Method for determining shale oil reservoir native porosity

Abstract

The invention relates to the technical field of characterization of unconventional shale reservoirs, and discloses a method for determining the primary porosity of a shale oil reservoir, which comprises the following steps of: measuring the oil-containing porosity of shale, namely testing the porosity of a fresh rock sample according to means such as a helium method, a nuclear magnetic method or a mercury intrusion method to obtain the porosity of the shale sample when the shale sample contains oil; acquiring the porosity of different hydrocarbon components in the shale sample, namely acquiring the porosity of a light oil component (S1-1), the porosity of a medium oil component (S1-2) and the porosity of a heavy oil component (S2-1); the shale primary porosity is quantified, that is, the primary porosity of the shale oil reservoir is quantitatively calculated by combining the porosity of the shale sample when the shale sample contains oil, the porosity of the light oil component, the porosity of the medium oil component and the porosity of the heavy oil component when the shale sample does not contain oil.

Description

Technical Field

[0001] The present invention belongs to the technical field of unconventional shale reservoir characterization, and in particular relates to a method for determining the primary porosity of a shale oil reservoir. Background Art

[0002] Shale oil resources are abundant and represent a crucial area for my country's energy transition. Shale oil has a complex composition, including light oil (C5-C12), medium oil (C12-C22), and heavy oil (C22 and above). Each component exists in different phases within shale reservoirs, including intergranular pores, intragranular pores, and microfractures. The degree of pore development directly determines the oil content that the shale can accommodate. Therefore, accurately evaluating the porosity parameters of shale oil reservoirs is of great practical significance for evaluating shale oil resource potential and selecting sweet spot intervals.

[0003] Because oil molecules are significantly heavier and less mobile than gas molecules, current porosity measurements of shale oil reservoirs are based on oil-containing conditions and fail to reveal the primary porosity characteristics of shale oil reservoirs in the absence of oil molecule saturation. Based on this, some researchers have proposed conducting oil-washing experiments on shale samples before conducting porosity tests. However, these processes often face challenges such as long washing times, difficulty, and some damage to the pore structure, resulting in poor application effectiveness and practicality. Other researchers have calculated the primary porosity of shale by deriving the weight of the oil component through conventional rock pyrolysis and converting this weight into volume and porosity based on density.

[0004] For example, patent CN111487176A converts free hydrocarbons S1 (measured in mg / g rock) obtained at 300°C into volume fraction (i.e., cm3 / g rock), quantifying the porosity of oil components per unit mass of shale samples. However, this method does not account for heavy oil components, and the free hydrocarbons represented by S1 are composed of light and medium oil components in varying proportions. When converted to volume fraction, significant density differences often lead to large errors in the obtained results, resulting in significantly lower estimated primary porosity.

[0005] Patent CN117571582A further obtains the difference in pyrolysis S2 (pyrolysis hydrocarbon content obtained at 650°C, in mg / g rock) before and after oil washing on this basis, and takes into account the influence of heavy oil components on porosity. However, it still faces the problems of being unable to distinguish the oil components represented by S1 and incomplete consideration of heavy oil components due to incomplete oil washing. The represented primary porosity has limited improvement in accuracy compared to the calculation results of patent CN111487176A. Based on this, the present invention adopts a step-by-step pyrolysis method to effectively distinguish the light oil, medium oil and heavy oil components in the shale reservoir by setting a fine temperature range, and calculates the porosity occupied by light oil, medium oil and heavy oil components in combination with the measured density of different oil components. On this basis, it further combines the measured oil-bearing porosity to quantify the primary porosity of the shale oil reservoir when it does not contain oil.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] In order to solve the above technical problems, the basic concept of the technical solution adopted by the present invention is:

[0008] A method for determining the primary porosity of a shale oil reservoir comprises the following steps:

[0009] Step S1: First, the collected fresh core sample is divided into two parts. The first part of the sample is prepared into a plug sample with a diameter of about 2.5 cm and a height of about 5 cm, and the plug sample is subjected to a porosity test to obtain the porosity of the shale sample when it contains oil;

[0010] Step S2: The second portion of the sample is hermetically crushed to approximately 100 mesh, and then the crushed sample is placed in a Rock-Eval 6 pyrolyzer. The instrument is gradually heated, and the pyrolysis products at the corresponding temperatures of 200°C, 350°C, and 450°C are measured to obtain the contents of the light oil component (i.e., S1-1, in mg / g rock), the medium oil component (i.e., S1-2, in mg / g rock), and the heavy oil component (i.e., S2-1, in mg / g rock), respectively;

[0011] Step S3: Simultaneously, density tests are performed on the collected light, medium, and heavy oil components to obtain the density values of the different oil components. The weight representation of the oil components is then converted to volume representation (i.e., cm3 / g rock). Based on this, the porosity occupied by the different oil components is calculated (i.e., the porosity Φ1-1 of S1-1, the porosity Φ1-2 of S1-2, and the porosity Φ2-1 of S2-1) by combining the volume content of the shale sample per unit mass.

[0012] Step S4: Finally, the measured oil-bearing porosity is combined to quantitatively calculate the primary porosity of the shale oil reservoir.

[0013] As a preferred embodiment of the present invention, the porosity in step S1 includes any one of helium porosity, nuclear magnetic porosity and mercury intrusion porosity.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The present invention compares the calculation results with the evaluation results of other established methods to verify the superiority and rationality of the present invention. Figure 3 As shown in Figure A, the original porosity calculated by CN111487176A is about 21% lower than that calculated by the method proposed in the present invention because only light and medium oil components are considered and the influence of heavy components on porosity is ignored. Although the heavy oil components are considered in CN117571582A, it still faces the problems of being unable to distinguish the oil components characterized by S1 and incomplete consideration of heavy oil components due to incomplete oil washing. Therefore, although the accuracy of the original porosity calculated by this method is improved, it is still about 14% lower than that calculated by the method proposed in the present invention. Figure 3 B) In addition, Figure 4 It is shown that the primary porosity calculated based on the present invention ( Figure 4 C) Compared with patent CN111487176A ( Figure 4 A) and patent CN117571582A( Figure 4 B) As for the calculation results, there is an optimal degree of fit with the shale oil content, which further verifies the rationality and accuracy of the present invention.

[0016] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the attached figure:

[0018] Figure 1 Flowchart for determining the porosity occupied by each oil component and determining the primary porosity of shale oil reservoirs;

[0019] Figure 2 This is a diagram of shale porosity and its step-by-step pyrolysis test results;

[0020] Figure 3 Comparison of shale primary porosity calculated by different methods;

[0021] Figure 4 This is a fitting relationship diagram between primary porosity calculated by different methods and shale oil content. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention.

[0023] A method for determining the primary porosity of a shale oil reservoir comprises the following steps:

[0024] (1) Determination of shale oil porosity, i.e., porosity testing of fresh rock samples by helium method, nuclear magnetic method or mercury intrusion method, etc., to obtain the porosity of shale samples when they contain oil; (2) Obtaining the porosity of different hydrocarbon components in shale samples, i.e., obtaining the porosity of light oil components (S 1-1 ), medium oil components (S 1-2 ) and heavy oil components (S 2-1 ) The porosity occupied by each of the three components; (3) quantify the primary porosity of shale, that is, combine the porosity of the shale sample when it contains oil, the porosity occupied by the light oil component, the porosity occupied by the medium oil component and the porosity occupied by the heavy oil component to quantitatively calculate the primary porosity of the shale oil reservoir when it does not contain oil.

[0025] Step S1, shale oil porosity:

[0026] This section mainly discusses porosity testing of fresh rock samples using methods such as helium, nuclear magnetic resonance, or mercury intrusion. Considering the advantages of the helium method over other testing methods, such as a wider detection range and higher accuracy, the present invention adopts the helium method for porosity determination.

[0027] Before the experiment, the collected shale core samples were prepared into plugs with a diameter of about 2.5 cm and a height of about 5 cm. Then, the prepared columnar core samples were placed in a drying oven at a temperature of 60°C and dried for more than 24 hours. After the shale samples were fully dried, they were taken out and placed in a KX-90F overburden porosity meter. Helium with a purity of 99.99% was selected as the working medium to carry out the porosity (i.e., Φ 含油 ) and density (i.e. ρ 页岩 )test.

[0028] Step S2, porosity occupied by different oil components:

[0029] This part can be divided into four parts: (1) Obtaining the light oil components (S) in the unit mass of rock samples according to the step-by-step pyrolysis method 1-1 ), medium oil components (S 1-2 ) and heavy oil components (S 2-1 ) their respective weights; (2) conducting density tests on the collected oil components to obtain the densities of different oil components; (3) based on the weight and density of each oil component, converting it into the volume content of each oil component per unit mass of rock sample; (4) combining the volume content of unit mass of shale sample to calculate the porosity occupied by each oil component.

[0030] In the step-by-step pyrolysis experiment, fresh rock samples were sealed and crushed to about 100 mesh. Then 50 mg of powdered rock samples were weighed and quickly placed in the Rock-Eval 6 pyrolyzer. The instrument was gradually heated at a heating rate of 25 °C / min. When heated to 200 °C, the temperature was kept constant for 1 min and the product was tested to obtain the light oil component content (i.e. parameter S 1-1 , the unit is mg / g rock, which means the weight of light oil components per unit mass of shale sample) and its corresponding density (i.e. ρ 1-1 ); Continue heating at a rate of 25°C / min to 350°C, then keep the temperature constant for 1min to detect the product and obtain the content of the medium oil component (i.e., parameter S 1-2 , the unit is mg / g rock, which means the weight of the medium oil component in the unit mass of shale sample) and its corresponding density (i.e. ρ 1-2 ); After the temperature reaches 350 ° C, it is heated to 450 ° C at the same heating rate and kept at this temperature for 1 min to obtain the heavy oil component (i.e., parameter S 2-1 , the unit is mg / g rock, which means the weight of heavy oil components per unit mass of shale sample) and its corresponding density (i.e. ρ 2-1 ). Further, based on the mass of the oil component and its density parameter, each oil component is converted into volume content according to Formula 1-Formula 3.

[0031] (1)

[0032] (2)

[0033] (3)

[0034] Where, Indicates the weight content of light oil components in unit mass of shale sample, mg / g rock; Indicates the weight content of medium oil components in unit mass of shale sample, mg / g rock; Expressed as single, mg / g rock; Indicates the density of light oil components, g / cm 3 ; Indicates the density of the medium oil component, g / cm 3 ; Indicates the density of heavy oil components, g / cm 3 ; Indicates the volume content of light oil components in unit mass of shale sample, cm 3 / g rock; Indicates the volume content of medium oil components in unit mass of shale sample, cm 3 / g rock; Indicates the volume content of heavy oil components per unit mass of shale sample, cm3 / g rock.

[0035] On the other hand, based on the shale density measured in 6.1, the volume of the shale sample per unit mass can be calculated using Equation 4. On this basis, by further combining the volume content of the different oil components, the porosity occupied by each oil component can be quantified (Equations 5-7).

[0036] (4)

[0037] (5)

[0038] (6)

[0039] (7)

[0040] Where, Indicates the shale density measured in 6.1 g / cm 3 ; Indicates the volume of shale sample per unit mass, cm 3 / g rock; Indicates the porosity occupied by light oil components, %; Indicates the porosity occupied by the medium oil component, %; Indicates the porosity occupied by heavy oil components, %.

[0041] Step S3, primary porosity of shale without oil:

[0042] Based on the Φ oil content determined in 6.1 and the 、 and , calculate the sum of the four to quantify the primary pore size of the shale sample before oil is contained. In this setting,

[0043] The implementation principle of the method for determining the primary porosity of shale oil reservoirs in this embodiment is as follows: The present invention has been successfully applied to the Shahejie Formation shale in the Bohai Bay Basin. The helium porosity test results show that the porosity distribution range of shale oil is 0.31%-7.99%, with an average value of 3.01% ( Figure 2 A). At the same time, based on the measured shale density value (the measured value of the present invention is 2.64g / cm3), the rock volume of the unit mass (1g) of the shale sample is calculated to be 0.38cm3. On the other hand, the step-by-step pyrolysis experiment revealed that the light oil component S1-1 of the shale sample is 0.03-1.38mg / g rock (the average is 0.35mg / g rock, Figure 2 B), S1-2 is 0.29-10.27 mg / g rock (the average is 3.43 mg / g rock, Figure 2C), S2-1 is 0.37-23.07 mg / g rock (the average is 6.29 mg / g rock, Figure 2 D). Based on the measured density of each oil component, it is converted into volume content through formula 1-formula 3. On this basis, combined with the unit mass (1g) of shale sample rock volume, using formula 5-formula 7, the total porosity of all oil components is calculated to be 0.20%-8.11%, with an average of 2.61% ( Figure 2 E). Based on this, the final calculation shows that the primary porosity of shale before oil is 0.63%-15.65%, with an average of 5.62% ( Figure 2 F). In addition, the present invention compares the calculation results with the evaluation results of other established methods to verify the superiority and rationality of the present invention. Figure 3 As shown in Figure A, the original porosity calculated by CN111487176A is about 21% lower than that calculated by the method proposed in the present invention because only light and medium oil components are considered and the influence of heavy components on porosity is ignored. Although the heavy oil components are considered in CN117571582A, it still faces the problems of being unable to distinguish the oil components characterized by S1 and incomplete consideration of heavy oil components due to incomplete oil washing. Therefore, although the accuracy of the original porosity calculated by this method is improved, it is still about 14% lower than that calculated by the method proposed in the present invention. Figure 3 B) In addition, Figure 4 It is shown that the primary porosity calculated based on the present invention ( Figure 4 C) Compared with patent CN111487176A ( Figure 4 A) and patent CN117571582A( Figure 4 B) As for the calculation results, there is an optimal degree of fit with the shale oil content, which further verifies the rationality and accuracy of the present invention.

Claims

1. A method for determining the primary porosity of a shale oil reservoir, characterized in that: The steps include: Step S1: First, the collected fresh core sample is divided into two parts. The first part of the sample is prepared into a plug sample with a diameter of about 2.5 cm and a height of about 5 cm, and the plug sample is subjected to a porosity test to obtain the porosity of the shale sample when it contains oil; Step S2: The second portion of the sample was hermetically crushed to approximately 100 mesh. The crushed sample was then placed in a Rock-Eval 6 pyrolyzer, and the temperature of the instrument was gradually increased. The pyrolysis products at the corresponding temperatures of 200°C, 350°C, and 450°C were measured to obtain the contents of the light oil component, medium oil component, and heavy oil component, respectively. Step S3: Simultaneously, density tests are performed on the collected light oil component, medium oil component, and heavy oil component to obtain density values for the different oil components. The oil components expressed in weight are converted to volume expressions. Based on this, the porosity occupied by the different oil components is calculated by combining the volume content of the shale sample per unit mass. Step S4: Finally, the measured oil-bearing porosity is combined to quantitatively calculate the primary porosity of the shale oil reservoir.

2. The method for determining the primary porosity of a shale oil reservoir according to claim 1, wherein: The porosity in step S1 includes any one of helium porosity, nuclear magnetic porosity and mercury intrusion porosity.

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