Wettability Change Related to Adsorption of Organic Acids on Calcite: Experimental and Ab Initio Computational Studies
- Christelle Legens (Inst. Français du Petrole) | Herve Toulhoat (Inst. Français du Petrole) | Louis Cuiec (Inst. Français du Petrole) | Frederic Villieras (Ecole Natl. Superieure de Geologie) | Thierry Palermo (Inst. Français du Petrole)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 328 - 333
- 1999. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.3.3 Aspaltenes, 5.8.7 Carbonate Reservoir, 4.3.4 Scale, 4.1.5 Processing Equipment, 4.3.1 Hydrates, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.4.10 Microbial Methods, 4.2 Pipelines, Flowlines and Risers, 5.1 Reservoir Characterisation, 5.3.4 Integration of geomechanics in models
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We have investigated the interactions between organic acids and a calcite powder by adsorption from an organic phase (benzoic acid and lauric acid in toluene) and from an aqueous phase (benzoic acid and lauric sodium salt).
In addition to the experimental study, ab initio quantum chemistry calculations were performed for cluster models, simulating the interactions of molecules with calcite surface sites.
At the toluene/calcite interface, acids chemisorb on surface calcium ions, replacing surface hydroxyls resulting from the previous surface hydration by water. This surface saponification phenomenon was predicted by ab initio calculations and confirmed by spectroscopic techniques. The adsorbed acids conferred a hydrophobic character to calcite, with the higher contact angles in water for the longest alkyl chains in the organic acid.
At the water/calcite interface, organic acids do not displace water.
Our results support the possible existence of mixed wettability patterns, related to spatial distribution of pore sizes, in the specific case of organic acids with calcite.
The wettability of reservoirs is an important parameter when evaluating oil recovery processes.1 Most of the carbonate reservoirs are considered oil-wet or mixed-wet, even if it is generally assumed that a pure carbonate rock is originally strongly water-wet. This wettability change is assigned to the adsorption of some crude oil compounds, mainly with carboxylic charged ends, on the mineral surface. Two mechanisms can be proposed depending on water film stability: (i) diffusion of acid molecules through the water film and (ii) direct adsorption from the bulk organic phase.
In order to evaluate these two possibilities, the adsorption of benzoic acid and lauric acid on calcite has been investigated, using ab initio quantum chemistry calculations and adsorption at solid/liquid interface from an organic phase (toluene) and from an aqueous phase.
The techniques used are thermogravimetry, infrared diffuse reflection, and thermal analysis linked to a mass spectrometer. Wettability changes induced by adsorption were evaluated by contact angle measurements.
Materials and Methods
The carbonate used in this study is a synthetic, pure, and well-crystallized calcite powder, provided by Rho?ne-Poulenc Chemicals (Calofort U.). Its specific surface area, determined by nitrogen adsorption, according to Villiéras' method,2 is 23.8 m2/g. The powder was kept in an oven at 105°C. The average density of surface calcium ions was evaluated to 28.1 Å2, according to Donnay-Harker morphology.3,4
The adsorbates, benzoic and lauric acids, were provided by Aldrich, with purities higher than 99.5%. Lauric sodium salt was synthesized from lauric acid and caustic soda.5
Adsorption experiments were carried out in two qualities of toluene and in pure water:
- toluene Rectapur (Prolabo) which contains 211 ppm of water at 99+% purity,
- toluene HPLC (Aldrich) which contains only 60 ppm of water, at 99.8% purity.
Ab initio calculations: a first-principles quantum chemistry method was used to compute adsorption enthalpies or binding energies: the density functional theory, as implemented in DMOL V.300.3 The isolated molecular models, or bound complex geometries were first optimized at the local density approximation level (VWN), and then the gradient corrections (Becke 88+Lin-Yang-Parr) were applied to energies. These calculations required up to several weeks on a Silicon Graphics Power Indigo 2 R8K workstation.
Calcite surface charge was measured with a Sephy zetaphoremeter II, where particles displacement speed is determined by analysis and comparison of successive images taken during application of a given voltage between two electrodes. This electrophoretic mobility is directly related to the zeta potential, via the Smoluchowski equation. Measurements were performed from 1 g L?1 calcite suspension using pure water or brines with different salinities: 0.01 M NaCl, 0.01 M NaCl 2, and 0.1 M NaCl. Suspensions were stirred by magnetic agitation during one night and pH was modified by addition of 0.01 M NaOH or HCl solutions.
Adsorption isotherms were carried out in a batch with 2 g of calcite in 100 cm 3 for toluene and 0.5 g of calcite in 200 cm3 for water. Equilibration time was 7 days for both solvents. Solid/liquid separation was then carried out using 0.45 ?m Millipore filters. Concentration of adsorbates in water was determined using UV spectroscopy (Spectrophotometer UV 5260 from Beckman) for benzoic acid and total carbon measurement for lauric acid. Adsorbed amounts onto the mineral surface were also measured by thermogravimetry which allowed the tracking of weight loss due to the thermal desorption of adsorbed molecules.6 The sample was put in a crucible and heated at 10°C/min, under helium, up to 900°C, using a Setaram TAG24 thermobalance.
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to determine the state of adsorbed organic acids on the mineral surface,7 using an IFS55 Brucker spectrometer.
The influence of adsorption on wettability was evaluated by contact angle (?) measurements on compacted powder. 1-cm-diam pellets were made with 100 mg of treated powder under pressure (200 kPa cm?2). The pure water drop profile was recorded in purified dodecane and the contact angle was calculated using a G40 KRUSS goniometer.
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