Preferential Adsorption of Hydrocarbons to Nanometer-Sized Clay on

Jun 25, 2013 - ... with the calcite of the coccolith elements, but rather with nanometer-sized authigenic clay crystals that decorate the surfaces of ...
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Preferential Adsorption of Hydrocarbons to Nanometer-Sized Clay on Chalk Particle Surfaces L. L. Skovbjerg,*,† D. V. Okhrimenko,† J. Khoo,‡ K. N. Dalby,† T. Hassenkam,† E. Makovicky,§ and S. L. S. Stipp† †

Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Kbh, Denmark Surface Measurement Systems Ltd., 5 Wharfside, Rosemont Road, London, HA0 4PE, United Kingdom § Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350, Kbh K, Denmark ‡

S Supporting Information *

ABSTRACT: The demand for oil is increasing, but many reservoirs are reaching the end of their productive lifetime. A clearer understanding of the fundamental chemical and physical controls on the wetting behavior of reservoir pore surfaces would provide clues for developing methods to improve, or enhance, recovery of the currently inaccessible oil (improved/enhanced oil recovery, IOR/EOR). In this work, the surfaces of chalk were investigated to understand hydrophobicity at nanometer scale spatial resolution. Chalk samples from the gas and water zones of the Danish sector in the North Sea Basin were used. With inverse gas chromatography (IGC), the surface characteristics were compared. Chalk from the gas zone has a lower surface energy, dispersive as well as specific, than chalk from the water zone, clearly indicating that the gas zone pore surfaces are more hydrophobic. X-ray photoelectron spectroscopy shows that the concentration of hydrocarbons is higher in gas zone chalk than in water zone chalk, which is consistent with IGC measurements. With combined atomic force microscopy and chemical force mapping, we demonstrated that the hydrophobicity of chalk is not correlated spatially with the calcite of the coccolith elements, but rather with nanometer-sized authigenic clay crystals that decorate the surfaces of the coccoliths. Our results suggest that clay and adsorbed organic material, not calcite, are responsible for wettability alterations in chalk during the introduction of hydrocarbons. Furthermore, we show that surface hydrophobicity is heterogeneous, even within single-clay laths.

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fluid injection, but it is important to know precisely what is in contact with the fluid phases at the pore surfaces. The physical properties of the rock can influence oil and water transport through the reservoir. For example, different pore shapes have been shown to result in different distributions of oil-wet surfaces.12 Theoretical wetting properties are included in models that place the rock in such categories as water-wet, oil-wet, fractional, or mixed-wet.13,14 However, it is recognized that the rock surface is complex and that these are crude approximations.15 Recent research shows that the chalk that forms oil reservoirs in the Danish North Sea Basin has a nonuniform surface with respect to hydrophobicity, even on the subpore scale and even before oil or gas enters the reservoir.16,17 Chalk is a biogenic sediment composed mainly of calcite coccoliths (Figure 1a), which are shields produced by unicellular algae, and minor percentages of silica, clay, and other silicates.18 In those previous studies, the observed hydrophobicity contrasts were attributed to a surface mosaic of calcite, organic material, and nanometer-sized clay. In chalk, two types of clay have been observed. One is detrital clay produced during rock weathering and deposited together with the coccoliths. This type of clay forms individual grains that are large enough to be observed using X-ray diffraction (XRD) on the undissolved residue from chalk, and

INTRODUCTION Around the globe, oil reservoirs hosted in sandstone and carbonate rocks are being depleted. However, a continuous supply of crude oil for energy and industrial purposes is still needed for many years to come. As the remaining reserves become progressively more difficult to produce, efforts are being made to increase yields from existing fields and apply new techniques to optimize oil recovery from future fields. One strategy involves changing the surface tension of the pore surfaces to make them more water-wet. Hydrophilic surfaces release more oil than oil-wet surfaces, leading to improved/ enhanced oil recovery (IOR/EOR). In reservoirs of both sandstone and chalk, EOR has been achieved by injecting ordinary and modified seawater.1−4 Finding a low cost, high yield injection fluid is the aim of much research in this field. If we could understand the chemical interactions that take place among the oil, the rock, and the aqueous phase in the reservoir, we could use this information to increase oil recovery, and we could also use the new knowledge to develop more effective ways to remove hydrocarbon contamination from aquifers and soil. Studies to elucidate the complex interactions among oil, water, and minerals are performed in model systems chosen to be either (1) as close to reservoir rock conditions as possible,3,5,6 which should give most realistic experimental values, or (2) more simple, model systems,7−11 where the rock phase is usually replaced by an ideal surface. This latter approach can increase the fundamental understanding of the chemical interactions relevant for wettability alterations upon © XXXX American Chemical Society

Received: November 10, 2012 Revised: April 28, 2013

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Figure 1. (a) Scanning electron microscopy (SEM) image of sample 10.5 from the Danian gas zone. (b) Atomic force microscopy (AFM) image (height with amplitude overlay; 1 μm2) of a chalk particle from a water zone sample. The profile from A to B shows a clay particle that is 2 nm thick. The white arrow points to a collection of clay laths with a total thickness of 15 nm.

Table 1. Samples and Analysesa rock saturation state gas water

sample

age

type

BET (m2/g)

calcite (%)

10.4 10.5 7−1 7−6 calcite calcite

Maastrichtian Danian Maastrichtian Maastrichtian

white chalk white chalk white chalk with few stylolites white chalk powder single crystal cleaved in air

1.67 1.69 1.68

97 99 99 99 100 100

0.22

CFM

IGC

XPS

×

× × ×

× × × × × ×

× × ×

a

Wt % calcite was obtained from XRD. Only 1−3 wt % of the samples were quartz and clay (XRD patterns are compared in Supporting Information, Figure 2). Crosses indicate which techniques were used: chemical force mapping (CFM), inverse gas chromatography (IGC), and X-ray photoelectron spectroscopy (XPS).

they can be observed with scanning electron microscopy (SEM).19 The other type is authigenic clay, which has grown directly in the sediment during diagenesis. Studies show that these authigenic clays can be mixed layer clays of mainly illite and smectite, chlorite, and in some cases, serpentine.20,21 It has been shown that some of the authigenic clay consists of thin, nanometer scale laths, typically 1−4 nm thick, and these partially cover the surface of the coccolith elements and interact strongly with both hydrophobic and polar functional groups on organic molecules.17 An atomic force microscopy (AFM) image of such clay particles is shown in Figure 1b. The presence of partially hydrophobic clay particles and organic material in chalk from the water zone, a part of the rock that has never seen oil or gas, contradicts the assumption that sediments deposited in water are completely hydrophilic initially and that the introduction of oil is needed to make surfaces hydrophobic and oil wetting.7,12,13,22 The aim of our study was to increase understanding about what makes a porous rock behave hydrophobic, or “oil-wet”. We investigated natural samples from a gas saturated and a water saturated zone of chalk from drill cores taken from the North Sea Basin. The gas saturated chalk was chosen as an analogue for oil saturated chalk because in the deep parts of the Basin, the gas contains condensates and thus has some of the same functional groups present as oil. We did not use oil saturated chalk because our strategy was to explore the behavior of oil in contact with the surface by using a functionalized AFM

tip as a model oil droplet and, if oil was already present, the samples would have had to be cleaned before imaging. Adding organic solvents to the rock risks changing the properties of the surface that are responsible for oil wettability. As previously suggested,17 when the nanometer-sized clay is effectively organoclay, which has very different wettability properties than purely inorganic clay,23,24 then cleaning could have changed this property.

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METHODS AND TECHNIQUES

The chalk samples came from core plugs drilled through the gas and water zones of Danian and Maastrichtian chalk from the Danish sector of the North Sea. This work was part of a larger project that had been designed so the exact geographic locations of the boreholes were not made available because of confidentiality. However, the cores had been carefully selected so that those termed “water zone chalk” came from water saturated layers in an area out of the migration route for oil or gas, as interpreted from geological evidence available to the oil company; those termed “gas zone chalk” came from a gas saturated layer where the geological evidence and rock properties indicated that oil had never passed through. Our results are completely consistent with the geological history that had been interpreted by the oil company. The cores had been stored dry for several years in a storage room where oil filled cores were also stored. The selected gas and water zone cores did not show traces of hydrocarbons in normal or UV light, and the smell of the samples did not indicate the presence of oil components. However, when a water droplet was placed on the outer surfaces of the dry core plugs, from both the gas zone and the water B

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zone chalk, the presence of hydrocarbons was clearly indicated by very slow water imbibition. The presence of hydrocarbons can be anticipated on the chalk from the gas saturated zone but is not expected on the chalk from the water saturated zone. To test whether the qualitatively assessed hydrophobic surface feature was an inherent property of both types of sample or was caused by contamination from the storage room, or drilling, or fingers, the same simple qualitative test was performed by breaking a piece of rock from the core and putting a droplet of pure water on the freshly broken surface of each types of dry sample. Here, we noticed a clear difference. A water droplet on a fresh fractured surface from the gas zone chalk remained as a semisphere for several minutes (photographs in Supporting Information, Figure 1), while a water droplet on a freshly fractured surface of the chalk from the water saturated zone imbibed immediately and completely, within seconds. The surfaces of the samples were equally rough and not suitable for reporting precise contact angles, but the difference in behavior compared to the outer surface of the core was significant. From a macroscopic point of view, mineral composition and surface area, there was no observable difference between the gas zone and water zone samples (Table 1). The only difference was their behavior toward water. To quantitatively study this property, we used inverse gas chromatography (IGC) to determine surface energy, X-ray photoelectron spectroscopy (XPS) to determine the chemical composition of the surface of the particles, and finally, AFM/chemical force mapping (CFM) to study surface morphology and chemical reactivity (adhesion) at the nanoscale. Inverse Gas Chromatography (IGC). Inverse gas chromatography provides information about the surface reactivity of a solid sample. At first, a probe gas with known surface tension is passed through a porous sample with unknown surface energy properties. The strength of the interaction energy is derived from the volume of carrier gas that must be flushed through the sample before the probe gas is completely desorbed. The vapors are selected to probe: (a) nonpolar surface groups that interact only through long-range Lifshitz-van der Waals forces or (b) polar surface groups that also interact through short-range, Lewis acid−base forces, termed polar or specific forces. The term, van der Waals forces, includes the three dipole forces: (1) Keesom (permanent−permanent), (2) Debye (permanent−induced), and (3) London (induced−induced). In the interaction between a nonpolar vapor and a surface, London dispersive forces are usually the most important, so the measured surface energy is often called the dispersive surface energy (γDS ). The part of the total surface energy that is from Lewis acid−base surface interactions is usually termed the specific or polar surface energy (γPS ). The surface energy of the total (γTS ) interaction on the surface is

γST = γSD + γSP

commercial calcite powder (Merck) for reference. Approximately 400−1000 mg of the porous material was packed into individual silanized IGC columns that were closed on both ends with silanized glass wool. To fit into the column, the chalk samples were crushed to 1−2 mm pieces. Methane was used as a noninteracting component to determine the dead volume of the column. The nonpolar contribution to surface energy was probed with n-heptane, n-octane, n-nonane, and n-decane. The polar contributions were assessed using dichloromethane and ethyl acetate. All experiments were run at 30 °C with helium carrier gas at a flow rate of 10 sccm (standard cubic centimeters per minute). Prior to the experiments, the samples were conditioned in situ with helium at 100 °C for 3 h to remove any preadsorbed gas molecules/impurities that could interact with the injected solvent vapors. Failure to remove surface contaminants can lead to artificially lower surface energy and surface area values because these adsorbates would occupy surface sites. Conditions were chosen to sufficiently remove these species without damaging the intrinsic properties of the samples. In particular, it is important to remove surface water, which is why 100 °C was chosen. All solvents used were of high purity; we used high-performance liquid chromatography (HPLC)/GC-grade solvents supplied by Sigma-Aldrich (Gillingham, U.K.) and high purity helium and methane supplied by BOC Gases Ltd. (Guildford, U.K.). X-ray Photoelectron Spectroscopy (XPS). XPS provides information about the chemical composition of the top ∼10 nm of surface.35 Measurements were performed on a Kratos Axis Ultra DLD instrument, equipped with a monochromatic Al Kα source (hν = 1486.6 eV, power =150 W). The base pressure of the ultrahigh vacuum chamber was 5 × 10−10 mbar; the pass energy was 160 eV for wide spectra and 10 eV for high resolution spectra. We selected an aperture that gave a rectangular analysis area of 300 by 700 μm2. The commercial CasaXPS software was used for data analysis. Calibration of binding energies was made using carbonate at 290.1 eV.36 Atomic Force Microscopy/Chemical Force Mapping. The technique used for AFM and CFM measurements on chalk cores is described in detail elsewhere.17 Briefly, chalk particles were chipped from the interior of larger pieces of core by poking with a syringe needle. We took samples from inside the core to avoid contamination from the drilling fluid and from air. Single coccoliths (about 10 μm in diameter) were fixed to a freshly cleaved muscovite substrate (SPI supplies) using epoxy (Araldite 2014−1, Danalim). In Skovbjerg et al.,17 Blaa epoxy (Danalim) was used, but this was not completely stable in water17,37 and in some cases altered the behavior of the surface of the coccoliths. A different epoxy with higher chemical resistance was used in the experiments for this work. The reason why the more chemical resistant epoxy, Araldite 2014, was not used in the earlier studies was that the two components, glue and resin, do not mix well at the micrometer scale. This makes it very tricky to find a spot where the glue mixes well and a coccolith platelet, with a diameter of about 10 μm, could be fixed. We did many experiments, but the coccoliths did not stick well enough to obtain useful data. The results presented here are from the samples where we did not observe artifacts caused by the glue. Freshly cleaved samples of Iceland spar calcite (Wards Natural Science) and muscovite were also examined for comparison. We used an MFP-3D AFM from Asylum Research. All tapping mode imaging was performed with OMCL-AC240TS-W2Micro Cantilevers from Olympus (nominal stiffness, 2 nN/nm; resonant frequency, 70 kHz). Image sizes were 2−10 μm. Three different parameters were recorded: height, with Ångstrøm (sub nanometer) resolution in the Z dimension; amplitude, where edges are emphasized; and phase, which is sensitive to differences in physical properties such as adhesiveness and softness of the surface. During imaging in air, the temperature was from 26 to 30 °C and the humidity was from 30 to 60%. Force mapping was conducted with functionalized biolevers (Olympus; nominal stiffness, 30 pN/nm). The spring constant of each cantilever was determined by fitting a Lorentzian function to the thermal spectrum of the cantilever. The detector was calibrated by fitting a first order polynomial to the contact regime of a force distance curve made on a hard, flat surface, in this case, muscovite. To

(1)

A solid, imperfect surface contains a range of sites with slightly varying surface energy. To study the heterogeneity, in addition to quantifying the surface energy, we use an approach of incremental surface coverage.25,26 The surface coverage at each increment is specified as the amount of probe vapor injected (n) relative to the monolayer capacity (nm), which was determined using the BET method27 (Table 1). Adsorption isotherms were determined using the peak maximum method described by Thielmann and Pearse28 and YlaMaihaniemi and Williams.29 In the present work, dispersive surface energy profiles were calculated using the approach by Yla-Maihaniemi et al.25 based on the Dorris−Gray method.30 The dispersive surface energy, γSD, for each surface coverage (0.01−0.07 n/nm) was determined graphically from the net volume of retention of a series of n-alkanes. The specific surface energy distribution, γpS, was assessed by measuring the polar (or specific) free energies of desorption for two monopolar probe molecules that interact with only positive or negative surface sites.26,31−34 Further details on the background can be found in the Supporting Information. Measurements were performed on an IGC surface energy analyzer (SEA; Surface Measurement Systems Ltd., London, U.K.) equipped with a flame ionization detector. Four samples were measured: two from the gas zone of chalk, one from the water zone, and one of C

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functionalize the tips, the gold coated biolevers were cleaned in ozone for 20 min before submersion for at least 18 h in 0.1−2 mM ethanol (99.8%, Fluka) solutions containing the thiol molecules. A carboxylate functionalized tip was made using 11-mercaptoundecanoic acid (purity 99%, Aldrich) and an alkyl functionalized tip was made using 1hexadecanethiol (purity 99%, Aldrich). Just before use, the tips were soaked in ethanol to remove excess or loosely bonded thiol molecules. The liquid sample holder was a semiclosed PEEK fluid cell with a glass bottom. We used Milli-Q water (18 MΩcm) saturated with calcite so the coccoliths would not dissolve. The pH of the solution was ∼8.3; the ionic strength, 1.4 × 10−3, and the temperature, 28 to 31 °C. Data sets consisting of 100 × 100 (10 000) force curves were recorded over areas of 4 and 25 μm2. Each force curve was analyzed quantitatively using software developed by Atomic Force F&E GmbH in which maps of height and adhesion were generated. The applied force was 500 pN. The contact area of this type of tip was estimated to be ∼365 nm2.16 It takes about 1 h to record the set of force−distance data, and it is not trivial to come back to the same imaging area after changing to a tip functionalized with a different end group. Several maps must usually be obtained before finding the same 4 μm2. Many experiments were performed but only one coccolith was successfully imaged using two different functionalized tips as well as a standard tip needed to get high resolution images. In the other experiments, the coccoliths were either flushed out of the hardened glue when water was added or the surface suffered from glue artifacts, which resulted in very limited adhesion to any surface. However, the adhesion patterns that we observed in this work were consistent and reproducible (i.e., in maps obtained several hours later). Gas Adsorption (BET Surface Area). For specific surface area measurements, we used a Quantachrome Nova 2200 Sorption Analyzer. Prior to measurements, the samples were degassed by heating to 120 °C in vacuum ( 10.5 > 10.4 (> calcite). Only the dispersive force was measurable for calcite because the specific interaction is too strong. The calcite profile shows a large decrease in dispersive surface energy with surface coverage above 0.02. This could be a reflection of the two very dissimilar sites in calcite: the higher energy kinks, steps, and corners and the lower energy terraces. The 2% error bars show differences between the samples. Error bars of 2% on the specific surface energy profiles fall within the symbols and are therefore not visible on this figure.

so strongly that no meaningful results could be obtained, indicating that the surface of precipitated calcite is different from the surface of chalk, as was also shown by Keller and Luner.38 Hence, only specific adsorption profiles for chalk are shown. The heterogeneity in surface energy is expressed in these profiles as the difference between adsorption across a range of surface coverage (0.01−0.07 n/nm). The higher the difference between adsorption at low coverage (0.01 n/nm) and at higher coverage (0.07 n/nm), the more heterogeneous is the surface energy. In a homogeneous sample, all the sites have similar energy so there would be no difference between high or low loading of vapor. The dispersive surface energy differences between 0.01 and 0.07 n/nm for calcite and the water zone chalk (Sample 7−1) are 9.3 and 7.4 mJ/m2; for the gas zone chalks (Samples 10.5 and 10.4), the differences are 4.5 and 2.8 mJ/m2. This means that calcite and water zone chalk are energetically more heterogeneous. The same trend is observed for the specific energy distribution. In Figure 3, the data presented in Figure 2 have been plotted as a function of sample number. From left to right, for all studied aspects of surface energies, the trends follow each other. The two samples from the gas zone (10.5 and 10.4) are energetically less heterogeneous; they have lower total, dispersive, and specific surface energies. Notably, the specific energy percentages of the total surface energies are lower for the gas zone samples. This is significant because water interacts mainly through Debye and Keesom forces39 (i.e., through specific interactions), and this clearly indicates that the gas zone samples have a more hydrophobic surface than the water zone sample. According to IGC results, the hydrophobicity increases in the order: sample 7−1 < 10.5 < 10.4. X-ray Photoelectron Spectroscopy. Figure 4 shows XPS spectra obtained on all of the samples measured with AFM and IGC. The spectra have been normalized to the intensity of the carbonate peak. The table in Figure 4 shows the results of

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RESULTS AND DISCUSSION Sample Characterization. There is no significant difference in mineral composition of the four samples. They all contained predominantly biogenic calcite as coccolith elements and very minor clay and SiO2. Table 1 lists the characteristics of the samples and the techniques with which they have been examined. Inverse Gas Chromatography. IGC results are presented in Figure 2. The total surface energy has been divided into the parts that come from the dispersive interaction and the parts that come from the specific interaction. The dispersive interaction is the result of Lifshitz/London-van der Waals forces that act between all species, polar and nonpolar. These forces generally increase with surface area and the number of electrons in the interacting molecules (i.e., molecular size). The specific interactions take place between polarized molecules and surfaces. This interaction is well described by Lewis acid− base interactions where the Lewis base is an electron donor and the acid is an electron acceptor. On commercial calcite powder, the gas vapor probes used to assess the specific forces interacted D

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noticeable is that the organic carbon consists mainly of hydrocarbons (CC and CH bonds) with a smaller percentage of carbon singly bonded to oxygen such as in alcohol groups and even less carbon coordinated with two oxygens, such as in carboxylic acid groups (see ref 36 and references therein). The concentration of OCO and C O(H) bonds is more or less the same in all chalk samples and significantly higher than for the commercial calcite powder and the cleaved Iceland spar calcite. This suggests that the compounds containing these bonds are inherent to the chalk and that their concentration is not affected by the presence or absence of hydrocarbons. Coccolithophorids, the species of algae that make coccoliths, are known to control the morphology of their coccoliths using acidic polysaccharides to control calcite growth to favor their preferred shape and orientation rather than the thermodynamically most stable form,40,41 and recently, it was demonstrated that polysaccharides are still present in chalk today.42,43 It is therefore likely that it is remnants of polysaccharide that we observe in our chalk samples. The concentration of C−C and C−H bonds, that represent hydrocarbons, is the same on the two chalk samples from the water zone (7−6 and 7−1). The hydrocarbon peaks on the gas zone chalks are higher than on the water zone chalk and the peak from 10.4 is almost twice as high as the peak from 10.5. These data are inversely proportional to the data from IGC (Figure 3). Not surprisingly, our results indicate that the concentration of hydrocarbons at the surface of the chalk is directly responsible for the hydrophobicity observed macroscopically. The organic material might uniformly cover the surface of the chalk, or it might be preferentially associated with some structures or minerals on the pore surfaces. Knowing the spatial distribution of the strong interaction sites for organic molecules on the chalk surface in a real reservoir (whether gas or oil saturated) is essential for fully understanding wettability and its alteration during water injection. Atomic Force Microscopy and Chemical Force Mapping. AFM imaging performed on many chalk samples from gas and water saturated zones of the North Sea chalk, as well as from outcrop samples, has already shown that the surfaces of chalk particles are decorated with nanometer-sized clay laths.17 Sometimes they appear as nearly full coatings, such as in Figure 1b, while at other sites the clay is present as isolated particles. The typical thickness of the clay laths ranges from 1 to 4 nm, which represents only one or a few phyllosilicate unit layers. In the North Sea Basin formations, the gas is “wet” in the sense that it contains condensates, which are longer chained alkanes, and other organic components that are typically present in oil. The wet gas thus consists of nonpolar and polar molecules. We used a nonpolar tip, functionalized with alkyl groups (CH3), to map nonpolar areas on a coccolith from the gas zone, sample 10.5. Another tip, functionalized with carboxylate groups (COO−), was used to probe the polar areas. The coccolith was also imaged at high resolution using a standard, high resolution tip so that areas of high tip adhesion could be correlated with surface features, such as clay laths or the topography of the coccolith elements. Figure 5 is a collection of images from the coccolith. Figure 5a is a tapping mode image, showing morphology. On the large coccolith elements, some smaller flat particles are present (white arrows). The particle at arrow A is 3 nm thick, while that at arrow D is ∼10 nm thick. The shape and thickness of particle A is similar to those observed previously on chalk particles from

Figure 3. Surface free energy (left y-axis) at surface coverage (n/nm = 0.04) plotted against sample number. The total (Tot), dispersive (D), and polar/specific (P) surface energies all decrease from right to left in the plot. The heterogeneity of the surface energy (Het), shown with small open circles, also decreases (left y-axis). Finally, the percentage of the total energy that can be related to specific (acid−base) interactions (P%) and thus hydrophilicity is shown with large open circles (with scale on the right y-axis) and also decreases.

Figure 4. C1s XPS spectra normalized to the intensity of the carbonate peak. The organic carbon peak becomes increasingly intense moving from the calcite single crystal and powder to chalk from the water zone to chalk from the gas zone of the North Sea Basin. The table summarizes the results of the curve fitting.

fitting the C 1s peak with four organic carbon contributions: adventitious carbon and hydrocarbons (CC(H)), alcohol groups (CO(H)), carboxylic groups (OCO), and carbonate (CO3). Although all samples have organic carbon on the surface, we note that freshly cleaved calcite has more than 10% in the form of adsorbed adventitious carbon. Also E

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Figure 5. AFM tapping mode images and force maps of the same areas on a coccolith from the gas zone (sample 10.5). Parts a and b are identical tapping mode images with amplitude overlay on 3-D height. In b, the yellow outlines indicate where an edge of clay is identified. Clay A is 3 nm thick, and clay D is ∼10 nm thick. Part c is from the same recording as parts a and b but with phase overlain on 3-D height. The phase signal is sensitive to changes in physical properties such as hardness and stickiness. Thus, clay A has a different physical nature than the substrate beneath. (d, f, and g) Adhesion overlay on height (e) with a COO− tip (f) and a CH3 tip (d and g). Parts d and g are the same images but on part d, the yellow markings copied from part b show that the high adhesion areas correlate very well with the clay areas.

the water zone. Particle D is thicker but has the same lath-like, flexible shape. Therefore, both are interpreted to be clay particles. In Figure 5b, a slightly modified version of Figure 5a is shown, now with outlines of where we are confident that there is an edge of a clay lath. In two places, A and C, the right and left edges of clay laths are visible. In two other places, B and D, only the left edge of a clay particle can be clearly distinguished. Figure 5c is made from data obtained from the same area as Figure 5a and b, but showing phase contrast overlain over topography (Figure 5e). The variation in color on a phase contrast image shows the variation in energy dissipation, usually caused by differences in hardness or stickiness of the surface.

Here, hardness refers to the resistance of the surface to indentation by the AFM tip; stickiness refers to the adhesion between the surface and an object in contact with the surface, in this case, the tip. The color of the particle labeled clay A is homogeneous and different from the background, supporting the interpretation that this is not the same material as the coccolith. The color of the plateau labeled B is also very homogeneous and no right edge is visible, so we interpret that this entire plateau is covered by clay. Clay D has a sharp left edge, but it is very difficult to determine the right boundary, perhaps because it coincides with the termination of the coccolith element on which it is situated. The phase contrast F

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Figure 6. Tapping mode images and force maps of an area expanded from that shown in Figure 5. The sites labeled A−D on Figure 5 are also labeled here for orientation. The blue square on part a is magnified in part b. One corner of the coccolith element is completely covered with clay (E). (c) 3D height information, which is overlain by adhesion in part d. The force maps (d and e) are from approximately the same area, but one shows data obtained with a COO− tip (d) and the other, data obtained with a CH3 tip (e). The yellow markings (b, d, e) indicate the edges of two clay particles. The adhesive areas (pink and dark blue) are present on clay, at corners and at edges of the coccolith particles (comparison with part c).

image does not suggest a right edge either, such as it does for clay A. Force maps were obtained on these surfaces. Figures 5d and g (which is the same image as d but without markings) show where the nonpolar tip (CH3) sticks. There are relatively sharp borders between adhesive areas (dark blue, red, and pink) and nonadhesive areas (light blue), and these correspond very well with the presence of clay (Figure 5d compared with b). Figure 5f shows where the polar tip (COO−) sticks. Comparison of parts f and g of Figure 5 shows that the alkyl and the carboxylate tip stick to almost the same areas. The pink areas on both maps are rounded, some more elongated than others. A few of the rounded areas are only adhesive in response to one or the other of the tips’ functional groups, but most are adhesive to both tips. Figure 6 is another collection of images from the coccolith but from an area (area 2) bordering the right edge of area 1 (Figure 5). The sites labeled in Figure 5 have also been labeled in Figure 6, for orientation. One corner (E) of a coccolith element is covered with the nanometer thick, lath-shaped clay particles. The adhesion force maps (Figure 6d and e), obtained using the carboxylate and the alkyl tip, reveal similar rounded, high adhesion areas on the clay as in area 1. What can further be observed in these maps is that there are rounded areas also along edges of the coccolith elements. Looking back at Figure 5f and g, there is also a high concentration of adhesive areas along edges in area 1. In summary, the data from this coccolith from the gas zone show many high adhesive areas that interact both with the polar and the nonpolar tips. These areas are

concentrated on clay particles and in tight intergranular spaces of coccolith elements. This is different from the adhesion patterns observed previously on coccoliths from the water zone.17 The adhesion on nano-sized clay particles in that work was not of a patchy nature but much more homogeneous over each clay lath. Figure 7 shows that, in addition to displaying patchy adhesion, the thickness of the laths from the gas zone is not uniform. This is in contrast with the adhesive clay particles observed previously.17 The general crystal form is very similar, and the laths are likewise interpreted to be illite or illite/ smectite based on their shape, size, and the diagenetic history of the sediment. However, the thickness of the edges of the clay laths from the gas zone chalk is greater than the central parts and the increase in thickness is ∼0.15 to 0.2 nm, seemingly independent of the thickness of the whole particle. Comparison of each row of images in Figure 7 shows that the high adhesion areas correspond to the depressed parts of the clay particles. It indicates that the clay minerals have been altered by a later reaction, that was initiated from the edges. This morphology of the clay laths was observed on other chalk samples from the gas zone, but was not observed on any of the chalk samples from the water zone or from outcrop samples. Although the introduction of hydrocarbons into sedimentary rocks has been shown to affect diagenetic mineral reactions, including illitization of kaolinite and smectite,44−46 there are, to our knowledge, no reports on illite reactions observed locally, on and within single crystals. There are three possible pathways that can explain the AFM observations: (1) another phase has G

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Figure 7. (a, b, d, and e) Magnification of images shown in Figures 5 and 6. The high adhesion areas on clay laths are correlated precisely with depressions in the clay (white arrows, top row; black arrows, bottom row). (c and f) Tapping mode height images showing the variation in topography. The insets are height profiles showing that the edges are thicker than the central parts of these clay laths. The images were flattened before making the profile because the underlying coccolith element is sloping; so unfortunately, this has induced an uncertainty of several Ångstrøm in the z direction.

edges, one would expect an increase proportional to the initial thickness, but this is not what we observe. Unfortunately, it is outside the scope of this paper to pursue the possibility of topotactic transformation experimentally. The third possibility, incorporation of larger compounds, implies that there are expandable layers (i.e., smectite) in the laths. The chemical affinity of swelling clays for organic compounds suggests that, in an initially water saturated sediment, compounds from the intruding gas phase are likely to be incorporated in the interlayers of the clay.48 Many studies in the research field of organoclays have shown that clay interlayer spacing can increase when organic molecules are incorporated (see refs 23, 49, and 50 and the references therein). Morphological changes of the edges of another type of layered compound that produces microcrystals, namely green rust, has been observed previously with AFM.51,52 In those studies, the structural transformation was attributed to reaction with redox active anions at the edges and in the interlayers of the layered structure. Similarly, in the present work, if there are smectite layers in the lath structure, incorporation of larger organic molecules from the gas phase is a plausible explanation

overgrown the clay minerals preferentially on the edges, (2) the clay mineral has transformed locally to a different mineral, or (3) the layers have expanded as a result of incorporation of larger compounds in the interlayer. The first possibility is less likely because the edges are rather sharp and overgrowth would presumably alter such a boundary, at least to some extent. The second possibility is a topotactic transformation of the clay minerals to a different mineral at the edges. However, we have found no reports on transformation of illite in the diagenetic regime, let alone such local transformation as our observations suggest. Transformation of illite to muscovite, for example, is reported47 but this requires high grade diagenesis or low grade metamorphism, at temperatures higher than those of the North Sea chalk. Furthermore, transformation would not increase the thickness of the layers. Transformation to ammonium tobellite is a possibility,44 but it would not increase thickness to the degree that we observe. One can speculate that introduction of hydrocarbon gases with reducing capacity could have transformed iron rich illite to chlorite by reducing Fe(III) to Fe(II), which would increase the basal spacing from 10 Å to 12−14 Å. However, if this is the reason for the increased thickness of the H

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hydration layer structure is distorted at edges, then collections of organic compounds that are less polar than water would decrease their surface energy most by adsorbing along the lower surface of an edge or in corners where it would be in contact with two or three surfaces. Ethanol has been observed to adhere on calcite surfaces through its OH functional groups, preferentially over water, in a nicely ordered structure, but the structure is disrupted enough at step edges to allow water access.54 This could also partly explain adhesion increase at edges where no clay is observable.

for the increased thickness of the edges. However, to unambiguously establish the nature of the edge alteration, more fundamental work on model systems would be needed, such as examining the behavior of synthetic nanometer-sized illite/smecite laths under reservoir conditions, that is, at gas saturation and at relevant temperature and pressure. Figure 8 shows histograms that represent the adhesion displayed as pink and dark blue areas of clay D (Figure 5) as

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SUMMARY We studied chalk from a gas zone in the North Sea Basin that had been equilibrated for geologic time with pore fluids containing high concentrations of organic compounds. The surface of the chalk had retained its micrometer scale heterogeneity, as has previously been observed for water zone chalk.17 Authigenic nanometer-sized clay laths, which had grown on the coccolith elements, are adhesive and provide attachment sites for organic compounds on pore surfaces in the chalk. In contrast, the calcite surfaces of the coccoliths are essentially inert, except perhaps at corners and at edges where organic compounds can adsorb more easily. The organic compounds from the gas phase are possibly intercalated into the interlayers of the nanometer-sized clay, starting from the edges. Thus, they might be a natural parallel to the organoclays that are synthesized for industrial purposes. The existence of natural organoclays is rarely mentioned in the literature,55 although such minerals could play an important role in biogeochemical cycles in the natural environment because of their ability to effectively adsorb polar and nonpolar compounds. In the research community of oil recovery, it is often assumed that reservoir rocks were originally water-wet before oil migrated into them, but our data demonstrate that a rock can have a highly hydrophobic surface even if it has previously only been exposed to natural gas, which probably contains longer hydrocarbon chains. Thus, in cases where hydrocarbon gas has entered a reservoir ahead of the oil, the resulting hydrophobicity of the pore surface would cause the oil to adsorb. This explanation reduces the need to explain wettability alterations by adsorption of crude oil components such as asphaltenes and acidic fractions.10,56 Our results also show that pure calcite, either as single crystals or as powder, is not a good model for the complicated North Sea chalk, and neither is outcrop or water zone chalk. Even though nanometer scale clay particles contribute less than 1% of the mass, their presence at surfaces gives them an extensive effect on wettability. The polar groups on a functionalized AFM tip, which serve as our model oil droplet, have a higher affinity for the nano-sized clay than for calcite, suggesting that the tiny clay laths serve as anchor points for hydrocarbons, and these, in turn, serve as attachment points for other hydrocarbons. This new understanding will aid in designing model systems that more closely resemble reservoir rocks for exploring how to develop better methods for improved or enhanced oil recovery (IOR/EOR).

Figure 8. Histogram of the adhesion force distribution of pixels in selected areas of images in Figure 5. The left histogram shows data obtained with a carboxylate (COO−) tip. The right histogram shows data obtained with an alkyl (CH3) tip. Black bars represent the low adhesive (light blue) area left of clay D, dark gray bars represent the medium adhesive (dark blue) area on clay D, and the light gray bars represent the high adhesive (pink) areas on clay D.

well as the light blue areas attributed to surface that is not covered with clay next to clay D. For the nonpolar (CH3) tip, the average adhesion on dark blue areas is 175 pN and on the adhesive, pink areas, 838 pN. For the polar (COO−) tip, the average adhesion on dark blue and pink areas is 236 pN and 940 pN. The unaltered clays observed previously in water zone and outcrop samples,17 using similar tips, all have average adhesion below 750 pN and some even below 200 pN. This shows that clay minerals that have been in contact with gas during millions of years remain very sticky for organic carbon. This strongly suggests that also in the oil saturated zone, hydrocarbons and polar groups from the oil would be able to stick to clay laths and ensure direct linkage between the oil phase and the rock surface. In contrast, the calcite coccolith surface next to the clay is largely nonadhesive. An exception is tight intergranular space such as corners and edges of the chalk particles, which also show rounded areas of increased adhesion (Figures 5 and 6). These parts could be covered with clay, but there is no evidence to either verify or dismiss this possibility. Height profile measurements, such as the insets in Figure 7, were performed, but because the flattening procedure was complicated by the closeness of the edge, the measurements are not reliable. For example, incrementally small differences in how the data were flattened would lead to different conclusions about the presence of a clay lath. An alternative explanation for the high adhesion at sharp changes in topography is disruption of the hydration layer. In a study of hydrophobic and hydrophilic rough model surfaces, it was shown that an adsorbed water film was discontinuous at the highest edges on the surface.53 If the

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ASSOCIATED CONTENT

S Supporting Information *

(1) Photographs of a water droplet on gas zone chalk showing qualitatively that surfaces are hydrophobic. (2) More detailed description on the theory of inverse gas chromatography I

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(13) Salathiel, R. A. Oil recovery by surface film drainage in mixed wettability rocks. J. Pet. Technol. 1973, 25, 1216−1224 (OCT). (14) Dixit, A. B.; Buckley, J. S.; McDougall, S. R.; Sorbie, K. S. Empirical measures of wettability in porous media and the relationship between them derived from pore-scale modelling. Transp. Porous Media 2000, 40 (1), 27−54. (15) Skauge, A.; Spildo, K.; Hoiland, L.; Vik, B. Theoretical and experimental evidence of different wettability classes. J. Pet. Sci. Eng. 2007, 57 (3−4), 321−333. (16) Hassenkam, T.; Skovbjerg, L. L.; Stipp, S. L. S. Probing the intrinsically oil-wet surfaces of pores in North Sea chalk at subpore resolution. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (15), 6071−6076. (17) Skovbjerg, L. L.; Hassenkam, T.; Makovicky, E.; Hem, C. P.; Yang, M.; Bovet, N.; Stipp, S. L. S. Nano-sized clay detected on chalk particle surfaces. Geochim. Cosmochim. Acta 2012, 99, 57−70. (18) Fabricius, I. L. Chalk: Composition, diagenesis, and physical properties. Bull. Geol. Soc. Den. 2007, 55, 97−128. (19) Strand, S.; Hjuler, M. L.; Torsvik, R.; Pedersen, J. I.; Madland, M. V.; Austad, T. Wettability of chalk: Impact of silica, clay content, and mechanical properties. Pet. Geosci. 2007, 13 (1), 69−80. (20) Lindgreen, H.; Drits, V. A.; Jakobsen, F. C.; Sakharov, B. A. Clay mineralogy of the central North Sea upper Cretaceous-Tertiary chalk and the formation of clay-rich layers. Clay Clay Min. 2008, 56 (6), 693−710. (21) Lindgreen, H.; Drits, V. A.; Sakharov, B. A.; Jakobsen, H. J.; Salyn, A. L.; Dainyak, L. G.; Kroyer, H. The structure and diagenetic transformation of illite-smectite and chlorite-smectite from North Sea Cretaceous-Tertiary chalk. Clay Min. 2002, 37 (3), 429−450. (22) Basu, S.; Sharma, M. M. Measurement of critical disjoining pressure for dewetting of solid surfaces. J. Colloid Interface Sci. 1996, 181 (2), 443−455. (23) Yariv, S.; Cross, H. Organo-clay Complexes and Interactions; Marcel Dekker, Inc: New York, 2002; p 688. (24) Lee, S. Y.; Kim, S. J.; Chung, S. Y.; Jeong, C. H. Sorption of hydrophobic organic compounds onto organoclays. Chemosphere 2004, 55 (5), 781−785. (25) Yla-Maihaniemi, P. P.; Heng, J. Y. Y.; Thielmann, F.; Williams, D. R. Inverse gas chromatographic method for measuring the dispersive surface energy distribution for particulates. Langmuir 2008, 24 (17), 9551−9557. (26) Das, S. C.; Larson, I.; Morton, D. A. V.; Stewart, P. J. Determination of the polar and total surface energy distributions of particulates by inverse gas chromatography. Langmuir 2010, 27 (2), 521−523. (27) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309−319. (28) Thielmann, F.; Pearse, D. Determination of surface heterogeneity profiles on graphite by finite concentration inverse gas chromatography. J. Chromatogr. A 2002, 969 (1−2), 323−327. (29) Yla-Maihaniemi, P. P.; Williams, D. R. A comparison of frontal and nonfrontal methods for determining solid−liquid adsorption isotherms using inverse liquid chromatography. Langmuir 2007, 23 (7), 4095−4101. (30) Dorris, G. M.; Gray, D. G. Adsorption of n-alkanes at zero surface coverage on cellulose paper and wood fibers. J. Colloid Interface Sci. 1980, 77 (2), 353−362. (31) Dong, S.; Brendle, M.; Donnet, J. B. Study of solid surface polarity by inverse gas chromatography at infinite dilution. Chromatographia 1989, 28 (9−10), 469−472. (32) Schultz, J.; Lavielle, L.; Martin, C. The role of the interface in carbon-fiber epoxy composites. J. Adhes. 1987, 23 (1), 45−60. (33) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Interfacial lifshitzvanderwaals and polar interactions in macroscopic systems. Chem. Rev. 1988, 88 (6), 927−941. (34) Della Volpe, C.; Siboni, S. Some reflections on acid−base solid surface free energy theories. J. Colloid Interface Sci. 1997, 195 (1), 121−136. (35) Hochella, M. F. Auger-electron and X-ray photoelectron spectroscopies. Rev. Miner. 1988, 18, 573−637.

(IGC). (3) XRD spectra of the samples. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Karen Henriksen, Maersk Oil and Gas A/S, for selecting the samples, Karina Sand for taking the photographs for Supporting Information Figure 1, and the members of the NanoGeoScience Research Group for daily discussions, especially Keld West. We are particularly grateful to Knud Dideriksen for discussions and comments on an earlier version of the manuscript. We thank Finn Engstrøm, Maersk Oil and Gas A/S, for deep engagement and lively discussions about chalk in general, Nader Payami, KU, for SEM assistance and Stefan Vinzelberg, Atomic Force F&E GmbH, for guidance with force mapping software. The paper was improved by comments from the anonymous reviewers. This work was funded by the Danish National Advanced Technology Foundation and Maersk Oil and Gas A/S through the NanoChalk Venture.

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K

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