Sandstone Surface Research Articles

Enhanced oil recovery using silica nanoparticles in the presence of salts for wettability alteration

This study focuses on the performance of nanosilica particles in the injected brine with different salt concentrations. For investigating the effect of SiO2-water-salt nanofluid in improving oil recovery, a set of experiment was conducted in both static and dynamic conditions. Interfacial Tension (IFT) measurement, contact angle measurements on glass surfaces, adhesion tests on sandstone grains and micromodel flooding tests were conducted. IFT results showed that although presence of nanoparticles can decrease the interfacial tension between injected brine and oil, but this change is not significant. The effect of nanofluid and salt concentration on the surface/fluid interaction was investigated by contact angle tests and new adhesion tests using grains of rocks. Both tests confirm that presence of nanofluid and salt can shift the wettability of rock surfaces towards more water wet condition. A glass micromodel was constructed for conducting flooding test to compare the capability of the nanofluid solution with/without salt, for improving oil recovery in dynamic conditions. The obtained results of all experiments demonstrated that silica nanoparticles are able to improve the wettability of both glass and sandstone surfaces towards more water-wet conditions. Wettability alteration by electrical double layer (EDL) and multicomponent ion exchange (MIE) are the dominant mechanisms in glass and sandstone surfaces during LSW injection and disjoining pressure is the major mechanism during nanofluid flooding. The presence of salts in the nanolfuids also showed to have a pronounced positive effect on the level of wettability alteration due to more adsorption of nanoparticles on the rock surface.

Investigation of sandstone erosion by continuous and pulsed water jets

The study compares the sandstone erosion manifestations (depth, width, and volume of the removed material) caused by continuous and pulsed water jets (WJs) with a frequency f = 20 kHz and 40 kHz. The WJ head follows an inclined trajectory at an angle of 45° to the horizontal surface. The starting point of the head was positioned 5 mm above the sandstone surface. The traverse speed of the head was varied from v = 0.25 mm/s to 1 mm/s. The structural changes and plastic deformation caused by the mechanical effect of the continuous and pulsed WJs were analysed using SEM and optical profilometer. The results showed that an optimal standoff distance was responsible for the maximum erosion effect under both continuous and pulsed regimes. The PWJ at f= 40 kHz showed better erosion performance in terms of depth (6.8 mm), width (6.83 mm) and volume removed (27.31 mm3) as compared to PWJ at f = 20 kHz and CWJ. The microstructural topography at varying standoff distance (z = 10 mm to 45 mm) showed erosion features such as micro-pits, cracks, perforated holes responsible for different erosion stages. On-line monitoring via means of acoustic emission was measured to evaluate the erosion phenomenon on the sandstone surface during the impact of the PWJ and CWJ. The correlation between the acoustic emission results and the erosion parameters showed the progression of the erosion process under different technological conditions.

Mechanism of Wettability Alteration of an Oil-Wet Sandstone Surface by a Novel Cationic Gemini Surfactant

A novel cationic gemini surfactant was prepared by chlorination and quaternization in this paper. The target product was successfully synthesized, which was confirmed by infrared (IR) spectrometer ...

Antiquity of the giant inselberg Burringurrah (Mount Augustus), Western Australia, inferred from oxygen isotope dating of kaolinitic weathering

Oxygen isotope-inferred ages as old as the Jurassic have been derived from weathered monzogranite underlying the folded Mount Augustus Sandstone, which forms the massive anticlinal Burringurrah (or Mount Augustus) inselberg, in the Gascoyne district of Western Australia. This large inselberg stands ~700 m above the surrounding duricrusted Neogene Gascoyne planation surface at ~400 m. The 1150 m thick Mount Augustus Sandstone (c. 1620 Ma), buried by some 10 km of rock of the Edmund and Collier Groups, underwent folding during the Edmundian Orogeny (1030–950 Ma). Abundant Permian glacial deposits throughout western and southern Australia suggest that the landscape was extensively impacted by a continental ice mass during the Gondwanan Permian glaciation, and which provides a maximum age for most recorded chemical weathering profiles within Australia.Nine samples of kaolinite were collected from a 30 m deep exposure of weathered monzogranite cropping out in the core of the anticline beneath the overlying Mount Augustus Sandstone within the topographic amphitheatre of ‘The Pound’. Four samples from the profile were analysed, revealing two ages of weathering. The higher samples immediately below the unconformably overlying boulder ferricrete (δ18OVSMOW values of +12.0 and +14.0‰) imply a Jurassic to early Cretaceous weathering age, while the lower samples (δ18OVSMOW values of +17.6 and +18.3‰) are indicative of a Neogene age. These results suggest downward ‘younging’ of the profile consistent with a top-down advancing weathering front that developed after uncapping of the anticlinal inselberg and exposure of the underlying monzogranite to surficial chemical weathering.A long-term rate of landscape denudation of ~11 m/Ma is estimated from established geological events, rock ages and thicknesses. Dykes and quartz veins suggest that Burringurrah was still deeply buried 500 Ma ago, with the denudation rate indicating exposure of the upper surface of the Mount Augustus Sandstone by ~100 Ma. The oxygen isotope data suggest that weathering of the monzogranite beneath the Mount Augustus Sandstone occurred during Jurassic to early Cretaceous times (~200 to 100 Ma), ages broadly coincident with those derived from the application of denudation rates. This suggests that Burringurrah initially developed as an inselberg prior to at least the past 100 Ma. The younger ages from lower parts of the profile suggest that weathering continued into the Neogene/Quaternary (23 Ma to present), during which time the surrounding, now dissected, planation surface was also weathered.

Condensate blockage alleviation around gas-condensate producing wells using wettability alteration

In the current survey, a novel fluorocarbon-based wettability modifier chemical is proposed to alter the wettability of sandstone rock surface from liquid wetting to preferentially gas wetting condition. Several experimental tests describing wettability condition of the rock surface including static contact angle measurements, spontaneous imbibition and dynamic core flooding using water and n-decane fluids were conducted on untreated and treated sandstone rock to investigate the effect of the proposed chemical on surface wetting behavior. Adsorption of fluorinated chemical on sandstone surfaces was characterized using FTIR and SEM. Elemental analysis of rock surface after treatment was determined by EDX analysis and EDX map. After chemical treatment of sandstone thin section, contact angles of water and n-decane in air-liquid-rock system were altered from 0° and 0° to 151° and 101°, respectively. Spontaneous imbibition of water and n-decane as imbibing liquid fluids into the core sample saturated with dry air at room temperature on untreated and treated core showed that the ultimate amounts of liquid imbibitions were decreased to factors of 0.03 and 0.16 PV, demonstrating wettability alteration from strongly condensate-and water-wet to preferentially gas-wet condition, respectively. Also, the results of core flooding experiments demonstrated the improvement of liquid phase mobility as a result of treatment with proposed chemical fluid by factors of 3.85 and 3.5 for water and n-decane, respectively. The outcome of this integrated study proposes that fluorochemical agents can be considered as a promising candidate for possible field applications to alleviate both condensate and water blockage in gas condensate reservoirs by wettability alteration technique.

A laboratory experiment on salt weathering by humidity change: salt damage induced by deliquescence and hydration

A laboratory experiment on salt weathering was carried out under changing humidity conditions. Three types of rock (tuff and dense and porous sandstone) with sodium chloride, sodium sulfate, or magnesium sulfate were subjected to humidity oscillations from 20%RH to 98%RH during 6 h periods at 20 °C. Prior to the weathering experiment, the rock specimens were oven-dried, and the water supply was restricted to only that from the air during the experiment. Salt on the rock specimens was deliquesced/hydrated at almost 100%RH during the high-humidity period and crystallized/dehydrated during the low-humidity period. Sodium chloride, which has high deliquescence ability, caused the most intensive weathering. Porous sandstone with sodium chloride was completely broken down after 100 cycles of humidity changes. The other two types of rock with sodium chloride showed flaking or swelling on their surfaces. For dense sandstone with sodium chloride, the Equotip rebound value decreased as the humidity cycles increased. Sodium chloride has a high susceptibility of deliquescence at humidities of more than 80%RH. Weathering by sodium chloride would occur extensively on the surfaces of stone heritages and geoheritages in humid coastal spray zones. Magnesium sulfate induced a weight loss in porous sandstone, flaking from the surface of tuff, and cracks on the surface of dense sandstone. Sodium sulfate had almost no effect in the humidity-change experiment, although salt efflorescence was obvious. The results for magnesium sulfate, salt deliquescence, hydration, and crystallization in the humidity-change experiment were easily repeated. Magnesium sulfate might have more influence than sodium sulfate when air humidity fluctuates in a short period.

Changing interfacial tension and wettability using new generation chemicals and nano metal particles at elevated temperatures and pressures: An analysis through a new experimental design for heavy-oil recovery applications

Capillary imbibition tests, contact angle and interfacial tension measurements were combined to understand the mechanism of alteration surface interplay (wettability and interfacial tension) amongst heavy crude oil, rock (sandstone and carbonate) surfaces, and seven different chemical agents at elevated temperatures and pressures. The results showed that the ionic liquid is thermally stable and has the ability to decrease contact angle in both limestone and sandstone samples indicating heavy-oil recovery increase potential during thermal methods. is also thermally stable and can reduce the interfacial tension at high temperature and high-pressure conditions but showed relatively insignificant effect on the recovery factor in the limestone due to limited wettability alteration capability at higher temperatures. nanoparticles did not increase thermal recovery in limestones but showed a good performance increase in sandstones. was observed to be another good candidate for thermal application in sandstones because of its capability to improve wettability.

Wettability Alteration of an Oil-Wet Sandstone Surface by Synergistic Adsorption/Desorption of Cationic/Nonionic Surfactant Mixtures

Various experimental methods including atomic force microscopy, scanning electron microscopy, zeta potential measurement, and contact angle measurement were used to analyze the mechanisms of wettab...

Assessment of natural radioactivity levels in the Ajali Formation, Enugu, South Eastern Nigeria

Activity concentrations of the radionuclides (238U, 232Th and 40K) were analysed using high resolution co-axial HPGe gamma ray spectrometer system to know the implication where groundwater is sourced from Enugu, South-East Nigeria. The activity concentration of 238U ranges from 37 ± 4 to 74 ± 6 Bq kg−1 with the highest value of 74 ± 6 Bq kg−1 noted in the thin coal sample. The 232Th activity level in the rock samples ranges from 58 ±5 to 85 ± 7 Bq kg−1 with the higher value of 89 ± 7 Bq kg−1 reported in the coal sample. For 40K, the activity concentration varies from 140 ± 19 to 293 ± 25 Bq kg−1 with the highest value of 293 ± 25 Bq kg−1 reported in coal samples whereas lowest value of 140 ± 19 Bq kg−1 was noted in Ajali sandstone in the study area (Enugu State). The higher activity levels of 238U, 232Th and 40K reported in coal samples may be due to the closer clusters of weathered surface of Ajali sandstone that overlays the thin coal layer with an escarpment being spread out, implying higher permeability towards deeper direction. All the values are within the recommended level when compared with United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000). Further research on groundwater activity concentration and rock geochemistry is required within the study area. This will allow for a comprehensive conclusion to be drawn on the level of exposures to the inhabitants relying on groundwater for consumption.

Macro- and micro-scale observations of a surface-functionalized nanocellulose based aqueous nanofluids in chemical enhanced oil recovery (C-EOR)

Nanocellulose was surface-functionalized for applications in chemical enhanced oil recovery (C-EOR) processes as a green alternative to synthetic polymers in order to reduce environmental impact. This paper focuses on the adsorption isotherms and oil displacement behaviors of an amphiphilic nanocellulose (S-NFC) based nanofluid in sandstone porous media. Particular attention was given to the oil displacement mechanisms of this nanofluid at pore level. The isotherms indicated that the equilibrium adsorption of S-NFC on a sandstone surface was approximately 5.08 mg/mm2 under our experimental conditions. The micrographs of Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) clearly showed the micro morphological change of the surface induced by S-NFC adsorption and aggregates, which, in turn, rendered oil-wet surfaces water-wet as confirmed previously. The injection of S-NFC nanofluid after waterflooding could further increase the oil recovery even for highly heterogeneous models. The oil displacement dynamics observed in a visual micromodel revealed that the nanofluid flooding improved the sweep efficiency and also reduced the residual oil saturation (Sor) after waterflooding process largely due to its bulk viscosity and surface-activation. Emulsification, dragging and wettability alteration were found to be three dominant pore level EOR mechanisms for this nanofluid. These results promise nanocellulose a candidate material for C-EOR applications, but more efforts are needed to further improve its dispersity in electrolytes.

Wettability of Hybrid Nanofluid-Treated Sandstone/Heavy Oil/Brine Systems: Implications for Enhanced Heavy Oil Recovery Potential

In this work, we investigated heavy oil/brine systems on oil-wet sandstone surfaces to quantify the performance of hybrid nanofluids (HNFs) for wettability alteration. In the first step, nanofluid stability analysis was conducted to screen effective single nanoparticles for formulating HNFs and ensure that the properties of the formulated HNFs did not change during the experiments. Then, the ability of HNFs to change the wettability of oil-wet sandstone surfaces to a water-wet state was systematically examined and compared with five types of single nanofluids by contact angle measurements. Then, the effects of HNF composition, hybrid nanoparticle concentration, salinity, and exposure time on the wettability change of sandstones were investigated. Finally, the mechanisms for the wettability shift by HNFs were proposed and verified by scanning electron microscopy visualizations. The results showed that the SiO2 + Al2O3, SiO2 + TiO2, and Al2O3 + TiO2 nanofluids could maintain their stability in the harsh res...

A novel nanofluid based on sulfonated graphene for enhanced oil recovery

In this study a novel nanofluid based on the sulfonated graphene (G-DS-Su) for enhanced oil recovery is developed. Nano porous graphene (NPG) synthesized by chemical vapor deposition method is sulfonated by a new route using 4-sulfobenzenediazonium salt and then chlorosulfonic acid. Presence of SO3H group in G-DS-Su structure is confirmed by the Fourier transform infrared spectroscopy (FTIR) result. This stable functionalized graphene dispersion is applied in core flood experiments. Using G-DS-Su at concentrations of 0.5 and 2 mg/mL results in 16 and 19% increase in the oil recovery. Prior experiments show that G-DS-Su tends to migrate to the oil/water interface and form emulsions. Moreover, the IFT decreases about 11% in presence of G-DS-Su but an ultralow IFT is not achieved. The contact angle measurements show that the wettability of sandstone surface is altered from the oil-wet to the intermediate condition in presence of G-DS-Su. The wettability alteration may be due to both forming a wedge film in the oil/nanofluid/solid three-phase contact region and the adsorption of G-DS-Su onto the sandstone due to the π-π and n-π interactions between the functional groups of sand and G-DS-Su. Hence, the wettability alteration plays a dominant role in the oil displacement mechanism.

Measurement and estimation of CO2–brine interfacial tension and rock wettability under CO2 sub- and super-critical conditions

Saline aquifer storage is considered to be a promising method of carbon dioxide (CO2) mitigation. The CO2–brine interfacial tension (IFT) and the caprock wettability under reservoir temperature and pressure conditions are essential for storage capacity estimation. In this study, the CO2–brine (NaCl + KCl) IFTs were obtained by using the pendant drop method under 298–373 K temperature, 3–15 MPa pressure, and 1.0–4.9 mol·kg−1 salinity. A detailed analysis of the relationship of IFT with temperature, pressure, and salinity was conducted. In addition, an empirical equation was developed to estimate the CO2–brine IFTs in a wide range of temperatures, pressures, and salt molality. The contact angles (CAs) of brine on quartz, Berea Sandstone, and limestone surfaces in the presence of supercritical, liquid, and gaseous CO2 were measured by using the sessile drop method, and the wettability alteration of the rock surfaces in the presence of supercritical CO2 was systematically investigated. According to the results, the CO2–brine IFTs increased with salinity and temperature and decreased with pressure until reaching a plateau. For a CO2–mixed brine system, a linear relationship between the IFT increase (Δγ) and molality was observed. The CAs of the different rock samples varied with temperature and pressure. However, all the three rock samples became less water-wet when the CO2 phase state changed from subcritical to supercritical.

Microbiological community of the Royal Palace in Angkor Thom and Beng Mealea of Cambodia by Illumina sequencing based on 16S rRNA gene

Angkor temples in Cambodia, an icon of Khmer civilization, display the ancient culture by bas-relief on the sandstone surface of different temples, which are being destroyed by physical, chemical and biological processes for more than a thousand years. To investigate the bio-erosion of temple sandstone at the Royal Palace of Angkor Thom and Beng Mealea in Cambodia, Next Generation Sequencing (NGS) Illumina sequencing technology based on 16S rRNA gene was performed on samples of biofilm and exfoliated sandstone materials to identify the microbial community composition. After quality filtering the raw data, 678,115 quality reads were obtained for bacterial 16S rRNA gene from a total of 13 samples with high Goods coverage and satisfactory rarefaction curves. Higher bacterial diversity was detected in exfoliated sandstone materials than the biofilms, but the lowest in the lower layers of the biofilm than the top layers. At the phylum level, 4 phyla, namely Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi, were the most common and dominant bacterial groups in these samples with each contributing to greater than 3.7% of the total abundance. Both Firmicutes and Gemmatimonadetes were the dominant phyla detected only in exfoliated materials, while Cyanobacteria, Chloroflexi, and unassigned bacteria were more abundant in the biofilms. Hierarchical cluster analysis at the genus level showed that the distribution of bacterial community composition between exfoliated materials and biofilms was significantly different. The microbiota of Beng Mealea and the Royal Palace was different, especially for the biofilm samples. The correlation of environmental factors and bacterial community structure suggested that the nitrification process was more active at Ben Mealea, which might contribute to biodeterioration. This analysis of microbiota in these biofilms and sandstone exfoliation materials provides further information on the responsible microorganisms involved in geobiochemical processes at Angkor monuments and preservation strategies under tropical climate conditions.

A comprehensive study of asphaltene fractionation based on adsorption onto calcite, dolomite and sandstone

In this study, based on a new procedure, asphaltene fractions of Sarvak oil samples were determined based on their adhesion to calcite, dolomite, and sandstone surfaces and their properties were evaluated. In irreversibly adsorbed asphaltenes, the highest concentrations of elements were 4.23 wt.% nitrogen (N) on the calcite surface, 7.12 wt.% sulfur (S) on dolomite surface, and 6.18 wt.% S and oxygen (O) on the sandstone surface. The dispersion of asphaltene fraction constituents on the surface, is approximately the same across the calcite sample, but in the dolomite sample, the highest adsorption of S correlates to the presence of magnesium (Mg). However, in sandstone samples, the highest S adsorption is achieved by potassium (K). The most dispersion of O occurs at the surface of the sandstone samples; according to matching through EDS mapping, due to the presence of silicon (Si), O has considerable dispersion. Asphaltene fractions also make lighter carbon compounds. It is important to note that in all samples, C17 is the highest peak seen in irreversibly adsorbed asphaltenes. In addition, the zeta potential is negatively affected by increasing the adsorption properties of asphaltene fractions; this depends on the concentration of asphaltene elements adhering to the surface. The O–H polar peak in the irreversibly absorbed asphaltene/sandstone sample is higher due to the presence of O, compared to the other samples. In all three mineral samples, bulk asphaltene and adsorbed fractions have roughly the same values for alkyl groups. The concentration of carbonyl, carboxylic acid, or derivative groups in adsorbed fractions in calcite and sandstone samples show a higher intensity.

Microscopic Failure Mechanism Analysis of Sandstone Under Triaxial Compression

Mineral composition and microstructure changes under loading greatly influence the macroscopic mechanical strength of rocks. In this study, the chemical and mineral composition of sandstone from a coal mine in China were analyzed with energy dispersive spectrometry and X-ray diffraction. Triaxial compression tests of three sandstone samples with different compositions were carried out by using a servo-controlled rock mechanics testing system to obtain macroscopic failure characteristics and complete stress–strain curves. Fracture morphologies of the sandstone samples were observed by scanning electron microscopy (SEM), and the fracture origins, defects, grain size, pore structure and distribution, microfracture characteristics and grain boundaries were determined. Finally, the fracture mechanism of the rocks was analyzed, and the microstructure and fracture characteristics of three samples were obtained. The results showed that the stress–strain curve of samples under triaxial compression can be divided into four stages: microfractures compressed-closed, elastic deformation, plastic-failure and post-failure. Under triaxial compression, shear failure occurred in the samples, and the slip occurred along the plane of maximum shear stress. The SEM results showed grooves and steps on the fracture surface, where the debris was piled up in angular and granular shapes. Higher quartz and feldspar content is positively correlated with sample strength. There were some microcracks in the fracture surface of the clastic sandstone with higher hydrophilic clay minerals, which indicates the fracture was easily extended during the shear process. Thus, tensile fractures are more likely to be generated along the bedding plane, resulting in the low strength of clastic sandstone.

Erosional surfaces in the Upper Cretaceous Castlegate Sandstone (Utah, USA): Sequence boundaries or autogenic scour from backwater hydrodynamics?

Erosional surfaces in the Upper Cretaceous Castlegate Sandstone (Utah, USA): Sequence boundaries or autogenic scour from backwater hydrodynamics?

Dynamic tensile properties of sandstone subjected to wetting and drying cycles

Rocks are commonly used as building stone and construction materials in many engineering applications. They usually undergo cyclic wetting and drying processes due to the periodical variations in moist conditions. In this paper, to understand the deterioration on the tensile strength of sandstone induced by wetting and drying cycles, dynamic Brazilian disc tests were conducted on sandstone specimens after every 10 cycles (for a total of 50 cycles) under a wide range of loading rates using a modified split Hopkinson pressure bar (SHPB) technique. Test results revealed that at the same loading rate, the dynamic tensile strength of specimens decreased with the number of wetting and drying cycles. Microscopic morphological structure on the sandstone surface were also obtained by scanning electron microscope (SEM), the characteristics and fractal analysis indicated that cyclic wetting-drying treatments would result in accumulative crack damage in rock, which was considered as the primary reason for the reduction in strength. Moreover, a decay model considering the effects of loading rate and cyclic wetting and drying deterioration was established to predict the dynamic tensile strength of sandstone after wetting and drying cycles. The forecast data via the decay model matched the trends of the experimental results well, meaning that the decay model was valid and applicable for the strength prediction.

Combustion Characteristics of Tight Sandstone

As a promising unconventional energy source, tight sandstone gas has attracted increasing attention. Because of the extremely low porosity/permeability of tight sandstone, large scale exploration and development are still challenging. Recently, high-temperature combustion or pyrolysis has been employed to increase sandstone porosity/permeability. In this study, the sandstone samples obtained from Xinjiang Province in China were heated for 30 min in air atmosphere at temperatures of 350, 500, 700, 800, and 900 °C respectively. The combustion characteristics and property changes of sandstone were investigated. In the beginning, thermal decomposition process of sandstone and gas emission were tested using thermogravimetry combined with Fourier transform infrared spectroscopy. Then, the mineral composition and pore structure of different combusted sandstones were tested and analyzed. Moreover, the effects of combustion temperature on particle size of sandstone were also investigated. Finally, scanning electron microscopy (SEM) technology was performed to study the surface appearance change of sandstone. According to the experiment results, organic matters started releasing at 350–500 °C, and most of the minerals such as carbonate decomposed at 600–870 °C. During the combustion process, carbon dioxide and water vapor were the main product gases. With the increase of combustion temperature, the mean pore diameter increased from 8.0 to 22.6 nm, while the particles size remained almost constant. In addition to the increase of pore size, it can be found from SEM photos that the compact surface of sandstone became smooth and some new pores and small cracks appeared on the surface of sandstone after high-temperature combustion, especially at 900 °C. As a result, high-temperature combustion is one of the feasible methods to improve the porosity/permeability of sandstone.

Comparison of Linear and Branched Molecular Structures of Two Fluorocarbon Organosilane Surfactants for the Alteration of Sandstone Wettability

The aim of this work is to build a molecular dynamics model to represent the experimental evaluation of the wettability alteration of sandstone surfaces due to the action of partially fluorocarbonated organosilane surfactants with linear (perfluorodecyl) and branched (fluorocarbamate) chains using the contact angles of water and n-decane droplets as the evaluation parameters. The thickness obtained for a monolayer coating with each surfactant was ∼13 A, in excellent agreement with the experimental measurements of the thickness of a monolayer of fluorocarbonated surfactants in similar systems. The contact angles obtained with molecular dynamics simulations deviate, in most cases, by less than 3° with respect to experimental measurements. The phenomenological model reveals that for the coating with a monolayer, the fluorocarbonated chains of perfluorodecyl were mostly oriented away from the sandstone surface, while the branches of fluorocarbamate are distributed between being adsorbed on the surface and ori...