Read Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing text version

Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing

C.N. Fredd, SPE, and H. Scott Fogler, SPE, U. of Michigan

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 production1 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. These treatments may also be 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 the acid penetration distance. The injection of hydrochloric acid (HCl) into carbonate formations at low rates results in face dissolution, or complete dissolution of the carbonate matrix near the wellbore. This face dissolution consumes 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: weak acids, such as acetic and formic acid, which have relatively low H concentrations and therefore react with carbonates at a slower rate than HCl3,4; 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 foamed acids (nitrogen gas and aqueous HCl) that prevent acid from spending outside the primary dissolution channel, thereby promoting the growth of wormholes.5 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.6 Thus, adequate corrosion protection becomes more essential. Acetic acid (HAc), an iron chelating agent, does not reduce sludging tendencies in the presence of ferric and ferrous iron.6 A variety of acid additives (antisludging agents, corrosion inhibitors, and iron-reducing agents) have been used to prevent the sludging problem. However, their effectiveness is limited by the need to obtain 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 that of HCl in that hydrogen ions are not required. The dissolution, however, is enhanced at low pH through a combination of hydrogen ion 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.7,8 Conventional acid treatments could dissolve the scale, but scale reprecipitation from the spent acid caused rapid productivity decline. EDTA effectively

Copyright 1998 Society of Petroleum Engineers Original SPE manuscript received for review 14 June 1996. Revised manuscript received 8 December 1997. Paper peer approved 15 December 1997. Paper (SPE 31074) first presented at the 1996 SPE Formation Damage Control Symposium held in Lafayette, Louisiana, 14­15 February.

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 boilers9,10 and the removal of sulfate and carbonate minerals from clay assemblages.11 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 HCl 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.12 The Damkohler number, NDa, 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 ¨

NDamt

aD2/3 L/q , e

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)

where De is the effective diffusion coefficient, q is the flow rate, L is the pore length, and a is a constant that depends on the carbonate core. The dissolution of limestone by HCl, Eq. 2, is mass transfer limited at temperatures above 0°C,13 while the dissolution of dolomite by HCl is mass transfer limited above about 75°C.14

2H

CaCO3 º Ca

2

CO2

H2 O .

. . . . . . . . . . . . . . . (2)

Typical dissolution structures formed during the stimulation of limestone by HCl, Fig. 1, range from face dissolution (or complete dissolution of the core starting from 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 intermediate Damkohler num¨ bers. Hoefner and Fogler12 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.12 Other investigators have reported the existence of an optimum injection rate for carbonate acidizing.15 (This observation is consistent with that of Hoefner and Fogler12 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

SPE Journal, March 1998

34

Fig. 1--Neutron radiographs of wormholes formed during the dissolution of Texas cream chalk by 0.5 M HCl.

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 number of pore volumes ¨ (PV's) 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.16 The stability constants for various metal/ligand chelates of EDTA are listed in Table 1. Notice that EDTA forms stable chelates (log KMY of 8 or greater) with ferric and ferrous iron as well as with calcium. The chemical structure of EDTA is shown in Fig. 2. The structure of EDTA is typically represented by H4Y, where the four hydrogens are those of the carboxylic acid groups. Aminopolycarboxylic acids undergo a stepwise loss of protons to reach their fully ionized state,

Fig. 2--Chemical structure of EDTA.

as shown by Eqs. 3 through 6 for EDTA.

H4Y º H3Y H3Y H2Y HY

3 1 2

1 2 3

H , H , H , H .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . (3) . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)

º H2Y º HY

4

ºY

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6)

TABLE 1--STABILITY CONSTANTS FOR EDTA CHELATES AT 20°C AND IONIC STRENGTH 0.1.16 Metal Ion Iron III Iron II Manganese II Calcium Magnesium Strontium Barium Sodium SPE Journal, March 1998 Log KMY 25.1 14.45 13.5 10.59 8.69 8.63 7.76 1.66

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 EDTA at 25°C was calculated using the pK values obtained from the literature17 and is shown in Fig. 3. At a pH of 4.5, EDTA is in the form of H2Y 2. At higher pH values of 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.9 ­11 The dissolution of calcium carbonate by EDTA at a pH between 4 and 5 has been reported as a combination of hydrogen-ion attack and free-calcium-ion sequestering.7 Note that in this pH range EDTA is predominantly the H2Y 2 form.

Ca

2

H2Y

2

º CaY

2

2H . . . . . . . . . . . . . . . . . . . . . (7)

Combining Eqs. 2 and 7 leads to the overall reaction:

H2Y

2

CaCO3 º CaY

2

H 2O

CO2 .

. . . . . . . . . . . . (8)

The reaction rate of EDTA at pH 5 with calcium carbonate Eq. (8) was reported to be 4 10 7 mol/cm2 s at 93°C.7 This rate was measured using a rotating disk apparatus with 500-psi CO2 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

35

Fig. 5--Schematic of film neutron radiography system.

Fig. 3--Distribution of ionic species of EDTA at 25°C.

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/L of solvent, respectively.11 At pH 13, the dissolution is analogous to calcium sulfate scale removal9,18 and is given by

Y

4

CaCO3 º CaY

2

CO3 . . . . . . . . . . . . . . . . . . . . . . (9)

Similarly, at pH 8.8,

HY

3

CaCO3 º CaY

2

HCO3 . . . . . . . . . . . . . . . . . . (10)

The diffusion coefficient of EDTA at room temperature is 6 10 6 cm2/s, which is about an order of magnitude lower than that of HCl (4 10 5 cm2/s). Based on these diffusion coefficients and the rate of dissolution being comparable to HCl, the dissolution of limestone by EDTA is assumed to be influenced by the rate of mass transfer. This assumption is supported by results presented in the following sections that revealed the formation of wormholes in limestone when EDTA was injected at pH values between 4 and 13.

Asphaltic Sludge Precipitation

significant sludge precipitation.6 Sludge precipitation has been found to increase dramatically with acid concentration6 and has also been reported during caustic flood projects associated with enhanced oil recovery (EOR) because of the high pH of the system.19 Sludge precipitation is considerably worse in the presence of ferric iron,6 which may be due to oxidative polymerization processes in the resin layer.21 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 because of 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, antisludging agents, and iron-reducing agents (to convert ferric iron to the less damaging ferrous iron), along with other additives such as 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

Formation damage caused by the precipitation of asphaltic sludge when crude oil is contacted by acid is a common 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.6,19 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 HCl, the colloidal dispersions are destabilized, causing the formation of asphaltene precipitates (sludge) and rigid film emulsions.20 Weaker acids, such as acetic acid, do not cause

Coreflood Experiments. Linear coreflood experiments were performed with the apparatus shown schematically in Fig. 4. Texas cream chalk and Indiana limestone cores 1.5 in. in diameter and 2.5, 4, or 5 in. long were studied. The cores had porosities between 15 and 20% and permeabilities of 0.8 to 2 md. Experiments were performed by first vacuum saturating a core with deionized water

Fig. 4 --Schematic of linear coreflood apparatus. 36

Fig. 6 --Neutron radiographs of wormholes formed during the dissolution of limestone by 0.25 M EDTA injected at 0.3 cm3/min (NDamt 0.29). SPE Journal, March 1998

and mounting it in a standard Hassler cell. An overburden pressure of at least 2,200 psi was applied to ensure that flow did not bypass the core. Fluid was injected axially through the core at a constant rate with an FDS-210 pump. Deionized 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. These data were used to calculate the permeability as a function of fluid volume injected using Darcy's law. Gaseous reaction products, specifically CO2, were kept in solution by maintaining a system pressure of at least 1,000 psi with a backpressure regulator. 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 HCl. 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 EDTA. Addition to deionized water resulted in a pH of 4.6. Sodium hydroxide and HCl 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.22 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 technique12 utilized a vacuum oven to dry and evacuate the acidized cores and then to inject molten Wood's metal at 100°C and 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. Because 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 AzoTM black and white film. An exposure

time of 40 seconds was required, and the photographic film was developed using standard procedures. Neutron radiography is described in more detail elsewhere in the literature.22,23 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% HCl. The crude oil was obtained from a field in west Texas. The acid system was prepared by adding 3000 ppm of FeCl3 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 No. 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. The blank was an oven-dried filter used to filter 100 mL of crude oil that had been contacted with deionized water using the above procedure.)

Results and Discussion

Wormhole Formation with EDTA and HAc. Neutron radiographs of wormholes formed by 0.25 M EDTA injected into limestone cores at 0.3 cm3/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 ion attack, as evidenced by the formation of wormholes when injected at pH values of 8.8 and 13.0. Injection of EDTA at pH 4.0 required a fewer number of PV's to break through than the higher pH stimulations (4.8, as opposed to 10.0 or 12.7 PV). This increased efficiency at the low pH is because of the rate of dissolution being enhanced by a combination of calcium chelation and hydrogen-ion 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 through 9 for Damkohler numbers spanning ¨ about three orders of magnitude. (The Damkohler number was ¨ determined by setting the product, a L, equal to 0.88 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 Hoefner and Fogler12 in that decreasing the Damkohler number (or increasing the injection rate) increases the ¨ amount of channel branching.

Fig. 7--Neutron radiographs of wormholes formed during the dissolution of limestone by 0.5 M HAc. SPE Journal, March 1998 37

Fig. 8 --Neutron radiographs of wormholes formed during the dissolution of limestone by 0.25 M EDTA injected at pH 4.

Fig. 9 --Neutron radiographs of wormholes formed during the dissolution of limestone by 0.25 M EDTA injected at pH 13.

Wormhole Formation at Low Injection Rates. Typical permeability responses for linear coreflood experiments 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 cm3/min. After injection of 43.1 PV of 0.5 M HCl, negligible increases in permeability were observed because 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 4-in. limestone cores after injection of 1.7, 3.3, and 8.1 PV's, respectively. Injection of these fluids resulted in the formation of single wormhole channels. Thus, improved acid penetration can be obtained during matrix stimulations with EDTA and HAc without the costly near-wellbore face dissolution associated with HCl. 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. Breakthrough was defined as the point at

38

Fig. 10 --Permeability responses during linear coreflood experiments with HAc, HCl, and EDTA injected at 0.1 cm3/min. SPE Journal, March 1998

Fig. 11--Neutron radiographs of wormholes formed during the dissolution of limestone by HAc, HCl, and EDTA injected at 0.1 cm3/min.

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.12,15 Notice that below 0.03 cm3/min EDTA injected at pH 4 required a fewer number of PV's 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 HCl. One can see that as the diffusion coefficient is decreased from HCl (4 10 5 cm2/s) to HAc (1 10 5 cm2/s) to EDTA (6 10 6 cm2/s), 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 EDTA and HAc induced only trace amounts of sludge to precipitate ( 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 because HAc has been reported to have virtually the same sludging tendencies as HCl in the presence of ferric and ferrous iron.6 HAc chelates ferric iron, thereby preventing it from interacting with asphaltene particles. However, the HAc solution (HAc plus FeCl3) had a pH of 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 because of the ability of HAc to chelate ferric iron and to the inability of the acid to destabilize the asphaltene particles in the crude oil. Additional Issues. 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°C (with possible exceptions when copper, tin, and aluminum are present),18 and EDTA chelates both ferric and ferrous irons (Table 1). There-

Fig. 12--Optimum injection rate for the dissolution of limestone by HAc, HCl, and EDTA at pH 4 and 13. SPE Journal, March 1998

Fig. 13--Mass of asphaltic sludge precipitated when crude oil was contacted by 15% HCl, 15% HAc, and 0.25 M EDTA at pH 6. 39

fore, 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 problems. Because these additives are not required, preliminary estimates indicate that the cost of an EDTA fluid system is comparable to that of an HCl fluid system that includes typical additives. The lack of corrosion problems with EDTA suggests additional applications for stimulating high temperature formations. The effectiveness of EDTA at high temperatures will be demonstrated in a future publication. A potential environmental concern associated with the use of EDTA is the increased mobility of heavy metals after being chelated. However, this increased mobility can be offset by a reduction in the amount of heavy metals released by the stimulation if an effective stimulation treatment is design (i.e., wormhole formation, as opposed to face dissolution).

Conclusions

this work. Contributing companies include Aramco Services, Arco E&P Technology, Chevron Petroleum Technology Co., Conoco Production and Research Div., Dowell Schlumberger Inc., Halliburton Services, Intevep, Mobil Technology Co., and Unocal Corp.

References

1. Williams, B.B., Gidley, J.L., and Schechter, R.S.: Acidizing Fundamentals, Monograph Series, SPE, Richardson, TX (1979). 2. Hoefner, M.L. and Fogler, H.S.: "Effective Matrix Acidizing in Carbonates Using Microemulsions," Chem. Eng. Prog. (May 1985) 40. 3. Abrams, A. et al.: "Higher-pH Acid Stimulation Systems," JPT (December 1983) 2175. 4. Harris, F.N.: "Applications of Acetic Acid to Well Completion, Stimulation and Reconditioning," JPT (July 1961) 637. 5. Bernadiner, M.G., Thompson, K.E., and Fogler, H.S.: "Effect of Foams Used During Carbonate Acidizing," SPEPE (November 1992) 350; Trans., AIME, 293. 6. Jacobs, I.C.: "Chemical Systems for the Control of Asphaltene Sludge During Oilwell Acidizing Treatments," paper SPE 18475 presented at the 1989 SPE International Symposium on Oilfield Chemistry, Houston, 8­10 February. 7. Shaughnessy, C.M. and Kline, W.E.: "EDTA Removes Formation Damage at Prudhoe Bay," JPT (October 1983) 1783. 8. Tyler, T.N., Metzger, R.R., and Twyford, L.R.: "Analysis of Treatment of Formation Damage at Prudhoe Bay, Alaska," JPT (June 1985) 1010. 9. Moore R.E. et al.: "One-Step Anhydrite Scale Removal," Materials Protection & Perf. (March 1972) 41. 10. Jamialahmadi, M. and Muller-Steinhagen, H.: "Reduction of Calcium Sulfate Scale Formation During Nucleate Boiling by Addition of EDTA," Heat Trans. Eng. (1991) 12, No. 4, 19. 11. Bodine, M.W. and Fernalld, T.H.: "EDTA Dissolution of Gypsum, Anhydrite, and Ca-Mg Carbonates," J. Sedimentary Petrology (December 1973) 43, No. 4, 1152. 12. Hoefner, M.L. and Fogler, H.S.: "Pore Evolution and Channel Formation During Flow and Reaction in Porous Media," AIChE J. (January 1988) 34, No. 1, 45. 13. Lund, K. et al.: "Acidizing-II. The Dissolution Of Calcite In Hydrochloric Acid," Chem. Eng. Sci. (1975) 30 825. 14. Lund, K., Fogler, H.S., and McCune, C.C.: "Acidizing-I. The Dissolution Of Dolomite In Hydrochloric Acid," Chem. Eng. Sci. (1973) 28, 691. 15. 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 SPE Annual Technical Conference and Exhibition, Houston, 3­6 October. 16. Martell, A.E. and Calvin, M.: Chemistry of Metal Chelate Compounds, Prentice-Hall Inc., Englewood Cliffs, New Jersey (1956). 17. Welcher, F.J.: The Analytical Uses of Ethylenediamine-tetraacetic Acid, D. Van Nostrand Co. Inc., New York City (1958). 18. Cikes, M. et al.: "A Successful Treatment of Formation Damage Caused by High-Density Brine," SPEPE (May 1990) 175. 19. 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 Production Operations Symposium, Oklahoma City, Oklahoma, 10­12 March. 20. 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, Montana, 10­11 June. 21. Jacobs, I.C. and Thorne, M.A.: "Asphaltene Precipitation During Acid Stimulation Treatments," paper SPE 14823 presented at the 1986 SPE Symposium on Formation Damage Control, Lafayette, Louisiana, 26­28 February. 22. Jasti, J.K. and Fogler, H.S.: "Application of Neutron Radiography to Image Flow Phenomena in Porous Media," AIChE J. (April 1992) 38, No. 4, 481. 23. Lindsay, J.T., et al.: "Neutron Radiography Applications at the University of Michigan, Phoenix Memorial Laboratory," Neutron Radiography (3), Proc., Third World Conference, J.P. Barton (ed.), Gordon and Breach Science Publishers, Boston, Massachusetts (1990) 621. SPE Journal, March 1998

1. EDTA is capable of forming wormholes in limestone when injected at pH values between 4 and 13. Hydrogen ions are not required because 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) HCl. 3. EDTA and HAc both stimulate more efficiently than HCl when injected at rates greater than 0.1 cm3/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 because the diffusion ¨ coefficients of EDTA and HAc are lower than that of HCl. 4. EDTA, HAc, and HCl exhibit an optimum Damkohler number ¨ (or injection rate) at which the number of PV's required to breakthrough is minimized. As the diffusion coefficient is decreased from HCl to HAc to EDTA, the optimum injection rate decreases. 5. Significant amounts of asphaltic sludge precipitated when crude oil was contacted by 15% HCl. However, only trace amounts of sludge were 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 because of 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 EDTA up to 204°C, and reducing agents are not required because 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, cm2/3/s1/3 NDa Damkohler number ¨ effective diffusion coefficient, L2/t, cm2/s De k permeability, L2, md K stability constant L length scale, L, cm pore volumes Vp q injection rate, L3/t, cm3/min

Subscripts BT breakthrough inj injected mt mass transfer limited i initial

Acknowledgments

The authors acknowledge the Industrial Affiliates Program on Flow and Reaction in Porous Media at the U. of Michigan for support of

40

SI Metric Conversion Factors

cp ft ft2 ft3 °F in. in2 md psi

1.0* 3.048* 9.290 304* 2.831 685 (°F 32)/1.8 2.54* 6.451 6* 9.869 233 6.894 757

E E E E E E E E

03 01 02 02 00 00 04 00

Pa s m m2 m3 °C cm cm2 m2 kPa SPEJ

.

is currently employed by Dowell Schlumberger in Houston. Photograph is unavailable. H. Scott Fogler is Vennema Professor of Chemical Engineering at the U. of Michigan and has been a consultant for Chevron Petroleum Technology Co. He holds MS and PhD degrees from the U. of Colorado at Boulder. Fogler is the author or coauthor of numerous articles and reviews in the areas of flow and reaction in porous media, colloid stability, bioremediation, and dissolution kinetics in microelectronic fabrication.

*Conversion factor is exact.

Christopher N. Fredd holds a BS degree in chemical engineering at Clarkson U., Potsdam, New York and MS and PhD degrees in chemical engineering at the U. of Michigan in Ann Arbor. He

Fogler

SPE Journal, March 1998

41

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