C. Lira-Galeana

Molecular size of asphaltene fractions obtained from residuum hydrotreatment

Previously, fluorescence depolarization techniques (FD) have been shown to measure asphaltene molecular size, thereby establishing the substantial difference between asphaltenes derived from crude oil vs from coal. Here, FD is used to track the changes of the asphaltenes from a petroleum atmospheric resid feedstock that has been subjected to increasing thermal severity of catalytic hydrothermal cracking. Changes in asphaltene properties with increasing cracking are readily observed and understood. In addition, asphaltene molecular size is measured for various asphaltene solubility fractions in binary solvent mixtures of toluene with either n-heptane or acetone; a strong dependence is found of asphaltene properties on the particular solvent mixtures in accord with recent publications.

A microbial consortium isolated from a crude oil sample that uses asphaltenes as a carbon and energy source

A microbial consortium capable of mineralizing asphaltenes was obtained from the Maya crude oil. The enrichment system was built with a glass column reactor containing mineral medium supplied with asphaltenes as energy and carbon source. The consortium growth was evaluated in Casoy agar during 40 weeks. The steady-state phase of the enriched bacterial community was observed after 10 weeks when the culture reach 105 to 106 CFU ml−1. The isolates belong to bacterial genus reported for degradation of other hydrocarbons and they were identified as Corynebacterium sp., Bacillus sp., Brevibacillus sp. and Staphylococcus sp. The bacterial consortium growth was evaluated by a viable counts during 14 days exposed to different aeration, temperature, salinity, and pH conditions. The ability of the consortium to mineralize asphaltenes was evaluated using the method of ISO 9439 in glass column reactors of 20 × 3.2 cm during 13 days. Temperatures of 55°C and salinity of 1.8% were growth limiting. The respiration of the microbial consortium using asphaltenes as a sole carbon source (800 μmoles CO2 in 13 days) was significantly higher than those of the samples containing only the microbial consortium (200 μmoles CO2) or only asphaltenes (300 μmoles CO2). These results indicated the existence of asphaltenes-degradating microbes in the crude oil and confirmed that the consortium could mineralize asphaltenes in conditions of room temperature, salinity of 100 ppm, aeration of 1 l min−1 and pH of 7.4.

CHARACTERIZATION OF ASPHALTENES AND RESINS FROM PROBLEMATIC MEXICAN CRUDE OILS

Asphaltenes and resins have been separated from four mexican oils suffering from the deposition of asphaltenic material during recovery operations. A SARA separation of the oils was performed and the resins and asphaltenes further analyzed. Characterization methods employed were FTIR, elemental composition both of CHNSO, and trace metals and molecular weight determination using size exclusion chromatography. NMR techniques were applied to two asphaltene samples. The overall scope of the work was to get a better understanding of the nature of the asphaltene stability at a molecular level in these problematic oils. Separation of resin in two fractions indicates that there is no long alkyl chains in these as given by FTIR, which may be the cause of the lack of stability along with the large difference in bubble point and reservoir pressure. *On sabbatical leave from Dept. Chem. Eng., Technical University of Denmark, DK-2800 Lyngby, Denmark.

Modeling Wax Deposition in Pipelines

Abstract A multicomponent liquid-wax hydrodynamic model that incorporates phase equilibria and a full non-Newtonian behavior is proposed. In this model, molecular diffusion through the boundary layer induced by a temperature gradient between the liquid and the exterior pipe wall is assumed to be the dominant mechanism for deposition. Numerical solutions to the conservation equations for Newtonian and non-Newtonian flow regimes in a model pipe are presented, and results on calculated radial mass flux and wax deposition profiles as a function of time and position in a vertical pipeline are discussed in detail. The results are compared with predictions from a previous model developed by Svendsen (Svendsen, J. A. (1993). Mathematical modeling of wax deposition in oil pipeline systems. AIChE J. 39(8):1377–1388.) and with experimental flow data for a binary mixture reported by Cordoba and Schall (Cordoba A. J., Schall C. A. Application of a heat method to determine wax deposition in a hydrocarbon binary mixture. Fuel 2001 80:1285–1291). Good agreement in both cases is found.

Computation of compositional grading in hydrocarbon reservoirs. Application of continuous thermodynamics

Abstract In a reservoir fluid column, the composition of the reservoir fluid varies from one depth to another owing to gravity forces. This problem is known as compositional grading. In the case of the plus fractions, the gravity field promotes a change in their overall composition and also their average characterization parameters will differ from one depth to another. We present a continuous thermodynamic framework for compositional grading calculations in hydrocarbon reservoirs using an equation of state. The effect of the gravity field on the segregation characteristics of the (continuous) heavy fractions of the oil is established analytically using the method of moments. This allows the molecular weight distributions of the heavy fractions to be described as a function of depth for both the oil and gas regions of a reservoir fluid column. Such monitoring is important for the case of an extreme segregation of the heavy fractions. The validity of the proposed method is demonstrated for a reservoir-fluid column where measured data is available.

INTERACTION ENERGY IN MAYA-OIL ASPHALTENES: A MOLECULAR MECHANICS STUDY

The interaction energy in vacuum between large-scale (630 atom) Maya-oil asphaltenes and that for an asphaltene-resin dimer has been calculated using molecular mechanics for various relative orientations and distances, keeping a frozen-structure scheme. In the case of asphaltene-asphaltene interactions, it is found that the anti-aliphatic conformation (i.e., with the monomer alkyl branches far away from each other) with parallel well-defined aromatic regions constitutes the most stable structure for an asphaltene dimer. For the asphaltene-resin case, a set of stable configurations is found mainly associated with the exposed asphaltene aromatic regions. In this paper, a survey of the interaction energy for a particular asphaltene-resin configuration is reported and compared with the asphaltene-asphaltene case. The interaction energies as a function of relative distance are parameterized through an analytical expression distinguishing among Coulomb, dispersion, and London contributions. Finally, an effective-medium approach to estimate the effect of the embedding medium on the interaction energy is proposed through a dielectric scaling of the Coulomb and London contributions.

Comparisons Between Asphaltenes from the Dead and Live-Oil Samples of the Same Crude Oils

Abstract Asphaltenes precipitated from pressure-preserve bottomhole oil samples have been obtained for three oils at different pressures, using a bulk high-pressure filtration apparatus. The precipitates captured on the filter were recovered, the asphaltenes defined by the n-heptane insolubility were extracted and analyzed. These pressure-driven asphaltenes found on the filter were found to make up in the range between 50 and 100 ppm of the whole crude oil. Opening of the cell did not reveal asphaltenes retained due to wall adhesion. Size exclusion chromatography tests performed on both the live-oil-derived asphaltenes and the standard asphaltenes as precipitated by atmospheric titration on the same crude oil, revealed that the live-oil asphaltenes had apparent smaller hydrodynamic volume and narrower distributions than the standard asphaltenes for two oils. Further FTIR tests also showed large differences between standard asphaltenes and the asphaltenes obtained at high pressure filter. The latter appeared to contain more functional groups and be less saturated. Implication of these structural differences on precipitation modeling is discussed.

Application of the SPHCT model to the prediction of phase equilibria in CO2-hydrocarbon systems

Abstract Ponce-Ramirez, L., Lira-Galeana, C. and Tapia-Medina, C., 1991. Application of the SPHCT model to the prediction of phase equilibria in CO 2 -hydrocarbon systems. Fluid phase Equilibria , 70: 1-18. A simplified version of the Perturbed Hard-Chain Theory (SPHCT) of Kim et al. is applied to the computation of the vapor-liquid equilibrium of binary systems of the type CO 2 -hydrocarbon at different temperatures of interest to the oil industry. The molecular parameters required for 70 components of the four homologous series of hydrocarbons (paraffins, olefins, naphthenes and aromatics) and other fluids of industrial interest are reported. Very satisfactory results are obtained with this model.

Critical point calculations for oil reservoir fluid systems using the SPHCT equation of state

Abstract Garcia-Sanchez, F., Ruiz-Cortina, J.L., Lira-Galeana, C. and Ponce-Ramirez, L., 1992. Critical point calculations for oil reservoir fluid systems using the SPHCT equation of state. Fluid Phase Equilibria , 81: 39-84. The ability for predicting the critical points of reservoir fluids using the Simplified Perturbed Hard-Chain Theory (SPHCT) equation of state proposed by Kim et al. is analyzed. The computational procedure developed by Heidemann and Khalil was used for locating the critical points in any multicomponent mixture described by this equation of state. Experimental binary vapor-liquid equilibrium data of interest in the petroleum industry have been used to evaluate the interaction coefficients in this equation for 164 binary mixtures. The performance of the SPHCT EOS to predict critical points is demonstrated on four oil reservoir fluid systems containing up to forty-eight components. Convergence to all the critical points existing in these large systems was obtained in a few iterations without any difficulty.

ON THE MASS BALANCE OF ASPHALTENE PRECIPITATION

In the evaluation of experimental data as well as in calculation of phase equilibria the necessity of the application of mass balances is obvious. In the case of asphaltenes the colloidal nature of these compounds may highly affect the mass balance. In the present paper several experiments are performed in order to check the consistency of mass balances within asphaltene precipitation. Asphaltenes are precipitated in two step processes either by changing temperature or by changes in precipitant with increasing precipitation power. This has been performed for three different oils. The data indicates that in temperature experiments as well as in solvent series experiments the precipitation of heavy asphaltenes affects the following precipitation of lighter asphaltenes. In both cases the mass balance using standard separation techniques cannot be closed, as less material is precipitated in a two step process than in the direct process either at low temperature or by direct precipitation with one precipitant. The different fractions were subject to HPLC size exclusion chromtagraphy showing that the material remaining in solution in the stepwise process was of low molecular weight, and that the material in the second precipitation step was often of higher apparent molecular weight and had an increased overall absorbance coefficient.