Adsorption of Carboxylic Acids on Reservoir Minerals From Organic and Aqueous Phase
- Lene Madsen (Technical U. of Denmark) | Lind Ida (Technical U. of Denmark)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 1998
- Document Type
- Journal Paper
- 47 - 51
- 1998. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.4.10 Microbial Methods, 5.8.7 Carbonate Reservoir, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics
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Adsorption of organic acids onto North Sea chalk and pure minerals from a hydrocarbon phase and an aqueous phase show that the maximum adsorption is larger for calcite than for silicate surfaces in the hydrocarbon phase. The opposite is observed, however, in the aqueous phase. This suggests that the available silicate surfaces and oil/water ratio may play a role in the wettability of chalk.
The wettability of a hydrocarbon reservoir depends on the specific interaction in the oil/rock/brine system, and it is generally accepted that the wettability of a reservoir can be changed by adsorption of polar compounds.1-3
The hydrocarbon-bearing Cretaceous and Palaeogene chalk reservoirs in the North Sea are predominantly composed of calcite with minor amounts of quartz and clay minerals.4 Thus, the wettability of the hydrocarbon reservoirs depends on how and to what extent the polar compounds are adsorbed onto the surfaces of calcite, quartz, and clay. Based on the work of Thomas et al.,5 we expect organic acids to play an important role in changing the wettability of chalk reservoirs, but there is only limited knowledge of the influence of organic acids on the three-phase system oil/brine/rock.6 Analysis of formation waters from various hydrocarbon reservoirs in the North Sea for short-chain fatty acid anions shows acetate to be the most abundant, with concentrations up to 0.7 g/L.7 In accordance with these results, the total content of acetate, propionate, and butyrate in produced water from North Sea oil reservoirs has been measured to about 1 g/L, corresponding to [75% of the total carbon content.8 Analysis for fatty acids ranging from hexanoic acid (C6) to stearic acid (C18) in North Sea crude oil from the Tyra field shows the presence of all these fatty acids with acids in the range C6 to C13 as the most abundant with concentration up to 0.03 g/L for C12.9
To study the influence of organic acids on reservoir wettability, the three phase system oil/brine/chalk is simplified into the two-phase systems: hydrocarbon/water, water/mineral, and hydrocarbon/mineral. The interaction between oil and water is well understood in terms of partition coefficients. Partition coefficients of various organic compounds in different oil/water systems at room temperature have been reviewed10 and show that fatty acids are transferred increasingly into a nonpolar alkane phase with increasing chain length. Previous work5,11-19 indicates that the properties of the systems water/mineral and hydrocarbon/mineral are dependent on the mineralogy of the actual reservoir rock.
In the present work, we study the adsorption properties of North Sea chalk from the water zone of the gas/oil producing Tyra field and Danish aquifer chalk by carboxylic acid adsorption in the systems water/mineral and hydrocarbon/mineral. To assess the role of the rock-forming minerals, we also studied the adsorption properties of pure minerals: calcite, quartz, and kaolinite. Because of its nonswelling properties, we chose kaolinite to represent the content of clay minerals in the natural chalk.
Most experiments were carried out with benzoic acid because, in contrast to aliphatic acids, it is soluble in both an aqueous and an organic phase. Short-chain fatty acids have only a limited solubility in the organic phase and were not included in this study, whereas longer chain fatty acids have only a limited solubility in the aqueous phase. Thus, we chose benzoic acid to represent crude oil components with similar polar group but with an aliphatic hydrocarbon chain. We performed simple experiments with C6 and C18 to observe whether any difference exists between the adsorption of the aromatic and aliphatic acid that contain the same number of carbon atoms and to cover the wide range of fatty acids found in crude oils.
We crushed the natural chalks by hand and gently treated these and the mineral powders by washing with deionized water, freeze drying, and storing in a desiccator over P2O5. The washed powders were characterized in terms of specific surface area (BET), total organic carbon (TOC), and phase content (Table 1).
We determined the specific surface area (BET) of the powders by standard N2 adsorption/desorption. The specific surface area of the calcium carbonate and quartz powders were determined after degassing at 200°C in a helium atmosphere; for the kaolinite, the degassing temperature was increased to 250°C. In all cases, the degassing time was 2 hours. These conditions gave the highest and most reproducible specific surface area data.
We measured the specific surface areas before and after the ultrasonic treatment of the organic suspensions. During ultrasonic treatment, aggregates can disintegrate such that the specific surface area increases. For the two natural chalks and the quartz powder, we used the BET measured before ultrasonic treatment because we did not observed any differences in BET (Table 1). For synthetic calcite and kaolinite, specific surface area increases by the ultrasonic treatment, and these higher values should be used. We carried out the experimental work with the synthetic calcite on ˜100 g subsamples extracted from a 25 kg batch of powder. The difference in BET for calcite (Table 1) reflects the large variation in the specific surface area through the original 25 kg batch.
We performed the adsorption experiments as batch experiments at 32°C where solutions with a known concentration of test molecules were mixed with a known amount of powder. The amount of adsorbed test molecules was expressed as the difference between the initial and final concentration of the test molecule relative to the specific surface area of the powder.
The washed and dried powder (0.5 g or 1 g) was weighed into 15 mL screw-capped centrifuge tubes and dried at 200°C or 250°C (for kaolinite) in a flow of N2 and cooled to room temperature in a desiccator over P2O5. In the hydrocarbon phase, we added the desired dry solution (10 mL) of carboxylic acid (10-2-10-4 M) in cyclohexane or hexane to these tubes. We treated the resulting samples ultrasonically for 45 minutes to ensure contact between the grains and the organic solvent, and the samples were subsequently rotated at 40 rounds per minute (rpm) for 20 hours in a thermostated water bath (t=32±0.3°C ). Increasing the equilibrium time did not alter the adsorption. Finally the suspension was centrifuged (15 minutes, 4,000 rpm) and the supernatant (˜8 mL) removed for recentrifugation (10 minutes, 4,000 rpm). The final concentration of carboxylic acid in the supernatant was determined by UV-spectroscopy and corrected for background absorbance.
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