogeneous formations 
with zones of low-conductivity (which need stimulation the 
most) that accept acid at low rates. It is at these low injection 
rates that the problem of rapid acid spending severely limits 
21 
the acid penetration distance. The injection of hydrochloric acid 
into carbonate formations at low rates results in face 
dissolution, or complete dissolution of the carbonate matrix 
near the wellbore. This face dissolution requires large volumes 
of acid and provides negligible increases in the conductivity of 
the formation. 
Various acid systems have been used to reduce the 
limitations of rapid acid spending at low injection rates. A few 
of the acids include: 1) weak acids, such as acetic and formic 
acid, which have relatively low H+ concentrations and therefore 
react with carbonates at a slower rate than HCJ,3 2) chemically 
retarded acids, such as oil external microemulsion systems 
containing HCl, that retard acid diffusion to the carbonate 
surface and thus allow deeper penetration of live acid2, and 3) 
foamed acids (nitrogen gas and aqueous HCl) that prevent acid 
from spending outside the primary dissolution channel thereby 
promoting the growth of wormholes.4 
Although retarded and foamed acid systems can stimulate 
carbonate formations at lower injection rates, strong acids such 
as HCl induce the precipitation of asphaltic sludge from crude 
oil. This sludge can plug the formation and restrict production 
after an acidizing treatment. When ferric ions are present, this 
problem is even more severe.5 Thus, adequa~e corrosion 
protection becomes more essential. Acetic acid, an iron 
chelating agent, does not reduce sludging tendencies in the 
presence of ferric and ferrous iron.5 A variety of acid additives 
(anti-sludging agents, corrosion inhibitors, and iron reducing 
agents) have been used to prevent the sludging probl~m. 
However, their effectiveness is limited by the need to obtam a 
compatible combination of additives and a lack of 
understanding of the complex chemistries involved in the 
precipitation reactions. These limitations demonstrate the need 
for an alternative stimulation fluid that combines the ability to 
stimulate at low injection rates with fluid properties that are 
not conducive to asphaltic sludge precipitation or corrosion 
problems. . . . 
Ethylenediaminetetraacetic acid (EDTA) Is an alternative 
fluid that is capable of stimulating carbonate porous media. 
EDTA is a chelating agent that stimulates by means of 
sequestering the metal components of the carbonate matrix. 
The dissolution mechanism is different from HCl in that 
hydrogen ions are not required. The dissolution, however, is 
2 ALTERNATIVE STIMULATION FLUIDS AND THEIR IMPACT ON CARBONATE ACIDIZING SPE 31074 
enhanced at low pH through a combination of hydrogen attack 
and chelation. Although EDTA has not been used for carbonate 
acidization, it has been used successfully for the removal of 
calcium carbonate scale from the sandstone formations of the 
Prudhoe Bay field.6,? Conventional acid treatments could 
dissolve the scale, but scale reprecipitation from the spent acid 
caused rapid productivity decline. EDTA effectively chelated or 
sequestered the metal ions of the dissolved scale, thus 
preventing their reprecipitation. Other related applications of 
EDTA include the removal of calcium sulfate anhydrite scale 
from brine heater tubes and boilers8•9 and the removal of 
sulfate and carbonate minerals from clay assemblages. 10 This 
paper describes applications of EDTA in carbonate acidizing 
and compares results from coreflood experiments and from 
sludge tests for EDTA, HAc, and HCl. 
Wormhole Formation in Carbonates 
The flow and reaction of hydrochloric acid in carbonate porous 
media results in the formation of highly conductive flow 
channels or wormholes. Wormholes form because of the 
natural heterogeneity of the porous matrix and the rapid, mass 
transfer limited and almost complete dissolution of the mineral 
in acid. During stimulation, the acid preferentially flows to the 
regions of highest permeability (the largest pores, vugs, or 
natural fractures). These initial flow paths are enlarged by rapid 
dissolution of the matrix material, causing these regions to 
receive even more of the flow. A dominant channel quickly 
forms and continues to propagate while diverting flow from 
other regions. Once formed, the wormhole channels provide 
negligible resistance to flow and carry essentially all the 
injected fluid. 
Previous studies have shown that the phenomenon of 
wormhole formation is governed by the Damkohler number for 
flow and reaction. 11 The Damkohler number (Da) is defined as 
the ratio of the net rate of dissolution by acid to the rate of 
convective transport of acid. When the rate of reaction is very 
rapid compared to the rate of mass transfer, the net rate of 
dissolution is mass transfer limited and the Damkohler number 
is given by 
Da = a De Z/3 Lj Q . . . . . . . . . . . . . . . . . . . . . . . ( 1) 
where De is the effective diffusion coefficient, Q is the flow 
rate, L is a length scale, and a is a proportionality constant 
that depends on the carbonate sample. The dissolution of 
limestoneby HCl, Eq. 2, is mass transfer limited at 
temperatures above 0°C, 12 while the dissolution of dolomite 
by HCl is mass transfer limited above 50°C. 13 
Typical wormhole structures for the dissolution of limestone 
by HCl, Fig. 1, range from face dissolution (or complete 
dissolution of the core starting form the inlet flow face) at 
high Damkohler numbers to uniform dissolution resulting in 
ramified wormhole structures at low Damkohler numbers. 
Single dominant wormhole channels are obtained at 
22 
intermediate Damkohler numbers. Hoefner and Fogler11 
observed this general trend of increasing channel branching 
with decreasing Damkohler number. 
The efficiency of acidizing treatments was observed to go 
through a maximum (i.e., minimum volume of acid required 
for channel breakthrough) with respect to changes in the 
Damkohler number. 11 Other investigators have reported the 
existence of an optimum injection rate for carbonate 
acidizing. 14 (This observation is consistent with that of 
Hoefner and Fogler11 for a constant diffusion coefficient, 
because the Damkohler number is inversely proportional to the 
injection rate.) At high Damkohler numbers (low injection 
rates) acid is consumed on the inlet flow face of the core, thus 
permeability increases are negligible and the stimulation is 
inefficient. At lower Damkohler numbers acid can penetrate 
into the porous matrix and enlarge flow channels. Live acid 
reaches the advancing tip of the channel and a wormhole 
forms. The wormhole provides significant permeability 
increases and requires a minimum volume of acid to permeate 
the rock matrix, thus providing an efficient mechanism of 
stimulation. As the Damkohler number is reduced further, flow 
channels become more highly branched. Dissolution occurs 
over a high surface area which results in a decrease in 
stimulation efficiency. Thus, there is an optimum Damkohler 
number at which the fewest pore volumes of acid are required 
for channel breakthrough. 
Chelation Chemistry 
Chelating agents have the ability to combine with metal ions 
M+n by surrounding them with one or more ringed structures. 
The process of chelation, or sequestering, results in the 
formation of metal/ligand chelates with exceptionally high 
stability. This stability makes chelating agents valuable for 
applications such as water softening, inactivation of metal 
ions, and titration of metal ions. Chelates of transition metals 
(such as iron) typically have the highest stability, while many 
chelates of alkaline-earth metals (such as calcium) have low 
stability with most chelating agents. Essentially all positive 
metal ions form stable chelates with some type of chelating 
agent. Aminopolycarboxylic acids, such as EDTA, are capable 
of forming stable chelates with alkaline-earth metals. 15 The 
stability constants for various metal/ligand chelates of EDTA 
are listed in Table 1. Notice that EDTA forms stable chelates 
(log K of approximately 8 or greater) with ferric and ferrous 
iron, as well as with calcium. 
The chemical structure of the ethylenediaminetetraacetic acid 
is shown in Fig. 2. The structure of EDTA is typically 
represented by H4 Y where the four hydrogen's are those of the 
carboxylic acid groups. Aminopolycarboxylic acids undergo a 
stepwise loss of protons to reach their fully ionized state, as 
shown by Eq. 3 through Eq. 6 for EDTA. 
H4Y~H3Y- 1 +H+ ................... (3) 
H3 y-1 ~Hz y-Z + H+ .................. (4) 
Hz y-z ~ HY-3 + H+ ................... (5) 
HY-3 ~ y-4 +H+ ..................... (6) 
SPE 31074 C.N. FREDD, H.S. FOGLER 3 
The distribution of ionic species is dependent upon the 
equilibrium constants for each of the dissociation reactions and 
on the pH of the solution. The distribution of ionic species for 
EDT A at room temperature was calculated using the pK values 
obtained from Welcher16 and is shown in Fig. 3. At a pH of 
approximately 4.5, EDT A is in the form of H2 y-2. At higher 
pH values of about 8.5 and 13, EDTA successively 
deprotonates to the HY-3 and y-4 species. This distribution is 
consistent with results presented by other investigators.8-10 
The dissolution of calcium carbonate by EDT A at a pH 
between 4 and 5 has been reported as a combination of 
hydrogen attack and free calcium ion sequestering.6 Note that 
in this pH range, EDTA is predominantly the H2 Y-2 form. 
Combining Eqs. 2 and ?leads to the overall reaction; 
The reaction rate of EDT A at pH 5 with calcium carbonate 
(Eq. 8) was reported to be approximately 4x10-7 mol/cm2/s at 
93°C.6 This rate was measured using a rotating disk apparatus 
with 500 psi C02 partial pressure. At these conditions, the 
reaction rate is about a factor of two lower than that of HCl 
with limestone. 
EDTA has been used to dissolve calcium carbonate and other 
minerals from clay assemblages. This reaction is carried out 
between pH 10 and 13 to avoid altering or destroying various 
clay species as usually happens with conventional acid 
dissolution. EDTA was reported to dissolve gypsum, 
anhydrite, calcite, dolomite, magnesite, and apatite in amounts 
of 43, 34, 23, 21, 19, and 1 g per liter of solvent, 
respectively. 10 At pH 13, the dissolution is analogous to 
calcium sulfate scale removal8·17 and is given by: 
y-4 + CaC03 ~ Ca y-
2 + C03 - 2 ............. (9) 
Similarly, at pH 8.8, 
HY-3 + CaC03 ~ Ca y-
2 + HC03- .......... ( 1 0) 
The diffusion coefficient of EDTA at room temperature is 
approximately 6x 1 o-6 cm2/s which is about an order of 
magnitude lower than that of HCl (4x10-5 cm2/s). Based on 
these diffusion coefficients and the rate of dissolution being 
comparable to HCl, the dissolution of limestone by EDT A is 
assumed to be mass transfer limited. This assumption is 
supported by results presented in the following sections that 
revealed the formation of wormholes in limestone when EDT A 
was injected at pH values between 4 and 13. 
Asphaltic Sludge Precipitation 
Formation damage caused by the precipitation of asphaltic 
sludge when crude oil is contacted by acid is a common 
23 
problem during acidizing treatments. Asphaltenes are present 
in crude oil in the form of colloidally dispersed particles. These 
particles consist of an aggregate of polyaromatic molecules 
surrounded and peptized by lower molecular weight neutral 
resins and paraffinic hydrocarbons.5·18 The asphaltenes will 
flocculate and precipitate from the crude oil when the 
asphaltene micelles are depeptized by any chemical, electrical, 
or mechanical means. In the presence of strong acids such as 
hydrochloric acid, the colloidal dispersions are destabilized 
causing the formation of asphaltene precipitates (sludge) and 
rigid film emulsions. 19 Weaker acids, such as acetic acid, do 
not cause significant sludge precipitation. 5 Sludge 
precipitation has been found to increase dramatically with acid 
concentrationS and has also been reported in EOR caustic flood 
projects due to the high pH of the system. 18 Sludge 
precipitation is considerably worse in the presence of ferric 
iron5 which may be due to oxidative polymerization processes 
in the resin layer. 19 Ferrous iron also contributes to asphaltic 
sludge precipitation but to a considerably less extent than ferric 
iron. 
The formation of asphaltic sludge and rigid film emulsions 
can lead to partial or complete plugging of the formation after 
an acidizing treatment. While this damage can be removed 
from the .tubing and casings by aromatic solvents, it is 
extremely difficult to remove from a formation. This difficulty 
is due to the inability to inject fluids into the formation such 
that they can contact the sludge particles. For this reason, it is 
essential to prevent the precipitation of asphaltic sludge. 
Various acid additives have been used to control sludge 
precipitation, such as acid corrosion inhibitors, anti-sludging 
agents, iron reducing agents (to convert ferric iron to the less 
damaging ferrous iron), along with other additives suchas 
mutual solvents, wetting agents, and iron chelating agents. 
These additives can be costly and require testing to ensure 
compatibility of the various components as well as their 
effectiveness for prevention of asphaltic sludge precipitation. 
The alternative, described in this paper, is to use a stimulation 
fluid that will not induce asphaltic sludge precipitation. 
Experimental Procedures 
Coreflood Experiments. Linear coreflood experiments 
were performed using the apparatus shown schematically in 
Fig. 4. Texas cream chalk and Indiana limestone cores of 
approximately 1.5 in. diameter and 2.5, 4, or 5 in length were 
studied. The cores had porosities between 15 and 20 percent 
and permeabilities of 0.8 to 2 md. Experiments were performed 
by first vacuum saturating a core with deionized (DI) water and 
mounting it in a standard Hassler cell. Overburden pressures of 
at least 2200 psi were applied to ensure that flow did not 
bypass the core. Fluid was injected axially through the core at 
a constant rate using an ISCO syringe pump. DI water was 
first injected through the core at the desired flow rate. When 
the flow stabilized, acid injection was started. To avoid 
contacting the pump with acid, the acid was displaced by water 
from a piston accumulator. The pressure drop across the length 
of the core was monitored by a differential pressure transducer 
and recorded by a personal computer. This data was used to 
calculate the permeability as a function of fluid volume 
pc
Realce
4 ALTERNATIVE STIMULATION FLUIDS AND THEIR IMPACT ON CARBONATE ACIDIZING SPE 31074 
injected using Darcy's law. Gaseous reaction products, 
specifically C02, were kept in solution by maintaining a 
system pressure of at least 1000 psi with a back pressure 
regulator. (This high back pressure was not required for 
experiments with EDTA injected at pH values greater than 8.8 
since no gaseous products were generated.) The experiment was 
terminated when the wormhole broke through the core, as 
evidenced by a negligible pressure drop. 
Experiments were conducted at room temperature with 0.25 
M EDTA, as well as 0.5 M HAc and HCI. Note that all of 
these solutions have the same effective acid capacity, or 
dissolving power. EDTA solutions were prepared from the 
reagent grade disodium salt dihydrate of ethylenediamine-
tetraacetic acid. Addition to DI water resulted in a pH of about 
4.6. Sodium hydroxide and hydrochloric acid were used to 
adjust the pH to the desired value. 
Neutron Radiography. Neutron radiography was used to 
image the wormhole structures formed during the various 
coreflood experiments. This technique is ideally suited for 
imaging structures within consolidated porous media because 
the matrix is virtually transparent to thermal neutrons. 21 To 
image the wormholes, cadmium-containing Wood's metal was 
injected into the dissolution channels. Cadmium is an 
excellent neutron absorber and thus, provides high contrast 
between the dissolution channels and the consolidated porous 
matrix. The Wood's metal casting technique11 utilized a 
. vacuum oven to dry and evacuate the acidized cores and then to 
inject molten Wood's metal at 100°C and at atmospheric 
pressure. The injection pressure was controlled to ensure that 
the metal invaded only the pore spaces that were enlarged by 
dissolution. Once invaded, the metal was allowed to solidify 
and the Wood's metal-filled cores were placed in a beam of 
thermal neutrons for imaging. The film radiography method 
was used to record the neutron flux onto a photographic film 
as shown in Fig. 5. Since thermal neutrons cannot directly 
expose the film, an intermediate screen was used to absorb the 
neutrons and generate a secondary form of radiation (such as 
electrons, gamma-rays, or visible light). In this study, a 
gadolinium oxisulphide screen was used to expose Kodak 
Azo ™ black and white film. An exposure time of about 40 
seconds was required and the photographic film was developed 
using standard procedures. Neutron radiography is described in 
more detail elsewhere in the literature.21,22 
Sludge Tests. Sludge tests were conducted to determine the 
relative amounts of asphaltic precipitates formed when crude 
oil was contacted with 0.25 M EDTA, 15% HAc, and 15% 
HCI. The crude oil was obtained from a field in west Texas. 
The acid system was prepared by adding 3000 ppm of FeC13 to 
100 ml of acid and heating to 85°C. The acid system was then 
mixed with 100 ml of crude oil (also at 85°C) and vigorously 
stirred for 30 minutes. The mixture was allowed to sit for at 
least one hour and was then vacuum filtered through a 
Whatman #41 filter. The filter was dried in an oven at 100°C 
and the mass of sludge was determined. (The mass of the 
sludge was found by subtracting the mass of the filter and the 
mass of oil in a blank from the mass of the filter with sludge. 
24 
The blank was an oven dried filter used to filter 100 ml of 
crude oil that had been contacted with DI water using the above 
procedure.) 
Results 
Wormhole Formation with EDT A and HAc. 
Neutron radiographs of wormholes formed by 0.25 M EDTA 
injected into limestone cores at 0.3 cc/min and pH values of 
4.0, 8.8, and 13.0 are shown in Fig. 6. EDTA is capable of 
stimulating the carbonate matrix without the typical hydrogen 
attack (Eq. 2) as evidenced by the formation of wormholes 
when injected at pH values of 8.8 and 13.0. Injection of EDT A 
at pH 4.0 required fewer pore volumes to breakthrough than 
the higher pH stimulations ( 4.8 as opposed to 10.0 or 12.7 
pore volumes). This increased efficiency at the low pH is most 
likely due to the rate of dissolution being enhanced by a 
combination of calcium chelation and hydrogen attack. 
The dependence of wormhole structures on the Damkohler 
number was investigated over a wide range of flow rates for the 
dissolution of Indiana limestone by 0.5 M HAc and 0.25 M 
EDTA injected at pH 4 and 13. The neutron radiographs of the 
wormholes are shown in Figs. 7, 8, and 9 for Damkohler 
numbers spanning about three orders of magnitude. (The 
Damkohler number was determined by setting aiL equal to 1.0 
in Eq. 1.) For comparison, refer to Fig. 1 which shows the 
wormhole structures formed by 0.5 M HCl over a similar 
range of Damkohler numbers. The wormhole structures for all 
fluids investigated are consistent with the results of Haefner 
and Fogler10 in that decreasing the Damkohler number (or 
increasing the injection rate) increases the amount of channel 
branching. 
Wormhole Formation at Low Injection Rates. 
Typical permeability responses for linear corefloods with 
EDTA, HAc, and HCl are shown in Fig. 10. The 
corresponding neutron radiographs are shown in Fig. 11. 
These experiments were conducted with an injection rate of 0.1 
cc/min. After injection of 43.1 pore volumes of 0.5 M HCl, 
negligible increases in permeability were observed since rapid 
acid spending led only to face dissolution. This type of 
dissolution requires large volumes of acid and results in limited 
acid penetration. In contrast, 0.5 M HAc and 0.25 M EDTA 
injected at pH 4 and 13 broke-through a 4 inch limestone core 
after injection of 1.7, 3.3, and 8.1 pore volumes, respectively. 
Injection of these fluids resulted in the formation of single 
wormhole channels. The ability of EDTA and HAc to 
wormhole at low injection rates is consistent with the 
dependence of the wormhole structure on the Damkohler since 
the diffusion coefficients of EDTA (6x1o-6 cm2/s) and HAc 
(lx10-5 cm2/s) are lower than that of HCl (4x1o-S cm2/s). 
Thus, improved acid penetration can be obtained during matrix 
stimulations with EDT A and HAc without the costly near 
wellbore face dissolution associated with HCI. 
The dependence of the number of pore volumes to 
breakthrough on the injection rate is shown in Fig. 12. The 
data are for the dissolution of limestone by 0.5 M HCl, 0.5 M 
HAc, and 0.25 M EDTA at pH 4 and 13. (Data for wormhole 
structures shown in Figs. 1,7, 8, and 9 are included in the 
pc
Realce
pc
Realce
pc
Realce
SPE 31074 C.N. FREDD, H.S. FOGLER 5 
plot.) Breakthrough was defined as the point at which the 
permeability ratio reached at least 100. The data are consistent 
with the existence of an optimum Damkohler number (or 
injection rate) as observed by previous investigators. 11 •14 At 
low injection rates, a large number of pore volumes were 
- required to breakthrough due to face dissolution. As the flow 
rate was increased, the pore volumes to breakthrough decreased 
and reached a minimum as an efficient wormhole was obtained. 
At higher injection rates, the pore volumes to breakthrough 
increased with the amount of channel branching. Notice that 
below about 0.03 cc/min, EDTA injected at pH 4 required 
fewer por0 volumes to breakthrough than both HCl and HAc. 
This efficiency is due to the dependence of the wormhole 
structure on the Damkohler number and the relatively low 
diffusion coefficient of EDTA compared to those of HAc and 
HCI. One can see that as the diffusion coefficient is decreased 
from HCl to HAc to EDT A, the optimum injection rate 
decreased. Thus, as the diffusion coefficient is decreased, the 
injection rate must also be decreased to maintain a relatively 
constant Damkohler number and an efficient wormhole. 
Sludge Precipitation. The mass of asphaltic sludge 
precipitated when crude oil was contacted with 0.25 M EDTA 
(pH 6), 15% HAc, and 15% HCl is shown in Fig. 13. 
Despite each stimulation fluid containing 3000 ppm ferric 
iron, only HCl induced significant amounts of asphaltic sludge 
to precipitate (25 g). This precipitation was due to the high 
acid concentration and the presence of ferric iron. Contact with 
EDT A and HAc induced only trace amounts of sludge to 
precipitate (about 0.5 g). The lack of sludge precipitation with 
EDTA was attributed to moderate acidity and the ability to 
form stable chelates with iron. The lack of sludge precipitation 
with HAc was somewhat surprising since HAc has been 
reported to have virtually the same sludging tendencies as HCl 
in the presence of ferric and ferrous iron. 5 HAc chelates ferric . 
iron thereby preventing it from interacting with asphaltene 
particles. However, the HAc solution (HAc plus FeC13) had a 
pH of about 1.1 which should have been low enough to induce 
sludge precipitation. The sensitivity of the crude oil to 
hydrogen ions was tested by contacting it with plain 15% HCl 
(i.e., no FeCl3). The result was the precipitation of only trace 
amounts of asphaltic sludge. Therefore, the asphaltenes in this 
particular crude oil were destabilized by ferric ions and not by 
hydrogen ions. The apparent effectiveness of HAc for not 
inducing sludge precipitation was due to the ability of HAc to 
chelate ferric iron and to the inability of the acid to destabilize 
the asphaltene particles in the crude oil. 
In addition to EDTA not inducing asphaltic sludge 
precipitation, costly acid additives such as corrosion inhibitors 
and reducing agents may not be necessary. Corrosion is 
negligible for alkaline solution of EDTA below 204 OC (with 
possible exceptions when copper, tin, and aluminum are 
present) 16 and EDTA chelate_s both ferric and ferrous irons 
(Table 1). Therefore, EDTA provides the properties necessary 
for a matrix stimulation fluid (wormholes formed in carbonates 
at low injection rates) while not requiring additives to control 
corrosion or asphaltic sludge precipitation. 
25 
Conclusions 
1. EDT A is capable of forming wormholes in limestone 
when injected at pH values between 4 and 13. Hydrogen ions 
are not required since EDTA can directly chelate calcium from 
the carbonate matrix. 
2. Neutron radiographs reveal that as the Damkohler number 
is decreased (injection rate increased) the amount of channel 
branching increases when limestone is dissolved by EDTA, 
HAc, and (as previously reported) HCI. 
3. EDTA and HAc both stimulate more efficiently than HCl 
when injected at rates less than about 0.1 cc/min. At these low 
rates, HCl is rapidly consumed on the inlet flow face and only 
face dissolution occurs. The ability of EDTA and HAc to 
wormhole at low injection rates is consistent with the 
dependence of the wormhole structure on the Damkohler 
number since the diffusion coefficients of EDTA and HAc are 
lower than that of HCI. 
4. EDTA, HAc, and HCl exhibit an optimum Damkohler 
number (or injection rate) at which the pore volumes required 
to breakthrough is minimized. As the diffusion coefficient is 
decreased from HCl to HAc to EDT A, the optimum injection 
rate decreases. 
5. Significant amounts of asphaltic sludge precipitated when 
crude oil was contacted by 15% HCI. However, only trace 
amounts of sludge was observed to precipitate when the same 
oil was contacted by pH 6 EDTA and 15% HAc, despite the 
presence of 3000 ppm of ferric iron. The lack of sludge 
precipitation with EDTA is attributed to its moderate acidity 
and ability to chelate iron. The apparent effectiveness of HAc 
is due to the ability of HAc to chelate ferric iron and to the 
inability of the acid to destabilize the asphaltene particles in 
the crude oil. 
6. An additional benefit of EDTA is that corrosion 
inhibitors may not be necessary for alkaline solution of EDT A 
up to 204°C and reducing agents are not required since EDTA 
chelates both ferric and ferrous irons. Thus, the use of EDTA 
as a stimulation fluid may eliminate the need for complex and 
costly acid additives. 
Nomenclature 
a = proportionality constant 
frl = Damkohler number 
De =effective diffusion coefficient, L2/t, cm2/s 
k = permeability, L 2, md 
K = stability constant 
L = length scale, L, em 
P V = pore volumes 
Q =injection rate, L3/t, cc/min 
p =density of material, MfL3, g/cm3 
Subscripts 
BT =breakthrough 
in} = injected 
o =initial 
Acknowledgments 
We would like to acknowledge the Industrial Affiliates 
Program on Flow and Reaction in Porous Media at the 
6 ALTERNATIVE STIMULATION FLUIDS AND THEIR IMPACT ON CARBONATE ACIDIZING SPE 31074 
University of Michigan, Aramco, ARCO Exploration and 
Production Services, Chevron Petroleum Technology 
Company, Conoco Production and Research Division, Dowell 
Schlumberger, Halliburton Services, Mobil Research and 
Development, Texaco Inc., and Unocal Science and 
Technology Division for their support of this work. 
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9. Jamialahmadi M. and Moller-Steinhagen H.: "Reduction of 
Calcium Sulfate Scale Formation During Nucleate Boiling by 
Addition of EDTA," Heat Trans. Eng. (1991) 12, No.4, 19-
26. 
10. Bodine M.W. and Fernalld T.H.: "EDTA Dissolution of 
Gypsum, Anhydrite, and Ca-Mg Carbonates," J. Sedimentary 
Petrology (Dec. 1973) 43, No. 4, 1152-1156. 
11. Hoefner M.L. and Fogler H.S.: "Pore Evolution and Channel 
Formation During Flow and Reaction in Porous Media,"A/ChE J. (Jan. 1988) 34, No. 1, 45-54. 
12. Lund K. et al.: "Acidizing- II. The Dissolution Of Calcite In 
Hydrochloric Acid," Chern. Eng. Sci. (1975) 30, 825-835. 
13. Lund K., Fogler H.S., and McCune C.C.: "Acidizing- I. The 
Dissolution Of Dolomite In Hydrochloric Acid," Chern. Eng. 
Sci. (1973) 28, 691-700. 
14. Wang Y., Hill A.D., and Schechter R.S.: "The Optimum 
Injection Rate for Matrix Acidizing of Carbonate 
Formations," paper SPE 26578 presented at the 1993 Annual 
Technical Conference and Exhibition, Houston, TX, October 
3-6. 
15. Martell A. E. and Calvin M.: Chemistry of Metal Chelate 
Compounds, Prentice-Hall, NJ (1956). 
16. Welcher F.J.: The Analytical Uses of Ethylenediamine-
tetraacetic Acid, D.Van Nostrand Company, Inc., New York 
(1958). 
1 7. Cikes M. et al.: "A Successful Treatment of Formation 
Damage Caused by High-Density Brine," SPE Prod. Eng. 
(May 1990) 175-179. 
18. Newberry, M.E. and Barker, K.M.: "Formation Damage 
Prevention Through the Control of Paraffin and Asphaltene 
Deposits," paper SPE 13796 presented at the 1985 SPE 
26 
Production Operations Symposium, Oklahoma City, OK, 
March 10-12. 
19. Moore, E.W., Crowe, C.W., and Hendrickson, A.R.: 
"Formation, Effect and Prevention of Asphaltene Sludges 
During Stimulation Treatments," paper SPE 1163 presented 
at the 1965 SPE Rocky Mountain Regional Meeting, 
Billings, MT, June 10-11. 
20. Jacobs, I.C. and Thorne, M.A.: "Asphaltene Precipitation 
During Acid Stimulation Treatments," paper SPE 14823 
presented at the 7th SPE Symposium on Formation Damage 
Control, Lafayette, LA, 1986. 
21. Jasti J.K. and Fogler H.S.: "Application of Neutron 
Radiography to Image Flow Phenomena in Porous Media," 
A/ChE J. (April 1992) 38, No. 4, 481-488. 
22. Lindsay, J.T. et al.: "Neutron Radiography Applications at 
the University of Michigan, Phoenix Memorial 
Laboratory," Neutron Radiography (3), Proceedings of the 
Third World Conference, J.P. Barton (ed.), Gordon and 
Breach Science Publishers, Boston (1990) 621-636. 
Sl Metric Conversion 
cp x 1.0* 
ft X 3.048* 
ft2 X 9.290 304* 
ft3 X 2.831 685 
OF CF-32)/1.8 
in. x 2.54* 
md x 9.869 233 
psi x 6.894 757 
*Conversion factor is exact. 
Factors 
E-03 = Pa·s 
E-01 = m 
E-02 = m2 
E-02 = m3 
=oC 
E+OO =em 
E-04 = Jlm2 
E+OO = kPa 
Table 1. - Stability constants for EDTA chelates 
(20°C and ionic strength= 0.1).15 
Metallon Log K 
Iron Ill 25.1 
Iron II 14.2 
Manganese II 13.5 
Calcium 10.59 
Magnesium 8.69 
Strontium 8.63 
Barium 7.76 
Sodium 1.66 
Q = 0.04 cc/min 
Da = 1.90 
PVinj = 43.1 
Q = 0.11 cc/min 
Da=0.64 
PVinj = 10.0 
Q = 0.3 cc/min 
Da = 0.23 
PVBT = 3.3 
Q = 1.05 cc/min 
Da = 0.067 
PVBT =0.8 
Q = 10 cc/min 
Da = 0.0070 
PVBT = 2.1 
Q = 60 cc/min 
Da = 0.0012 
PVBT = 6.7 
Fig. 1 - Neutron radiographs of wormholes formed during the dissolution of Texas cream chalk by 0.5 M HCI. 
0 0 
II II 
H-O-C-CH2 ""' ~ CHTC-0-H 
~ N-CH2-CH2-N ""' 
H-O-C-CH2 CH2-C-O-H 
II II 
0 0 
Fig. 2 - Chemical structure of EDTA. 
27 
0.8 
Cll 
Q) 
·u 
Q) 
0.6 0.. 
Cl') 
4-o 
0 
c:: 
0.4 .Q 
t) 
~ 
'""' ~ 
0.2 
0 
0 2 4 6 8 10 12 
pH 
Fig. 3 - Distribution of ionic species of EDTA at 
room temperature. 
14 
ISCO syringe 
pump 
Hassler cell 
Differential 
pressure 
gauge 
Inlet pressure 
gauge 
Fig. 4 - Schematic of linear coreflood apparatus. 
Thermal 
neutrons 
Wood's metal-
filled core 
Cassette 
.., __ Gd02S screen 
t41-+-- Photographic film 
-
Fig. 5 - Schematic of film neutron radiography system. 
28 
pH=4.0 
PVBT = 4.8 
pH= 8.8 
PVBT = 10.0 
pH= 13.0 
PVBT = 12.7 
Fig. 6 - Neutron radiographs of wormholes formed during the 
dissolution of limestone by 0.25 M EDTA injected at 0.3 cc/min 
(Da = 0.066). 
Q = 0.03 cc/min 
Da = 1.37 
PVBT=5.3 
Q = 0.07 cc/min 
Da=0.59 
PVBT = 2.2 
Q = 0.10 cc/min 
Da = 0.41 
PVBT = 1.7 
Q = 0.5 cc/min 
Da = 0.082 
PVBT = 2.2 
Q = 13.5 cc/min 
Da = 0.0031 
PVinj = 11.1 
Fig. 7- Neutron radiographs of wormholes formed during the dissolution of limestone by 0.5 M HAc. 
29 
Q = 0.025 cc/min 
Da = 0.79 
PVBT = 3.7 
Q = 0.07 cc/min 
Da = 0.28 
PVBT = 3.3 
Q = 0.15 cc/min 
Da = 0.13 
PVBT = 3.6 
Q = 1.0 cc/min 
Da = 0.020 
PVBT = 10.6 
Q = 3.0 cc/min 
Da = 0.0066 
PVBT = 16.2 
Q = 8.0 cc/min 
Da = 0.0025 
PVinj = 24.1 
Fig. 8- Neutron radiographs of wormholes formed during the dissolution of limestone by 0.25 M EDTA 
injected at pH 4. 
Q = 0.009 cc/min 
Da = 2.30 
PVBT = 20.2 
Q = 0.05 cc/min 
Da = 0.42 
PVBT = 6.4 
Q = 0.15 cc/min 
Da = 0.14 
PVsT= 8.9 
Q = 0.3 cc/min 
Da = 0.070 
PVBT = 12.7 
Q = 1.0 cc/min 
Da = 0.021 
PVsT= 25.5 
Q = 3.0 cc/min 
Da = 0.0070 
PVinj = 25.1 
Fig. 9- Neutron radiographs of wormholes formed during the dissolution of limestone by 0.25 M EDTA 
injected at pH 13. 
30 
100 
10 
0 4 8 
-+--HAc 
____,.____ EDTA, pH 4 
--o- EDTA, pH 13 
--+-- HCI 
12 16 
Pore Volumes Injected 
Fig. 1 0 - Permeability response during linear corefloods 
with 0.5 M HAc, 0.5 M HCI, and 0.25 M EDTA injected at 
0.1 cc/min. 
0.5 MHAc 
Da = 0.41 
PVBT = 1.7 
0.25 M EDTA, pH=4 
Da = 0.28 
PVBT = 3.3 
0.25 M EDTA, pH=13 
Da=0.29 
PVBT = 8.1 
0.5 MHCI 
Da = 0.70 
PVinj = 43.1 
Fig. 11 - Neutron radiographs of wormholes formed during the dissolution of limestone by 
HAc, HCI, and EDTA injected at 0.1 cc/min. 
31 
-& 100 
;:::l 
! 
~ 
~ 
B 10 
~ 
Q) 
8 
;:::l 
0 
> g 
0-t 1 
Injection Rate [cc/min] 
EDTA, pH 13 
EDTA,pH4 
HAc 
Fig. 12 - Optimum injection rate for the dissolution of 
limestone by 0.5 M HCI, 0.5 M HAc, and 0.25 M EDTA at 
pH 4 and 13. 
30 ....... 
0 
8 25 
0 
0 - 20 1-o 
Q) 
c. 
:§ 15 
Q) 
01) 
"'0 
10 ;:::l c;; ._ 
0 
5 ~ 
~ 
~ 
~ 
0 
HCI HAc EDTA /· 
Fig. 13 - Mass of asphaltic sludge precipitated when 
crude oil was contacted by 15% HCI, 15% HAc, and 
0.25 M EDT A at pH 6. 
32 
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