Method for calculating maturity of heavy oil reservoir

Abstract

The invention relates to a method for calculating the maturity of a heavy oil reservoir, which comprises the following steps of: (1) collecting a typical coal sample, extracting asphaltene in the typical coal sample, and analyzing to obtain the air reflectivity of the asphaltene and the vitrinite reflectivity Ro of the typical coal sample; (2) establishing a fitting relation between asphaltene air reflectivity in the typical coal sample and vitrinite reflectivity Ro of the typical coal sample; (3) enabling the fitting relationship established in the step (2) to be equivalent to a corresponding relationship between the asphaltene air reflectivity of the heavy oil reservoir and the equivalent maturity of the heavy oil reservoir; and asphaltene in the heavy oil reservoir in the target area is extracted and analyzed to obtain the air reflectivity of the asphaltene, and the maturity of the heavy oil reservoir in the target area is calculated in combination with the corresponding relation. According to the method, the maturity of the target heavy oil reservoir can be accurately calculated, the required experimental technology is simple, data acquisition is easier, cost is low, practicability is high, and large-scale application and popularization are easier.

Description

Technical Field

[0001] The present invention relates to a method for calculating the maturity of heavy oil reservoirs, belonging to the technical field of oil and gas exploration and development. Background Art

[0002] As of the end of 2020, the external dependence on oil in China has reached as high as 73.5%, and it is necessary to increase the exploration and development efforts of domestic oil resources. Heavy oil is an important part of China's crude oil production, and the resources account for more than 20% of China's total oil resources. At the same time, heavy oil is also an important raw material for processing high-grade asphalt, high-end engine oil, aerospace fuel, etc.

[0003] China is the fourth largest heavy oil country in the world, and the heavy oil resource volume is second only to the United States, Canada, and Venezuela. At present, more than 70 heavy oil production areas of different sizes have been discovered in more than a dozen basins in China. At present, the proven reserves of heavy oil in China are about 4 billion tons. In recent years, due to technological breakthroughs, the output has basically stabilized at the scale of 15 million to 16 million tons per year. In this situation, realizing the efficient exploration and development of heavy oil resources is of great practical significance for reducing the external dependence on oil and ensuring national energy security. In the process of evaluating the efficient exploration and development of heavy oil resources, the key scientific problem of whether heavy oil is autogenous or secondary is often encountered. A key technical problem in solving this problem is the determination of the maturity of heavy oil reservoirs.

[0004] To evaluate the maturity of traditional normal crude oil, the evolution degree of organic matter is generally represented by saturated hydrocarbon chromatography and gas chromatography-mass spectrometry. However, for heavy oil, due to biodegradation, a large amount of saturated hydrocarbons in the crude oil are lost. Using saturated hydrocarbon chromatography and gas chromatography-mass spectrometry to represent the evolution degree of organic matter, its credibility is greatly reduced. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for calculating the maturity of heavy oil reservoirs to solve the problem of low credibility in judging the maturity of heavy oil in the prior art.

[0006] In order to achieve the above purpose, the technical solution of a method for calculating the maturity of heavy oil reservoirs in the present invention is as follows:

[0007] A method for calculating the maturity of heavy oil reservoirs includes the following steps:

[0008] (1) Collect typical coal samples and extract the asphaltenes therein, and analyze to obtain the air reflectance of asphaltenes and the vitrinite reflectance Ro of the typical coal samples;

[0009] (2) Establish a fitting relationship between the air reflectance of asphaltenes in the typical coal samples and the vitrinite reflectance Ro of the typical coal samples;

[0010] (3) Equivalently regard the fitting relationship established in step (2) as the corresponding relationship between the asphaltene air reflectivity of the heavy oil reservoir and the equivalent maturity of the heavy oil reservoir; extract the asphaltene in the heavy oil reservoir in the target area and analyze to obtain its asphaltene air reflectivity, and calculate the maturity of the heavy oil reservoir in the target area in combination with the above-mentioned corresponding relationship.

[0011] The beneficial effects of the above technical solution are as follows: Vitrinite emissivity is currently the most reliable parameter for evaluating the maturity of hydrocarbon source rocks. As the main component of soluble hydrocarbons, asphaltene begins to form and dissolve in hydrocarbons during the early evolution of organic matter and migrates from the source rock to the reservoir together. It exists in large quantities in the hydrocarbon source rock and heavy oil. Due to its relatively stable properties, measuring the emissivity of heavy oil asphaltene can reflect the evolution degree of soluble hydrocarbons in geological bodies and provide a basis for oil-source correlation. The present invention starts from the experimental method. First, pre-treat the heavy oil in the heavy oil reservoir to extract asphaltene and purify the asphaltene, and then measure the air reflectivity of the purified asphaltene to determine the magnitude of the asphaltene air reflectivity of different heavy oil reservoirs; in addition, extract the asphaltene in typical coal samples, and measure the asphaltene air reflectivity (%) and the vitrinite reflectivity (%) of the typical coal samples to establish a fitting relationship between the two; finally, obtain the asphaltene air reflectivity (%) of the heavy oil reservoir, and use the fitting relationship established by the asphaltene air reflectivity (%) and the vitrinite reflectivity (%) of the typical coal samples to infer the maturity of the heavy oil reservoir, providing technical support for the exploration and development of heavy oil reservoirs.

[0012] As a further improvement, the extraction of the asphaltene in steps (1) and (3) includes: subjecting the typical coal sample to n-hexane precipitation, leaching, extraction, and cleaning with petroleum ether to obtain the asphaltene.

[0013] As a further improvement, in step (2), establish the fitting relationship according to y = ax + b, where y is the vitrinite reflectivity of the typical coal sample, x is the asphaltene air reflectivity, and a and b are fitting parameters. Description of the Drawings

[0014] Figure 1 It is a flow block diagram of the calculation method for the maturity of the heavy oil reservoir of the present invention;

[0015] Figure 2 It is a schematic diagram of the asphaltene treatment process of the present invention;

[0016] Figure 3 It is a correlation diagram of the asphaltene air reflectivity and the vitrinite reflectivity Ro of the typical coal sample of the present invention. Detailed Embodiments

[0017] In the detection of the maturity of normal crude oil in the prior art, the chromatogram of saturated hydrocarbons and the technology of chromatography-mass spectrometry are usually used to represent the evolution degree of organic matter, and then the maturity of crude oil is judged. However, due to biodegradation in heavy oil, a large amount of saturated hydrocarbons in the crude oil are lost. Using the above methods to judge the maturity of crude oil will obviously lead to a weak credibility of the judgment result due to the degradation of organic matter. Vitrinite reflectance is currently the most reliable parameter for evaluating the maturity of hydrocarbon source rocks. As the main component of soluble hydrocarbons, asphaltene begins to be generated when organic matter evolves in the early stage, dissolves in hydrocarbons and migrates from the source rock to the reservoir together. It exists in large quantities in hydrocarbon source rocks and heavy oil. Because of its relatively stable properties, measuring the reflectance of asphaltene in heavy oil can reflect the evolution degree of soluble hydrocarbons in geological bodies and provide a basis for oil-source correlation. Therefore, carrying out the measurement of asphaltene reflectance has positive significance for evaluating the maturity of heavy oil reservoirs.

[0018] Starting from the experimental method, the present invention first performs asphaltene pretreatment and asphaltene purification on the heavy oil in the heavy oil reservoir, and then measures the air reflectance of the purified asphaltene to determine the magnitude of the air reflectance of asphaltene in different heavy oil reservoirs; in addition, asphaltene in typical coal samples is extracted, and the air reflectance (%) of asphaltene and the vitrinite reflectance (%) of typical coal samples are measured to establish a fitting relationship between the two; finally, the air reflectance (%) of asphaltene in the heavy oil reservoir is obtained, and the fitting relationship established by the air reflectance (%) of asphaltene in typical coal samples and the vitrinite reflectance (%) of typical coal samples is used to infer the maturity of the heavy oil reservoir, providing technical support for the exploration and development of heavy oil reservoirs.

[0019] The following further describes the present invention in combination with specific embodiments. It should be noted that, on the premise of no conflict, the following-described embodiments or technical features can be arbitrarily combined to form new embodiments. The equipment and raw materials used can be purchased from the market or are commonly used in the art. The methods in the following embodiments are conventional methods in the art unless otherwise specified.

[0020] In the following embodiments, unless otherwise specified, the experimental operations are conventional operations in the art.

[0021] In the following embodiments, unless otherwise specified, the raw materials used are all conventional commercial products in the art.

[0022] Specific Embodiment 1 of the Calculation Method for the Maturity of a Heavy Oil Reservoir of the Present Invention

[0023] In the embodiment of the present invention, the Baiyinchagan Sag in Inner Mongolia is taken as an example for illustration.

[0024] In this embodiment, the heavy oil in the heavy oil reservoir is first subjected to asphaltene pretreatment and asphaltene purification, and then the air reflectivity of the purified asphaltene is measured to determine the air reflectivity of asphaltenes in different heavy oil reservoirs. In addition, asphaltenes in typical coal samples are extracted, and the air reflectivity (%) of the asphaltenes and the vitrinite reflectivity (%) of the typical coal samples are measured to establish a fitting relationship between the two. Finally, the air reflectivity (%) of the asphaltenes in the heavy oil reservoir is obtained, and the maturity of the heavy oil reservoir is inferred using the fitting relationship established between the air reflectivity (%) of the asphaltenes in the typical coal samples and the vitrinite reflectivity (%) of the typical coal samples. The process is as shown in Figure 1 and the specific operation steps are as follows:

[0025] 1. Sources and physical property parameters of heavy oil samples

[0026] Two heavy oils from Well 1 (233 - 241.50 m) and Well 2 (239.40 m) in a certain heavy oil reservoir were selected as the objects of this study. The physical property characteristics of the heavy oil are shown in Table 1.

[0027] Table 1 Physical property parameters of heavy oil samples

[0028] Hash sign Well section, m Stratigraphic horizon <![CDATA[Specific gravity, g / cm 3 > Viscosity, mPa.s Pour point, °C Well 1 233-241.50 d 0.9661 1862.70 24 Well 2 239.40 d 0.926 186.47 3

[0029] 2. Asphaltene treatment of heavy oil samples

[0030] The process of asphaltene treatment of heavy oil samples is as shown in Figure 2 and the specific steps are as follows:

[0031] 2.1 Pretreatment of asphaltenes in heavy oil

[0032] (1) The heavy oil samples selected in Step 1 are dehydrated. The specific process refers to the petroleum and natural gas industry standard "Testing Method for the Performance of Crude Oil Demulsifiers (Cake Test Method)".

[0033] (2) According to the asphaltene content in the heavy oil group composition, a certain amount of treated heavy oil (generally, the asphaltene content is greater than 50 mg) is selected, and the asphaltene is precipitated with n - hexane. After standing overnight, it is leached with n - hexane to remove most of the saturated hydrocarbons, aromatics, and non - hydrocarbons, etc. The specific process refers to the Ministry of Geology and Mineral Resources of the People's Republic of China standard "Analysis Method for the Group Components of Crude Oil and Organic Extracts" issued on May 23, 1989.

[0034] (3) The leached sample is wrapped with filter paper and extracted with a rapid extractor. During the extraction process, the fluorescence of the filtrate is checked. The fluorescence of the bottom liquid of the rapid extraction should reach below level 1. The specific results of the n - hexane extraction time and fluorescence detection are shown in Table 2.

[0035] Table 2 Comparison table of n - hexane extraction time and fluorescence level

[0036]

[0037] From the purified asphaltene, the filtrate reaches fluorescence level below 2. The original heavy oil asphaltenes in Well 1 and Well 2 have become fragmented and are easily broken to form cross-sections, indicating that the content of some light hydrocarbons and resins with large molecular weights is very small, and the next process can be carried out.

[0038] 2.2. Purification of Asphaltene

[0039] Wrap the asphaltene obtained from the pretreatment in Step 2.1 with filter paper and place it in a 100 mL beaker. Add petroleum ether (analytical pure) with a boiling range of 60 - 90 °C and soak for 5 - 10 h, then place it in an ultrasonic cleaner for 2 - 3 h to check the fluorescence of the clear petroleum ether solution. If there is still fluorescence, replace the petroleum ether and wash repeatedly until the washing solution has no fluorescence. Finally, transfer the treated asphaltene to a petri dish after filtration (naturally air-dried or dried in an oven at 50 °C). The obtained asphaltene is in flakes, brittle, has an approximate luster, and has good reflectivity on the natural cross-section, suitable for reflectivity measurement.

[0040] 3. Determination and Data Processing of Asphaltene Air Reflectivity

[0041] 3.1. Measurement of Asphaltene Reflectivity

[0042] The instrument used is an MPV-sp microspectrophotometer, and the analysis conditions are: medium: air; wavelength: 546 nm; objective lens: 20x dry objective lens; temperature: 23 °C ± 3 °C environment; darkroom.

[0043] Using the spreading method: Shatter the fragmented asphaltene to form a natural cross-section, sprinkle it on the glass slide, and then calibrate it using the MPV-sp microspectrophotometer. When measuring the asphaltene air reflectivity, select the brightest particle on the cross-section of the asphaltene under the microscope for measurement.

[0044] 3.2. Result Analysis

[0045] Since the sample preparation for asphaltene air reflectivity uses the spreading method, the angles of the natural cross-sections of the asphaltene fragments are different. Only the cross-section perpendicular to the optical axis can have its measured value represent the true value of the asphaltene air reflectivity. Therefore, when processing the results, use the maximum value to represent the asphaltene air reflectivity, and its analysis results are shown in Table 3.

[0046] Table 3 Measurement Results of Heavy Oil Asphaltene Air Reflectivity

[0047]

[0048] As can be seen from Table 3, the repeatability of the measurement of the asphaltene air reflectivity of heavy oil is good, and the measured results are reliable.

[0049] 4. Establishment of the Intrinsic Relationship between the Air Reflectivity of Asphaltene (%) and the Vitrinite Reflectance Ro of Typical Coal Samples

[0050] (1) Select coal samples containing both asphaltene and vitrinite as typical samples. According to the operations in step 2 above, prepare the asphaltene of the typical coal samples and measure the air reflectivity of asphaltene. The specific test results are shown in Table 4.

[0051] (2) Measure the vitrinite reflectance (%) of the typical coal samples. The specific test results are shown in Table 4.

[0052] Table 4 Comparison Table of Air Reflectivity of Asphaltene and Vitrinite Reflectance of Typical Coal Samples

[0053] Sample Lithology Air reflectance of asphaltene (%) Ro reflectance of vitrinite in rock sample (%) 1 Coal 5.26 0.40 2 Coal 5.46 0.44 3 Coal 5.48 0.46 4 Coal 5.69 0.52 5 Coal 7.9 0.85

[0054] (3) Conduct fitting analysis on the data in Table 4 to establish the intrinsic relationship between the air reflectivity of asphaltene (%) and the vitrinite reflectance Ro of typical coal samples, and establish the fitting relationship formula between the air reflectivity of asphaltene (%) and the vitrinite reflectance Ro of typical coal samples. See the specific content in Figure 3 as shown.

[0055] The fitting relationship formula between the air reflectivity of asphaltene (%) and the vitrinite reflectance Ro of typical coal samples is y = 0.165x - 0.4491, R 2 = 0.989, where y is the vitrinite reflectance of typical coal samples and x is the air reflectivity of asphaltene.

[0056] 5. Identification of the Maturity of Heavy Oil

[0057] Using the fitting relationship formula y = 0.165x - 0.4491, R 2 = 0.989 established in step 4 for the air reflectivity of asphaltene (%) and the vitrinite reflectance Ro of typical coal samples, and based on the test results of the asphaltene reflectivity in two heavy oil samples from Well 1 (233 - 241.50 m) and Well 2 (239.40 m) obtained in step 3.2, the air reflectivity of asphaltene in Well 1 of heavy oil is 7.118 (%), and the air reflectivity of asphaltene in Well 2 is 7.414 (%). Since the test methods are the same and asphaltene only exists in different samples, its response to thermal evolution is the same. Under the same thermal evolution conditions, the value of the air reflectivity of asphaltene (%) in coal samples is the same as that in heavy oil. According to the equivalent principle, that is, when oil migrates out of the source rock, the maturity of its rock sample is equal to the maturity of the crude oil, that is, the vitrinite reflectance of the rock sample is consistent with the maturity of the crude oil. Thus, it is determined that the equivalent vitrinite reflectance of the crude oil in Well 1 of heavy oil is 0.73%, and that in Well 2 is 0.77%. The specific results are shown in Table 5.

[0058] Table 5 Comparison Table of Reflectivity of Heavy Oil Asphaltene and Equivalent Vitrinite Reflectivity

[0059] Crude oil type Well number Air reflectance of asphaltene (%) Equivalent vitrinite reflectance (%) Heavy oil Well 1 7.118 0.73 Heavy oil Well 2 7.414 0.77

[0060] According to the industry standard that for the thermal evolution degree of source rocks, Ro less than 0.7% is immature, Ro between 0.7 - 1.3% is mature, Ro between 1.3 - 2.0% is highly mature, and Ro greater than 2.0% is over - mature. It can be seen from Table 5 that the equivalent maturity Ro of the heavy oil in this oilfield is 0.73% and 0.77% respectively, indicating that the maturity of the heavy oil in this oilfield is not high, but it has the characteristics of relatively mature crude oil, ruling out the possibility that the heavy oil in this area comes from the shallow Du 1 section. Thus, it is speculated that the heavy oil in this area comes from the relatively highly mature sag zone, mainly from the Tengge'er Formation. The heavy oil is mainly transported from the deep - layer Tengge'er Formation to the shallow Du 1 section through fault communication. After being transported to the shallow layer, it forms heavy oil through biodegradation. Therefore, it is speculated that the near - sag zone of this oilfield has great exploration potential. The exploration in recent years has also confirmed this understanding. The oilfield has obtained nearly ten million tons of reserve - increasing positions in the Tengge'er Formation of the near - sag zone, effectively supporting the exploration and development of this oilfield.

[0061] As mentioned above, it is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the embodiments of the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for calculating the maturity of heavy oil reservoirs, characterized in that: It includes the following steps: (1) Collect typical coal samples and extract the asphaltenes therein, and analyze to obtain the air reflectivity of asphaltenes and the vitrinite reflectance Ro of the typical coal samples; (2) Establish a fitting relationship between the air reflectivity of asphaltenes in the typical coal samples and the vitrinite reflectance Ro of the typical coal samples; (3) Equivalent the fitting relationship established in step (2) to the corresponding relationship between the air reflectivity of asphaltenes in the heavy oil reservoir and the equivalent maturity of the heavy oil reservoir; Extract the asphaltenes in the heavy oil reservoir in the target area and analyze to obtain their air reflectivity of asphaltenes, and calculate the maturity of the heavy oil reservoir in the target area in combination with the said corresponding relationship.

2. The method for calculating the maturity of a heavy oil reservoir according to claim 1, characterized in that: The extraction of the asphaltenes in steps (1) and (3) includes: subjecting the typical coal samples to n-hexane precipitation, leaching, extraction, and petroleum ether washing to obtain asphaltenes.

3. The method for calculating the maturity of a heavy oil reservoir according to claim 1 or 2, characterized in that: In step (2), the fitting relationship is established as y = ax + b, where y is the vitrinite reflectance of the typical coal sample, x is the air reflectivity of asphaltenes, and a and b are fitting parameters.

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