580
Ind. Eng. Chem. Process Des. Dev. 1982, 21, 580-583
Generation of Surface Active Acids in Crude Oil for Caustic Flooding Enhanced OH Recovery Mankin Chan, Mukul M. Sharma, and T. F. Yen' Department of Chemical Engineering, University of Southern Californ&, Los Angeles, Callfornia 90007
The process of recovering residual oil from an oil reservoir by alkaline flooding relies on soap formation by the reaction of the alkali and free long-chain organic acids in the crude oil. In this study, dilute mineral acids were
used to liberate additional new organlc acids from their nonreactive compounds such as esters, amides, and ackl-base complexes present in the crude 011. This was accompanied by an observed enhanced or regenerated interfacial acthrlty of the oil. This acM pretreatment method may be applied to oil reservoirs which have been exhausted by previous alkaline floodings to regenerate activlty in the residual oil or to reservoirs never exposed to any alkali contact to give the subsequent alkaline flooding a better first-round recovery. The major technical problem anticipated in applying this process to actual reservoirs is the consumption of acid by the reservoir rock. An analysis was done that showed that sandstone reservoirs with low clay and carbonate content are amenable economically to such an acid treatment. Slug sizes for the acid arid water buffer slugs have been estimated using a simple diffusion, convective dispersion model to ensure that the acid and alkaline slugs do not mix.
Introduction The lowering of interfacial tension (IFT) between acid containing oil and alkaline water was recognized by Donnan in 1899. Subsequently, patents have been granted to Squires (1921), Atkinson (1927), and others on the use of alkaline agents in the recovery of oil by the method of alkaline or caustic flooding. Johnson in 1976 and recently Mayer et al. (1980) have reviewed the current status of the recovery process. It has been generally accepted that the acids present in the crude oil interact with the alkali to form surface active soap. Seifert et al. (1969a,b,c) have extracted acids from crude oil and showed that carboxylic acids are primarily responsible for the observed surface activity. Not all long-chain acids in the crude oil, however, exist in free form which would readily react with alkali to form soap. Some exist as natural esters, amides, and other acid-base complexes. The presence of these compounds in crude oil has been identified by many investigators including Snyder (1969a,b) and Latham (1974). In this paper, the use of mineral acid to liberate surface active acids from these inactive compounds is investigated. The technical feasibility of the application of such a process to an actual reservoir is also explored. Experimental Section Material. Three crude oil samples were used in this study. They were from Long Beach, CA, Huntington Beach, CA, and Smackover Field, AFt, respectively. Oleic acid, oleic acid methyl ester, and egg lecithin were from Sigma Chemical; oleic acid monoethanolamide was from Pfaltz-Bauer Chemical. Interfacial Tension. Interfacial tensions (IFT) of the oil-aqueous systems were measured using a spinning drop interfacial tensiometer. All aqueous solutions mentioned in this paper contain 7500 ppm sodium chloride. Except for crude oil which was measured undiluted, all other oil systems are 1 wt % solution of a given compound in toluene. In each case, the IFT of the oil-aqueous system is monitored until the equilibrium value is reached. The latter is the IFT value we refer to in our results. Extraction of Acids and Bases. Acids were extracted from oil by shaking moderately for 10 min a 1:2 mixture of oil and a 1wt % NaOH aqueous solution. The mixture 0 196-4305/82/1121-0580$01.25/0
was centrifuged for 2 min at 3000 ppm and the alkaline solution was then withdrawn. The extraction was repeated using fresh alkaline solution every time until no visible trace of acids, either as a white precipitate or as a turbid soluble material in the aqueous phase, was observed. At this point, the IFT of the oil was very high (above 10 dyn/cm) at all concentrations of alkali used; i.e., all the surface active components in the crude have been extracted out by the alkali. The aqueous portions were combined, pH adjusted to slightly acidic, extracted with methylene chloride, and dried by vacuum. Bases, primarily amino compounds, were similarly extracted using a 0.2 N HCl solution in place of the alkaline solution. Identification of the acids and the bases was done by infrared spectroscopy and elemental analyses of oxygen and nitrogen content. Model Compound Studies. We believe that the acid works by hydrolyzing compounds including esters such as the ubiquitous natural lipids, amides, and other amino acid complexes to yield the kind of free long-chain acids which are needed to give the interfacial activity. Other than the already proven presence of these compounds in petroleum, the following results gave additional support to the proposed mechanism. The 1 wt % toluene solutions of (1) egg lecithin, a naturally occurring lipid, (2) oleic acid methyl ester, and (3) oleic monoethanolamide, respectively, were exhausted with alkaline solution to practically no interfacial activity; i.e., all IFTvalues are more than 10 dyn/cm, to get rid of originally present free or alkali hydrolysable acids. When these samples were subjected to acid treatment, as described earlier, their interfacial activity was restored. This regenerated interfacial activity is represented well in the IFT results shown in Figure 1. The two minima seen in the curve for the egg lecithin are a reflection of the fact that hydrolysis of each egg lecithin molecule yields two fatty acid chains which may have different ionization constants. The close similarity between the response of the known oleic acid sample and those of the acids liberated from the oleic esters and amide is good evidence for the proposed hydrolysis reaction. The molecules extracted every time by the alkali from the crude as well as the above model compounds were identified, by infrared spectrscopy, to be carboxylic acids. 0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol.
ds LL c“
z
21,
No. 4,
1982
581
“E 01
ti10
I 100
,,I 1,000
I
lop00
ppm NaOH
Figure 1. Interfacial tension vs. alkali concentrations for toluene solutions of 0.02% oleic acid ( O ) , acid (0.2 N HCl) activated 1% solutions of oleic acid methyl ester (e),oleic monoethanolamide (A), and egg lecithin (A). All solutions subjected to acid activation treatment had their interfacial activity first removed by shaking with a 1%NaOH solution.
In the crude used, the relative weight ratio of original free or alkali hydrolyzable acids and those liberated by the acid treatment were found to be about 2:l. This figure suggests that a substantial amount of new acids can be liberated by an acid treatment of the crude. Indication that some of these long-chain acids may be liberated from some kind of acid-base complex present in the crude comes from the observation that the activation of an interfacially inactive crude sample by acid treatment was accompanied by the extraction of soluble basic molecules identified by infrared spectroscopy and elemental analysis to be rich in -NHgroups. Therefore, it is possible that these acid-base complexes are also broken up at acidic pH to yield basic amino molecules which are extracted out into the aqueous phase, and long-chain acids which remain in the oil phase. Core Flooding Experiments. An oil reservoir was simulated by a 1 2 X 1in. cylindrical core packed with 100 mesh Ottawa sand which had been predigested with 0.2 N HC1, washed, and dried. The predigestion with acid was done to eliminate the possibility of enhanced oil recovery through increased porosity and permeability due to any acid digestion of the core sand. All flooding was done at a flow rate of 10 mL/h. The average porosity of the sandpack was 22%. The cores were saturated with saline water followed by the Long Beach crude oil. Secondary water flooding was carried out by flooding the cores with saline water until it had attained a residual oil saturation. The oil recovery during water flooding was approximately 65%. At this point, three different tertiary recovery schemes were initiated: (1)regular alkaline flooding followed by a preflush of 5 pore volumes of saline water, followed by (2) four pore volumes of 0.2 N HC1 solution then a rinse flush of 5 P.V. of saline water, followed by regular alkaline flooding, and (3) first flood with four pore volumes of 0.2 N HC1 then the regular alkaline flooding. All core flood runs were done in duplicate to ensure consistent results. Since the three crude oils showed very similar response, the results of the Long Beach crude only are presented here. Figure 2 showed the IFT response curves of the crude, crude treated with 0.05 N HC1, crude treated with 0.2 N HC1, and crude deactivated, i.e., with free acids depleted, by alkaline treatment, and then activated by treatment with 0.2 N HC1, respectively. The result indi-
01
1 10
,
I
1
, l , , l
1
1
I
IIIIII
1
I
I 1 l l l l
lop1 0
1,000
100 ppm NaOH
Figure 2. Interfacial tension vs. alkali concentrations for Long Beach crude ( O ) , crude treated with 0.2 N HC1 (@), and crude of which the interfacial activity was removed first by 1%NaOH solution and was regenerated by treatment with 0.05 N HCl (A)or 0.2 N HC1 (A). Table 1. Sand Packed Core Flooding Results flooding agent recovery alkali al kali-acid-a1 kali acid-alkali
32 32 48
%a
+ 26
a Weight percent recovered based o n residual oil after secondary water flooding of the core. The figures are recovery % brought about by each round of alkali flooding (500, 3000 and 10 000 ppm NaOH). N o oil was recovered during the acid flooding (0.2 N HCl). Cores are flooded with plenty of saline water between each alkaline and acid flood.
cated that new acids are being liberated from the crude by treating the crude with a 0.05 N or stronger acid solution. This is not due to any oxidation reaction because the dilute concentration of acid used cannot cause such oxidation. Core flood experiments showed recovery efficiencies parallel to the observed low IFT (see Table I), indicating that for the sand packed cores we used, low interfacial tension is responsible for good oil recovery, and that the acid treatment can be used to either reactivate an alkali exhausted oil reservoir or pretreat an oil reservoir unexposed to alkali to make way for a better first round alkali flooding oil recovery. Both the re-lowering of IFT and the renewed oil recovery from cores can be repeated over and over again with cyclic treatment of acid and alkali. Of course, the amount of residual oil will be less each time, making recovery more and more uneconomical. Sulfuric acid was found to work just as well as hydrochloric acid in this treatment method. The application of such a process to an actual reservoir still poses serious technical problems. The consumption of acid by the rock and the possibility of the mixing of acidic and alkaline slugs are the two major matters of concern. In the following sections these two problems are addressed and the severe limitations of such a process are emphasized. Consumption of Acid by Reservoir Rock. Of the minerals present in sandstone, the carbonates, feldspars, and clays have a high reactivity whereas the reactivity of a-quartz is very small. This is the reason why HF and HF-HC1 mixtures are used in acidizing, seldom HC1 alone. Reaction rate studies have been done using the rotating
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982
582
disk and the flat plate methods, by Lund et al. (1973,1974, 1975). For dolomite, CaMg(C03)z,the dissolution rate at 600 psig was measured at different temperatures. At 50°C -rHCI
9.8
X
10-7c~cp1
where the activation energy = E, = 14.7 kcal/mol, the rate is in g-mol/cm2 s, and the concentration in g-mol/L. For calcite (CaC03),the rate of HCl consumption is given by the following expression.
21-
-rHCI = 8.8 X 10+cHcp63 at 1 "C 0.0
E , = 15 kcal/g-mol The consumption of acids by Na and K feldspars of composition (Nh.,2Ko.oaSio.8) (CaAl)o.2A1Si208 and (K0.76' N ~ . l s S ~ . w ) ( C a A l ) o . ~ S iwas z O smeasured , at 40 psig and 32 "C. The rate for HF/HC1 mixtures is of the form racid
= K1(l - GICHC1)CHF
However, for pure HC1 the rates are very small. Studies for silica or quartz show similar results. The interaction of clays with HCl is very complex indeed. According to Grim (196% very limited studies show that HC1 is much less reactive with kaolinite and illite than H2S04. At low acid concentrations (0.05 to 0.2 N) the "solution" of clays is essentially a silicate hydrosol similar in composition to the clay itself. The higher the element in the electrochemical series (K > Na > Ca), the more easily it goes into solution in an acid. Well-crystallized chlorite was unaffected by 1N HCl over extended periods. Illite was unaffected by even 10 N HC1. In mixed-layer illite-montmorillonite assemblages 1.9 N HCI was sufficient to alter diffraction data, but no effect was produced by 0.36 N HC1. Well-crystallized kaolinite was unaffected after treatment with 10 N HC1 for 70 h. Brindley (1961) found that fine grained chlorites were decomposed by warm dilute HC1, but kaolinite was unaffected. Using a lumped parameter model, we may represent the rate of dissolution of all the dissolvable minerals by an expression of the kind -rHC1 = KdCHCI(Cd - Cdi) where Kd is the reaction rate constant, Cd is the concentration of dissolvable rock, and c d i is the amount inaccessible to the acid. The nondimensional mass balance equations are
where E = tu/+&; Cd
=
(cd
z = x/L;
c*a = CHC,/CHClO
- Cdi)/ (CdO - Cdi); ND, = [&(Cdo - cdi)L]/u = Dahmkohler number
NAC= '$oCHCI,/[W(l - d'O)(cdo - Cdi)] = acid capacity number
+
where u is the velocity, is the porosity, L is the reservoir length, and the subscript 0 refers to initial conditions. The solution as obtained by Fogler (1976) is
0.2
04
ii
06
08
IO
Figure 3. Concentration profiles along the length of the reservoir at f = 1 for different values of the Damkohler number.
For a typical sandstone, q50 = 0.2; (Cdo - c,) = 0.636 g-mol of rock/L; w = 19.8 g-mol of acid/g-mol of rock; N A C = 0.00397 (if CHclo = 0.2 N). At f = 1, ?i = 1 1/e~c= l 1 + eXp(1.004N~,- 1) The concentration profiles at f = 1for different values of the Dahmkohler number are plotted in Figure 3. It is seen that for values of ND, L 5 the effluent concentrations of acid are very small (
