System and method for determining the asphaltene content of crude oil

A system for determining the asphaltene content of crude oil includes a first optical flow cell, a first spectrometer operably associated with the first optical flow cell, and a mixer in fluid communication with the first optical flow cell. The system further includes a crude oil injection/metering device configured to receive the crude oil, the crude oil injection/metering device being in fluid communication with the first optical flow cell; a titrant injection/metering device in fluid communication with the mixer, the titrant injection/metering device configured to receive a titrant; and a filtration unit in fluid communication with the mixer. The system further includes a second optical flow cell in fluid communication with the filtration unit, and a second spectrometer operably associated with the second optical flow cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for determining the asphaltene content of crude oil.

2. Description of Related Art

Asphaltenes are a solubility class of components of crude oil. Commonly, asphaltenes are defined as such components that are insoluble in pentane or heptane but that are soluble in toluene or dichloromethane. Asphaltenes are of particular interest to the petroleum industry because of their depositional effect in production equipment, such as in tubular members in oil wells. Additives are sometimes used to inhibit these deleterious effects. In addition, asphaltenes impart high viscosity to crude oils, negatively impacting production. The variable asphaltene concentration in crude oils within individual reservoirs can create a myriad of production problems. Accordingly, it is often desirable to determine the amount of asphaltenes in crude oil and a variety of methods exist for making such determinations.

Conventional methods for determining the asphaltene content of crude oil rely upon precipitating by a titrant and filtering the asphaltenes from the crude oil, then weighing the asphaltenes. Methods such as this, however, (1) are often not sufficiently repeatable and reproducible; (2) are well-suited for laboratory operations only; (3) require significant time to complete, often as long as two days or more; (4) require large volumes of samples that are inherently hazardous in nature; (5) are dependent upon controlled humidity environments for reliable results; and (6) are dependent upon operator skill for reliable results.

Although there are methods for determining the asphaltene content of crude oil that are well known in the art, considerable shortcomings remain.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention provides a system for determining the asphaltene content of crude oil. The system comprises a first optical flow cell, a first spectrometer operably associated with the first optical flow cell, and a mixer in fluid communication with the first optical flow cell. The system further comprises a crude oil injection/metering device configured to receive the crude oil, the crude oil injection/metering device being in fluid communication with the first optical flow cell; a titrant injection/metering device in fluid communication with the mixer, the titrant injection/metering device configured to receive a titrant; and a filtration unit in fluid communication with the mixer. The system further comprises a second optical flow cell in fluid communication with the filtration unit and a second spectrometer operably associated with the second optical flow cell.

In another aspect, the present invention provides a system for determining the asphaltene content of crude oil. The system comprises an injection/metering device configured to receive the crude oil and a titrant, a mixer in fluid communication with the injection/metering device, and a filtration unit in fluid communication with the mixer. The system further comprises an optical flow cell in fluid communication with the filtration unit and a spectrometer operably associated with the optical flow cell.

In yet another aspect, the present invention provides a method for determining the asphaltene content of crude oil. The method comprises obtaining a crude oil sample, determining an optical spectrum of the crude oil sample, and removing asphaltenes from the crude oil sample. The method further comprises determining an optical spectrum of maltenes of the crude oil sample, subtracting the optical spectrum of the maltenes of the crude oil sample from the optical spectrum of the crude oil sample to yield an optical spectrum of asphaltenes of the crude oil sample, and comparing the optical spectrum of the asphaltenes of the crude oil sample to predetermined calibration data.

In another aspect, the present invention provides a method for determining the asphaltene content of crude oil. The method comprises determining an optical spectrum of a first sample of the crude oil, removing asphaltenes from a second sample of the crude oil, and determining an optical spectrum of maltenes of the second sample of the crude oil. The method further comprises subtracting the optical spectrum of the maltenes of the second sample of the crude oil from the optical spectrum of the first sample of the crude oil to yield an optical spectrum of asphaltenes of the crude oil and comparing the optical spectrum of the asphaltenes of the crude oil to predetermined calibration data.

The present invention provides significant advantages, including (1) providing a method for determining asphaltene content of crude oil that is repeatable and reproducible; (2) providing a system and a method for determining asphaltene content of crude oil that are suitable for use at a wellsite; (3) providing a system and a method for determining asphaltene content of crude oil that are suitable for use on offshore platforms; (4) providing a system and a method for quickly determining asphaltene content of crude oil; (5) providing a system and a method for determining asphaltene content of crude oil that utilizes small sample volumes; (6) providing a system and a method for determining asphaltene content of crude oil that are not significantly affected by humidity; and (7) providing a system and a method for determining asphaltene content of crude oil that do not rely upon highly skilled operators.

Additional objectives, features, and advantages will be apparent in the written description which follows.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system and method for determining the asphaltene content of crude oil. Generally, crude oil is made up of asphaltenes, which are insoluble in pentane or heptane, and maltenes, which are soluble in pentane and heptane. The asphaltene content of a sample of crude oil is determined by determining the optical spectrum of the asphaltenes in the crude oil and comparing the optical spectrum to predetermined calibration data, which provides a correlation between asphaltene optical spectra and asphaltene content. The asphaltene optical spectrum of the crude oil sample is determined by subtracting the optical spectrum of the maltenes of the crude oil sample from the optical spectrum of the crude oil sample.

FIG. 1provides a flow chart representing an illustrative embodiment of a method for determining the asphaltene content of crude oil. In the illustrated embodiment, a crude oil sample is obtained (block101). It should be noted that the obtained sample may be a sample that is retrieved and transported to another location, such as a laboratory, for analysis, or a sample that is retrieved and analyzed in the field, as is discussed in greater detail herein. The system and method of the present invention are also capable of being installed and used in a downhole tool. The scope of the present invention is not limited by the means by which the crude oil sample is obtained. Returning toFIG. 1, the optical spectrum of the crude oil sample is measured (block103). The crude oil is then mixed with a titrant (block105) to precipitate the asphaltenes from the crude oil. In one embodiment, the titrant is n-heptane, mixed at a ratio of one part crude oil to 40 parts n-heptane. However, other titrants, such as n-pentane or the like, and other mixing ratios are contemplated by the present invention. The precipitated asphaltenes are then removed from the crude oil-titrant mixture (block107). The portion of the crude oil remaining after the precipitated asphaltenes are removed comprises maltenes, which are species having lower molecular weights than asphaltenes and are soluble in the titrant. The optical spectrum of the maltenes is measured (block109), which is then subtracted from the optical spectrum of the crude oil prior to the asphaltenes being removed (block111). The resulting optical spectrum corresponds to the optical spectrum of the asphaltenes in the sample of crude oil.

FIG. 2depicts a graphical representation of an exemplary optical spectrum of a crude oil, represented by line201, and of the maltenes in the crude oil, represented by line203. The difference between these optical spectra is due to the optical spectrum of the asphaltenes in the crude oil. In one embodiment, the optical spectrum at one or more longer wavelengths of the maltenes of the crude oil sample, such as at wavelengths of about 800 nanometers, is subtracted from the optical spectrum at one or more shorter wavelengths of the maltenes of the crude oil sample, such as at wavelengths of about 600 nanometers, to reduce the error from a spectral offset introduced by light scattering and the effect of variation of refractive index in the measuring instrument. Returning toFIG. 1, the optical spectrum of the asphaltenes, i.e., the result from block111, wherein the optical spectrum of maltenes of the crude oil is subtracted from the optical spectrum of the crude oil prior to the asphaltenes being removed, is compared to calibration data (block113), such as a calibration curve. The calibration data correlates the optical spectrum of the asphaltene molecules to the asphaltene content measured using another technique, such as a conventional gravimetry technique, in which a series of crude oil samples are collected and tested.

FIGS. 3 and 4illustrate the improvement in correlation between asphaltene optical density and asphaltene content when the optical spectrum of the maltenes in the crude oil is subtracted.FIG. 3depicts a graphical representation of one example of the optical density of various samples of crude oil and their asphaltene contents. Line301represents a linear model generated using the optical density at a particular wavelength and the asphaltene contents of the samples. In this example, the linear model exhibits a coefficient of determination (R2) of 0.83. Note that models exhibiting coefficients of determination that approach 1.00 fit the data well, while models having coefficients of determination that are less than 1.00 do not represent the data as well.FIG. 4depicts a graphical representation of an example of the optical density of various samples of crude oil in which the optical density of the maltenes of the crude oil samples have been subtracted from the optical density of the crude oil samples. Line401represents a linear model generated using the resulting optical density and asphaltene contents of the samples. In the example ofFIG. 4, the linear model exhibits a coefficient of determination of 0.95. Thus, the correlation between asphaltene content and the optical density ofFIG. 4, i.e., the optical density of samples in which contributions by maltenes have been removed, is significantly better than the correlation between asphaltene content and the optical density ofFIG. 3, i.e., the optical density of the base crude oil samples.

FIG. 5depicts a stylized, graphical representation of a first illustrative embodiment of a system501for determining the asphaltene content of crude oil. Specifically, in reference toFIG. 1, system501is configured to accomplish measuring an optical spectrum of a sample of crude oil (block103), mixing the crude oil sample with a titrant (block105), removing precipitated asphaltenes from the crude oil-titrant mixture (block107), and measuring an optical spectrum of the maltenes of the crude oil sample (block109). In the illustrated embodiment, system501comprises a first optical flow cell503that is in fluid communication with a crude oil sample505via a crude oil injection/metering device509and is in fluid communication with a mixer507. In one embodiment, mixer507is a microfluidic mixer, such as those available from The Dolomite Centre Limited of Royston, UK. A first spectrometer511is operably associated with the first optical flow cell503. A titrant injection/metering device513is in fluid communication with a titrant source515and mixer507. In the illustrated embodiment, crude oil injection/metering device509and titrant injection/metering device513are pumps, such as syringe pumps available from Thermo Fisher Scientific Inc. of Pittsburgh, Pa., USA. Mixer507is in fluid communication with a second optical flow cell517via a filtration unit519. In the illustrated embodiment, optical flow cells503and517are optical flow cells such as those available from Ocean Optics, Inc. of Dunedin, Fla., USA. Filtration unit519, in the illustrated embodiment, is a microfluidic membrane filtration unit, such as those available from The Dolomite Centre Limited. A second spectrometer521is operably associated with second optical flow cell517. In the illustrated embodiment, spectrometers511and521are spectrometers such as those available from Ocean Optics, Inc. First spectrometer511and second spectrometer521are operably associated, in the illustrated embodiment, with a comparator523, such as a computer, although certain embodiments of system501omit comparator523, wherein the functions of comparator523are performed by human or other means.

Still referring toFIG. 5, an exemplary operation of system501for determining the asphaltene content of crude oil is disclosed. At least a portion of crude oil sample505is transmitted to first optical flow cell503by crude oil injection/metering device509. First spectrometer511analyzes the portion of crude oil sample505disposed in first optical flow cell503and determines an optical spectrum of the portion of crude oil sample505, represented by graph525. The crude oil from crude oil sample505is further urged to mixer507by crude oil injection/metering device509. A titrant, such as heptane, pentane, or the like, is transmitted from titrant source515to mixer507by titrant injection/metering device513. Crude oil and the titrant are mixed in mixer507at a predetermined ratio, such as at a ratio of about one part crude oil to about 40 parts titrant. Once the crude oil and titrant are mixed, the titrant causes the asphaltenes in the crude oil to be precipitated in a channel, represented by arrow527. The crude oil-titrant mixture is then filtered by filtration unit519, which retains precipitated asphaltenes529and allows the remaining fluid, i.e., the maltenes of the sample of crude oil, to pass therethrough to second optical flow cell517. Second spectrometer521analyzes the maltenes in second optical flow cell517and determines an optical spectrum of the maltenes, represented by graph531. The optical spectrum of the crude oil, i.e., represented by graph525, and the optical spectrum of the maltenes of the crude oil, i.e., represented by graph531, are fed to comparator523, where the optical spectrum of the maltenes of the crude oil is subtracted from the optical spectrum of the crude oil, resulting in the optical spectrum of the asphaltenes in the crude oil, represented by graph533. The optical spectrum of the asphaltenes in the crude oil is then compared to predetermined calibration data, such as a predetermined calibration curve, as discussed herein, to measure the asphaltene content of crude oil sample505.

FIGS. 6A and 6Bdepict a stylized, graphical representation of a second illustrative embodiment of a system601for determining the asphaltene content of crude oil. In this embodiment, injection/metering devices509and513of system501are replaced with an injection/metering device603, optical flow cells503and517of system501are replaced with an optical flow cell605, and spectrometers511and521of system501are replaced with a spectrometer607. In one embodiment, injection/metering device603corresponds to one of injection/metering devices509and513of system501, optical flow cell605corresponds to one of optical flow cells503and517of system501, and spectrometer607corresponds to one of spectrometers511and521of system501. Otherwise, the elements of system601correspond to those of system501. As in the embodiment ofFIG. 5, the embodiment ofFIGS. 6A and 6Bis configured to accomplish measuring an optical spectrum of a sample of crude oil (block103), mixing the crude oil sample with a titrant (block105), removing precipitated asphaltenes from the crude oil-titrant mixture (block107), and measuring an optical spectrum of the maltenes of the crude oil sample (block109), shown inFIG. 1.

In the embodiment illustrated inFIGS. 6A and 6B, mixer507is in fluid communication with crude oil sample505and titrant source515via injection/metering device603. Mixer507is in fluid communication with optical flow cell605via filtration unit519. Spectrometer607is operably associated with optical flow cell605. In the illustrated embodiment, spectrometer607is operably associated with comparator523, although certain embodiments of system601omit comparator523, wherein the functions of comparator523are performed by human or other means.

Still referring toFIGS. 6A and 6B, an exemplary operation of system601for determining the asphaltene content of crude oil is disclosed. InFIG. 6A, a first portion of crude oil sample505is transmitted through mixer507and filtration unit519to optical flow cell605by injection/metering device603. Spectrometer607analyzes the crude oil disposed in optical flow cell605and determines an optical spectrum of the crude oil, represented by graph525. In the illustrated embodiment, the optical spectrum of the crude oil, i.e., represented by graph525, is fed to comparator523for use in determining the asphaltene content of crude oil sample505. The flow path of crude oil in system601is then cleaned.

Referring toFIG. 6B, a second portion of crude oil sample505and a titrant, such as heptane, pentane, or the like, is transmitted to mixer507by injection/metering device603. The second portion of crude oil sample505and the titrant are mixed in mixer507at a predetermined ratio, such as at a ratio of about one part crude oil to about40parts titrant. Once the second portion of crude oil sample505and the titrant are mixed, the titrant causes the asphaltenes in the crude oil to precipitate in the channel represented by arrow527. The crude oil-titrant mixture is then filtered by filtration unit519, which retains precipitated asphaltenes529and allows the remaining fluid, i.e., the maltenes of the sample of crude oil, to pass therethrough to optical flow cell605. Spectrometer607analyzes the maltenes in optical flow cell605and determines an optical spectrum of the maltenes, represented by graph531. In the illustrated embodiment, the optical spectrum of the maltenes of the crude oil, i.e., represented by graph531, is fed to comparator523, where the optical spectrum of the maltenes of the crude oil is subtracted from the optical spectrum of the crude oil, resulting in the optical spectrum of the asphaltenes in the crude oil, represented by graph533. The optical spectrum of the asphaltenes in the crude oil is then compared to predetermined calibration data, such as a predetermined calibration curve, as discussed herein, to measure the asphaltene content of the crude oil. As discussed herein concerning the embodiment ofFIG. 5, certain embodiments of system601omit comparator523, wherein the functions of comparator523are performed by human or other means.

It should be noted that, in certain embodiments, the modifications made to system501(shown inFIG. 5) resulting in system601may be incorporated singly or in any combination. For example, system601may be modified such that injection/metering devices509and513of system501are replaced with injection/metering device603, but optical flow cells503and517and spectrometers511and521are not replaced. In another example, system601may be modified such that optical flow cells503and517of system501are replaced with optical flow cell605but injection/metering devices509and513and spectrometers511and521are not replaced. In yet another example, system601may be modified such that spectrometers511and521of system501are replaced with spectrometer607but injection/metering devices509and513and optical flow cells503and517are not replaced. The present invention further contemplates various combinations of these embodiments.

FIG. 7depicts a graphical representation of one example of the optical density of various samples of crude oil, such as crude oil sample505, as determined by first spectrometer511and their asphaltene contents. Line701represents a linear model generated using the optical density at a particular wavelength and asphaltene contents of the samples. In this example, the linear model exhibits a coefficient of determination (R2) of 0.84, which is comparable to that shown inFIG. 3.FIG. 8depicts a graphical representation of an example of optical density at a particular wavelength of asphaltenes of various samples of crude oil in which the optical spectra of the maltenes of the crude oil samples have been subtracted from the optical spectra of the crude oil samples. Line801represents a linear model generated using the resulting optical density and asphaltene contents of the samples. In the example ofFIG. 8, the linear model exhibits a coefficient of determination of 0.96, which is comparable to that shown inFIG. 4. Thus the correlation between asphaltene content and the optical density ofFIG. 8, i.e., the optical density of samples in which contributions by maltenes have been removed, is significantly better than the correlation between asphaltene content and the optical density ofFIG. 7, i.e., the optical density of the base crude oil samples.