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Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 1–4

Adsorption of nonionic surfactants in sandstones

Fabıola D.S. Curbelo a,∗, Vanessa C. Santanna a, Eduardo L. Barros Neto a,Tarcılio Viana Dutra Jr. a, Tereza N. Castro Dantas b,

Afonso A. Dantas Neto a, Alfredo I.C. Garnica c

a Departamento de Engenharia Quımica, Universidade Federal do Rio Grande do Norte, Campus Universitario, 59072-970 Natal (RN), Brazilb Faculdade Natalense para o Desenvolvimento do Rio Grande do Norte, Tirol, 59014-540 Natal (RN), Brazil

c Departamento de Tecnologia Quımica e de Alimentos, Universidade Federal da Paraıba, Cidade Universitaria, Campus I, Joao Pessoa (PB), Brazil

Received 28 April 2005; received in revised form 16 February 2006; accepted 28 June 2006Available online 3 July 2006

bstract

Adsorption of surfactants from aqueous solutions in porous media is very important in enhanced oil recovery (EOR) of oil reservoirs becauseurfactant loss due to adsorption on the reservoir rocks impairs the effectiveness of the chemical slurry injected to reduce the oil–water interfacialension (IFT) and renders the process economically unfeasible. In this paper, two nonionic surfactants with different ethoxylation degrees were

tudied, ENP95 with ethoxylation degree 9.5 and ENP150 with ethoxylation degree 15. The experiments were carried out in a surfactant floodingpparatus, with a pressure gradient of 30 psi. The concentration of the injected solutions were 30% above the critical micelle concentration, tossure micelle formation. The results from the flow experiments of surfactant solutions in porous media showed that the adsorption extent wasigher for ENP95 than for ENP150 because the previous surfactant has a smaller ethoxylation degree, that is, a smaller polar part. 2006 Elsevier B.V. All rights reserved.

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eywords: Nonionic surfactant; Adsorption; EOR; Reservoir rock; Surface ten

. Introduction

Oil is the most used nonrenewable source of energy in theorld and this has induced the study of new processes for

xploration and production of oil reservoirs. Oil and serviceompanies have been investing heavily in increasing the recov-ry and productivity of old reservoirs, mainly in processes ofnhanced oil recovery (EOR).

Considering only conventional processes, it is believed thatust about 30% of the original oil in place is recoverable. These of more advanced methods come to attend the demand fornergy when the reserves based on conventional methods starto decline. Therefore, the target for EOR processes correspondso 70% of the original oil in place, which will remain in theeservoirs after conventional production.

The enhanced recovery methods are processes that seekn additional recovery of the reservoirs, which have not beenxploited in a fully efficient manner. As a consequence, they

∗ Corresponding author. Tel.: +55 84 3215 3827; fax: +55 84 3215 3827.E-mail address: fabiola@eq.ufrn.br (F.D.S. Curbelo).

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927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.06.038

etain a large amount of hydrocarbons after their natural energys depleted.

These methods have been developed with the objective ofllowing for a larger production than that provided just by theatural energy of the reservoir and consist basically on the flood-ng of fluids aiming to move the oil out of the pores of the rock.hus, the injected or displacing fluid should push the oil, knowns displaced fluid, out of the rock and, at the same time, shouldccupy the space left.

Among the methods used in the advanced recovery ofetroleum are the chemical methods, which comprise surfactantooding. The surfactant flooding may be used with the pur-ose of reducing the interfacial tensions between oil and water,ncreasing the displacement efficiency [1,2].

According to Agharazi-Dormani et al. [3], the surfactantooding is considered to be a method of additional recoveryf oil from partially depleted reservoirs. The mechanism ofction of the surfactant in a porous medium, partially filled with

il and brine, is still not very well understood. In wet waterystems, for example, the oil in place, after water flooding, con-ists mainly of isolated oil drops within the pores. In order toobilize the residual oil trapped by capillary forces in oil reser-

2 s A: Physicochem. Eng. Aspects 293 (2007) 1–4

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Fig. 1. Schematic of the core flood system for adsorption and oil recovery tests.

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F.D.S. Curbelo et al. / Colloids and Surface

oirs, many enhanced oil recovery methods rely on reducinghe oil–water interfacial tension (IFT) to extremely low values,ften to 10−2 dyn/cm or less [4]. Therefore, it is important toeep low interfacial tensions for large periods of time.

The complexity of the system increases with the effects ofther parameters, such as: heterogeneity of the rock, interactionf the surfactants with reservoir fluids, coalescence of the oilrops and surfactant adsorption. The surfactant flooding process,owever, encounters problems due to loss of high cost surfactantn the form of adsorption and retention in the porous media.

The loss of surfactant from the main chemical slurry duringurfactant flooding is a major determinant in process perfor-ance and economy. It has been shown that the nature of the

dsorption isotherm depends to a large extent on the type ofurfactant used, the morphological and mineralogical charac-eristics of the rock, the type of electrolytes present in solutionnd, also, of cosurfactants and alcohols. The adsorption of sur-actants can be affected by the charge on the rock surface anduid interfaces [5,6].

In this work, the understanding of the mechanism wherebyonionic surfactants adsorb onto rocks has been investigated,ith particular attention to the effect of the surfactants’ ethoxy-

ation degree.

. Experimental/materials

The chemicals used to prepare the surfactant solutions werewo commercial nonionic surfactants, namely nonyl phenolxyethylenes, both supplied by Oxiteno (Brazil). The surfac-ants, ENP95 and ENP150, have ethoxylation degrees of 9.5nd 15.0, respectively. The HLB of ENP95 is 13.0 and that ofNP150 is 15.0. KCl used in the brine (purchased from Isofar-razil) has 99.99% purity. The solid adsorbent (core) was a

ample of Acu sandstone, Brazil. All chemicals were used aseceived.

. Preparation of cores

The cores used were coated with resin and had the follow-ng dimensions: 3.8 cm diameter, 8.7 cm length and an averageorosity of 24%. Before coating with resin, all cores were roastedt 700 ◦C during 18 h, to remove humidity and increase per-eability. Two distributors were attached to the edges of the

ore plug, featuring two ends connected to the injection androduction lines. The assembly formed by the core plug, twoistributors and two ends was fixed together with a coating ofesin as show in Fig. 1.

. Properties of surfactant solutions

A specific property of surfactant solutions is the concentra-ion (CMC) above which the first aggregates of monomers or

icelles are formed [7]. Above this concentration, the surfac-

ant solutions display particular solubilizing properties used in

any applications [8,9].The CMC of the solutions were determined at 26 ◦C by a

onventional method, from surface tension measurements taken

6

t

Fig. 2. Apparatus for the surfactant flooding experiments.

t different concentrations, with a Surface Tensiometer, Sensa-yne Instrument Div. The surfactant was diluted in KCl brine

2 wt.%).

. Procedures/adsorption tests

The adsorption experiments were conducted at room temper-ture with a known concentration of surfactant in the core plug,sing the surfactant flooding apparatus showed in Fig. 2. Forach surfactant, the flooding was carried out with a concentration0% above the CMC, assuring the formation of micelle solu-ions. The flooding experiments were performed in two steps:

First step: surfactant flooding in the core at a constant pressureof 30 psi.Second step: collection of samples in different periods of time,with the purpose of determining the surfactant loss in the rock.The surfactant solution was continuously injected into the coreuntil the surfactant concentration in the effluent approachedthat of the surfactant solution injected. The surfactant concen-trations in the effluent were determined by absorbance, usinga UV–visible Spectrophotometer.

. Results and discussion

The variation of surface tension with surfactant concentra-ion, for ENP95 and ENP150, is shown in Figs. 3 and 4, respec-

F.D.S. Curbelo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 1–4 3

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Fig. 3. Determination of the CMC for surfactant ENP95.

ively. The break in the curves corresponds to the CMC. It cane seen that ENP150 has a higher CMC than ENP95, becauset has a larger polar chain, i.e., higher ethoxylation degree and,onsequently, higher solubility in the aqueous phase. Therefore,higher concentration of ENP150 than ENP95 is required in

rder to form micelles [10].For nonionic surfactants, polyoxyethylene (POE) polar

hains have a typical size well above the alkyl chain dimen-ion and fill an extended polar corona. An average volume ofnteraction per polar chain must replace the average surface peronic head. During the transfer of a POE chain inside this polarorona, two important contributions should be considered: stericepulsion and stretching deformation [11].

From the curves used to obtain the CMC, the area of theurfactant polar chain can be calculated from the Gibbs isotherm,ccording to the equations bellow:

= 1.66 × 10−4 × 1

Γ(1)

1

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here S is the area of the surfactant polar chain, in A2; Γ thexcess surface concentration, in mol/m2; γ the surface tension,n mN/m; RT given in J/mol and d(ln C)/dy is the slope of theurve before the CMC is reached.

The total area (St) filled with surfactants in the rock wasetermined by Eq. (3), considering monolayer adsorption:

t = n × S (3)

here St is given in A2; n the number of moles of surfactantnjected and S is the area of the surfactant polar chain, in A2.

Fig. 4. Determination of the CMC for surfactant ENP150.

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ig. 5. Normalized ENP95 concentration as a function of the pore volume ofuid produced in the equilibrium adsorption test.

The structure of the adsorption layer of nonionic surfactantsn surfaces was examined by using fluorescence decay tech-iques [12,13], made possible due to the existence of hydropho-ic domains on the surface. Indeed, adsorption occurs by forma-ion of surfactant aggregates on the surfaces. The size of theseggregates was measured and found to depend on the structuref the surfactant and on the coverage of the surface.

The thermodynamic behavior of the system has been studiedy microcalorimetric techniques [14–16]. All of these workshow that the adsorption mechanism of nonionic surfactant iseversible and that a plateau in the isotherm is reached just afterhe critical micelle concentration, as shown in Figs. 5 and 6.his corresponds to the stabilization of the chemical potentialf the monomer in equilibrium with the adsorbed phase. Non-onic surfactant monomers adsorb as individual ions throughydrogen bonding between the polyoxyethylene chain and theydroxyl groups on the surface [17]. This corresponds to regionin Figs. 5 and 6, without any interaction among the monomers.

The mechanism that governs the adsorption in region IIFigs. 5 and 6) is due to the association of adsorbed surfac-ants inside the pores of the rock at the liquid–solid interface.his association is attributed to the tail–tail interactions of sur-

actants, which are the same hydrophobic interactions by whichicelles are formed. The break point between regions I and IIay be taken as an estimate of the critical surface micellization

s formed at the surface and a monolayer begins to form.

ig. 6. Normalized ENP150 concentration as a function of the pore volume ofuid produced in the equilibrium adsorption test.

4 F.D.S. Curbelo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 293 (2007) 1–4

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Table 1Surfactant loss in the rock

Surfactant Concentrationof solutioninjected(mol/l)

Molecularweight(g/mol)

Surfactant loss(mg surfactant/grock)

Area filledwith surfactantin rock (m2)

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[[17] T.H. Van Den Boomgard, T.H.F. Trados, J. Lyklema, J. Colloids Surf. Sci.

116 (1987) 8.[18] T. Austad, T.A. Hansen, G. Staurland, Adsorption of ethoxylated surfac-

tants on reservoir minerals—an experimental study, in: Proceedings of the

Fig. 7. Multilayer adsorption.

In region III, the decrease in slope is ascribed to inversion ofharge at the surface due to adsorbed surfactants ions, wherebybilayer is formed, in the beginning of region II, and prolongs

o region III. However, this occurs at a different rate due to amaller surface energy in region III. This region, also regardeds the adsorption landing, usually begins near the CMC and isharacterized by little or almost no increase in the adsorptionith the increase in surfactant concentration. In this region theredominance of micelles occurs.

Figs. 5 and 6 show the normalized surfactant concentrationn the effluent samples as a function of the pore volume (PV) ofuid produced. The normalized surfactant concentration is theatio between the surfactant concentration in the effluent samplesnd the original concentration. In Figs. 5 and 6, the normalizedoncentration reached 0.96 at about 74 pore volume produced,or ENP95, and 0.88 at 16 PV produced, for ENP150, remainingonstant for another 6 and 9 PV, respectively. It was believed thathe equilibrium monolayer adsorption had been reached and thathe continued loss of surfactant after 75 PV for ENP95 and 16V for ENP150 was the result of multilayer adsorption (Fig. 7).he test was stopped at 80 PV for ENP95 and 25 PV for ENP150.

The amount of surfactant adsorbed was obtained by subtract-ng the amount of surfactant remaining in the effluent samplesrom that of surfactant injected. The surfactant adsorption inhe core under the test conditions was found to be 13.15 mgNP95/g rock and 8.47 mg ENP150/g rock (Table 1). According

o Austad et al. [18], there is a direct dependence of the extent ofurfactant adsorption with the ethoxylation degree. The adsorp-ion decreases with the increase of the ethoxylation degree. Itas been verified that ENP95 has a higher adsorption loss than

NP150, due to the surface area of the polar part of ENP95

27.7 A2) being lower than that of ENP150 (77.5 A2). In otherords, the amount of ENP95 required to cover the internal sur-

ace of the rock is larger than that of ENP150.

NP95 0.0023 640 13.15 651NP150 0.0053 882 8.47 841

. Conclusion

The results of the injection tests suggested for the influence ofhe ethoxylation degree on the adsorption of ENP95 and ENP150n the rock. ENP95 presents a higher adsorption extent thanNP150 due its ethoxylation degree being lower (9.5) than thatf ENP150 (15.0). This is an important factor in enhanced oilecovery, because a more adsorbed surfactant has stronger inter-ctions with the rock than with the oil, hence allowing for a betteril recovery factor. However, it is necessary to carry out an eco-omic analysis for an improved, final surfactant selection. It iselieved that the surfactant adsorption forms multilayer and theolar part area is important to determine the amount of surfac-ant lost in the rock after adsorption. This demonstrates that thequilibrium between isolated and aggregated surfactants insidehe adsorption layer can be considered.

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4th European Symposium on EOR, 1987, p. 231.