A method for determining shale porosity using rock density and fluid mass difference

Abstract

This invention relates to the field of geological exploration and discloses a method for determining shale porosity using the difference in rock density and fluid mass. First, the apparent density is calculated by measuring the mass and apparent volume of a blocky rock sample. Then, the rock sample is crushed, extracted, and dried, the fluid mass loss is measured, and the skeleton density of the dried powder is measured. Finally, the volume difference per unit mass of rock is estimated using the reciprocal difference between the apparent density and the skeleton density, and then corrected for fluid loss mass to obtain the total porosity of the rock sample. This invention calculates porosity by using the density and fluid mass differences of rock samples with different particle sizes, and is not limited by the size of the rock sample or the fluid it contains. It can measure closed pores in the rock, and by using the fluid loss mass during crushing, washing / extraction, and drying processes for correction, a more accurate total shale porosity can be obtained.

Description

Technical Field

[0001] This invention relates to the field of geological exploration, and more particularly to a method for determining shale porosity using the difference between rock density and fluid mass. Background Technology

[0002] Rock pores are the spaces where hydrocarbon-containing fluids reside, and can be divided into open pores and closed pores. Currently, there are many methods for determining rock pore volume, commonly used ones include mercury intrusion porosimetry, helium porosimetry, gas adsorption, nuclear magnetic resonance (NMR), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAX). However, methods that inject fluids such as mercury, helium, and nitrogen into the rock sample can only measure the volume of open pores. Furthermore, methods using NMR pulses, neutron beams, or X-rays cannot capture the vacancy signals of fluids that have escaped during drilling. In addition, porosimetry methods that combine fluid injection with X-ray detection, such as saturated fluid SSX and saturated fluid NMR, also struggle to fully inject fluid into dense shale samples, especially nanoscale open and closed pores.

[0003] Unlike conventional sandstone, organic-rich shale is characterized by nanoscale pores, extremely complex pore structures, and a large number of closed pores. Measuring these closed pores is a prerequisite for accurately obtaining the total porosity of shale. The GRI (Geometric Intensive Research) method for shale porosity, developed by the Gas Research Institute (GRI), involves pulverizing rock samples down to a minimum of 200 mesh to maximize the opening of these closed pores. It is important to note that the apparent volume of the pulverized sample cannot be directly measured using the GRI method. Instead, it is calculated by assuming the density of the rock plunger and the powder sample are consistent, and then using the mass ratio of the plunger to the powder sample. However, during the rock pulverization process, the closed pores are gradually opened, and the density of the rock sample gradually increases. This leads to a significant deviation in the GRI method's calculation of the total volume of the powder sample. Summary of the Invention

[0004] The purpose of this invention is to propose a method for determining shale porosity using the difference between rock density and fluid mass, thereby solving the problem that existing methods for determining shale porosity using the difference between rock density and fluid mass are not accurate.

[0005] Specifically, the present invention provides a method for determining shale porosity using the difference between rock density and fluid mass, comprising the following steps:

[0006] S1. Obtain the mass m of the blocky rock sample to be tested. i ;

[0007] S2. Measure the apparent volume v of the rock sample. i ;

[0008] S3. Calculate the apparent density ρ based on the mass and apparent volume of the rock sample. i ;

[0009] S4. Dry the rock sample to obtain the pretreated rock sample;

[0010] S5. Measure the mass m of the pretreated rock sample. d1 ;

[0011] S6. Calculate the fluid loss per unit mass of the blocky rock, m, based on the mass of the rock sample and the mass of the pretreated rock sample. f1 ;

[0012] S7. After crushing the pretreated rock sample to a preset mesh size, take the mass of the crushed sample as m. d2 Extraction experiments were conducted, followed by further drying to obtain pulverized extracted rock samples. The mass m of the pulverized rock samples was then used to determine the optimal extraction method. d2 The mass m of the rock sample after crushing and extraction d3 Calculate the fluid loss m per unit mass of powdered rock. f2 ;

[0013] S8. Weigh a certain mass of the dried powder sample from step S7 and measure its helium density ρ. d ;

[0014] S9. Based on the helium density ρ d Apparent density ρ i Fluid loss per unit mass of blocky rock (m) f1 Fluid loss per unit mass of powdered rock (m) f2 Determine the total porosity of the rock sample.

[0015] The beneficial effects provided by this invention are: porosity can be calculated by the density and fluid mass differences of rock samples with different particle sizes, without being limited by the size of the rock sample and the fluid it contains, and can measure the closed pores in the rock. At the same time, the mass loss of fluid during the crushing, washing / extraction and drying processes can be used for correction, so as to obtain a more accurate total porosity of shale. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0017] Figure 2 This is a schematic diagram comparing the measured porosity and the logging porosity of well XY10. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0019] Before formally describing the present invention, a general description of the solution of the present invention will be given first to facilitate understanding.

[0020] Please refer to Figure 1 The present invention provides a method for determining shale porosity using the difference between rock density and fluid mass, comprising the following steps:

[0021] S1. Obtain the mass m of the blocky rock sample to be tested. i ;

[0022] S2. Measure the apparent volume v of the rock sample. i ;

[0023] It should be noted that the methods for measuring the apparent volume of the rock sample in step S2 include: contact measurement method and non-contact measurement method.

[0024] In this invention, contact measurement methods can include particle compaction, mercury intrusion porosimetry, and water displacement mass difference methods, while non-contact measurement methods can include direct measurement and laser three-dimensional volume scanning. Furthermore, if a contact apparent volume measurement method is used, to avoid errors caused by contact measurement, a parallel sample with a similar mass to the rock sample can be taken for measurement.

[0025] S3. Calculate the apparent density ρ based on the mass and apparent volume of the rock sample. i ;

[0026] It is easy to understand that in this invention, the apparent density ρ i The calculation formula is as follows:

[0027] S4. Dry the rock sample to obtain the pretreated rock sample;

[0028] It should be noted that before the drying pretreatment in step S4, an oil washing treatment is also performed according to the maturity of the rock sample. If the rock sample is of high maturity, the oil washing treatment is not performed; otherwise, the oil washing treatment is required. Specifically, a rock sample of high maturity refers to one whose corresponding maturity index exceeds 1.3%Ro of the equivalent vitrinite reflectance.

[0029] It should be noted that there are many methods for measuring the maturity of shale, such as rock pyrolysis analysis, vitrinite reflectance, and other geochemical indicators. Any of these methods can be used for measurement and assessment in this invention.

[0030] As an example, taking vitrinite reflectance as an example, the principle is that the reflectance of vitrinite (organic matter formed from plant remains) increases with increasing temperature, directly reflecting the degree of thermal evolution.

[0031] The main methods for determining maturity are as follows: Ro < 0.5% indicates an immature biogas stage; 0.5% ≤ Ro ≤ 1.3% indicates a moderately mature oil-producing window stage; Ro > 1.3% indicates a highly mature biogas-producing stage. Of course, other indicators can also be used to determine maturity, and this invention does not limit this to any particular indicator.

[0032] In addition, the temperature of the drying pretreatment in step S4 of the present invention is not lower than 110°C, and the drying time is based on the reduction of the rock sample mass to a constant value.

[0033] S5. Measure the mass m of the pretreated rock sample. d1 ;

[0034] It should be noted that the sample should be kept away from air for an extended period during the weighing process, as the sample will absorb water upon contact with air, leading to deviations in mass.

[0035] S6. Calculate the fluid loss per unit mass of the blocky rock, m, based on the mass of the rock sample and the mass of the pretreated rock sample. f1 ;

[0036] In step S6 of this invention, the fluid loss per unit mass of the blocky rock is m f1 The calculation formula is as follows:

[0037] S7. After crushing the pretreated rock sample to a preset mesh size, take the mass of the crushed sample as m. d2 Extraction experiments were conducted, followed by further drying to obtain pulverized extracted rock samples. The mass m of the pulverized rock samples was then used to determine the optimal extraction method. d2 The mass m of the rock sample after crushing and extraction d3 Calculate the fluid loss m per unit mass of powdered rock. f2 ;

[0038] It should be noted that in this invention, the rock sample is crushed to a particle size of less than 200 mesh, and then Soxhlet extraction is carried out on it. It is recommended to use a ternary solvent of chloroform-methanol-acetone as the extraction solvent. After the extraction experiment, the solvent needs to be fully evaporated by a high drying temperature and time.

[0039] It is easy to understand that the fluid loss per unit mass of powdered rock in step S7 of this invention is m f2 The calculation formula is as follows:

[0040] It should be noted that steps S7 and S4 can also be combined to obtain the total fluid loss m of the rock sample. f (i.e. m) fl and m f2The sum of the two is the sum of the rock samples, which is obtained by weighing the rock samples at one time after sequentially performing oil washing, drying pretreatment, crushing, Soxhlet extraction and drying treatment.

[0041] S8. Weigh a certain mass of the dried powder sample from step S7 and measure its helium density ρ. d ;

[0042] It should be noted that, considering the potential influence of air adsorption during the powder drying process in the oven, it is recommended to discard the initial test data of helium density. This step also requires ensuring a certain saturation pressure and equilibrium time to allow the helium molecules to be fully charged, and to perform leakage pressure correction for the corresponding equilibrium time.

[0043] S9. Based on the helium density ρ d Apparent density ρ i Fluid loss per unit mass of blocky rock (m) f1 Fluid loss per unit mass of powdered rock (m) f2 Determine the total porosity of the rock sample.

[0044] The formula for calculating the total porosity of the rock sample in step S9 of this invention is as follows:

[0045] This invention uses measured data as an example for explanation and illustration.

[0046] First, determine the mass (m) of the blocky rock sample to be tested at a depth of 2554m in well XY10. i =8.7316g), and at the same time, a parallel sample (m) was selected at this depth. i` =8.1929g). Measure the apparent volume (v) of parallel samples. i` =3.0566cm 3 To determine the apparent density of a parallel blocky sample equivalent to the original rock sample. Then, the original blocky rock samples were pretreated by washing and drying, and the mass of the pretreated rock samples was weighed (m). d1 =8.6943g), calculate the fluid mass loss per unit mass of rock during the pretreatment process. The pretreated rock samples were further pulverized into powder with a particle size of 200-400 mesh and subjected to extraction and drying treatment. The fluid loss per unit mass of rock was measured (m). f2 =0.0002g / g), a certain mass of dry powder sample was weighed and its helium density (ρ) was measured. d =2.7239g / cm 3 Finally, use the formula The total porosity of the shale sample at a depth of 2554m in well XY10 can then be calculated.

[0047] On the cross-section of the Lianggaoshan Formation and Dongyuemiao Section of Well XY10, by comparing the variation trends of the measured porosity and the logging porosity of this invention, it can be seen that the two have good consistency. Figure 2 ).

[0048] The beneficial effects of this invention are: it calculates porosity by using the density and fluid mass differences of rock samples with different particle sizes, without being limited by the size of the rock sample or the fluid it contains, and can measure the closed pores in the rock. At the same time, it can use the fluid loss mass during the crushing, washing / extraction and drying processes for correction, so as to obtain a more accurate total porosity of shale.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining shale porosity using the difference between rock density and fluid mass, characterized in that: Includes the following steps: S1. Obtain the mass m of the blocky rock sample to be tested. i ; S2. Measure the apparent volume v of the rock sample. i ; S3. Calculate the apparent density ρ based on the mass and apparent volume of the rock sample. i ; S4. Dry the rock sample to obtain the pretreated rock sample; S5. Measure the mass m of the pretreated rock sample. d1 ; S6. Calculate the fluid loss per unit mass of the blocky rock, m, based on the mass of the rock sample and the mass of the pretreated rock sample. f1 ; S7. After crushing the pretreated rock sample to a preset mesh size, take the mass of the crushed sample as m. d2 Extraction experiments were conducted, followed by further drying to obtain pulverized extracted rock samples. The mass m of the pulverized rock samples was then used to determine the optimal extraction method. d2 The mass m of the rock sample after crushing and extraction d3 Calculate the fluid loss m per unit mass of powdered rock. f2 ; S8. Weigh a certain mass of the dried powder sample from step S7 and measure its helium density ρ. d ; S9. Based on the helium density ρ d Apparent density ρ i Fluid loss per unit mass of blocky rock (m) f1 Fluid loss per unit mass of powdered rock (m) f2 Determine the total porosity of the rock sample.

2. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: The methods for measuring the apparent volume of rock samples in step S2 include: contact measurement and non-contact measurement.

3. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 2, characterized in that: When measuring apparent volume using the contact measurement method, a parallel sample with a mass similar to that of the rock sample should be taken for measurement.

4. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: Before the drying pretreatment in step S4, an oil washing treatment is also performed according to the maturity of the rock sample. If the rock sample is highly mature, the oil washing treatment is not performed; otherwise, the oil washing treatment is required. Specifically, a highly mature rock sample refers to one whose corresponding maturity index exceeds 1.3%Ro of the equivalent vitrinite reflectance.

5. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: In step S4, the temperature of the drying pretreatment shall not be lower than 110℃, and the drying time shall be based on the reduction of the rock sample mass to a constant value.

6. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: In step S6, the fluid loss per unit mass of the blocky rock is m f1 The calculation formula is as follows:

7. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: The fluid loss per unit mass of powdered rock in step S7 is m f2 The calculation formula is as follows:

8. The method for determining shale porosity using the difference between rock density and fluid mass as described in claim 1, characterized in that: The formula for calculating the total porosity of the rock sample in step S9 is as follows:

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