Studies On Structural Stabilities Of Petroleum Dispersion System And On Recipe Of Combined Flood

It is an increasingly important subject for petroleum and chemistry researchers to explore some general and essential aspects of the complexed petroleum system from molecular level and to present theoretical basis for the development of new techniques of oil recovery. It is well-known that crude oil can be considered to be a special colloidal dispersion system of alsphaltenes and resins, which constitute the discrete and polar components, dispersed in a continuous phase made of non-polar compounds, paraffins, naphthenes and aromatics. Asphaltene acts as the core of an asphaltene micelle. Resins act as peptizing agents keeping asphaltenes in suspension and maintaining the stability of crude oil. The stability related asphaltene precipitation, interfacial interaction, and thermal cracking runs through the whole process of petroleum industry and its study may take directive effects on the theory and practice of petroleum science and engineering. In the present work, some advanced theoretical methods and instrumental techniques are used to study on the structural stabilities particularly related to asphaltenes and on the optimization of recipe of combined flooding. The main contents and results are as follows.Firstly, the composition and structure of petroleum asphaltenes and resins are investigated. ¹H and ¹³C NMR spectra of seven kinds of petroleum asphaltenes separated from different crude oils with n-hexane as solvent are determined with a Bruker Avance DMX500 NMR spectrometer. A series of average chemical structural parameters are obtained on the basis of the ¹H and ¹³C NMR information. By combining the experimental relative molecular weight and the elemental analysis results, the average molecular formula of the asphaltene and resin unit sheets are calculated and the hypothetical model molecules are evaluated. It can be found that the asphaltene or resin units are mainly composed of the polycyclic aromatics linked with naphthalene rings and side aliphatic chains. There obviously exist heteroatoms, O, N and S, in the asphaltene or resin units. The units can be associated each other with the association number from 3 to 6 for asphaltenes and that of about 2 for resin. These results provided an efficient structural description for the study of petroleum asphaltenes and resins at the molecular level. It is also tested that NMR is a good method to study the average structure of a complexed system such as asphaltenes.Secondly, the thermal stabilities of asphaltenes and resins are carried out.. Thermal pyrolysis kinetics of the asphaltenes and resins has been measured usingthermogravimetric analysis (TGA) with three or four different heating rates. It is clearly observed that there are different temperature ranges of weight losses between asphaltenes and resins. The pyrolysis process of the asphaltenes mainly occurs at a narrow temperature range with only single weight loss peak and that of the resins, however, occurs at a wide temperature range with two-peak weight loss feature. The pyrolysis stages of the asphaltenes and resins can be reasonablely explained by the model molecule assumptions from NMR measurements. Three different kinetic methods are used to quantitatively treat the TGA data. Two methods, overall first-order reaction method and two-stage first-order reaction method, are both not very well to interpret the characteristics of pyrolysis processes. A new distributed activation energy model (DAEM) is introduced to correlate the experimental TGA data. The kinetic parameters, active energy (E) and preexponential factor (ko), of thermal pyrolysis for asphaltenes or resins at different conversion levels are calculated. It follows that many parallel reactions with different rate parameters occur simultaneously during the thermal pyrolysis of asphaltenes or resins. The distribution functions, fiJE) and ko(E), are then estimated by DAEM satisfactorily. The compensation effect is clearly observed from a straight line of the plot of ln((Ao) versus E, which is discussed from the transition state theory combining with the hypothetical model molecules.Thirdly, the stability of asphaltenes and resins in solutions is discussed. Evidence of the aggregation behavior of asphaltene-toluene and resin-toluene solutions is investigated by UV-vis and fluorescence spectroscopy. The stability and aggregation tendency are discussed from various experimental results such as absorbance, absorptivity, florescence peak position, fluorescence peak shift, and total florescence intensity. Similar absorption spectra and fluorescence spectra for the asphaltenes and resins in toluene are observed due to the similar type of chromospheres and fluorophores. The experiments allow concluding that asphaltenes or resins in toluene solutions start to aggregates at considerably low dilution. The aggregation appears at higher concentration values as lOOmg/L for Daqing asphaltenes and at at lower concentration values as 40mg/L for Daqing resins. Therefore, the effects from the aggregation of asphaltenes or resins in solutions on the stability of crude oil usually should be considered.In chapter 5, the interfacial stability of the alkaline-surfactant-polymer (ASP) system is studied. With the crude oil from No. 2 Oil Production Company, Daqing Oil Field Co. Ltd as the object, the interfacial tensions between oil and water for manyalkaline-surfactant-polymer systems with different compositions are determined. The effects of the addition of polar compounds on the values of the interfacial tension are investigated. It follows that the ultra low interfacial tension can be obtained only when a surfactant is selected with appropriate molecular chain length and that obvious influences of polar compounds on the interfacial tension can be observed. Combining the principles of phase equilibrium and chemical equilibrium with the theory of surface thermodynamics, a simple model is abstracted from the complexed crude oil-alkali liquor-surfactant system and then a quantitative expression is proposed. From the model, the effects of the concentrations of petroleum acid, alkaline, and surfactant on the production of interfacial active acidic anion and the form of ultra low interfacial tension are explained. The surface thermodynamic model can correlate the interfacial tension values with results in reasonable agreement with the observed data. It can effectively express, to a certain extent, the tendency of interfacial tensionchange for the ASP combined flooding systems.In chapter 6, the prescription of ASP combined flooding are designed and optimized. With Nan'erxi polymer flooding of industrial scale area in Daqing oilfield as the object, several main factors, including interfacial tension between oil and water, stability of the combined system, adsorption of chemicals, recovery of physical simulation flooding experiment, etc. are evaluated to screen out the suitable chemicals, the prescription of combined system and the separate injection projects for polymer and alkaline-surfactant system. Both the selected alkaline-surfactant (AS) system and the ASP system can achieve allowable interfacial tension in a wide alkaline and surfactant concentration range and have good interfacial tension stability for a long time. Physical simulation flooding experiments indicates that the method of separate injection of polymer and alkaline-surfactant system has higher recovery enhance rate than only polymer flooding. The method of separate injection can enhance oil recovery about 5 percent above the usual polymer flooding. This new method needs no more construction than the usual ASP flooding and can make use of sewage well at the same time. Therefore, a considerable economy benefit can be obtained.