A method for determining the porosity of a shale oil sample of effective pore space

Abstract

A method for determining the porosity of shale oil samples with effective pore space. This method primarily addresses the problem of inaccurate determination of effective storage space porosity caused by fractures resulting from poor oil and gas extraction in oil-bearing shale and loss of clay water. The method is characterized by the following steps: S1, preparing a shale sample; S2, immersing the sample in a saturated solution for self-saturation and recording the self-saturation time; S3, measuring the mass of the sample in the saturated solution using a hanging scale and the wet sample mass filled with saturated solution, determining the density of the saturated solution, and calculating the total sample volume; S4, removing oil, ventilating, and dehumidifying the sample; S5, measuring the dry sample mass and sample skeleton volume, and measuring the mass of particles and dust inside the sample bag; S6, calculating the porosity of the shale sample. This method eliminates the influence of fractures caused by sample dehydration, accurately determines the effective porosity of shale oil samples by retaining clay-adsorbed water in situ while removing pore water, and features a short experimental cycle and high accuracy.

Description

Technical Field

[0001] This invention relates to the field of experimental testing in oil and gas exploration and development, specifically a method for determining the porosity of shale oil samples with effective pore space. Background Technology

[0002] Shale oil and gas exploration and development has become an important area of ​​global oil and gas exploration and development. Due to the special characteristics of shale, many previous exploration and development technologies are difficult to adapt to the requirements of shale. Porosity is an indispensable parameter for shale oil and gas exploration and development, and its analysis methods are also affected by lithology. Shale oil reservoir samples have well-developed bedding fractures, making them prone to fractures and breakage. Moreover, they will generate fractures when exposed to water or dehydrated. In previous methods, the incomplete saturation method and the method of "measuring the total volume first and then removing oil and moisture" were used to solve the problem of inaccurate porosity measurement of shale oil samples due to deformation caused by dehydration during shale sample fracture. The incomplete saturation method can solve the problem of fractures in shale samples due to saturation. This method stipulates that the total volume of the sample should be measured before it is completely saturated, avoiding inaccurate porosity measurements caused by fractures after complete saturation. However, it only considers the problem of inaccurate porosity measurements due to increased total volume caused by fractures after complete saturation. It does not consider the problem of saturated liquid (such as ethanol) entering the clay adsorption layer, displacing the adsorbed water in the clay, causing the adsorbed water in the clay to be released, resulting in an underestimation of the skeleton volume and an overestimation of the porosity, thus losing accuracy. The problem of inaccurate porosity measurements caused by saturated liquid entering the clay adsorption layer is becoming increasingly obvious, and this problem needs to be studied and solved as soon as possible.

[0003] Shale lithology is dense, with oil and gas contained within nano- and micro-nano-pores. Oil and gas seepage is controlled by pore distribution. In shale oil reservoir pressurized core samples, due to the use of freezing technology, oil and gas are sealed within the pores. When the pressurized samples thaw slowly, some samples experience impeded oil and gas release. Under the influence of internal fluid pressure, shale oil samples may fracture along bedding planes, creating fissures. These fissures also affect the accuracy of porosity measurements. Currently, there is no effective method to solve the problem of inaccurate porosity measurements caused by fissures resulting from impaired oil and gas release.

[0004] Another characteristic affecting porosity measurement is the clay mineral composition of shale and mudstone. Clay minerals are rich in water, containing not only pore water but also clay-adsorbed water. This clay-adsorbed water is retained in the interlayer domain through adsorption, with most of it adsorbed on the surface of clay particles and not occupying effective pore space. Clay has a large specific surface area and a high content of clay-adsorbed water, which, like pore water, can be released at around 100℃. The porosity of the same shale and mudstone sample differs significantly between those containing and releasing clay-adsorbed water. In shale oil and gas exploration and development, porosity parameters containing and releasing clay-adsorbed water have different applications. In particular, the porosity of shale with retained clay-adsorbed water, i.e., effective pore space, is indispensable in reservoir studies and reserve evaluation.

[0005] There is no good experimental method for removing pore water while retaining clay-adsorbed water in situ in oil-bearing shale. Currently, there is no proprietary experimental method. The method used to remove pore water in non-oil-bearing shale is borrowed. This involves drying the sample to constant weight under vacuum at 60°C for 4 hours, cooling it to room temperature in a desiccator, and then weighing it. This process is repeated until the sample reaches constant weight. For non-oil-bearing shale, it usually takes 3-4 cycles to dry a batch of samples to constant weight. Due to the long experimental cycle, clay water is lost, resulting in a large experimental error. For shale oil reservoir samples, due to the presence of oil and the loss of different oil and gas, it is more difficult to achieve constant weight under vacuum at 60°C, the experimental cycle is longer, and the loss of clay water is greater, which will produce greater errors in the determination of effective porosity. Therefore, the application of pore water removal technology from oil-free shale to shale oil reservoir samples will produce greater errors, and the accuracy of sample analysis cannot be guaranteed. There is also a lack of effective methods for determining the effective pore space porosity of shale oil reservoir samples by retaining clay adsorbed water in situ and removing pore water. Summary of the Invention

[0006] To overcome the problem of inaccurate porosity determination in effective storage space caused by fractures and clay water loss due to poor oil and gas extraction in existing oil-bearing shale samples, this invention provides a method for determining the porosity of shale oil samples with effective pore space. This method eliminates or partially eliminates the impact of rapid release of fluid pressure within the pores of shale oil samples causing fractures in the core, thus avoiding the drawback of inaccurate porosity determination due to clay water loss caused by various reasons during the effective pore space porosity determination process of shale oil reservoir samples. It can accurately determine the effective porosity of shale oil samples with in-situ retention of clay-adsorbed water and removal of pore water, with a short experimental cycle and high accuracy.

[0007] The technical solution of this invention is: a method for determining the porosity of shale oil samples with effective pore space, comprising the following steps:

[0008] S1. Prepare shale samples;

[0009] S2. Place the sample in the saturated solution for self-absorption saturation and record the self-absorption saturation time;

[0010] S3. Measure the mass M2 of the sample in the saturated liquid using a hanging scale and the mass M1 of the wet sample filled with saturated liquid, and determine the density ρ1 of the saturated liquid. Calculate the total sample volume V.

[0011]

[0012] Where: V—total volume of the sample being tested, cm³ 3 M2—Mass of the test sample in the saturated liquid after being filled with saturated liquid, g; M1—Mass of the wet sample after being filled with saturated liquid, g; ρ1—Density of the saturated liquid, g / ml;

[0013] S4. Degrease, ventilate, and dehumidify the sample;

[0014] S5. Measure the dry sample mass m1 and the sample skeleton volume V. m1 And measure the mass m2 of particles and dust in the sample bag;

[0015] S6. Calculate the porosity φ of the shale sample:

[0016]

[0017] In the formula: V m1 —Sample skeleton volume, cm³ 3 m2—mass of particles and dust that detached from the sample bag, g; m1—mass of the dry sample, g; φ—effective porosity of the sample being tested, %.

[0018] Furthermore, for samples with visible cracks, a total volume correction for cracked samples is performed: the sample is separated along the crack, and a large, crack-free piece is taken as the cracked sample. After self-absorption saturation, the weight of the cracked sample (m3) and the wet sample weight (m4) are measured. After degreasing, ventilating, and dehumidifying the cracked sample, the dry sample weight (m5) is measured, and the total volume V of the cracked sample is calculated. t :

[0019]

[0020] Where: m3—mass of the fractured sample on the crane scale, g; m4—wet mass of the fractured sample, g; m5—dry mass of the fractured sample, g; V t —Total volume of the fractured sample, cm³ 3 ;

[0021] At this time, the porosity φ of the sample is:

[0022]

[0023] Furthermore, step S1 includes:

[0024] S1.1. Freezing is required to prepare a full-diameter sample of the shale oil reservoir to be tested;

[0025] S1.2 Cut full-diameter sheet samples of the required thickness from the full-diameter sample of the shale oil reservoir;

[0026] S1.3. Cut the full-diameter sheet sample into shale samples of the required size using a cutting tool along a direction perpendicular to the bedding and the end face of the bedding.

[0027] Furthermore, in step S1, the sample is prepared by freezing with liquid nitrogen.

[0028] Furthermore, in step S2, the saturated liquid is anhydrous ethanol or kerosene.

[0029] Furthermore, in step S2, the saturated solution for oil-free shale samples is anhydrous ethanol, while the saturated solution for oil-containing shale samples is anhydrous ethanol or kerosene.

[0030] Furthermore, in step S2,

[0031] t < T p (5)

[0032] In the formula, t represents the longest time for the sample to self-absorb and saturate; T p —Time it takes for the saturated liquid to penetrate the clay adsorption layer or for cracks to form.

[0033] Furthermore, in step S2,

[0034]

[0035] In the formula: t is the longest self-absorption saturation time, and t0 is the sample self-absorption saturation time corresponding to a certain stable time interval (Δt). The average time used to test each sample is given by n, which is the number of self-absorption saturated samples.

[0036] Furthermore, in step S4, chloroform is used as the degreasing agent when degreasing the sample. After degreasing, the sample and sample bag are ventilated together for 4-8 hours. Then, the sample and sample bag are placed in a vacuum drying oven and dried at 60°C under vacuum for 8 hours before being taken out.

[0037] The present invention has the following beneficial effects: Due to the above-mentioned scheme, (1) the present invention controls or limits the self-absorption saturation time of the sample to be tested, so that the total volume of the sample is determined before the saturated liquid of the sample penetrates into the clay adsorption layer of the shale sample. This avoids the drawback of inaccurate porosity (total volume) measurement caused by water flowing out of the clay adsorption layer or cracks generated by partial / complete saturation of the sample during the saturation process. (2) The present invention adopts the method of first measuring the total volume of the sample and then measuring the sample skeleton volume after degreasing and dehumidifying. This avoids the drawback of the previous method of first degreasing and dehumidifying and then measuring the total volume, which caused cracks in the mudstone and shale sample and brought errors to the porosity measurement results. This improves the accuracy of the effective porosity measurement of shale oil samples. (3) Taking advantage of the characteristic that chloroform is soluble in oil but insoluble in water, chloroform was selected as the oil removal reagent to ensure that the pore water and clay adsorbed water in the sample were not damaged. The total volume measurement technology of limiting the self-absorption saturation time to avoid the saturated liquid from entering the clay layer and the dehumidification technology of "vacuum 60℃ 8h" to remove pore water can effectively remove pore water while retaining clay adsorbed water in situ. This ensures the accuracy of the skeleton volume measurement and avoids the disadvantage of reduced accuracy of porosity (skeleton volume) measurement due to the outflow of clay water. It improves the accuracy and reliability of the effective porosity measurement of shale oil samples with retained clay adsorbed water and the work efficiency. (4) In addition, the total volume correction measurement was carried out using a part of the sample without visible cracks to eliminate the influence of cracks in the shale caused by poor drainage of fluid in the sample on the porosity. (5) The "vacuum 60℃ 8h" dehumidification technology provided by the method of this invention offers an effective experimental technique for removing pore water while retaining it in situ for oil-bearing shale. Furthermore, dehumidification and drying for 8 hours under vacuum at 60℃ avoids repeated evacuation and heating, weighing, and other experimental work, shortening the experimental cycle, reducing labor intensity, and improving work efficiency. Simultaneously, this invention aligns with the health, safety, and environmental protection principles of HSE (Health, Safety, and Environment) analysis and testing. Attached Figure Description

[0038] Figure 1 This is a flowchart of the present invention;

[0039] Figure 2 This is a graph showing the change in porosity over different dehumidification times. Detailed Implementation

[0040] The present invention will be further described below:

[0041] Depend on Figure 1 As shown, a method for determining the porosity of shale oil samples with effective pore space includes the following steps:

[0042] S1. Preparation of shale samples, specifically:

[0043] S1.1 For preparing parallel shale oil reservoir samples of full diameter using liquid nitrogen, dry ice, or a freezer, liquid nitrogen freezing is preferred. Shale oil samples have well-developed bedding; under external forces, the samples will fracture or break along the bedding planes. For fresh samples (i.e., where there is not a significant loss of oil and water), the water within the sample acts as a binding agent, adhering and consolidating the particles and bedding planes together, maximizing resistance to external forces. Therefore, frozen samples have increased hardness and enhanced resistance to breakage, improving the success rate of sample preparation.

[0044] S1.2 At the sampling depth of the frozen shale oil reservoir full-diameter sample, under liquid nitrogen cooling, use a core cutting tool to cut full-diameter sheet samples of the required thickness along the core bedding plane. The sample thickness is generally 15mm-20mm, depending on the different analytical samples.

[0045] S1.3. Cut the full-diameter sheet sample into shale samples of the required size using a cutting tool along a direction perpendicular to the bedding planes and bedding end faces. Using a tool to cut the sample avoids crushing it during extraction, greatly improving the success rate and allowing for the preparation of samples at any desired location. The cutting tool should be perpendicular to the bedding end face to prevent cracks or splits along the bedding planes.

[0046] S2. Immerse the shale samples to be tested in the saturated solution in the container according to their numbers, allowing the samples to self-saturate. The saturating solution is anhydrous ethanol or kerosene. Oil-free samples (such as oil-free mudstone and shale, shale gas, and de-oiled shale oil samples) are generally saturated with anhydrous ethanol. If kerosene is used for oil-free samples, the oil must be completely removed during porosity analysis, which is more time-consuming and labor-intensive than using saturated anhydrous ethanol, significantly extending the experimental cycle and greatly reducing work efficiency. Furthermore, the de-oiling process uses chemical reagents, all of which are toxic and harmful substances, posing a risk to the environment and human health. Therefore, ethanol saturation is preferred for oil-free samples whenever possible. Oil-containing samples are generally saturated with kerosene, but anhydrous ethanol saturation can also be used.

[0047] Record the self-absorption saturation time: Select a sample of suitable mass and volume, place the sample in the hanging net of the beaker in the buoyancy measuring device, use a stopwatch to time the time, observe the balance reading at the same time, and record the self-absorption saturation time of the sample.

[0048] The purpose of determining the self-absorption saturation time is to minimize or avoid the saturated liquid from entering the clay adsorption layer, while also preventing the sample from cracking or breaking after reaching a certain level of saturation. If the saturated liquid enters the clay adsorption layer, it will extract water from the layer. During sample dehumidification, the saturated liquid will escape, creating pores in the adsorption layer and leading to an overestimation of porosity, thus reducing the accuracy of the effective porosity measurement. To accurately determine the porosity of the sample within the effective pore space, it is necessary to avoid ethanol entering the clay adsorption layer or causing cracks or even breakage in the sample. Therefore, the maximum self-absorption saturation time t should meet the following requirements:

[0049] t < T p (5)

[0050] In the formula, t represents the longest time for the sample to self-absorb and saturate; T p —Time it takes for the saturated liquid to penetrate the clay adsorption layer or for cracks to form.

[0051] Meanwhile, the longest self-absorption saturation time t of the sample

[0052]

[0053] In the formula: t is the longest self-absorption saturation time, and t0 is the sample self-absorption saturation time corresponding to a certain stable time interval. The average time used to test each sample is given by n, which is the number of self-absorption saturated samples.

[0054] To satisfy t < T p Appropriate values ​​for t0 and the number of self-absorption saturated samples n need to be selected. At T... p In smaller cases, i.e., when the pore-permeability conditions of the tested mudstone and shale sample are good (such as shallow to medium-depth mudstone), or when the pore water content of the sample is low due to loss or the sample itself has a low pore water content (such as deep shale samples that have been stored for a long time), T p The value of t0 will decrease. In this case, a smaller t0 should be selected as much as possible, or the number of self-absorption saturated samples should be reduced to satisfy the above relationship. p The self-absorption saturation time can be determined using methods such as NMR experiments with different self-absorption saturation times. However, since shale lithology is relatively dense, when a smaller stable time interval is selected for measurement, the time required is much shorter than the self-absorption saturation time t0 corresponding to the maximum stable time interval measured in the experiment to determine the self-absorption saturation time. Therefore, the maximum self-absorption saturation time (denoted as t0) can be used. 0最大 As T p That is, when At that time, T P ≈t 0最大 , t 较小 To minimize the self-absorption saturation time, t 0最大 The experiment yielded the longest self-absorption saturation time.

[0055] Generally, for tight shale with minimal water loss, the longest self-saturation time t is within 1-2 hours. The self-saturation time t0 can be selected based on a stable time interval (from a few seconds to several minutes, or even tens of minutes or more) where the balance reading remains constant. The maximum number of saturated samples can be 40-50. For tight shale samples with significant pore water loss and low pore water content, the self-saturation time t0 can be selected based on experimental data and the specific sample conditions, with a stable time interval of several minutes, tens of seconds, or even a few seconds. The number of self-saturated samples can be 10-20, or even just a few samples, or the total volume of a single sample can be directly determined using the method for determining the self-saturation time. The situation is similar for shallow and medium-depth mudstone, mudstone shale, and tight shale with low pore water content, determined based on experiments and lithological conditions. The same self-saturation time can be used for the same type of mudstone and shale in the same region and reservoir, requiring only one experiment.

[0056] S3. Using an Archimedes buoyancy measuring apparatus, after self-saturation, measure the mass M2 of the sample in the saturated liquid and the wet sample mass M1 of the saturated liquid. The entire process, from reading the balance to removing the sample and wiping away any remaining liquid, takes approximately 5 seconds. The measuring balance is a 0.1% balance. To improve the accuracy of sample measurement, a dynamic rapid total volume determination method can be used, where the sample is self-saturated while being measured, and both masses are determined before the 0.1% balance reading changes.

[0057] Measure the density ρ1 of the saturated liquid using a hydrometer, and calculate the total sample volume V:

[0058]

[0059] Where: V—total volume of the sample being tested, cm³ 3 M2—Mass of the test sample in the saturated liquid after being filled with saturated liquid, g; M1—Mass of the wet sample after being filled with saturated liquid, g; ρ1—Density of the saturated liquid, g / ml.

[0060] S4. Remove oil, ventilate, and dehumidify the sample.

[0061] S4.1 Oil Removal. Place the sample into a dry sample bag of known mass and corresponding number. Use chloroform (a low-boiling-point, water-insoluble solvent) as the oil removal agent, ensuring it does not damage the clay-adsorbed water and pore water within the sample. Remove the oil using a Soxhlet extractor according to the prescribed operating procedure. After oil removal, remove the sample (along with the sample bag) from the Soxhlet extractor and place it in a fume hood for 4-8 hours to allow some of the reagent in the sample bag and sample to evaporate.

[0062] S4.2 Dehumidification. Place the sample to be tested (together with the sample bag) in a vacuum drying oven and dry it at 60°C under vacuum for 8 hours. Then remove it and place it in a desiccator. Alternatively, remove the sample from the sample bag and place both the sample and the sample bag in a vacuum drying oven. Dry it at 60°C under vacuum for 8 hours. Then remove the sample and the sample bag and place them in a desiccator.

[0063] S5. Measure the dry sample mass m1 and the sample skeleton volume V. m1 The mass (m2) of particles and dust within the sample bag was measured. The sample skeleton volume was determined using a gas injection method, the purpose of which was to combine the total volume with the pore volume or porosity. During the oil removal process, some samples experienced particle and / or dust shedding; the volume of the dust and particle skeleton can be calculated from the particle density, therefore it does not need to be directly measured. For broken samples, the volume of the broken sample skeleton can be directly measured.

[0064] S6. Calculate the porosity φ of the shale sample based on the data from steps S2 and S5:

[0065]

[0066] In the formula: V m1 —Sample skeleton volume, cm³ 3 m2—mass of particles and dust that detached from the sample bag, g; m1—mass of the dry sample, g; φ—effective porosity of the sample being tested, %.

[0067] In addition, for samples with visible fractures, a fractured sample total volume correction is performed, measuring the total volume of the shale sample after removing the fractured portion. For shale samples with good homogeneity, the total volume of the sample can be obtained from the density of a portion of the sample, which can eliminate the influence of fractures. This method has a certain calibration effect on the inaccurate total volume caused by fluid leakage in the original shale sample before it was separated along the fractures. This method is suitable for samples with visible fractures.

[0068] The sample was separated along the crack, and a large, crack-free piece was taken as the crack sample. After self-absorption saturation, the mass m3 of the crack sample and the wet mass m4 of the crack sample were measured according to steps S3-S5. After degreasing, ventilating, and dehumidifying the crack sample, the dry mass m5 of the crack sample was measured, and the total volume V of the crack sample was calculated. t :

[0069]

[0070] Where: m3—mass of the fractured sample on the crane scale, g; m4—wet mass of the fractured sample, g; m5—dry mass of the fractured sample, g; V t —Total volume of the fractured sample, cm³ 3 .

[0071] At this time, the porosity φ of the sample is:

[0072]

[0073] The following experiment will further illustrate this point.

[0074] Samples from mudstone and shale oil reservoirs were selected, and some experiments were conducted to determine the technical conditions for porosity measurement.

[0075] Experiment 1: Pore Water Removal Experiment

[0076] The researchers in this application discovered that although both pore water and clay-adsorbed water can be released at around 100℃, the regions where pore water and clay-adsorbed water exist in the rock core are different. Clay-adsorbed water is mostly adsorbed on the surface of clay particles, while pore water exists within the rock pores. The surface tension (i.e., attraction) of the water on the rock particle surface gradually increases from the center of the rock pores towards the surface of the rock particles. Under the influence of temperature and vacuum conditions, the release of pore water and clay-adsorbed water occurs sequentially, with pore water releasing first. To verify this finding, an experimental method was studied to retain clay-adsorbed water in situ while releasing only pore water. The specific experimental methods and results are as follows.

[0077] The experimental sample was selected from the shale oil layer of well XX1. After the core was brought to the surface from the wellbore, the required sample was taken from the middle part of the core. The sample thickness was about 15 mm, with an average length and width of 25 mm and 19 mm, respectively, and a mass of 17.753 g. As it was a conventional core sampling method, some light hydrocarbons were lost from the sample during the process of bringing the core to the surface from the wellbore, but the pore water and clay-adsorbed water in the core were basically distributed under formation conditions.

[0078] To prevent water loss from the sample, it was transported back to the laboratory using liquid nitrogen freezing. After thawing and slow melting, NMR measurements were immediately performed. The sample was then placed in a vacuum drying oven, evacuated, and heated to 60°C to dry it under vacuum and at 60°C for 10 hours.

[0079] To understand the changes in clay-adsorbed water and pore water within the samples, two-dimensional nuclear magnetic resonance (NMR) analysis was performed every 2 hours to obtain NMR distribution spectrum data at different times. Two-dimensional NMR analysis can separate pore water from structural water / crystallization water / clay-adsorbed water, oil from organic matter / asphalt, and oil from water. Therefore, four sets of data were obtained, as shown in Table 1 below.

[0080] Table 1. Two-dimensional NMR analysis data of mudstone and shale.

[0081]

[0082] As can be seen from the experimental data in Table 1, the measured data for structural water / crystallization water / clay-adsorbed water and organic matter / asphalt remained basically unchanged, with slight differences at different times. No clay-adsorbed water flowed out of the structural water / crystallization water / clay-adsorbed water. The pore water content in the sample gradually decreased with increasing experimental time, and the rate of decrease became smaller and smaller. When the drying time reached 8 hours, the pore water content was less than 0.02 mL. Moreover, with the same increase in drying time, the change in pore water content in the tested sample was very small. Therefore, after drying the tested shale oil sample at 60℃ under vacuum for 8 hours, the pore water content was basically removed, and the pore water content was less than 0.02 mL, which meets the accuracy requirements of geological experiments. However, the oil content decreased with increasing drying time. For oil-bearing shale samples, the constant mass method for determining pore water affects the degree of sample mass change due to oil loss. This indicates that the constant mass method is not suitable for determining pore water in oil-bearing shale samples during the heating and dehydration process due to the loss of oil and gas.

[0083] Repeated experiments using shale oil samples of different geometric sizes have demonstrated that the optimal extraction time for pore water under vacuum and 60°C conditions is 8 hours.

[0084] Experiment 2: Porosity Measurement Experiment at Different Self-Absorption Saturation Times

[0085] To understand the differences in porosity of effective storage space for samples with different self-absorption packs and time periods, mudstone and shale samples from the exploration area were selected, and two sets of parallel samples were prepared using a cutting method. Preparation method: On full-diameter mudstone and shale samples, a full-diameter sheet sample with a thickness of 14mm–22mm was prepared by using a specialized tool along the sedimentary bedding plane of the core under liquid nitrogen cryocooling. This sample was then cut into approximately cuboid samples with a length of 40mm–60mm and a width of 18mm–25mm using a cutting tool along a direction perpendicular to the bedding plane and bedding end face. The sample was then cut into two parallel samples, which were randomly divided into two groups, A and B, with a total of 11 samples in each group. Analysis method: The samples were simultaneously immersed in anhydrous ethanol. For group A samples, the total volume was measured 15 minutes after self-absorption saturation, with a total measurement time of approximately 10 minutes. For group A samples, the saturation and measurement time was 25 minutes. The samples were then sealed in a specific container. The total volume of sample B was measured after 150 minutes of self-absorption saturation. The total measurement time was approximately 160 minutes. After the measurement, the two groups of samples were placed into cloth bags corresponding to the sample numbers. The two groups of samples were mixed together and placed into the same Soxhlet extractor for 72 hours of degreasing with chloroform. After degreasing, the samples were placed in a vacuum drying oven for dehumidification and kept at 60°C for 8 hours. Then, the sample skeleton volume was measured by gas injection method to obtain the porosity of the sample.

[0086] The experimental results are shown in Table 2.

[0087] Table 2. Experimental data on porosity determination at different self-absorption saturation times.

[0088]

[0089] As can be seen from Table 2, the porosity of the samples in group B is greater than that of the samples in group A. Except for the difference in self-absorption saturation time, the test conditions and test methods of the two groups are the same. The samples in group B have a larger porosity because the self-absorption saturation time is too long, which causes ethanol to penetrate into the clay adsorption layer.

[0090] Experiment 3: Porosity Measurement Experiment under Different Dehumidification Times

[0091] Three samples from mudstone and shale oil reservoirs were selected for porosity measurement experiments. The method was as follows: after sample preparation, the samples were soaked in anhydrous ethanol for 15 minutes and the total volume of the samples was measured. Then, the samples were degreased with chloroform, dehumidified under vacuum at 60℃ for 8 hours, and then dehumidified at 105℃ to constant weight. In order to study the porosity change characteristics at different times, the sample skeleton volume was measured every hour during the 8 hours of dehumidification at 60℃ under vacuum. The initial skeleton volume measurement time was 3 hours of dehumidification.

[0092] The experimental results are shown in Figure 2 Before 6 hours of vacuum drying at 60℃, the porosity of the samples increased linearly over time. After 6 hours, the change in porosity slowed down and became relatively uniform. Between 7 and 9 hours, the porosity curves of each sample showed a smooth segment, indicating approximately the same porosity. After 10 hours, the porosity increased significantly, and the porosity after dehumidification at 105℃ to constant weight increased substantially. Combined with the pore water removal experiment in Experiment 1, the pore water in the samples was almost completely removed between 7 and 9 hours, while the clay-adsorbed water remained in situ. The porosity during this period represents the effective porosity after removing only the pore water and retaining the clay-adsorbed water. Therefore, drying at 60℃ under vacuum for 8 hours is the optimal time for removing pore water, and the obtained porosity accurately reflects the effective pore space.

[0093] Experiment 4: Sample Analysis

[0094] Currently, shale oil is one of the hot topics in oil and gas exploration and development. The porosity of the effective pore space of shale oil reservoir samples is an indispensable data for reservoir research and understanding. Therefore, a large number of sample analyses have been carried out. Table 3 shows the analysis results according to the method of this invention. In order to illustrate the influence of clay adsorbed water on porosity, after analysis according to the method of this invention, the samples were dehumidified to constant weight in a drying constant temperature oven at 105℃. After removing the clay adsorbed water from the samples, the sample skeleton volume was measured to obtain the effective porosity after removing the clay adsorbed water.

[0095] The analysis data (see Table 3) shows that the porosity measured by the method of this invention is generally lower than that after removing clay-adsorbed water, with a maximum difference of 3.3%, a minimum difference of 1.8%, and an average of 2.5%. This indicates that clay-adsorbed water has a significant impact on the porosity of shale oil rock samples. In actual testing, experimental techniques that control or limit the removal of clay-adsorbed water should be used as much as possible to determine the effective pore space porosity of shale oil rock samples. It is evident that the porosity measurement method for limiting the removal of clay-adsorbed water from shale oil rock samples provided by this invention can better reflect the true porosity changes in the effective pore space of shale oil rock samples, and has important value for shale oil and gas exploration and development.

[0096] Table 3 Comparison of Sample Analysis Data

[0097]

[0098]

[0099] The embodiments described above are merely illustrative of implementation methods of this disclosure, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent disclosure. It should be noted that those skilled in the art can make various modifications, equivalent substitutions, and improvements without departing from the concept of this disclosure, and these all fall within the protection scope of this disclosure.

Claims

1. A method for determining the porosity of shale oil samples with effective pore space, characterized in that... Includes the following steps: S1. Prepare shale samples; S2. Place the sample in the saturated solution for self-absorption saturation and record the self-absorption saturation time; S3. Measure the mass M2 of the sample in the saturated liquid using a hanging scale and the mass M1 of the wet sample filled with saturated liquid, and determine the density ρ1 of the saturated liquid. Calculate the total sample volume V. , In the formula: V —Total volume of the sample being tested, in cm³ 3 ; M 2 —The mass of the sample on a hanging scale in the saturated liquid after it is filled with the saturated liquid, in g; M 1 —Wet sample mass after the sample is saturated with liquid, in g; —Saturated liquid density, g / ml; S4. Degrease, ventilate, and dehumidify the sample; S5. Measure the dry sample mass m1 and the sample skeleton volume V. m1 And measure the mass m2 of particles and dust in the sample bag; S6. Calculate the porosity φ of the shale sample: , In the formula: —Sample skeleton volume, cm³ 3 ; —The mass of particles and dust that detached from the sample bag, in grams; —Sample dry mass, g; —Effective porosity of the sample under test, % S7. For samples with visible cracks, perform a total volume correction for the cracked sample: Separate the sample along the crack, take the largest uncracked sample as the cracked sample, and after self-absorption saturation, measure the weight of the cracked sample (m3) and the wet sample weight (m4). After degreasing, ventilating, and dehumidifying the cracked sample, measure the dry sample weight (m5) and calculate the total volume V of the cracked sample. t : , In the formula: —The mass of the fractured sample on the crane scale, in grams; —Wet sample mass of the fracture sample, g; —Dry sample mass of the fracture sample, g; —Total volume of the fractured sample, cm³ 3 ; At this time, the porosity φ of the sample is: 。 2. The method for determining the porosity of shale oil samples with effective pore space according to claim 1, characterized in that: Step S1 includes: S1.

1. Freezing is required to prepare a full-diameter sample of the shale oil reservoir to be tested; S1.2 Cut full-diameter sheet samples of the required thickness from the full-diameter sample of the shale oil reservoir; S1.

3. Cut the full-diameter sheet sample into shale samples of the required size using a cutting tool along a direction perpendicular to the bedding and the end face of the bedding.

3. The method for determining the porosity of shale oil samples with effective pore space according to claim 2, characterized in that: In step S1, the sample is prepared by freezing with liquid nitrogen.

4. The method for determining the porosity of shale oil samples with effective pore space according to claim 1, characterized in that: In step S2, the saturated liquid is anhydrous ethanol or kerosene.

5. The method for determining the porosity of shale oil samples with effective pore space according to claim 4, characterized in that: In step S2, anhydrous ethanol is used as the saturated solution for shale samples that do not contain oil, while anhydrous ethanol or kerosene is used as the saturated solution for shale samples that contain oil.

6. The method for determining the porosity of shale oil samples with effective pore space according to claim 5, characterized in that: In step S2 ， In the formula, t represents the longest time for the sample to self-absorb and saturate. —Time it takes for the saturated liquid to penetrate the clay adsorption layer or for cracks to form.

7. The method for determining the porosity of shale oil samples with effective pore space according to claim 6, characterized in that: In step S2 ， In the formula: t is the longest time for self-absorption saturation. Let be the sample self-absorption saturation time corresponding to a certain stable time interval (Δt). The average time used to test each sample is given by n, which is the number of self-absorption saturated samples.

8. The method for determining the porosity of shale oil samples with effective pore space according to claim 1, characterized in that: In step S4, chloroform is used as the degreasing agent when removing oil from the sample.

9. The method for determining the porosity of shale oil samples with effective pore space according to claim 8, characterized in that: In step S4, after the oil removal is completed, the sample and sample bag are ventilated together for 4-8 hours. Then, the sample and sample bag are placed in a vacuum drying oven and dried at 60°C for 8 hours before being taken out.

10. The method for determining the porosity of shale oil samples with effective pore space according to claim 2, characterized in that: In step S1.2, the thickness of the full-diameter sheet sample is 15mm-20mm.

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