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Adsorbed organic material and its control on wettability Jesper Matthiesen, Tue Hassenkam, N. Bovet, K. N. Dalby, and Susan L. S. Stipp Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00627 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 5, 2016
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Adsorbed organic material and its control on wettability J. Matthiesen, T. Hassenkam, N. Bovet, K.N. Dalby and S.L.S. Stipp Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen 2100, Denmark
Abstract: Laboratory core flood and field scale tests have demonstrated that 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 surfaces, as a result of the decrease in divalent cation concentration. We have previously demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material and this affects wettability and response to changed salinity. Here we quantified the response of sandstone core plug material in its preserved state (i.e. after storage in kerosene) and after the same core plug material was treated with ethanol and ozone to remove adsorbed organic compounds. We used the chemical force microscopy (CFM) mode of atomic force microscopy (AFM) to directly measure the adhesion force for two types of molecules on pore surfaces of individual sand grains that were plucked from an oil reservoir core plug. We functionalized AFM
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tips with alkane or carboxylate, so they resembled tiny oil droplets and measured adhesion to the sand grain surfaces in artificial seawater (ASW; 35,600 ppm) and in ASW diluted to ∼1,500 ppm (ASW-low). Both before and after the ethanol/ozone treatment, and for both the alkane and the carboxylate functionalized tips, the adhesion was lower in ASW diluted to ∼1,500 ppm than in ASW. For both alkane and carboxylate, the difference in adhesion between ASW and ASW-low was higher before the ethanol/ozone treatment. We attribute this change in response to the salinity dependent force caused mainly by the electric double layer (EDL) at the sand grain surfaces. We interpret the higher adhesion difference, before a very thorough, ethanol/ozone treatment, to be a result of the loss of the organic material that was originally adsorbed on these surfaces, which adds to the charge density and thereby to the salinity dependent EDL force. Investigating the same area on the same pore surface, before and after removal of the organic material, demonstrates without doubt, that it is organic material that causes the low salinity response, not the underlying mineral surface.
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1. Introduction When oil is produced from a reservoir, the first 5 to 10% comes from pressure reduction. To produce more, oil reservoirs are often flooded with water from a nearby source, such as 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 of > 200,000 ppm total dissolved solids (TDS). However, even after water flooding, significant quantities of oil remain. By decreasing the salinity of the water used for flooding ( 18 MΩ·cm (MilliQ). The composition of the solutions used for the experiments can be found in Table 1. The pH of the water in the sandstone oil reservoir is generally ∼5.5 so to mimic reservoir conditions we kept our solutions at pH 5.5. This was easy because it is the pH of NaCl
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solutions in equilibrium with the CO2 in air. Therefore, we omitted NaHCO3 from the artificial seawater (ASW) recipe. We also omitted SO42- because it is often removed before water injection 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. Table 1: Solution composition.
ppm Salt
CaCl2 MgCl2 KCl NaCl Total (ppm)
Artificial seawater (ASW)
1,332 5,428 917 27,951 35,628
(mmol/kg)
Diluted artificial seawater (ASW-low)
52 208 40 1,176 1,476
ASW
12.00 57.01 12.30 478.3
ASW-low
0.47 2.18 0.54 20.13
2.6 AFM imaging and chemical force mapping. We used an MFP-3D AFM from Asylum Research, Santa Barbara, USA. AFM images were acquired in tapping mode in air with an amplitude set point at ∼100 nm. The chemical force maps on core plug sand grains were made of 50 × 50 data points, recorded over an area of 5 × 5 µm2. The acquisition time for one map was ∼15 min. Chemical force maps on the model substrates were acquired on a 2 × 2 µm2 area in a 15 × 15 or 20 × 20 pixel grid. 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 away from the surface. It moves toward the surface, comes into contact and eventually stops when the surface resists with a predetermined force, in our case, 0.5 nN. The tip is then retracted
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from the surface and as it moves away, adhesion between tip and sample causes 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. For our cantilevers, it is in the range 10-30 ms. Many other properties can be extracted from the force curves, such as elasticity, plasticity and work of adhesion.19,20 In this paper, we focus on the adhesion force. The chemical force maps were acquired in solution. In the fluid cell, the sample and tip were initially covered by ∼3 mL of either ASW or ASW-low and the first chemical force map was acquired. The initial solution was then replaced by the alternate salinity solution. This was done by extracting ∼2 mL from the liquid cell and then injecting ∼2 mL of the new solution. This was repeated typically four times to dilute the original solution with the new one so the final composition was very close to the composition of the injected solution. At all times, the sample remained covered by the remaining liquid in the cell to keep the sample from drying and to avoid the tip loosing contact with the imaging location. After exchanging the solution, a new chemical force map was acquired, the solution changed again etc. In this way, maps could be generated at precisely the same site on the same sample, with the same tip in a series of solutions. At least four chemical force maps were acquired on each area in sequence: ASW – ASW-low – ASW – ASW-low or ASW-low – ASW – ASW-low – ASW. After a chemical force map series was acquired, a new area was chosen without removing the tip and sample from the liquid and a new chemical force map series was acquired. Typically, two chemical force map series were acquired on each sand grain.
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Figure 1. Force-distance curve measured with a functionalized AFM tip. The red curve marks the approach and the blue curve shows the tip withdrawal from the surface. 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. The small offset (10-20 pN) between the approach and withdraw curves is an artefact that results from viscous drag as the cantilever moves through the liquid. 2.7 Scanning electron microscopy (SEM). Images were acquired using an FEI Quanta 3D, field emission gun (FEG) SEM using a back-scattered electron detector. To avoid surface charging, we ran the instrument in low vacuum mode (atmosphere of 60 Pa H2O vapor). An Oxford X-Max detector was used for energy dispersive X-ray (EDXS) analysis. 2.8 X-ray photoelectron spectroscopy (XPS). This technique is optimized for analyzing the chemical composition of 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. All pumps on the system are oil free to avoid adventitious carbon deposition. A monochromatic AlKα X-ray source (energy = 1486.6 eV,
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power = 150 W) was used to generate 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 of 103.5 eV, consistent with literature values. 2.9 Theory for adhesion difference between solutions of high (ASW) and low (ASW-low) salinity. There are attractive components to the adhesion force between the AFM tip and the mineral surface that come from van der Waals forces and hydrophobic interactions. These forces depend on the local properties of the surface but do not depend on salinity.21 If the surface is charged, an electric double layer (EDL) forms in the solution at its interface with the pore surface. The force from the EDL felt by an approaching surface, such as our tip, depends on the ionic strength of the surrounding liquid. From the measured differences in adhesion between ASW and ASW-low, we can estimate the surface charge before and after sample treatment. We used an expression presented by Butt22 to derive the force exerted on an AFM tip by the EDL, assuming that the charge density does not change as the tip approaches the surface: =
+ ∙ ⁄ + 2 ∙ ⁄ ,
(1)
where R represents the tip radius, ε0, the dielectric permittivity of vacuum, ε, the relative permittivity of the medium (ε ≈ 80 for salt water), D, the tip/sample distance, σT and σS, the surface charge densities of the tip and substrate and λD, the Debye length, expressed as: = ∑
" " ! ! #!
.
(2)
In this case k is Boltzmann’s constant, T, temperature, e, the elemental charge, $%& , the number density in the bulk (m-3) of the ion, i, and z represents the valence of ion, i. Our -CH3 terminated
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tip carries no surface charge, i.e. σT = 0, so the EDL force between the tip and sample simplifies to: = +
⁄ .
(3)
Assuming the substrate surface charge density is the same in ASW and ASW-low, the adhesion difference for the -CH3 tip between ASW and ASW-low is: ∆ = ()* + ,-. =
/ /
0 123 ⁄456 + 78 ⁄9:;
