Jill S. Buckley

Asphaltene precipitation and solvent properties of crude oils

ABSTRACT Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The i...

Some mechanisms of crude oil/brine/solid interactions

Mechanisms by which crude oil components may adsorb on high energy mineral surfaces include polar, acid/base, and ion-binding interactions. Surface precipitation of asphaltic material can further alter surface wetting. Design of controlled experiments to investigate the contributions of different mechanisms is hindered by the lack of simple, well-characterized analogs for crude oils that show even qualitatively similar interfacial properties. As an alternative, we observe the wettability altering tendencies of a variety of crude oils with the aim of relating the fluid/solid interactions to crude oil composition. The mixtures of compounds in crude oils, variability of samples, and changes that can occur during storage, all add to the uncertainties of mechanistic studies with crude oils. Nevertheless, consistent trends can be identified.

Effective wettability of minerals exposed to crude oil

This review focuses on the macroscopic phenomena that can be used to assess effective wetting, especially the use of contact angles to quantify wetting conditions in the presence of brine and crude oil or on surfaces that have previously been exposed to brine and crude oil. Reservoir wettability has long been a puzzle. Wetting is almost certainly changed during all but the most careful core recovery processes and there is no guarantee that it can be preserved or recreated in the lab. Thus a great deal of effort has been directed at trying to understand reservoir wetting at a more fundamental level. The most important recent advances have demonstrated that there are multiple ways that crude oil components can adsorb to alter effective wetting, especially when an aqueous phase is also present.

Evaluation of Reservoir Wettability and its Effect on Oil Recovery

This project has three main goals. The first is to achieve improved understanding of the surface and interfacial properties of crude oils and their interactions with mineral surfaces. The second goal is to apply the results of surface studies to improved predictions of oil production in laboratory experiments. Finally, we aim to use the results of this research to recommend ways to improve oil recovery by waterflooding. In order to achieve these goals, the mechanisms of wetting alteration must be explained. We propose a methodology for studying those mechanisms on mineral surfaces, then applying the results to prediction and observation of wetting alteration in porous media. Improved understanding of the underlying mechanisms will show when and how wettability in the reservoir can be altered and under what circumstances that alteration would be beneficial in terms of increased production of oil. In the work reported this quarter, crude oil interactions with Berea sandstone have been used to prepare cores with mixed wettability.

Empirical Measures of Wettability in Porous Media and the Relationship between Them Derived From Pore-Scale Modelling

The wettability of a crude oil/brine/rock system is of central importance in determining the oil recovery efficiency of water displacement processes in oil reservoirs. Wettability of a rock sample has traditionally been measured using one of two experimental techniques, viz. the United States Bureau of Mines and Amott tests. The former gives the USBM index, IUSBM, and the latter yields the Amott–Harvey index, IAH. As there is no well-established theoretical basis for either test, any relationship between the two indices remains unclear.Analytical relationships between IAH and IUSBM for mixed-wet and fractionally-wet media have been based on a number of simplifying assumptions relating to the underlying pore-scale displacement mechanisms. This simple approach provides some guidelines regarding the influence of the distribution of oil-wet surfaces within the porous medium on IAH and IUSBM. More detailed insight into the relationship between IAH and IUSBM is provided by modelling the pore-scale displacement processes in a network of interconnected pores. The effects of pore size distribution, interconnectivity, displacement mechanisms, distribution of volume and of oil-wet pores within the pore space have all been investigated by means of the network model.The results of these analytical calculations and network simulations show that IAH and IUSBM need not be identical. Moreover, the calculated indices and the relationship between them suggest explanations for some of the trends that appear in experimental data when both IUSBM and IAH have been reported in the literature for tests with comparable fluids and solids. Such calculations should help with the design of more informative wettability tests in the future.

Crude oil and asphaltene characterization for prediction of wetting alteration

Abstract Measures of a crude oils polar functionality and asphaltene stability are key to predicting its wetting behavior. No publicly available data sources and few proprietary databases include all of the information necessary to characterize a crude oil with respect to its interactions with solid surfaces in the presence of an aqueous phase. At a minimum, this information includes both acid and base numbers, which are related to polar and ionic interactions between oil components and charged mineral surfaces, and refractive indeces (RI) of both the oil and oil mixtures at the onset of asphaltene flocculation, which is related to asphaltene stability. Data accumulated thus far illustrate a linear relationship between API gravity and refractive index that can be used to estimate refractive index from existing data. Acid and base numbers for many different crude oil samples show little evidence of correlation between the two, thus both numbers are needed for prediction of wetting alteration. Measurements of refractive index at the onset of precipitation with n-heptane are used to characterize asphaltene stability. Extensive data for seven crude oils are presented to illustrate how asphaltene stability can be predicted over a wide range of conditions from the n-heptane onsets.

MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

ABSTRACT A microscopic study of the onset of asphaltene precipitation is reported. The onset conditions can be quantified by measurement of mixture refractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added solvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at which precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in temperature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregates that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differentiate between those that preven...

Asphaltene Deposition on Metallic Surfaces

Abstract The potential for asphaltene deposition in wellbores and flowlines is a major concern during design of oil production and transportation facilities, especially in deep‐water environments. Understanding the processes that control asphaltene deposition, especially the relationship between precipitation and deposition, can help to reduce the risk and cost. Stainless steel capillary tubes were used to study the influences of factors including temperature, degree of asphaltene instability, and precipitant molar volume on asphaltene deposition from mixtures of stock‐tank oils and n‐alkanes. Temperature varied from 20°C to 60°C. Pressure drop across the capillary tube was used to estimate the amount and distribution of deposit formation. Existing asphaltic particles in stock‐tank oil samples did not create deposits. Below the wax appearance temperature, intermittent pressure spikes indicated deposition of wax. Above the wax appearance temperature, deposition occurred gradually from near‐onset mixtures created by co‐injection of oil and n‐alkane precipitants. Asphaltenes flocculated by addition of higher molar volume n‐alkanes deposited more material than those flocculated by lower molar volume n‐alkanes. Examination of the deposited materials showed that they contained not only asphaltenes but also waxes. Much different depositional characteristics were observed for solutions of asphaltenes in an aromatic solvent than for the stock‐tank oil from which they were derived.

Evolution of wetting alteration by adsorption from crude oil

Crude oils are complex mixtures of organic molecules, some of which can adsorb onto high-energy surfaces, altering mineral surface wettability. In cores, the extent of wetting alteration varies with oil and brine compositions and saturations as well as with aging time and temperature. Previous studies on flat surfaces have demonstrated rapid interaction between oil components and solid surfaces that can vary with the composition of the intervening brine phase. This study is designed to investigate interactions that occur after initial oil/solid contact. Wettability alternation is assessed by measurement of contact angles between pure fluids after removal of bulk crude oil. The influences of aging time, temperature, and fluid compositions have been considered. Adsorption appears to proceed in two stages. Initial exposure of wet surfaces to oil produces weakly water-wet conditions. Longer aging may produce oil-wet surfaces or, in some cases, a return to more water-wet conditions. This second stage of the adsorption process may continue for days or even weeks and is influenced by temperature and fluid compositions. Desorption of crude oil components can occur when a treated surface is re-exposed to brine.

Solubility of the Least-Soluble Asphaltenes

The key to understanding many asphaltene-related phenomena is a quantitative description of the solubility conditions at which the least-soluble asphaltenes begin to flocculate from a crude oil, often referred to as the onset of flocculation. Models that treat asphaltene flocculation as a liquid–liquid phase separation of large solute molecules dispersed in a solvent composed of much smaller molecules can successfully describe experimental observations in which solubility conditions vary due to changes in pressure and composition. Formation of small, well-dispersed asphaltene aggregates of colloidal dimensions (on the order of nanometers) does not invalidate the thermodynamic approach to modeling asphaltene phase behavior. The parameters needed to describe asphaltene phase behavior are solubility parameters and molar volumes of asphaltic and nonasphaltic portions of the oil. There are experimental barriers to accurate measurement of these important parameters, especially for the asphaltenes. We review several approaches to estimation of the solubility parameters of stock tank oil (STO) and mixtures with flocculating agents at the onset conditions, including the use of refractive index to estimate solubility parameters. We discuss the minimum data requirements for quantifying and predicting asphaltene instability from experiments with liquid alkane nonsolvents that define an asphaltene instability trend (ASIST) and we demonstrate application of STO ASIST data to prediction of asphaltene instability during depressurization of live oil. Finally, we apply the thermodynamic model to predict asphaltene instability in mixtures of petroleum fluids. Asphaltenes are defined, based on standardized tests, as the materials in petroleum products that are insoluble in n-heptane or n-pentane, but soluble in benzene or toluene (e.g., ASTM D2007). Asphaltene characterization techniques can be divided into two main groups: those based on determination of the amount of asphaltene using the standardized tests and those based on observations of