Long-term effectiveness of rock wettability alteration for water removal during gas production from tight reservoir depends on the surfactant adsorption on the pore surface of a reservoir. This paper selected typical cationic fluorosurfactant FW-134 as an example and took advantage of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM) to investigate its adsorption stability on the rock mineral surface under the oscillation condition at high temperature for a long time. The experimental results indicate that the F element content on the sample surface increases obviously, the surface structure of fluorine-carbonization also undergoes a significant change, and the fluorine surfactant exhibits a good interfacial modification and wettability alteration ability due to its adsorption on the pore surface transforming the chemical structure of the original surface. The adsorption increases indistinctly with the concentration of over 0.05% due to a single layer adsorption structure and is mainly electrostatic adsorption because the chemical bonding between the fluorosurfactant and the rock mineral surface, the hydrogen bonding, is weak and inconspicuous.
Tight gas reservoirs, one of main development objectives in the future dozens of years, are characterized by a high water phase trapping damage potential owing to the characteristic of water-wet and high capillary pressure. The water phase trapping damage can induce a fluid sensitivity damage, an aggravate stress sensitivity damage, and other damage [
Surfactant interface modification is increasingly attracting more attention of reservoir engineers because it can reduce interface intension and alter wettability of rocks. Compared with ordinary hydrocarbon surfactant, fluorosurfactant possesses the outstanding characteristic of a high surface activity, high thermal stability, and high chemical stability [
In this study, we select a kind of fluorine surfactant FW-134 which is able to successfully change water-wet tight sandstone into gas-wetness and improve the water flow rate by modifying the interface [
Only a small part of clay minerals and carbonate cementation play an important role in surfactant adsorption, and their main components are layered silicate minerals, containing oxygen, silicon, aluminum, calcium, sodium, and potassium. Mica is easily split into pieces to obtain a smooth atomic hydrophilic surface [
Experimental procedures are as follows. Wash with distilled water to remove impurities from the mica surface and put them into a constant temperature oven to dry to a constant weight at 60°C. Crush the mica into powder, sieve the powder with 100~200 meshes, and dry it to a constant weight at 60°C. Make up 0.05% and 0.10% FW-134 solution with distilled water and FW-134 according to a 1 : 50 ratio of solid to liquid into the conical flask, respectively [ Shake up the mixtures, cover tightly the conical flasks with a plug, and immerse the flasks into the water bath oscillator to oscillate at 90°C for 40 hours. The purpose of oscillation is to simulate the reservoir detention water flow scouring to the pore surface during gas production. In order to ensure that the FW-134 reaches saturation adsorption on solid surfaces, we test the adsorptive capacity Γ changing with time on the mica surface. We can see from the results (Figure Take out the conical flasks and leave them to rest for 30 minutes. Filter the precipitation separation of solid particles and soak up the residual solution on the solid particle surface with a filter paper. Dry solid particles to a constant weight at 60°C, number them sample A (distilled water), sample B (0.05% FW-134), and sample C (0.10% FW-134) according to the concentration of FW-134 from low to high, and seal them for WQF-520 Fourier infrared (FTIR) spectrometer and Thermo Scientific Escalab 250 X-ray photoelectron spectroscopy (XPS) tests. Select fresh natural dissociated mica (1 cm × 1 cm, thickness < 2 mm) and immerse them into 0.1% FW-134 solution according to a 1 : 50 ratio of solid to liquid at 90°C constant temperature water bath. Take out the mica plate after the oscillation for 40 hours, dry the surface with nitrogen flow to obtain the mica surface after the adsorption of FW-134, and fix the sample on a sample table of MMAFMLN 1728EX atomic force microscope (AFM) to test surface morphology. Nitrogen flow drying is to simulate gas flow scouring to the pore surface in the reservoir during gas production. Measure the surface contact angle.
FW-134 adsorption capability on mica versus time.
Sample A, sample B, and sample C were analyzed by FTIR. These samples were obtained from mica powder immersed into distilled water, 0.05% and 0.10% FW-134 solution under 90°C for 40 hours’ oscillation, as mentioned above. The FTIR results can be seen in Figure
The peak at 3,625 cm−1 is the O-H stretching vibration absorption peak of mica Al-OH and Mg-OH, 3,442 cm−1 the O-H stretching vibration absorption peak of mica surface adsorbing water, and 630~1,640 cm−1 the O-H bending vibration absorption peak; a high intensity peak at 1,024 cm−1 is the Si-O absorption peak of stretching vibration, the absorption peak at 950~600 cm−1 is O-H bending vibration, and a vibration zone at 800~500 cm−1 belongs to Si-O-Al spectral absorption peak. Using a comparative analysis of the FTIR diagram, methyl CH3 and methylene CH2 stretching vibration absorption peaks appeared obvious at 3,000~2,800 cm−1 in sample B and sample C, and the absorption peak of stretching vibration of fluoroalkyl groups CF3 and CF2 also appeared at 1,350~1,120 cm−1. These indicate that FW-134 produces a significant adsorption on mica surfaces under the conditions of high temperature and long-term oscillations, but there is basically no obvious change of displacement, strength, or shape of the absorption peak from the whole FTIR graph characteristics before and after adsorption, suggesting that the chemical bond between FW-134 and the mica surface is weak and the adsorption is mainly physical adsorption. The peaks 1800~1200 cm−1 in sample A and sample B are the same, which hardly appear in sample C. This shows that this peak is not related to the surfactant adsorption and will not affect the experiment results.
A mica surface has a strong capacity of bonding hydroxyl groups, polar atoms like F, N, and O are easy to form a hydrogen bond with the surface hydroxyl groups, and mica surfaces in aqueous solution are negatively charged, so FW-134, a kind of cationic fluorinated surfactant, is easy to be adsorbed on mica surfaces due to the electrostatic force, dispersion force, hydrogen bond, and other physical and chemical effects. However, it can be seen in Figure
FT-IR spectra of samples adsorbing FW-134.
We take an advantage of XPS to test the energy spectrum of sample A, sample B, and sample C, and the results are seen in Figure
XPS spectra of sample systems adsorbing FW-134.
The content and the binding energy of each element are calculated according to the peak area (Tables
XPS element content data of sample systems adsorbing FW-134.
Content (%) | Al2p | Si2p | C1s | K2p | O1s | F1s | N1s | S2p |
---|---|---|---|---|---|---|---|---|
Sample A | 13.245 | 17.383 | 11.470 | 4.950 | 51.768 | 0.598 | 0.577 | 0.502 |
Sample B | 10.269 | 13.458 | 7.588 | 4.689 | 43.194 | 17.545 | 2.092 | 1.166 |
Sample C | 9.3410 | 11.949 | 10.377 | 4.327 | 37.953 | 22.037 | 2.616 | 1.400 |
XPS
Binding energy (eV) | Al2p | Si2p | C1s | K2p | O1s | F1s | N1s | S2p |
---|---|---|---|---|---|---|---|---|
Sample A | 74.26 | 102.42 | 284.76 | 293.1 | 531.58 | 685.07 | 401.92 | 169.16 |
Sample B | 74.62 | 102.80 | 286.55 | 292.97 | 531.66 | 689.19 | 402.77 | 169.23 |
Sample C | 74.50 | 102.65 | 286.31 | 292.67 | 531.69 | 689.06 | 402.79 | 169.02 |
Sample A, with no FW-134 adsorption on the mica, only contains trace F, N, and S element, while the F, N, and S element contents in sample B and sample C with the adsorption of FW-134 increase significantly, and the content of the F element increases from 0.598% to 17.545% and 22.037%, the content of N element from 0.577% to 2.092% and 2.616%, and the S content from 0.502% to 1.166% and 1.400% (Table
The element binding energy (B.E.) in these samples can be seen in Table
XPS C1s spectra of sample systems adsorbing FW-134 and contrast diagram.
The F1s peaks of all samples are a single peak (Figure
The F1s peak positions of samples B and C are different from those of sample A, and the F1s peak binding energy of sample B and sample C is basically the same (Figure
The Gauss fitting spectra of the C1s XPS peaks of samples are shown in Figure
XPS C1s spectra of samples adsorbing FW-134 and contrast diagram.
The analysis results of the peak binding energy and peak area are shown in Table
XPS results of samples adsorbing FW-134 calculated by Gauss method.
Sample | Type of structure | Binding energy (eV) | Peak area (%) |
---|---|---|---|
A | C-H C-C | 285.0 | 88.80 |
C-O | 286.5 | 5.70 | |
C=O | 288.8 | 5.50 | |
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B | C-H C-C | 285.0 | 14.22 |
C-N C-C | 285.7 | 46.07 | |
C-F | 286.8 | 39.81 | |
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C | C-H C-C | 285.0 | 18.37 |
C-N C-C | 285.7 | 38.90 | |
C-F | 286.8 | 42.73 |
The water adsorption on modified surface of minerals depends on the surface structure [
AFM plane height map and the three-dimensional topography (before cleaning).
AFM plane height map and the three-dimensional topography (after cleaning).
As intuitively seen in Figure
Under the conditions of water oscillations, FW-134 on the mica surface is mainly physical adsorption, generally including the electrostatic and dispersion force adsorption. Because of characteristics of F atoms in fluorine surfactant molecules (the most electronegative and the minimum atomic polarizability) and the carbon chain of “shielding,” the C-F bond is not easy to be polarized, and dispersion forces of fluorine surfactant molecules are small. FW-134 adsorption on the mica surface force is electrostatic adsorption.
With an increase of concentration, FW-134 reaches a saturated adsorption state on the surface of mica. Because of “hydrophobic effect” [
FW-134 adsorption schematic diagram on mica surface.
Owing to an outstanding high surface activity of fluorine surfactant, its 0.05%~0.1% solution concentrations can decrease the surface tension of the aqueous solution to below 20 mN/m. Low surface energy of fluorine or a silicon treatment agent is widely used in the surface modification to enhance the surface hydrophobicity. Genzer and Efimenko [
By electrostatic adsorption on the mica surface, a cationic hydrophilic ionic bond of FW-134 molecules is inward and its fluorocarbon hydrophobic chain is outward. The maximum C-F peak and the highest percentage of the area (Figure
Fluorine surfactant adsorption on mica surfaces under the condition of oscillation at 90°C causes a hydrophilic surface to become hydrophobic structure with the strong ability of modifying interface and reversing wettability.
Fluorine surfactant adsorption on the mica surface is mainly physical adsorption and has a single layer structure when it reaches the saturated adsorption, which may be the reason that the period of interface modification validity is short.
0.05%~0.1% FW-134 can decrease the surface tension of the aqueous solution to below 20 mN/m and show high quality of wettability alteration effect.
There is not any conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge financial support from The National Basic Research Program (973) of China (no. 2010CB226705), China Scholarship Council fund, and The National Municipal Science and Technology Project (2011ZX05018).