We report extensive conformational searches of the gas-phase neutral nicotine, nornicotine, and their protonated analogs and the pathways and barriers for the interconversion between their various isomers that are based on ab initio second-order Møller-Plesset perturbation (MP2) electronic structure calculations. Initial searches were performed with the 6-31G(d,p), and the energetics of the most important structures were further refined from geometry optimizations with the larger aug-cc-pVTZ basis set. On the basis of the calculated free energies at T = 298 K for the gas-phase molecules, neutral nicotine has two dominant trans conformers, whereas neutral nornicotine is a mixture of several conformers. For nicotine, the protonation on both the pyridine and the pyrrolidine sites is energetically competitive, whereas nornicotine prefers protonation on the pyridine nitrogen. The protonated form of nicotine is mainly a mixture of two pyridine-protonated trans conformers and two pyrrolidine-protonated trans conformers, whereas the protonated form of nornicotine is a mixture of four pyridine-protonated trans conformers. Nornicotine is conformationally more flexible than nicotine; however, it is less protonated at the biologically important pyrrolidine nitrogen site. The lowest energy isomers for each case were found to interconvert via low (<6 kcal/mol) rotational barriers around the pyridine-pyrrolidine bond. These barriers are much lower than previous estimates based on lower levels of theory obtained without relaxation of the structure along the path. Nicotine was found to bind more strongly to tryptophan (Trp) than nornicotine, a finding that is consistent with nicotine's enhanced affinity in the nicotinic acetylcholide receptor.

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Calcium Carbonate (CaCO3) is one of the most common scales in oil field operations. Various fluids (brine, oil and gas) can mix in the reservoir and/or wellbores under drastically varying sets of thermodynamic, kinetic and hydrodynamic conditions that will affect the carbonate scale forming tendencies. Many of these critical conditions are ignored in today's oil field operations as far as calcium carbonate scale is concerned. Presently, the mechanisms of CaCO3 scale formation, as conceived by the industry, considers only the liquid brine phases. The flashing of gases from both the oil and the brine phases is critical for the CaCO3 scale formation. Pertinent aspects of this flash process are generally ignored. In addition, the complete mechanism of the CaCO3 must also consider the partitioning of gases (particularly CO2) between the liquid oil and brine phases during the entire gas flash process. This means, the formation of CaCO3 cannot be determined by considering only the brine phase as attempted by the industry. Instead, the entire three-phase PVT behavior (oil/brine/gas) and associated CO2 partitioning must be considered to determine the CaCO3 scale formation in an oil field. The existing and published models on the CaCO3 formation consider only the basic thermodynamics of the brine phase and totally ignore the critical effects of the oil phase behavior on this scale formation. This automatically means that the oil industry can not adopt any of the existing scale models without serious modifications to account for the unique effects of the oil phase. The main variables dictating the location and amount of calcium carbonate scale deposition in an oil field are as follows:Pressures and temperatures at any location within the entire system.The brine and oil compositions prior, during and after the reservoir fluids have been exposed to temperature and/or pressure changes.The bubble point and pertinent flash behavior of the three-phase oil/brine/gas system as a function of pressure and temperature.The distribution of CO2 between oil and brine phases and the drastic variations of this CO2 partitioning prior and during any production operation.The constant variation of the water(brine)/oil ratio (" WOR"), the gas/oil ratio (" GOR") and the gas/water(brine) ratio (" GWR") during any production operation. These reservoir and production variables at various locations in the production system may change constantly as a function of location and time during any type of production operation within a given field. These critical variables are not considered in their entirety in the existing and most frequently used CaCO3 scale prediction models. In the present paper, the problems of calcium carbonate scale formation in oil field operations (primary, secondary, and tertiary production modes) are critically discussed. The various effects of the gas distribution (especially CO2) between oil and brine phases under reservoir and various production conditions are delineated as far as CaCO3 scale is concerned. Some algorithms are given. Finally, the paper discusses a methodology which can be used to predict CaCO3 under all field conditions as a function of the water composition, pressure, temperature, "WOR", "GOR", "GWR", total CO2 in the system and CO2 partitioning between the various liquid phases. The methods and problems of obtaining all the required input information in order to apply this or similar models for site-specific oil field operations will be discussed in detail. PVT and CO2 partitioning data obtained through some recent work with three-phase fluid systems from California oil fields will be used to illustrate the complex CaCO3 scale behavior in an oil field. P. 307^

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Calcium carbonate scale, the most common oil field scale, normally forms from three-phase fluid systems at various downhole locations. The basic scale formation mechanism of CaC03 in oil field operations was explained in a recent publication [1]. However, only the very basic model considerations and only a few field application details were given in this recent publication related to CaCO3 scale modeling. Most practical field applications of the CaCO3 scale modeling efforts were ignored. In this present paper, we elaborate on the same mechanisms of CaCO3 scale formation in a typical oil field emphasizing some of the more practical aspects of this scale modeling for field operators. None of the theoretical but only some very practical aspects of this scale formation are stressed in this present paper. The outcome of some scale model runs is used to show some practical field applications of any serious model efforts. An attempt is made to show some field problems related to this common type of oil field scale and how a proper modeling can overcome some of these problems. A series of different produced fluids are used in fifteen (15) examples (representing 15 different wells) to illustrate the complexity and intricacy of various CaCo3 scale formations in a hypothetical oil field. This paper is a logical follow-up of a recently issued SPE paper Ill and is mainly written for a practicing petroleum engineer who is concerned mainly with practical field operations. Therefore, all of the pertinent thermodynamics and other basic physical and chemical correlations are kept at a bare-bone minimum. Instead, we are stressing the more practical aspects of fighting CaCO3 scale in a "typical" oil field based on some model considerations and some actual calculations. Any theoretical and any serious computational considerations are deemphasized. INTRODUCTION: A SHORT REVIEW OF THE CaCO3 SCALE FORMATION CaC03 scale will be generated from the water or brine phase during oil field operations. However, this CaCO3 scale formation normally takes place in the presence of other fluids such as oil and gas. Obviously, most CaC03 scale formations in oil fields will occur in the presence of three-phase fluid systems the CaCO3 scale formations originate from a water phase in the presence of an oil and a gas phase. In two recent SPE publications, we stressed the often ignored fact that even though CaCO3 scale forms from the aqueous phase, both the gas and oil phases have rather pronounced effects on the actual CaC03 scale formation. Ignoring these effects of the gas and oil phases on the CaC03 scale formation as commonly done in the oil industry will not allow the field operator to properly design and execute effective counter measures aimed at a removal and prevention of this CaCo3 scale. The most recent publications on this topic (for a more complete list of references see [l] and [2]) outline the rather complex and often intricate behavior of the water phase and its compositional dependency upon the coexisting oil and gas phases.