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" Wettability Alteration Mechanisms in Carbonate Rocks: An Experimental and Molecular-Scale Computational Study "
Bai, Shixun
Piri, Mohammad
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Latin Dissertation
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| Language of Document
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English
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| Record Number
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1057070
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| Doc. No
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TL56187
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| Main Entry
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Bai, Shixun
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| Title & Author
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Wettability Alteration Mechanisms in Carbonate Rocks: An Experimental and Molecular-Scale Computational Study\ Bai, ShixunPiri, Mohammad
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| College
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University of Wyoming
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| Date
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2020
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| Degree
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Ph.D.
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2020
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| Note
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193 p.
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| Abstract
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Wettability reversal is one of the most important processes involved in chemical enhanced oil recovery (EOR) from oil-wet carbonate reservoirs. Among the most effective chemical additives used to reverse the carbonate wettability are surfactants and Smart Water (water with specifically tuned ionic composition). While several mechanisms have been proposed for the wettability reversal by the surfactants and Smart Water, detailed molecular scale understanding is still lacking. The objective of this work is to advance the mechanistic understanding of the wettability reversal in carbonates by combining experimental and theoretical approaches. First, the wettability of calcite and dolomite surfaces in the presence of different types of surfactants and brines of various compositions was studied experimentally, confirming the effectiveness of cationic surfactants over anionic ones, as well as the pivotal role of SO42− ions in the wettability reversal process by the Smart Water on both calcite and dolomite surfaces. The experimental results were subsequently rationalized using quantum mechanics (QM) calculations and molecular dynamics (MD) simulations with a series of calcite and dolomite surface models. The simulations suggest that the oil-wetness of carbonate mineral surfaces is solely due to the oil carboxylates (disregarding the heavy fractions such as asphaltenes, which are not considered in this work). Consequently, the wettability reversal by various chemical agents is directly tied to their ability to weaken the carboxylate affinity to carbonate surfaces. For calcite, electrostatic interaction between the oppositely charged cationic surfactants and carboxylates facilitates the detachment of the latter from the point defects on the calcite surface, resulting in the wettability reversal. A combined application of QM and MD methods revealed that the positive charges distributed on the hydrogen atoms of the -CH2 of a quaternary ammonium group have the highest impact on the surfactant performance. The simulated Smart Water effect was achieved with the step vacancy structure on the calcite surface, which revealed that the wettability reversal occurs by SO42− approaching the surface with the pre-adsorbed Ca2+ or Mg2+, enabling the detachment of the Ca2+-carboxylate. Similar surface features were used to model the dolomite surface, and the wettability reversal mechanisms of cationic surfactants and Smart Water were explored and were compared with those for calcite. The findings in this work provide molecular-scale insights into the wetting behavior of carbonate rocks, which can facilitate the design and optimizations of chemical agents and formulations to enhance the oil recovery from carbonate reservoirs.
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| Descriptor
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Engineering
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Petroleum engineering
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| Added Entry
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Piri, Mohammad
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University of Wyoming
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