Article pubs.acs.org/EF
How Naturally Adsorbed Material on Minerals Affects Low Salinity Enhanced Oil Recovery J. Matthiesen,*,† N. Bovet,† E. Hilner,† M. P. Andersson,† D. A. Schmidt,‡ K. J. Webb,§ K. N. Dalby,† T. Hassenkam,† J. Crouch,§ I. R. Collins,§ and S. L .S. Stipp† †
Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen 2100, Denmark Department of Physics, North Carolina A&T State University, Greensboro, North Carolina 27411, United States § Upstream Technology, BP Exploration Operating Company, Sunbury-on-Thames TW16 7LN, United Kingdom ‡
ABSTRACT: Laboratory core flood and field scale tests have demonstrated that about 5 to 40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore wall, as a result of the decrease in divalent cation concentration. Using X-ray photoelectron spectroscopy, we have demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material. Thus, pore wall wettability is more likely dominated by tightly adsorbed organic material than by the character of the underlying minerals. To test this hypothesis, we used the chemical force mapping (CFM) mode of atomic force microscopy (AFM) to directly measure adhesion forces on individual quartz grains that were plucked from core plugs. We functionalized AFM tips with model oil compounds so they would represent tiny oil droplets, and we measured their ability to adhere to surfaces as salinity changed. We examined grains from a sandstone core plug that had been cut into segments, which had been stored in kerosene or solvent cleaned. On all samples, surfaces were more oil wet (higher adhesion) in artificial seawater (ASW; 35,600 ppm) than in ASW diluted with ultrapure deionized water to ∼1,500 ppm. XPS demonstrated that solvent cleaned surfaces had less adsorbed organic material than the kerosene stored sample. AFM measurements showed that the low salinity effect, namely the change in adhesion caused by decreasing salinity, was twice as high on kerosene stored samples as on solvent cleaned surfaces. The organic material that is adsorbed on the pore surfaces in the preserved sandstone offer very sticky anchor points for adhering oil molecules. This suggests that in reservoirs, even hydrophilic minerals located at the pore-fluid interface have tightly adhering hydrocarbons and the low salinity response depends on the behavior of this adsorbed material.
1. INTRODUCTION To recover oil from a reservoir, the pressure needed to push the oil toward the production well is generally maintained by injecting seawater, produced water or water extracted from a different rock formation. Seawater has a salinity of ∼35,600 ppm and, depending on its host rock, formation water can have a salt content ranging to 200,000 ppm total dissolved solids (TDS) or higher. Core flood experiments by Tang, Morrow, and co-workers1,2 showed that by decreasing the salinity of the water used for flooding a sandstone core plug, the amount of produced oil increased. Based on core flood tests from several sandstone reservoirs, Lager et al. reported an average recovery of about 14% more oil with low salinity water than from flooding with sea or formation water.3 The laboratory test results were confirmed in the field by single well tests4,5 and by interwell trials.6 Several mechanisms have been proposed to explain the observations. Tang and Morrow suggested that mixed wet, clay fines (particles less than 1 μm diameter) would be stripped from pore walls during low salinity water flooding. Oil attached to the clay would then be carried with the particles and be produced.2 However, Romero et al. found in a study of Berea sandstone core plugs that the release of fines was not a major factor for incremental oil production during low salinity flooding.7 Lager et al. also reported finding no evidence of mobile clay fines3 but rather suggested that the additional oil © 2014 American Chemical Society
was released by a change in wettability of the mineral surfaces in the sandstone, particularly the clay particle surfaces, so they became more hydrophilic. They proposed that a decrease in divalent cations and ionic strength in the floodwater would lead to an expansion of the electrical double layer at the mineral surfaces that make the pore walls and release adsorbed cations and thereby release the oil adsorbed on the pore walls. This is backed up by Brady et al., who using a surface complexation model showed that the number of electrostatic bridges between species in the oil and sites on kaolinite edges decrease when injecting low salinity water.8 An alternative mechanism, suggested by Austad et al.,9 was that during low salinity flooding, adsorbed divalent cations, such as Ca2+, would be exchanged by H+ derived from hydrolysis. Then reactions between the OH− that were produced and adsorbed acidic material would cause desorption of hydrocarbons, increasing water wettability and thus releasing oil.9 Common for all of the proposed mechanisms is that decreasing hydrophobicity increases oil recovery. The effects of salinity and pH on adhesion of various crude oils to glass surfaces were investigated by Buckley and coworkers.10 They used both numeric models and experiments to Received: January 22, 2014 Revised: June 24, 2014 Published: July 2, 2014 4849
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and toluene, and (iii) restored with dead crude oil, i.e. oil where CO2 and other gases have exsolved. By comparing adhesion during exposure to ASW and diluted ASW, we could determine the influence of the adsorbed organic material on the low salinity effect.
measure whether oil droplets would stick to glass surfaces while these were submerged in saline solutions. Such measurements are qualitative in nature but give good indications of the trends. More quantitative measurements on the low salinity effect have been performed using atomic force spectroscopy (AFS), otherwise known as the chemical force mapping (CFM) mode of atomic force microscopy (AFM). Hassenkam and colleagues measured adhesion using tips functionalized with carboxylic acid terminated molecules on outcrop and solvent cleaned reservoir sandstone during exposure to artificial seawater (ASW) and diluted ASW.11 Organic molecules with carboxylic acid functional groups adhere more strongly to quartz from both the outcrop and the cleaned sandstone during exposure to ASW than during exposure to ASW diluted to 1,400 ppm. This effect was reproducible on many sand grains and consistent through many cycles of changed salinity. In a recent paper, Hassenkam et al. also reported results from experiments on model surfaces of minerals commonly found in sandstone.12 Carboxylic acid functionalized tips also adhered less strongly on the amorphous silica and mica surfaces during exposure to diluted ASW. However, on illite, a clay common in sandstones, adhesion was the same in both high and low salinity solutions. Tips functionalized with nonpolar alkanes adhered less to mica in diluted ASW, whereas on amorphous silica and illite, adhesion was the same in ASW and diluted ASW. These results were unexpected because clays are thought to play an important role for the low salinity mechanism.3 These results raised the question if it is truly the bare mineral surface that defines wettability or rather organic material that is adsorbed from the air, gas, or the liquid phase in contact with the mineral surface that defines wettability. It is well-known in the sedimentology community that clay is an excellent scavenger of organic material in the water column, in lakes, streams, and the ocean and that carbon containing compounds associate with clay minerals.13,14 Reservoir rock that has been exposed to crude oil is known in some cases to change from initially waterwet to oil-wet.15 The presence of organic material on pore surfaces has been hypothesized based on coreflood studies where it was found that oil recovery is higher from aged cores than unaged ones.16 Where the organic material has been identified, it is generally assumed to be asphaltenes or resins.13 The purpose of this paper is to explore the effect of adsorbed organic material and its impact on wettability change with changing salinity, and consequently, on low salinity oil recovery. The mechanisms proposed by both Lager et al.3 and Austad et al.9 imply that mineral surfaces are rendered bare and clean once the releasable oil has left them. There are millions of millions of possible organic compounds in water, in the sediment, in the rock, and from oil that could stick to the pore mineral surfaces. The properties they render to the fluid−solid interfaces vary with the organic species, the mineral substrate, and the composition of the fluid in contact. So our question is does adsorbed organic material play a role in the low salinity effect. This paper investigates the impact of organic material that is naturally present on mineral surfaces in a sandstone core plug on the low salinity response. Using CFM, we measured adhesion between nonpolar, alkane functionalized tips (representing oil droplets), and the surfaces of quartz grains plucked from a core plug that had been (i) preserved in kerosene (CorePreserve) from the time it was removed from the reservoir, (ii) solvent cleaned (CoreClean) using methanol
2. EXPERIMENTAL DETAILS 2.1. Samples from Core Plugs. We used a core drilled from a consolidated sandstone oil reservoir where the low salinity response was known to be moderately high. X-ray diffraction data showed that the sample consisted mainly of quartz, some feldspar in various stages of weathering to clay, and both detrital and authigenic clay particles. The core plug, which had been stored in kerosene to minimize the effects of drying and oxidation, was cut into three sections. One, called CorePreserve, was used directly. The second portion, called CoreClean, had been solvent cleaned by flushing repeatedly with methanol followed by toluene, until the ejected toluene phase was optically clear. The third section, CoreRestore, had been restored with crude oil, according to the procedure used to prepare solvent cleaned core plug samples for core flood tests. There was a visible amount of carbon containing compounds on the pore surfaces of samples taken from this section, which adhered to the AFM tip as well as the sample, making all experiments with atomic force microscopy impossible. Therefore, the restored core was not investigated further in this study. The AFM samples were prepared as described previously.11 Briefly, small pieces were broken from each of the core plug sections. Single sand grains or small aggregates of grains were carefully plucked from the sandstone and attached to a microscope coverslip using epoxy (Danalim two component epoxy or EPON Resin 1002F). Care was taken to make a very thin strip of epoxy so the grain would not sink into the glue and to keep it from spreading up the sides of the mineral grains. A standard reflective optical microscope was used to choose the surfaces for AFM imaging. We chose grains that had relatively flat areas that would be accessible to the AFM tip, that were at least 15 × 15 μm2 and where the differences in height on the surface were not more than ∼1 μm over a lateral distance of 5 μm. To be able to compare the results of CorePreserve with CoreClean, we chose only quartz crystal surfaces for this study. We could easily identify quartz by the characteristic prismatic and pyramidal crystal faces on grains that had partially recrystallized during diagenesis. We chose these surfaces to represent the pores exposed in the sandstone fluid pathways. The fact that they are recrystallized provides good evidence that they had been exposed to pore fluids and were indeed part of the pore walls. 2.2. AFM Tips. For acquiring AFM tapping mode images, we used standard AC mode silicon tips from Olympus (OMCL-AC240). They had a resonance frequency of ∼80 kHz and a spring constant of 2 nN/ nm. For all force mapping, we used Olympus biolever AFM probes. The chips are equipped with two cantilevers, that have nominal spring constants of 30 and 6 pN/nm. Both are coated with 5 nm of chromium and 30 nm of Au. For each experiment, the deflection sensitivity was determined, and the actual spring constant for that cantilever was estimated from a fit to a thermal spectrum.17 The spring constant varied between 20 and 30 pN/nm for the stiff cantilevers and between 4 and 8 pN/nm for the soft cantilevers. Before use, the biolever AFM probes were prepared for the functionalization procedure by first rinsing them with ethanol, then drying them with N2, and treating them in UV/ozone for 20 min. After cleaning they were immediately submerged in an ethanol solution of ∼5 mM HS(CH2)10CH3 for at least 24 h. The thiol group bonds strongly with the gold on the tip, forming a self-assembled monolayer of hydrocarbon molecules with the alkane end protruding, as the bristles on a brush.18 We used these tips to represent tiny droplets of oil that could interact with the surface. 2.3. Solutions. The pH of the formation water (FW) in a sandstone reservoir is typically around 5.5 so we chose that as standard. This is also the pH of NaCl solutions at equilibrium with atmospheric CO2 so all of the experiments of this study were kept at pH 5.5 by omitting NaHCO3 from the artificial seawater (ASW) recipe. The ASW was diluted to form the low salinity solution. We also 4850
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Table 1. Composition of the Solutions Used for AFM Measurements ppm CaCl2 MgCl2 KCl NaCl total (ppm)
cation concentration (mmol/kg)
formation water (FW)
artificial seawater (ASW)
diluted artificial seawater (ASW-low)
FW
ASW
ASW-low
745 191 172 27507 28615
1332 5428 917 27951 35628
52 208 40 1176 1476
6.71 2.01 2.30 470.66
12.00 57.01 12.30 478.26
0.47 2.18 0.54 20.13
sample. All Raman spectra were fitted with a second order polynomial to subtract background fluorescence. 2.7. AFM Imaging and Force Mapping. We used an MFP-3D AFM from Asylum Research, Santa Barbara, USA. AFM images were acquired in tapping mode in air, usually with an amplitude set point of ∼100 nm. The force maps were made with 50 × 50 data points, recorded over areas that were 3 to 5 μm on a side. The acquisition time for each force map was ∼15 min. At each point, the tip and sample were brought into contact and then separated again, generating a force curve such as that shown in Figure 1. The tip starts ∼1 μm
omitted SO42− because it is often removed from the water used for flooding before it is pumped into the oil reservoir to decrease the risk of BaSO4 precipitation in pores and pipes and to avoid offering nutrients to the sulfate reducing bacteria that produce H2S, a hazard for the working environment on the rigs and for the quality of the oil. We used reagent grade or better NaCl, KCl, MgCl2, and CaCl2. Our recipe for artificial seawater is similar to those found in the literature.19 For the low salinity solution (ASW-low), we simply diluted the stock ASW by about 25 times, to produce a solution that was 1,500 ppm. We also used artificial formation water that reflected the composition of the water that was present in the oil reservoir where the core was taken, before the oil was produced. It was a NaCl brine, and its salinity was 28,600 ppm. The solutions were made by dissolving the salts in ultrapure deionized water (Milli-Q) with resistivity >18 MΩ·cm. Data for the composition of the solutions used for the experiments are listed in Table 1. 2.4. Scanning Electron Microscopy (SEM). Images were acquired using a FEI Quanta 3D field emission gun (FEG) SEM by collecting back scattered electrons. We used the low vacuum mode of this device in an atmosphere of 60 Pa H2O vapor so coating the samples was not necessary. The accelerating voltage was 5−10 kV. 2.5. X-ray Photoelectron Spectroscopy (XPS). This technique is optimized for analyzing chemical composition from the surface of solid samples. Information comes from the outer ∼10 nm of the sample. Photoelectron escape drops exponentially with depth so near surface composition dominates the spectra. Experiments were performed using a Kratos Axis UltraDLD in a vacuum chamber running at approximately 10−9 Torr. A monochromatic AlKα X-ray source (energy = 1486.6 eV, power = 150 W) was used to generate the photoelectrons from the samples. The data were fitted using the commercial software CasaXPS. Binding energies were calibrated to the adventitious carbon peak at 285 eV. The calibration gave a Si 2p binding energy for both CoreClean and CorePreserve of 103.5 eV consistent with literature values.20 The chemical mapping capability of the XPS instrument was used to identify the sandstone particles by observing the Si 2p peaks. Spectra were collected with a spot size of 110 μm, thus ensuring that only the quartz grains were analyzed, not the glue surrounding them. The analysis area was centered in the area where AFM measurement had been performed. Pass energy was set at 160 eV and step size at 0.5 eV. 2.6. Raman Spectroscopy. Spectra were acquired using a WITec (Ulm, Germany) Alpha300 RAS confocal Raman microscope. In this instrument, the excitation wavelength at 532 nm was provided by a frequency doubled Nd:YAG laser (29 mW power). The laser beam was collimated via an achromatic lens and then passed through a dichroic mirror. The beam was focused on the sample using a 100×/ 0.90 NA objective lens to a spot diameter of ∼360 nm. The sample was mounted on a scanning stage driven by a piezoelectric element. The reflected laser was blocked by a dichroic mirror and Rayleigh scattered light was blocked by an edge filter. The Raman scattered light was focused into a multimode optical fiber, which served as the entrance slit for the spectrometer. Detection was made by a back illuminated 1600 × 200 pixel EMCCD (electron multiplying, charge coupled device) camera operating at −60 °C. Imaging experiments were performed by raster scanning the laser beam over the sample and accumulating a full Raman spectrum at each pixel at a speed of 50 ms/pixel. The image size was in the range of 100 × 100 pixels over a square 40 μm on a side for the CoreClean sample and 100 × 100 pixels and 15 μm on a side for the CorePreserve
Figure 1. Force−distance curve measured with a functionalized AFM tip. The red curve marks the tip’s approach, and the blue curve is the withdrawal from the surface. The small offset (10−20 pN) between the approach and withdrawal curves results from viscous drag as the cantilever moves through the liquid. The tip is pushed toward the surface until the maximum force, 0.5 nN, is reached. The adhesion force is measured as the tip springs free of the surface during retraction. away from the surface (Figure 1). It moves toward the surface, comes into contact, and eventually stops its travel when the surface resists with a predetermined force, in our case, 0.5 nN. The tip is then retracted from the surface, and as it moves away, adhesion between tip and sample cause the cantilever to deflect. At some displacement, the tip snaps free. The force at that point is the adhesion force. Contact time between the tip and the surface depends on the spring constant of the cantilever that holds the tip. It is typically in the range of 10−30 ms. A number of other properties can also be extracted from the force curves, such as elastic and plastic deformation,21 but in this work, we focused on the adhesion maps acquired in saline solutions. The sample and tip were initially submerged in ∼3 mL of FW to mimic the initial reservoir conditions and the first force map was acquired. The FW solution was then replaced by either ASW or ASWlow. This was done by extracting ∼2 mL of the solution from the liquid cell and then injecting 2 mL of the ASW or ASW-low solution. Solution injection was usually repeated four times to bring the composition in the liquid cell close to the composition of the injected solution. We did not remove all of the solution during a single exchange because it is important to keep the sample covered to avoid evaporation and precipitation of salts and to keep the tip near the sample so we did not lose the imaging area. After exchanging the solution, a new force map was acquired. In this way, force maps could 4851
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Figure 2. Backscatter SEM images of quartz grains from (a-b) CoreClean and (c) CorePreserve.
Figure 3. AFM amplitude images from some representative quartz grains (a-b) CoreClean, (c) CorePreserve. (d-e) The line profiles show the step heights from the solvent cleaned samples (a-b). Their position and orientation are marked with red arrows in the AFM images. be generated at the same location on the same sample, with the same tip while changing the salinity conditions. Typically we measured at least five force maps in any sequence, such as FW − ASW − ASW-low − ASW − ASW-low or FW − ASW-low − ASW − ASW-low − ASW. In most cases, the results from the FW solutions were indistinguishable from the data from the ASW solutions so in this paper we only report the data from ASW and ASW-low.
grains. All three surfaces show evidence of recrystallization such as those shown in Figure 2b and c. Figure 3a and b show images of grains from CoreClean. The data have been rendered to show topography, as if illuminated from the left. The faint bright lines in Figure 3a and the bright lines going from the top left to the lower right corner in Figure 3b are atomic scale steps on the crystal surface. The corresponding line profiles are shown in Figure 3d and e. Step heights measured in the line profiles, 0.28 to 0.45 nm, are close to the unit cell spacing expected normal to the pyramidal {101̅1} face, namely 0.3341 nm and the prismatic {101̅0} face, 0.4252 Å.22 The step of 0.72 nm is about twice the unit cell spacing, i.e. a double step. A number of small features, in the range of tens to a few hundred nanometers, are nearly always observed on the sandstone particle surfaces such as we see in Figures 3a and b. These could be residual organic compounds from the oil,23 authigenic clay crystallites or weathered quartz or feldspar particles.11 Surfaces from the CorePreserve samples look different (Figure 3c). Steps are usually not visible, and the topography of the small features is damped, consistent with a surface covered with a continuous film that must be considerably higher than the atomic scale steps. The film is removed by solvent cleaning; XPS analysis before and after solvent treatment shows a significant decrease in adsorbed carbon compounds.
3. RESULTS AND DISCUSSION 3.1. Quartz Surface Topography and Chemical Composition. Figure 2 shows SEM images of some of the sand grains investigated in this study. Figure 2a is an example of a complete quartz grain from CoreClean. The grain is rounded, but at many sites the surface has recrystallized during diagenesis, producing many flat quartz crystal faces. Figure 2b and c are images of two of the surfaces investigated with AFM. Figure 2b is an image from the same grain shown in Figure 2a from CoreClean, while the image in Figure 2c is a grain from CorePreserve. The most noticeable difference between the two is the dark patches at grooves and rough areas on the samples from CorePreserve. An example is marked by the white arrow in Figure 2c. Energy dispersive X-ray spectroscopy (EDXS) showed that these patches were very rich in carbon. Figure 3 shows some representative atomic force microscopy (AFM) amplitude images from three of the investigated quartz 4852
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To characterize the composition of the upper ten nanometers of the sand grain surfaces, we used X-ray photoelectron spectroscopy (XPS) (Figure 4). We analyzed the samples after
Figure 4. XPS spectra from CoreClean (red) and CorePreserve (black). The inset shows a zoom in on the carbon peak region. There is significantly more carbon on the preserved sample than on the solvent cleaned sand grain. The photoelectrons that produce the silicon peak, which represents the underlying quartz, are attenuated by the adsorbed organic material. This picture is consistent for all samples examined.
the AFM studies to minimize modification of the surface by the X-ray beam. The AFM force mapping results are described later. The XPS spectra from CoreClean and CorePreserve show both silicon and carbon peaks. The silicon peaks are expected as the sandstone grains are formed of quartz, while the carbon peaks must be due to organic material on the surface of the samples. The carbon peak in CorePreserve is more intense than the silicon peaks indicating that most of the surface in this sample is covered with organic material. There is still a carbon peak present in CoreClean showing that even this sample has some organic material on its surface although the peak is less intense than those for silicon. The C peak is at a binding energy that represents −C−C and −C−H (285 eV) bonds so the material is composed mostly of hydrocarbons and possibly some carbohydrates (C−O is at 286 eV). There are no visible peaks with binding energy in the range of −COOH (289 eV) or CaCO3 (290 eV). This information agrees well with the AFM images (Figure 3), which show surfaces from CorePreserve to be covered with a layer of organic material. From the XPS spectra, we can estimate the average organic layer thickness to be several nanometers. We know that at least in some areas, the material is less than ∼10 nm, the probing depth of the technique, because the signal from silicon is not completely attenuated. Although vertical resolution with XPS is excellent, ∼10 nm, the spectra represent an average over an area ∼110 μm diameter. To get a better idea of the local distribution of the organic material, we used Raman spectroscopy imaging. 3.2. Raman Imaging. We chose samples for Raman imaging that had roughness similar to the samples imaged with AFM. Figure 5a and c show optical microscopy images of the sand grain surfaces from CoreClean and CorePreserve that were examined. The Raman signal for hydrocarbons was too weak to observe any organic material at ∼2.5 mW laser power, so we increased the power to 29 mW. This was enough to burn the organic material to amorphous carbon, which has a higher Raman activity. Amorphous carbon has a broad, intense, double
Figure 5. (a) Optical microscopy image and (b) Raman map of the carbon from a quartz grain from CoreClean. (c) Optical microscopy image and (d) Raman map from a quartz grain from CorePreserve. The color scale in (b) shows the relative intensities for the carbon maps where black is least. (e) Three example Raman spectra from the two samples. The spectrum with the least carbon from CoreClean is shown in red. That with the least carbon from CorePreserve is in blue. The spectrum in green is from a site with substantial carbon on CorePreserve. The spectra were normalized by the quartz peak at 465 cm−1.
peak centered at ∼1350 and 1600 cm−1. Quartz has its most intense peak at 465 cm−1. Quartz was present in all spectra. Figure 5e shows three example spectra from the two samples after burning the organic material to amorphous carbon. The spectrum from CoreClean, with the lowest amount of carbon, is shown in red and that from CorePreserve, with the least carbon, is shown in blue. The green spectrum shows an area from CorePreserve with a substantial amount of carbon. The probe depth of the laser is ∼1 μm so the signal coming from quartz underneath the carbon should be almost constant for the expected carbon thicknesses. The spectra were normalized so the quartz peak at 465 cm−1 had the same area for all. Figure 5b and d maps the intensity of the carbon over the two surfaces. White represents areas where there is no Raman data because of fluorescence or because the area is out of the focal plane of the objective or because there is no material. Black shows areas with very little or no carbon. The amount of carbon is significantly higher on CorePreserve; on average, carbon peaks 4853
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Figure 6. Height and adhesion force map series acquired under two cycles of salinity change from ASW to ASW-low on a 5 μm square area on a quartz grain taken from a CorePreserve sample. Each map is constructed from 50 × 50 force curves. In the top row (a-d) are the height maps, extracted from the force curves. In the bottom row (e-h) are the corresponding adhesion force maps. The change in adhesion that results from a change in solution can be seen by the difference in color and by the average adhesion determined over the entire area, which is given below the row of adhesion force maps.
Figure 7. A topography and adhesion force map series of a quartz grain from the CorePreserve sample during exposure to ASW and ASW-low solutions. (a) A height map with a flat area in the center left side of the map (dark area enclosed by red line) and topographically higher features (enclosed by green lines); (b-e) Adhesion force maps. Average adhesion for the topographically low (higher adhesion) and elevated (lower adhesion) areas are listed in the table.
are ∼13 times more intense. However, on both surfaces, the distribution is heterogeneous. 3.3. Adhesion Force between Tip and Core Plug Surfaces. A series of adhesion force maps from faces of a CorePreserve quartz grain are shown in Figure 6. The top row shows the height maps made from data extracted from 2,500 force curves taken during the solution sequence ASW − ASWlow − ASW − ASW-low. A comparison of the features on the height maps shows that we have successfully mapped approximately the same area of the surface each time the solution was replaced. The bottom row of images shows the corresponding adhesion force maps. The first map was acquired during exposure to ASW. On these maps, pale blue is very low adhesion. Dark blue is higher, then black, red, and pink is maximum, in this case, 0.3 nN. The average adhesion for the whole ASW map was 40 pN. The solution was then changed to
diluted ASW. Average adhesion dropped to 15 pN. A switch back to ASW increased average adhesion back to 42 pN and replacement by ASW-low produced a drop to 17 pN. Adhesion between the CH3 tip, which represents a tiny oil droplet, and the quartz grain, which represents the pore surface, is clearly higher when the pore fluid is ASW than when it is low salinity water. The effect is reversible and consistent through two cycles of salinity change. Figure 7 shows a series of force maps from another quartz grain from CorePreserve. The map of topography (Figure 7a) shows a much rougher surface than that imaged in Figure 6. We have delineated the higher adhesion area (red) and three low adhesion areas (green), both in the height maps and in the adhesion maps taken during the same ASW and ASW-low exchange sequence. At this location, there is a strong correlation between topography and adhesion. The flat area 4854
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in the center part of the height map coincides with high adhesion, and some very adhesive spots (red) are located on the flat area but near the topographically higher features. We interpret the flat area to be the quartz surface and the higher elevation, low adhesion features to be particles adsorbed on the quartz face. With AFM we cannot identify the chemical composition of these features. To compare adhesion in ASW and ASW-low, we determined the average adhesion for the two types of areas. They are listed in the table in Figure 7. For both the high and the low adhesion areas, the average adhesion decreases during exposure to ASW-low. Just as we observed in Figure 6, the change in adhesion is reversible and reproducible. There is some difference between the absolute adhesion measured in ASW in Figure 7b and d; however, for both maps, absolute adhesion is higher than for the ASW-low adhesion maps. 3.4. Adhesion Force Averages for CoreClean and CorePreserve Surfaces. The average adhesion for the force maps measured on the surfaces of quartz grains from CoreClean and CorePreserve samples are plotted in Figure 8a and b. The plot is semilog so a 10% adhesion change looks the same whether the absolute adhesion is 50 pN or 1000 pN. For CoreClean, we measured six force map series on three different quartz surfaces (C1, C2, and C3). There is a large spread in the adhesion measured on the three different surfaces and even for different sites on the same C1 sand grain. The
adhesion force values are lower in low salinity water and show an up/down pattern in three of the curves reflecting reversible adhesion changes with changing salinity. For the other three curves the response is more mixed. For CorePreserve, we measured 11 force map series. The adhesion force averages range widely, by a factor of nearly 30. Seven of the curves follow an up/down pattern, with lower adhesion in ASW-low than in ASW, while the last four show a more mixed response. The differences between the curves are likely caused by the heterogeneity of the surface, which was especially clear from the Raman spectroscopy images in Figure 5. The lack of reproducible up/down behavior in some curves, for example the two lower C1 curves in Figure 8a, could be caused by the following. Since the probed surfaces are inherently not clean, some of the organic material on the surface may stick to the tip and thereby may be removed from the surface. While we have not observed sudden large changes in the adhesion during the acquisition of the force maps smaller more subtle changes might lead to a lack of reproducibility. Examples of molecules sticking to the tip are shown in Figure 9. Of course there are
Figure 9. Examples of force curves where the tip does not separate cleanly from the surface. (a) The tip feels an adhesion force from the surface until its final release at ∼60 nm. The inset is an enlargement of the region where the tip is close to the surface. (b) Organic molecules are stretched further and further between the tip and surface until they are separated by ∼360 nm.
many other variables in the system like salt concentration, pH, and temperature. However, we do try to keep these fixed during the experiment. No error bars have been added to the averages since each data point in a force map represents a single measurement of the adhesion force in a specific spot. So the spread in adhesion values across a force map represent the heterogeneity of the area rather than uncertainty. The noise level in an individual force curve is typically around 10 pN, but since each average is calculated from 2500 curves this contributes a very small uncertainty in the average.
Figure 8. (a) Average adhesion force from maps of quartz surfaces from CoreClean. We examined between 1 and 3 sites on 3 different grains, C1, C2, and C3. (b) The same for the CorePreserve samples. A total of 11 force map series were acquired at 1 to 3 sites on 5 different grains. There is a large spread in the absolute adhesion, but there is a consistent drop in adhesion when high salinity solution is replaced by low salinity. 4855
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Figure 1 shows a force curve where the tip makes a clean release from the surface. However, not all force curves acquired on these samples look like this. Figure 9 shows two example force curves where release is not clean. A connection is maintained between the tip and sample for as much as several hundred nanometers until finally the tip snaps free. On the plot in Figure 9a, the tip is finally released ∼60 nm from the surface. We observed this type of force curve mainly on CorePreserve samples P1 and P2, which have relatively low average adhesion. As many as 30% of the curves from these surfaces resembled Figure 9a. In Figure 9b, final release takes place ∼360 nm from the surface. This type of force curve was observed on both CoreClean and CorePreserve samples but mainly on surfaces with high average adhesion, i.e. above ∼100 pN. About 10% of the force curves for CorePreserve resembled Figure 9b. Similar force curves have been reported in the literature, where lipid bilayers24 or single molecules25,26 have been investigated. Such curves are explained by organic molecules that are pulled between the surface and the tip. We observed several hundred such force curves in one force map. The tips are prepared with a monolayer of molecules that are only ∼1 nm long and bound strongly on the gold by the thiol group so it is unlikely that the tip molecules are responsible for, or contribute to, the organic material that is stretched. This material must come from the surface of some samples. Figure 10 shows the difference in adhesion on surfaces exposed to ASW and ASW-low, determined from the data of Figure 8. For CoreClean surfaces, two values are below zero, meaning that force increased in low salinity fluids; 15 values are positive, meaning that decreasing salinity decreases force; one value is zero, meaning there is no difference in adhesion, when salinity is changed. The average decrease in adhesion from ASW to ASW-low is 14 ± 10 pN, which is the average of the six plotted series of Figure 10a plus or minus the standard deviation divided by √n, where n is the number of values in the average. For CorePreserve surfaces, six values are negative and 27 are above zero. The average decrease in adhesion from ASW to ASW-low is 33 ± 9 pN. Especially for two sites on P5, there is a very large difference in adhesion as high salinity solution is replaced by low salinity (Figure 10b), but this is only partially reversible when salinity increases again. This is caused by a phenomenon we have observed several times, namely, that adhesion decreases gradually with the number of acquired force maps, no matter what the salinity. We can see this trend in Figure 8b as a slight overall downward slope from left to right. We are currently investigating the cause, but in this paper, to minimize the effect of this behavior, we always measure adhesion in each solution at least twice. For samples from CorePreserve we observe a clear decrease in adhesion with decrease in salinity (33 ± 9 pN), while for CoreClean we observe a small effect at best (14 ± 10 pN). Both XPS and Raman spectroscopy demonstrate that the difference in surface composition of the two samples is the amount of organic carbon so the more organic material there is at the mineral surface, the larger the change in adhesion between high and low salinity solutions. In Figure 11, we have plotted the ASW/ASW-low adhesion difference for each force map series versus average adhesion for that series. Data from experiments on CorePreserve samples are on average further to the right than data from CoreClean samples, i.e. we measured higher adhesion on samples from CorePreserve. This corresponds well with the XPS and Raman results. A clean quartz surface is hydrophilic, but adsorbed organic material
Figure 10. Average change in adhesion when salinity changes from ASW to ASW-low or from ASW-low to ASW. The values are all determined from the adhesion in ASW minus the adhesion in ASWlow. (a) Data from quartz surfaces from CoreClean. Two values are below zero, 15 are above, and one is zero. Average adhesion decrease from ASW to ASW-low is 14 ± 10 pN. (b) Data from quartz from CorePreserve. Six values are below zero and 27 are above. The average decrease in adhesion from ASW to ASW-low is 33 ± 9 pN.
Figure 11. Average adhesion force difference for each force map series shown in Figure 8.
increases its hydrophobic nature, thus leading to increased adhesion between the hydrophobic tip and the surface. The data are scattered, but this agrees well with the Raman results. The distribution of the organic material is very heterogeneous, and if adhesion correlates with organic material, it is logical that adhesion behavior would also be inhomogeneous. For average adhesion values below ∼100 pN, the adhesion difference (FASW − FASW‑low) is between roughly −10 and 40 pN. For larger adhesion differences, we see higher adhesion. So there is a 4856
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was −0.015 ± 0.012 C/m2, which is consistent with values measured for quartz at pH 5.5.31 Thus, for the most hydrophilic surfaces in the measurements reported here, a substantial part of the charge density probably comes from the quartz mineral surface. This could explain the measured adhesion difference of up to ∼30 pN for the most hydrophilic surfaces, i.e. the points in Figure 11 at the lowest average adhesion. To explain the higher absolute adhesion and the difference in adhesion as a result of salinity change, we propose that ionic functional groups in the adsorbed organic material contribute to differences in repulsion from the electric double layer at the sample surface. This is consistent with the model proposed by Kleber for the molecular structure of soil organic matter adsorbed at mineral surfaces.32 In this model, strong organomineral associations occur via ligand exchange (polar organic functional groups of amphiphilic molecules interact with singly coordinated mineral hydroxyls). However, entropic considerations dictate that exposed hydrophobic portions of amphiphilic molecules adsorbed directly to mineral surfaces are shielded from polar aqueous phases through association with hydrophobic moieties of other amphiphilic molecules. This process creates a pseudobilayer, a hydrophobic zone. The outer amphiphilic molecules of this layer then expose their charged groups to the solution, thereby allowing further organic material to be sorbed by such processes as cation bridging. If this process occurs in the adsorption of organic material to the mineral surfaces within oil reservoirs, then such processes as the low salinity effect could break the outer cation bridge leaving considerable organic material adhering to the mineral surface. To summarize, we have made observations which suggest that the organic material on the mineral surfaces in the experiments reported here influences (i) the affinity of the substrate for hydrophobic molecules, such as the alkanes attached to our tip, and (ii) the affinity change of the substrate for hydrophobic molecules when salinity is changed. This implies that for a core flood test to give results that are representative of the reservoir, it is important that the organic material adsorbed on the pore surfaces in the core plug are as close as possible to those present in the reservoir because the organic material that covers the pores plays an important role in controlling the properties within the core.
correlation between the local hydrophobicity of the quartz surfaces and local differences in the change in adhesion when ASW is replaced by ASW-low. An interesting question is what the properties of such an organic layer must be, to result in a salinity dependent adhesion. One possibility is that some of the molecules are ionic. According to DLVO theory, the forces between our neutral tip and a charged surface should change with changing salinity.24,27 At low ionic strength, the repulsion resulting from the electric double layer at the charged surface is stronger than at high ionic strength. Assuming that the other forces between the tip and surface (van der Waals, hydrophobic) do not change when salinity changes, this would result in the observed effect. The specifics of the interaction between the tip and the surface would likely depend on both the charge density and the overall hydrophobicity of the surface. If the hydrocarbon surface is completely hydrophobic, water would be excluded from the contact zone between the hydrophobic tip and the surface when the tip is pushed into contact (Figure 12a).
Figure 12. Schematic models of the fluid near the AFM tip in contact with the substrate for (a) a hydrophobic and (b) hydrophilic surface.
Several groups have claimed that a cavity can form around the contact area when two hydrophobic surfaces come into contact.28−30 For the water to be excluded, the contact zone would have to be neutral, because the organic molecules on the tip have a much smaller dielectric constant (ε ∼ 2) than the water (ε ∼ 78), that otherwise surrounds the surface, i.e. charge at the interface would be energetically costly compared to charge at the substrate-water interface. In the area just outside the contact zone, water remains so charges can still be present. These charges would contribute to the salinity dependent adhesion force because of changes in the electrical double layer. For more hydrophilic surfaces, a layer of water is expected to be present between the tip and the surface, even at close contact. This water layer would stabilize any charge at the surface (Figure 12b). In a previous set of experiments on CorePreserve reported elsewhere,31 we varied the ionic strength of the contacting solution from seawater salinity to about that of the diluted ASW in a succession of 13 steps. The salinity dependence of the adhesion was consistent with expansion of the double layer as the salinity decreased.31 The absolute adhesion was ∼60 pN for the highest salinities, with an adhesion difference between the ASW and ASW-low conditions of ∼30 pN. The data fitted a model where the closest tip−surface distance was ∼1 nm, i.e. a model such as that shown in Figure 12b. The charge density
4. CONCLUSIONS We have investigated core plug material from an oil reservoir using AFM, XPS, SEM, and Raman spectroscopy. We explored the behavior of two samples from the same core plug, CorePreserve, which had been stored in kerosene from the time it was drilled, and CoreClean, which had been initially stored in kerosene and then cleaned using methanol and toluene. Surfaces of single quartz grains that had been recrystallized during diagenesis to produce flat faces were chosen for investigation. The AFM analysis included force mapping in saline solutions using very soft cantilevers functionalized with a self-assembled monolayer of HS(CH2)10CH3, which produced a tip that served as a model for a tiny oil droplet. The topographic AFM images, XPS spectra, and Raman images revealed organic material on CorePreserve surfaces that was heterogeneous in thickness. Hydrocarbons were also observed on the surfaces of CoreClean, although to a much lesser extent. The average adhesion force across ASW and ASW-low measured on CorePreserve was 263 pN and on CoreClean was 114 pN. 4857
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(13) Pan, C.; Feng, J.; Tian, Y.; Yu, L.; Luo, X.; Sheng, G.; Fu, J. Org. Geochem. 2005, 36, 633−654. (14) Wu, L. M.; Zhou, C. H.; Keeling, J.; Tong, D. S.; Yu, W. H. Earth-Sci. Rev. 2012, 115, 373−386. (15) Anderson, W. G. J. Pet. Technol. 1986, 38, 1125−1144. (16) Alagic, E.; Spildo, K.; Skauge, A.; Solbakken, J. J. Pet. Sci. Eng. 2011, 78, 220−227. (17) Hutter, J. L.; Bechhoefer, J. Rev. Sci. Instrum. 1993, 64, 1868− 1873. (18) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1169. (19) Kester, D. R.; Duedall, I. W.; Connors, D. N.; Pytkowic, R. M. Limnol. Oceanogr. 1967, 12, 176−179. (20) Zakaznova-Herzog, V. P.; Nesbitt, H. W.; Bancroft, G. M.; Tse, J. S.; Gao, X.; Skinner, W. Phys. Rev. B 2005, 72, 205113. (21) Hassenkam, T.; Skovbjerg, L. L.; Stipp, S. L. S. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 6071−6076. (22) Schlegel, M. L.; Nagy, K. L.; Fenter, P.; Sturchio, N. C. Geochim. Cosmochim. Acta 2002, 66, 3037−3054. (23) Lebedeva, E. V.; Fogden, A. Colloids Surf., A 2011, 380, 280− 291. (24) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1− 152. (25) Hugel, T.; Seitz, M. Macromol. Rapid Commun. 2001, 22, 989− 1016. (26) Pirzer, T.; Geisler, M.; Scheibel, T.; Hugel, T. Phys. Biol. 2009, 6, 025004. (27) Israelachvili, J. N. Intermolecular and surface forces, 3rd ed.; Academic Press: 2011. (28) Yaminsky, V. V.; Yushchenko, V. S.; Amelina, E. A.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96, 301−306. (29) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96, 307−314. (30) Ishida, N.; Kusaka, Y.; Ushijima, H. Langmuir 2012, 28, 13952− 13959. (31) Hilner, E.; Andersson, M.; Matthiesen, J.; Hassenkam, T.; Stipp, S. S. L. Energy Fuels Submitted for publication. (32) Kleber, M.; Sollins, P.; Sutton, R. Biogeochemistry 2007, 85, 9− 24.
Thus, the larger amount of organic material correlated with a more hydrophobic surface. Adhesion force maps were acquired in both artificial seawater (ASW) and ASW diluted approximately 25 times (ASW-low). Less adhesion was observed when salinity was 1500 ppm than in solutions with the salinity of seawater (35,600 ppm). For CoreClean the average difference in adhesion between the low and high salinity solutions was 14 ± 10 pN and for CorePreserve, it was 33 ± 9 pN, i.e. we observed a larger difference in the adhesion between ASW and ASW-low on CorePreserve, where the surface concentration of adsorbed organic material was highest. We propose that ionic functional groups in the adsorbed organic material are partly responsible for the salinity dependent adhesion because of changes in repulsion resulting from the electric double layer at the charged surface. Because the tip was nonpolar, our results also show that the adhesion of nonpolar oil components to pore surfaces can be affected by a change in the water flood salinity. The influence of the adsorbed organic material on the measured low salinity effect underlines the importance of sufficient aging to obtain representative results from core plug tests.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to members of the BP Pushing Reservoir Limits Flagship team and the NanoGeoScience research group for discussion, particularly Henning Osholm Sørensen for commenting on the initial draft. We are grateful to Keld West for technical support, to Stephanie Houston in the BP team, for some of the sample selection, and to Tom Vosch for helpful discussions and the initial Raman spectroscopy measurements. This work is part of the Nano-Sand Project, funded by the BP Exploration Operating Company Limited, under the ExploRe Program.
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