A method of determining original porosity of a shale oil reservoir

Abstract

The present application relates to unconventional shale reservoir characterization technical field, disclose a kind of method for determining shale oil reservoir primary porosity, including the following steps shale oil-bearing porosity determination, i.e. according to helium method, nuclear magnetic method or mercury injection method etc. means to carry out porosity test to fresh rock sample, obtain the porosity of shale sample when containing oil;Different hydrocarbon components in shale sample account for porosity, i.e. obtain the porosity of light oil component (S 1‑1 ), middle oil component (S 1‑2 ) and heavy oil component (S 2‑1 ) respectively account for;Quantify shale primary porosity, i.e. combine the porosity of shale sample when containing oil, the porosity of light oil component, the porosity of middle oil component and the porosity of heavy oil component, quantitatively calculate the primary porosity of shale oil reservoir when not containing oil.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of unconventional shale reservoir characterization, and particularly relates to a method for determining the original porosity of a shale oil reservoir. BACKGROUND

[0002] Shale oil resources are abundant and are an important field for realizing energy replacement in China. Shale oil components are complex and include light oil components (C5-C12), medium oil components (C12-C22), and heavy oil components (C22 and above), which are present in different phases in intergranular pores, intragranular pores, and microfractures and other types of reservoir spaces in shale reservoirs. The degree of pore development directly determines the oil content that can be accommodated by shale. Therefore, accurately evaluating shale oil reservoir porosity parameters has important practical significance for shale oil resource potential evaluation and sweet spot layer selection.

[0003] Because oil molecules are significantly heavier than gas molecules and have poor mobility, the shale oil reservoir porosities tested today are all porosities under oil-containing conditions, and cannot reveal the original porosity characteristics of shale oil reservoirs under the condition of no oil molecule saturation. Based on this, some scholars propose to conduct oil washing experiments on shale samples and then test the porosity, but in the oil washing process, there are often problems such as long oil washing time, great difficulty, and some damage to the pore structure, and the application effect and practicality are poor. Some scholars also convert the weight of oil components obtained by conventional rock pyrolysis into volume and porosity by combining density, and then calculate the original porosity of shale.

[0004] For example, patent CN111487176A converts the free hydrocarbons S1 (in mg / g rock) obtained at 300℃ into volume content (i.e., into cm3 / g rock), quantifying the porosity occupied by oil components in unit mass of shale sample. However, this method does not consider heavy oil components, and the free hydrocarbons represented by S1 are composed of different proportions of light and medium oil components. When converted into volume content, the results obtained often have large errors due to significant differences in density, and the evaluated original porosity is significantly lower.

[0005] The patent CN117571582A further obtains the difference between the pyrolysis S2 (the pyrolysis hydrocarbon content at 650 DEG C, unit: mg / g rock) before and after the oil washing, considers the influence of the heavy oil component on the porosity, but still faces the problems that the oil component represented by S1 cannot be distinguished, the heavy oil component is not completely considered due to the incomplete oil washing, and the accuracy of the calculated primary porosity is limited compared with the calculation result of the patent CN111487176A. Based on this, the present application adopts the step-by-step pyrolysis method, effectively distinguishes the light oil component, the medium oil component and the heavy oil component in the shale reservoir by setting a fine temperature interval, calculates the porosity occupied by the light oil component, the medium oil component and the heavy oil component in combination with the measured density of different oil components, and further calculates the primary porosity of the shale oil reservoir when it does not contain oil in combination with the measured oil-containing porosity.

[0006] Therefore, the present application is provided. SUMMARY

[0007] To solve the above technical problems, the basic idea of the technical solution of the present application is:

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

[0009] Step S1: First, divide the collected fresh core sample into two parts, wherein the first part of the sample is prepared into a plunger sample with a diameter of about 2.5 cm and a height of about 5 cm, and the porosity of the shale sample when containing oil is obtained by testing the porosity of the sample;

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

[0011] Step S3: At the same time, the densities of the collected light oil component, medium oil component and heavy oil component are tested to obtain the density values of different oil components, and the oil components represented by weight are converted into volume representation (i.e. cm3 / g rock). On this basis, the porosities occupied by different oil components (i.e. the porosity Φ1-1 of S1-1, the porosity Φ1-2 of S1-2 and the porosity Φ2-1 of S2-1) are calculated in combination with the volume content of the shale sample per unit mass;

[0012] Step S4: Finally, the primary porosity of the shale oil reservoir is quantitatively calculated in combination with the measured oil-containing porosity.

[0013] As a preferred embodiment of the present application, 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 application has the following beneficial effects:

[0015] The present application compares the calculation results with the evaluation results of other established methods to verify the superiority and rationality of the present application. Figure 3 As shown in A, patent CN111487176A only considers light and medium oil components, ignoring the influence of heavy components on porosity, resulting in a calculation result of primary porosity that is about 21% lower than the calculation result of the method proposed in the present application; although patent CN117571582A considers heavy oil components, it still faces problems such as the inability to distinguish between oil components in S1 characterization, incomplete washing of oil, and incomplete consideration of heavy oil components, so although the primary porosity calculated by this method has improved accuracy, it is still about 14% lower than the calculation result of the method proposed in the present application Figure 3 B). In addition, Figure 4 It is shown that the primary porosity calculated based on the present application Figure 4 C) has the optimal fitting degree with shale oil content compared with the calculation results of patent CN111487176A Figure 4 A) and patent CN117571582A Figure 4 B), which further verifies the rationality and accuracy of the present application.

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

[0017] In the drawings:

[0018] Figure 1 Flowchart for determining the porosity of each oil component and determining the primary porosity of a shale oil reservoir;

[0019] Figure 2 Shale porosity and step-by-step pyrolysis test results chart;

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

[0021] Figure 4 Fitting relationship chart of primary porosities calculated by different methods and shale oil content. DETAILED DESCRIPTION

[0022] To make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments will be described clearly and completely below with reference to the drawings in the embodiments of the present application, and the following embodiments are used to illustrate the present application.

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

[0024] (1) Determination of shale oil-bearing porosity, i.e. porosity test of fresh rock sample according to helium method, nuclear magnetic method or mercury injection method, etc. to obtain the porosity of the shale sample when containing oil; (2) Obtain the porosity occupied by different hydrocarbon components in the shale sample, i.e. obtain the porosity occupied by light oil component (S 1-1 ), medium oil component (S 1-2 ) and heavy oil component (S 2-1 ); (3) Quantify the original porosity of shale, i.e. combine the porosity of the shale sample when containing oil, the porosity occupied by light oil component, the porosity occupied by medium oil component and the porosity occupied by heavy oil component to quantitatively calculate the original porosity of the shale oil reservoir when not containing oil.

[0025] Step S1, shale oil-bearing porosity:

[0026] This part mainly carries out porosity test of fresh rock sample by helium method, nuclear magnetic method or mercury injection method, etc. Considering that helium method has the advantages of larger detection range and higher accuracy compared with other test methods, therefore helium method is adopted when determining porosity in the present application.

[0027] Before the experiment, the collected shale core sample is prepared into a plunger sample with a diameter of about 2.5 cm and a height of about 5 cm. Then, the prepared columnar core sample is placed in a drying box with a temperature of 60℃ for drying for more than 24h. After the shale sample is fully dried, the sample is taken out and placed in a KX-90F type pressure-over-porosity measuring instrument, and helium with a purity of 99.99% is selected as the working medium to carry out porosity (i.e. Φ 含油 ) and density (i.e. ρ 页岩 ) test of the shale sample.

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

[0029] This part can be mainly divided into 4 parts: (1) Obtain the weight of light oil component (S 1-1 ), medium oil component (S 1-2 ) and heavy oil component (S 2-1 ) in unit mass of rock sample according to step-by-step pyrolysis method; (2) Carry out density test of each oil component collected to obtain the density of different oil components; (3) Based on the weight and density of each oil component, convert it into the volume content of each oil component in unit mass of rock sample; (4) Combine the volume content of unit mass of shale sample to calculate the porosity occupied by each oil component.

[0030] The step pyrolysis experiment selects a fresh rock sample, which is crushed to about 100 mesh, and then 50 mg of the powder rock sample is quickly placed into a Rock-Eval 6 pyrolysis instrument. The instrument is gradually heated at a heating rate of 25 ℃ / min, and when heated to 200 ℃, it is kept constant for 1 min to detect the product, and the light oil component content (i.e., parameter S 1-1 , unit: mg / g rock, meaning the weight of the light oil component in unit mass of the shale sample) and its corresponding density (i.e., p 1-1 ) are obtained; continue to heat at a heating rate of 25 ℃ / min to 350 ℃, and then keep constant for 1 min to detect the product, and obtain the medium oil component content (i.e., parameter S 1-2 , unit: mg / g rock, meaning the weight of the medium oil component in unit mass of the shale sample) and its corresponding density (i.e., p 1-2 ); after the temperature reaches 350 ℃, it is heated to 450 ℃ at the same heating rate and kept constant for 1 min to obtain the heavy oil component (i.e., parameter S 2-1 , unit: mg / g rock, meaning the weight of the heavy oil component in unit mass of the shale sample) and its corresponding density (i.e., p 2-1 ). Further, based on the oil component mass and its density parameters, each oil component is converted to a volume content according to Formulas 1-3.

[0031] (1)

[0032] (2)

[0033] (3)

[0034] In the formula, represents the weight content of the light oil component in unit mass of the shale sample, mg / g rock; represents the weight content of the medium oil component in unit mass of the shale sample, mg / g rock; represents the weight content of the heavy oil component in unit mass of the shale sample, mg / g rock; represents the density of the light oil component, g / cm 3 ; represents the density of the medium oil component, g / cm 3 ; represents the density of the heavy oil component, g / cm 3 ; represents the volume content of the light oil component in unit mass of the shale sample, cm 3 / g rock; represents the volume content of the medium oil component in unit mass of the shale sample, cm 3 / g rock; represents the volume content of the heavy oil component in unit mass of the shale sample, cm3 / g rock.

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

[0036] (4)

[0037] (5)

[0038] (6)

[0039] (7)

[0040] In the formula, represents the shale density determined in 6.1 g / cm 3 ; represents the volume of unit mass shale sample, cm 3 / g rock; represents the porosity occupied by light oil component, %; represents the porosity occupied by medium oil component, %; represents the porosity occupied by heavy oil component, %.

[0041] Step S3, shale oil-free primary porosity:

[0042] Based on the measured Φoil in 6.1, and ΦS1-1, ΦS1-2 and ΦS2 in 6.2, , and , the sum of the four is calculated to quantify the primary porosity of the shale sample without oil. In this setting,

[0043] The implementation principle of one method for determining the primary porosity of shale oil reservoirs in this embodiment is as follows: the present application has been successfully applied in the shale of Shahejie Formation in Bohai Bay Basin. The helium porosity test results show that the porosity of the shale containing oil ranges from 0.31% to 7.99%, with an average value of 3.01% (A). At the same time, based on the measured shale density value (the measured value of the present application is 2.64 g / cm3), the rock volume of unit mass (1 g) shale sample is calculated to be 0.38 cm3. On the other hand, the stepwise pyrolysis experiment reveals that the light oil component S1-1 of the shale sample is 0.03-1.38 mg / g rock (with an average value of 0.35 mg / g rock, Figure 2 B), S1-2 is 0.29-10.27 mg / g rock (with an average value of 3.43 mg / g rock, Figure 2 C) and S2 is 0.01-0.48 mg / g rock (with an average value of 0.15 mg / g rock, Figure 2C) S2-1 is 0.37-23.07 mg / g rock (average 6.29 mg / g rock, Figure 2 D) Based on the measured density of each oil component, it is converted into volume content by formula 1-3, on this basis, combined with the rock volume of unit mass (1g) shale sample, the total porosity occupied by all oil components is calculated by formula 5-7, which is 0.20%-8.11%, and the average is 2.61%( Figure 2 E) Based on this, the primary porosity of shale without oil is finally calculated to be 0.63%-15.65%, and the average is 5.62%( Figure 2 F) In addition, the calculation results of the present application are compared with the evaluation results of other established methods to verify the superiority and rationality of the present application. For example Figure 3 A) As shown in the patent CN111487176A, only considering light and medium oil components, the influence of heavy oil components on porosity is ignored, resulting in the primary porosity calculation result being about 21% lower than the calculation result of the method proposed in the present application; Although the patent CN117571582A considers heavy oil components, it still faces the problem that the oil components represented by S1 cannot be distinguished, and the heavy oil components are not completely considered due to incomplete washing, so although the primary porosity calculated by this method has improved accuracy, it is still about 14% lower than the calculation result of the method proposed in the present application( Figure 3 B) In addition, Figure 4 It is shown that the primary porosity calculated based on the present application ( Figure 4 C) has the optimal fitting degree with the shale oil content compared with the calculation results of the patent CN111487176A ( Figure 4 A) and the patent CN117571582A ( Figure 4 B), which further verifies the rationality and accuracy of the present application.

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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