WO2009048701A1

WO2009048701A1 - Methods for characterizing heavy oils

Info

Publication number: WO2009048701A1
Application number: PCT/US2008/075670
Authority: WO
Inventors: Abdel M. Kharrat
Original assignee: Prad Research And Development Limited; Schlumberger Technology B.V.; Schlumberger Holdings Limited; Schlumberger Technology Corporation
Priority date: 2007-10-12
Filing date: 2008-09-09
Publication date: 2009-04-16
Also published as: CA2606628A1

Abstract

Methods for characterizing heavy oils are described. The methods involve extracting one or more asphaltenes from the heavy oil and measuring the viscosity of the heavy oil. The extracted asphaltenes are characterized and correlated to the viscosity of the heavy oil. By identifying the asphaltenes that are responsible for the high viscosity in the heavy oil, solvent systems can be developed for reducing the viscosity of the heavy oil.

Description

METHODS FOR CHARACTERIZING HEAVY OILS BACKGROUND OF THE INVENTION

[0001] Heavy oils generally possess a very high viscosity. Due to the high viscosity, the removal of heavy oils from underground reservoirs is especially challenging. Heavy oils contain a number of different compounds that contribute to high viscosity. One class of such compounds is asphaltenes. Asphaltenes are generally polar compounds that are insoluble in hydrocarbon solvents such as pentane, hexane, or heptane. The precipitation of asphaltenes in underground reservoirs can damage the reservoir by plugging the pores. A number of factors can contribute to asphaltene precipitation, including reduction in pressure and temperature, the compatibility of the oils when mixed together, the presence of water, and the presence of inorganic materials that can act as a seed for precipitation.

[0002] The characterization of asphaltenes present in heavy oils is known. The techniques generally involve precipitating the asphaltenes from the oil followed by fractionating and characterizing the asphaltenes. Although this approach is useful for characterizing the asphaltenes, it is not possible to obtain useful information regarding the oil (e.g., viscosity) after the asphaltenes have been removed. Indeed, these techniques are performed outside the oil. Therefore, there is no way to correlate the types and amounts of asphaltenes present in the heavy oil to the viscosity of the oil using current techniques known in the art.

[0003] Thus, what are needed are methods for characterizing heavy oils. In particular, it would be desirable to identify the components in heavy oils that are responsible for increased viscosity. If it is possible to identify and quantify these components, then solvent systems can be developed in order to reduce the viscosity of the heavy oils upon injection of the solvent system into the underground reservoir, which ultimately increases the efficiency for removing the heavy oils from the reservoir.

BRIEF SUMMARY OF THE INVENTION

[0004] Described herein are methods for characterizing heavy oils. The methods involve extracting one or more asphaltenes from the heavy oil and measuring the viscosity of the heavy oil. The extracted asphaltenes are characterized and correlated to the viscosity of the heavy oil. By identifying the asphaltenes that are responsible for the high viscosity in the heavy oil, solvent systems can be developed for reducing the viscosity of the heavy oil. The advantages of the materials, methods, and articles described herein will be set forth in part in the description which follows, or may be learned by practice of the aspects described below. The advantages described below will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The accompanying Figure, which is incorporated in and constitutes a part of this specification, illustrates several aspects described below.

[0006] Figure 1 shows a schematic of a series of extraction steps and viscosity measurements of an oil residue.

DETAILED DESCRIPTION OF THE INVENTION

[0007] Before the present materials, articles, and/or methods are disclosed and described, it is to be understood that the aspects described below are not limited to specific compounds, synthetic methods, or uses as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

[0008] In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings:

[0009] Throughout this specification, unless the context requires otherwise, the word "comprise," or variations such as "comprises" or "comprising," will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0010] It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "an oil" includes a single oil or mixtures of two or more oils. [0011] "Optional" or "optionally" means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0012] Described herein are methods for characterizing heavy oils taken from underground reservoirs. The term "heavy oil" is any source or form of viscous oil. For example, a source of heavy oil includes tar sand. Tar sand, also referred to as oil sand or bituminous sand, is a combination of clay, sand, water, and bitumen. As will be discussed below, identifying and quantifying the components that contribute to elevated viscosity can lead to the development of solvent systems for reducing the viscosity of the heavy oil and increase recovery efficiency.

[0013] In one aspect, the method involves correlating the viscosity of an underground oil to the presence of one or more asphaltenes present in the underground oil, wherein the method comprises: distilling a sample comprising the underground oil to produce a first oil residue; measuring the viscosity of the first oil residue, wherein the first oil residue comprises one or more asphaltenes incorporated within a first matrix; extracting the first oil residue with an extraction solvent to remove one or more asphaltenes and produce a second oil residue; measuring the viscosity of the second oil residue; identifying and quantifying the asphaltenes extracted from the first oil residue; and correlating the identity and amount of asphaltenes to the difference in viscosity between the first oil residue and the second oil residue.

[0014] In certain aspects, the methods described herein permit the characterization of heavy oil samples taken from an oilfield and analyzed in the laboratory. Using the methods described herein, solvent systems are developed based upon the laboratory results for decreasing the viscosity of the heavy oil in the field. The methods are versatile in identifying the components responsible for increased viscosity, which is significant in that different oilfields possess heavy oils with different types and amounts of components responsible for increased viscosity.

[0015] In general, a sample of heavy oil is taken from an oilfield and characterized in the laboratory. By identifying in the lab the types and amounts of asphaltenes present in high viscosity oils, it is possible to develop solvent systems that can be introduced into an underground reservoir in the field that precipitate a selected fraction of asphaltenes responsible for high viscosity. By precipitating this fraction of asphaltenes from the oil matrix, the viscosity of the heavy oil is reduced, which increases the efficiency of removing the heavy oils from the reservoir. The term "oil matrix" is referred to herein as the oil solution where asphaltenes are dissolved and interact with the resins and other components present in the oil solution.

[0016] The first step involves distilling the heavy oil in order to remove low boiling components. By removing the low boiling components, a more precise mass balance of the asphaltenes present in the oil can be calculated during the extraction/fractionation steps. Techniques such as, for example, spinning band distillation can be used to remove the low boiling components. Spinning band distillation provides a high theoretical plate number, which ensures consistent distillate cuts are made.

[0017] After the distillation of the heavy oil, a residue containing asphaltenes is produced. Asphaltenes are complex hydrocarbons composed of aromatic hydrocarbons with sidechains up to Cjo, hetero-aromatic compounds, and metals such as, for example, iron, nickel, and vanadium. The residue is further characterized in order to correlate asphaltene content with viscosity. By characterizing the residue, a better understanding of the properties of the oil (e.g., the viscosity) in the underground reservoir can be achieved. This is not the case with current techniques, which do not permit the characterization of the oil matrix.

[0018] The viscosity of the residue produced after the distillation step, which is referred to herein as "the first oil residue," is measured using techniques known in the art. The viscosity measurement provides a baseline for subsequent viscosity measurements. The first oil residue is then extracted with an extraction solvent to remove one or more asphaltenes and produce a second oil residue. In certain aspects, the asphaltenes are precipitated from the first oil residue. After the extraction step, the viscosity of the second oil residue is calculated. The asphaltenes that are removed from the first oil residue are purified and characterized using techniques known in the art including, but not limited to, elemental analysis, mass spectrometry, metal content, nuclear magnetic resonance spectroscopy, and infrared spectroscopy. Identification and quantification of the asphaltenes that were removed from the first oil residue can be used to correlate the asphaltenes to the reduced viscosity observed in the second oil residue.

[0019] By performing a series of extractions, viscosity measurements, and characterization of the isolated asphaltenes, the asphaltenes responsible for increasing the viscosity of the heavy oil can be identified. Additionally, other components that can affect viscosity can be identified as well. For example, inorganics including, but not limited to, iron, nickel, and vanadium can be separated and quantified using the methods described herein. Referring to Figure 1 , which is an exemplary embodiment, the viscosity (η¹) of first oil residue Rl is measured. Rl is extracted with a solvent system to remove asphaltenes and to produce second oil residue Fl . Prior to subsequent extraction of Fl, the viscosity of Fl (η²) is measured. As shown in Figure 1, a series of extraction steps and viscosity measurements are performed. Each asphaltene fraction is purified and characterized as described above.

[0020] The types and amounts of asphaltenes present in heavy oils can vary from region-to-region and from field-to-field. Thus, the selection of the extraction solvent can vary as well. The asphaltenes are generally polar compounds and require the use of polar solvents to dissolve them. Examples of polar solvents include, but are not limited to, ethers (cyclic and acyclic), chlorinated solvents, alcohols, ketones, esters, and combinations thereof. Specific examples of polar solvents useful herein include, but are not limited to, THF, dimethyl ether, methylene oxide, methyl tertiobutyl ether, sulfur dioxide, and any combination thereof.

[0021] In certain aspects, the extraction solvent includes a mixture of two or more solvents. For example, the solvent can include a polar solvent and a non-polar solvent, where the asphaltenes are only partially soluble to insoluble in the non-polar solvent. Thus, depending upon the extraction solvent selected, the asphaltenes precipitate from the oil residue when the oil residue is contacted with the extraction solvent. The precipitated asphaltene in this aspect can be filtered and subsequently characterized as described above. Not wishing to be bound by theory, the polar solvent dissolves the asphaltenes and removes them from the oil matrix and the non- polar solvent precipitates the asphaltenes for easy removal. Examples of non-polar solvents include, but are not limited to, hydrocarbons (e.g., methane, propane, butane, pentane, hexane, heptane) or carbon dioxide.

[0022] In certain aspects, when a series of extraction steps are performed, the polarity of the solvent system decreases from extraction to extraction. For example, the extraction solvent could be 100 percent polar solvent in the first extraction, then a non-polar solvent can be added to decrease the polarity. Using routine extraction techniques, appropriate solvent systems with varying polarity can be developed based upon the identity and amount of asphaltenes present in the oil residue and the corresponding reduction in viscosity.

[0023] The methods described herein are also useful in identifying other components in oil besides asphaltenes that can contribute to high viscosity. In one aspect, the methods described herein can be used to isolate and characterize maltenes present in the oil. Maltenes generally contain aromatic hydrocarbons with or without oxygen, nitrogen, and sulfur, saturated straight-chain and cyclic unsaturated hydrocarbons, naphthenes, and straight- or branch-chain saturated hydrocarbons. Maltenes also include resins, which are smaller molecular weight versions of asphaltenes. Maltenes are generally soluble in non-polar solvents. Once removed from the oil residue, the maltenes can be characterized by techniques known in the art including chromatography (e.g., TLC, column chromatography, and HPLC).

[0024] The methods described herein identify the components (e.g., asphaltenes or maltenes) responsible for the high viscosity observed in heavy oils. For example, by correlating a series of viscosities to the corresponding asphaltene content, a visual aid correlating the asphaltene content, viscosity, and solvent system useful in precipitating the asphaltenes and subsequently reducing the viscosity of the heavy oil can be produced. The visual aid can be a table, chart, or graph. The visual aid is particularly useful when testing asphaltene samples from field-to-field, which can provide a useful guide for selecting solvent systems for reducing the viscosity of heavy oils in underground reservoirs. [0025] In one aspect, the methods described herein can be used to reduce the viscosity of an underground oil in an underground reservoir, wherein the method comprises: removing a sample of the underground oil from the underground reservoir; correlating the identity and amount of asphaltenes present in the underground oil responsible for the viscosity of the oil using the techniques described above; and introducing a solvent system into the underground reservoir based upon the correlation step, wherein the solvent system precipitates specific asphaltenes and reduces the viscosity of the underground oil.

[0026] Using the techniques described above, it is possible to identify and quantify the asphaltenes that are responsible for increased viscosity. The techniques also identify the solvent system for removing (e.g., precipitating) these asphaltenes from the oil matrix. Thus, by identifying the solvent system for removing the specific asphaltenes responsible for increased viscosity in the laboratory, solvent systems having similar properties (e.g., polarity) can be used in the field to reduce the viscosity of heavy oil in underground reservoirs. For example, if a solvent system composed of 30/70 by volume THF/hexane was discovered in the laboratory to precipitate the asphaltenes responsible for increased viscosity, a solvent system composed of dimethyl ether and methane in amounts sufficient to achieve the same properties (e.g., solubility parameters) as that of the THF/hexane solvent system can be injected into the underground reservoir to precipitate out asphaltenes from the oil matrix and reduce the viscosity of the oil. When selecting the solvent system used in the field, considerations such as toxicity should be considered.

EXAMPLES

[0027] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, and methods described and claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, desired solvents, solvent mixtures, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions.

[0028] A sample of about 10 grams of the oil residue was accurately weighed after spinning band distillation (274 plus) in a 1 liter flask. A mixture of 70 volumes of hexane and 30 volumes of tetrahydrofuran (sample to solvent ratio is 1:40) was added and the solution was refluxed (68-69° C) for 2 hours then filtered through a 0.45 micron filter. The asphaltene fraction was washed with the same solvent mixture under reflux using a Soxhlet apparatus. The washed asphaltene fraction was extracted with dichloromethane, dried, and weighed in a vial. The deasphalted oil fraction was dried in a rotary evaporator at 95° C under a vacuum of 26 inches of mercury. Viscosity measurements were then performed on the deasphalted fraction (Fl in Figure 1). The portion used for viscosity measurement was recovered with dichloromethane and combined with the original one, dried in the rotary evaporator, and weighed.

[0029] The same precipitation procedure described above was continued with the following solvent mixtures: 25/75, 20/80, 15/85, 10/90, 5/95 and 0/100 by volume THF/hexane. The precipitated asphaltenes were washed with the corresponding solvent mixture, and each asphaltene fraction was separated, dried, and weighed.

[0030] The viscosity of the residue (Rl in Figure 1) and each deasphalted oil (F1-F7) after each fractionation was measured using a Reologica rheometer. All fraction measurements were made using a parallel plate system with 0.3 mm gap with a small amount of sample (4 to 5 grams). Even though shear rate has no impact on the viscosity of these oils (Newtonian fluid), a constant shear rate of 0.5 s^-1 was chosen for all fractions. For whole oil samples, cup & bob measurements were used. All measurements were made at 25° C. In all cases, 100 data points were collected with a relative standard deviation of not more than 3.0 and the mean value was reported as the fraction viscosity. [0031] A known amount of C5 maltenes was loaded on a chromatography column packed with activated alumina. First, the saturate fraction was eluted with heptane followed by the aromatic fraction with toluene. A mixture of dichloromethane and methanol (50:50) was used to elute Resin 1. Further elution was continued by dichloromethane to collect the Resin 2 followed by methanol for Resin 3 (Figure 1). Each fraction was separated from the solvent, dried, and weighed.

[0032] Tables 1 and 2 show asphaltene content and viscosity measurements results based upon the characterization of a Canadian heavy oil sample. Each asphaltene fraction was characterized elemental, metals, nuclear magnetic resonance, infrared spectroscopy, and mass spectrometry.

[0033] Various modifications and variations can be made to the compounds, compositions, and methods described herein. Other aspects of the compounds, compositions, and methods described herein will be apparent from consideration of the specification and practice of the compounds, compositions, and methods disclosed herein. It is intended that the specification and examples be considered as exemplary.

Claims

CLAIMS What is claimed is:

1. A method for correlating the viscosity of an underground oil to the presence of one or more asphaltenes in the underground oil, comprising:

(a) distilling a sample comprising the underground oil to produce a first oil residue;

(b) measuring the viscosity of the first oil residue, wherein the first oil residue comprises one or more asphaltenes incorporated within a first matrix;

(c) extracting the first oil residue with an extraction solvent to remove one or more asphaltenes and produce a second oil residue;

(d) measuring the viscosity of the second oil residue;

(e) identifying and quantifying the asphaltenes extracted from the first oil residue; and

(f) correlating the identity and amount of asphaltenes to the difference in viscosity between the first oil residue and the second oil residue.

2. The method of claim 1, wherein the oil comprises a heavy oil or bitumen.

3. The method of claim 1, wherein the extraction step comprises a series of two or more extractions.

4. The method of claim 3, wherein the polarity of each extraction solvent decreases for each subsequent extraction step.

5. The method of claim 1, wherein the extraction solvent comprises a mixture of two or more solvents.

6. The method of claim 1, wherein the extraction solvent comprises a mixture of a first solvent and second solvent, wherein the first solvent dissolves one or more asphaltenes and the second solvent precipitates one or more asphaltenes.

7. The method of claim 1, wherein the extraction solvent comprises a mixture of a polar solvent and a non-polar solvent.

8. The method of claim 7, wherein the polar solvent comprises an ether, a chlorinated solvent, an alcohol, a ketone, an ester, or any combination thereof.

9. The method of claim 7, wherein the polar solvent comprises THF, dimethyl ether, methylene oxide, methyl tertiobutyl ether, sulfur dioxide, or any combination thereof.

10. The method of claim 7, wherein the non-polar solvent comprises a hydrocarbon.

11. The method of claim 7, wherein the non-polar solvent comprises methane, propane, butane, pentane, hexane, heptane, carbon dioxide, or any combination thereof.

12. The method of claim 1, wherein the extraction solvent comprises a mixture of THF and hexane.

13. The method of claim 1, wherein the extraction solvent comprises a mixture of dimethyl ether and methane.

14. The method of claim 1, further comprising after step (c), separating one or more maltenes from the second oil residue.

15. The method of claim 14, wherein the maltenes are separated from the second oil residue by a chromatographic technique.

16. The method of claim 1, wherein the method identifies the asphaltenes and amounts thereof responsible for high viscosity.

17. A method for reducing the viscosity of an underground oil in an underground reservoir comprising: removing a sample of the underground oil from the underground reservoir; correlating the identity and amount of asphaltenes present in the underground oil responsible for the viscosity of the oil using the method of claim l; and introducing a solvent system into the underground reservoir based upon the correlation step, wherein the solvent system precipitates specific asphaltenes and reduces the viscosity of the underground oil.

18. A visual aid for correlating the identity and amount of an asphaltene present in an underground oil to the viscosity of the underground oil.

19. The visual aid of claim 18, wherein the visual aid comprises a table, chart, or graph.