A high-precision porosity measurement method for rock based on lost-wax method

Abstract

This invention relates to the field of geological engineering and rock physics testing technology, and provides a high-precision method for measuring the porosity of rocks based on the lost-wax method. The method includes: S1, drilling a pre-drilled hole; S2, wax injection, where a wax solution containing surfactants is heated and melted, and wax is injected under multi-stage pressure, recording the total volume of the injected wax solution; S3, core sampling and core sealing, where a core drilling machine performs low-temperature core sampling of the wax-injected area; S4, thermal desorption and correction, recording the total mass; collecting and weighing the outflowing paraffin wax; S6, cleaning and dry weight measurement, where the core is completely dried to constant weight and its dry weight is measured; S5, porosity calculation, determining the effective porosity and total porosity of the target material. This invention utilizes surfactant modification, multi-stage pressure wax injection, shrinkage correction, and low-temperature core drilling to achieve a "modified filling-conformal sampling-correction analysis" process, simultaneously obtaining both effective porosity and total porosity, ensuring the authenticity and reliability of the measurement results.

Description

Technical Field

[0001] This invention relates to the fields of geological engineering and rock physics testing technology, and in particular to a high-precision method for measuring the porosity of rocks based on the lost-wax method. Background Technology

[0002] Porosity is a core parameter for evaluating the physical and mechanical properties, permeability, reservoir capacity, and durability of rocks, and it has significant application value in geotechnical engineering, geological exploration, oil development, and building materials evaluation. Currently, commonly used porosity measurement methods mainly include water saturation methods, gas displacement methods (such as the helium gravity method), mercury intrusion porosimetry, and image analysis methods (such as CT scans). However, these methods all have certain limitations in practical applications: water saturation methods cannot completely eliminate closed air bubbles, resulting in large measurement errors for low-permeability rocks, and are difficult to use for in-situ testing; gas displacement methods involve expensive equipment, are complex to operate, and have poor field adaptability; mercury intrusion porosimetry involves toxic metals, causing significant disturbance to the rock sample, and the mercury solution easily leaks along dominant channels; image analysis methods such as CT scans have limited resolution, making it difficult to quantitatively measure porosity, and the equipment is expensive.

[0003] The lost-wax casting method, a mature process in precision casting, works by melting down a wax model to create a precise cavity. Following this logic, low-melting-point wax is injected into the pores of a rock. After solidification, it forms a "negative model" of the pore structure. In-situ core sampling is then performed, and the volume of the in-situ pores can be accurately calculated by weighing the melted wax. However, in practical applications, the lost-wax method for measuring rock porosity faces a series of technical challenges, including wettability barriers (leading to significant interfacial tension between the wax and the pore walls), viscosity limitations (high-viscosity fluids experience extremely high flow resistance in micropores), shrinkage errors (significant volume shrinkage occurs during solidification and cooling), limited connectivity (the wax cannot enter isolated pores), thermal disturbance effects, operational complexity, and error accumulation. Currently, there is no existing method that can effectively overcome the inherent technical difficulties of the lost-wax method while simultaneously achieving rapid on-site sampling and high-precision measurement of rock porosity. Therefore, there is an urgent need to develop a new measurement technique that can solve technical problems such as wettability, viscosity, shrinkage, and thermal disturbance, while taking into account both the characterization of connected pores and total porosity. Summary of the Invention

[0004] This invention aims to apply the lost-wax method to rock porosity measurement and solve existing technical problems such as poor wettability, difficulty in filling micropores, measurement deviation caused by solidification shrinkage, loss of filler material due to thermal disturbance, and inability to characterize isolated pores. To this end, this invention proposes a high-precision rock porosity measurement method based on the lost-wax method. This is a systematic solution that, through key technologies such as surfactant modification, multi-stage pressure wax injection, shrinkage correction, and low-temperature core drilling, achieves for the first time an integrated process of "modification and filling - conformal sampling - correction and analysis," simultaneously obtaining effective porosity and total porosity, ensuring the authenticity and reliability of the measurement results.

[0005] This invention provides the following technical solution:

[0006] A high-precision method for measuring the porosity of rocks based on the lost-wax casting method includes the following steps:

[0007] S1. Drilling pre-drilled holes

[0008] Multiple pre-drilled holes (1) were drilled on the fractured rock surface in the area to be cored for injection of wax and cleaning of the hole walls;

[0009] S2, Wax Injection Filling

[0010] Low-melting-point wax liquid with added surfactant was heated and melted. The wax liquid was then injected into the pre-drilled hole (1) using a multi-stage pressure wax injection process. First, the wax liquid filled the macroscopic pores under low pressure. Then, the pressure was gradually increased to the set pressure and maintained for a certain period of time to allow the wax liquid to overcome capillary resistance and fully penetrate and fill the micropores inside the rock. The total volume V of the injected wax liquid was recorded. inj The wax is then corrected for by the solidification shrinkage rate and subsequently cooled and solidified to form a paraffin filler.

[0011] S3. Core Sampling and Core Preservation

[0012] A core drilling machine equipped with a coolant circulation system was used to drill core samples from the wax injection area. The working temperature of the drill bit was controlled to be lower than the melting point of the wax to prevent the wax from melting out and to obtain a core sample containing paraffin filler (2). After the sample was taken out, it was immediately put into a vacuum insulated protective sleeve for storage to prevent external heat from entering.

[0013] S4, Thermal Desorption and Correction

[0014] The core sample (2) was removed and its pores and surface paraffin were accurately weighed and recorded as the total mass M1. Then, the core sample (2) was placed on the rotary solidification apparatus (5) in the collection device (4), and the heating unit (8) was used to make the temperature inside the instrument higher than the melting point of the wax. The wax inside the sample was completely melted and flowed out through the filter screen (9) and the collection hole (10). The mass m of the outflowing paraffin was completely collected and weighed. wax ;

[0015] S6. Cleaning and Dry Weight Measurement

[0016] The rock core was cleaned using an organic solvent that has no dissolving effect on the rock to remove trace amounts of paraffin residue; the cleaned rock core was then completely dried to constant weight, and its dry weight M2 was obtained.

[0017] S5. Porosity Calculation

[0018] Based on the pre-determined wax solidification volume shrinkage rate η, η was calibrated through solidification experiments of the same batch of wax under simulated pore conditions. The value of η ranged from 5-8%. The melted wax volume was corrected to obtain the effective pore volume V. pore_effective :

[0019] V pore_effective = (m wax / ρ wax ) × (1+η) (1)

[0020] In the above formula, ρ wax The density of the wax;

[0021] Total volume V of the rock core total The volume of the core after washing and drying is obtained by directly measuring the volume using the drainage method, or by measuring the dry weight M2 and the rock skeleton density ρ. matrix calculate:

[0022] V total = V skeleton + V pore_effective = M2 / ρ matrix + V pore_effective (2)

[0023] In the above formula, V skeleton V represents the volume of the core skeleton. pore_effectivew Effective pore volume;

[0024] Effective porosity φ effective for:

[0025] φ effective = V pore_effective / V total × 100% (3)

[0026] Based on the total volume V of injected wax recorded during the pressure wax injection phase inj Considering potential surface leakage losses during the wax injection process (corrected for by the difference in quality between the wax storage tanks before and after wax injection), the total pore volume V of the rock can be estimated. pore_total :

[0027] V pore_total = V inj × k - Vsurface (4)

[0028] In the above formula, k is a correction coefficient that takes into account the diffusion of wax liquid to the periphery of the sampling area during the wax injection process, which was calibrated through previous experiments; V surface The total porosity φ is the volume of wax adhering to the sample surface, estimated by observation or weighing. total for:

[0029] φ total = V pore_total / V total × 100% (5)

[0030] The effective porosity φ of the target material is obtained. effective and total porosity φ total .

[0031] In the above embodiments, total porosity refers to the percentage of the total volume of all pore spaces (whether connected or not) in a rock out of the total rock volume. Effective porosity refers to the percentage of the total rock volume of interconnected pores that allow fluid flow under certain pressure conditions. The pressure wax injection process is similar to forced saturation, thus allowing for the measurement of total porosity, which is impossible with the drainage method.

[0032] Preferably, the multi-stage pressure wax injection process in step S2 specifically includes:

[0033] S2.1 Heat the low-melting-point wax to a molten state and add 1.0-1.5 wt% of a nonionic fluorocarbon surfactant;

[0034] S2.2 A multi-stage pressure wax injection process is adopted using a pressure injection device. First, according to the rock mechanical properties, the wax is injected at a low pressure of 0.05-0.1MPa for 5-10 minutes to fill macroscopic fractures and large pores. Then, the pressure is gradually increased to 0.2-0.5MPa and maintained for 20-40 minutes to allow the wax to overcome capillary resistance and enter the micropores. The multi-stage pressure wax injection process is used to inject multiple times until the wax can no longer be injected, to prevent the wax from flowing away through the dominant channels and to ensure that all pores are fully filled.

[0035] S2.3 After wax injection is completed, allow the paraffin wax to solidify naturally or with assisted cooling, and record the total volume V of the injected wax. inj .

[0036] In the above embodiments, the nonionic fluorocarbon surfactant can reduce the contact angle between the wax liquid and the quartz surface from 80-90° to 20-30°, significantly improving wettability and reducing the interfacial tension between the wax liquid and the rock mineral surface. Depending on the rock properties, the injection pressure may vary, for example, 0.01-0.05 MPa for soft rocks and 0.1-0.2 MPa for hard rocks, and the same applies to subsequent pressurization.

[0037] Preferably, the low-melting-point wax is paraffin or modified wax with a melting point range of 55-70℃, and its viscosity at 60℃ is less than 10 cP and its solidification shrinkage rate is less than 6%.

[0038] In the above embodiments, low-melting-point wax is key to ensuring good flowability and dimensional stability.

[0039] Preferably, step S3 specifically includes the following steps:

[0040] S3.1 uses antifreeze with a freezing point below -20°C as coolant, which circulates through the inner cavity of the hollow drill pipe to cool the cutting edge of the drill bit.

[0041] S3.2 adopts an alternating operation mode of "drilling-pause-cooling", pausing drilling for 1-2 minutes every 5-10cm of drilling;

[0042] S3.3 Control the drilling pressure of the core drill to be 5-15kN, the rotation speed to be 100-300r / min, and the advance rate to be no more than 3cm / min;

[0043] S3.4 integrates a miniature temperature sensor near the drill bit to monitor the temperature at the interface between the drill bit and the sample in real time. When the temperature approaches the melting point of the wax, it will automatically alarm or stop drilling.

[0044] After drilling S3.5 is completed, the sample is immediately placed into a vacuum-insulated protective casing to prevent external heat from entering and causing the wax to melt prematurely.

[0045] Preferably, the pre-drilled hole (1) has a diameter of Φ80-120 mm and a depth of 150-300 mm.

[0046] Preferably, in step S4, the heating unit (8) is a programmable temperature-controlled electric furnace or an oil bath device, with a controllable heating rate and temperature uniformity better than ±2℃.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) Targeted solution to the wettability problem: By adding surfactants, the interfacial tension between the wax liquid and the hydrophilic rock surface is significantly reduced, enabling the wax liquid to overcome capillary resistance and enter the micropores, effectively solving the technical bottleneck of insufficient wax liquid filling in the traditional lost wax method.

[0049] (2) Innovative solution to viscosity limitation: The multi-stage pressure wax injection process is adopted. First, the macroscopic pores are filled under low pressure, and then the pressure is gradually increased to allow the wax liquid to enter the micropores. This not only ensures the filling effect, but also avoids the rock from cracking due to excessive pressure, and achieves uniform filling of pores at all scales.

[0050] (3) Accurate correction of shrinkage error: By pre-calibrating the solidification shrinkage rate of wax, the volume of melted wax is corrected, eliminating the systematic deviation of wax solidification shrinkage on pore volume measurement and significantly improving measurement accuracy.

[0051] (4) Achieve total porosity characterization: By recording the total amount of wax injected and combining it with the correction coefficient, the total porosity of rock was calculated for the first time within the framework of the lost wax method, which made up for the limitation of traditional methods that can only measure connected pores, and provided more comprehensive parameters for reservoir evaluation and other applications.

[0052] (5) Effectively eliminate thermal disturbance: Low temperature core drilling technology is adopted. Through multiple means such as coolant circulation, intermittent drilling, and temperature monitoring, the temperature of the drill bit is always lower than the melting point of the wax during the sampling process, which prevents the filling wax from melting away and ensures the original state of the pore structure.

[0053] (6) High level of intelligence: It integrates sensors and databases to realize full-process data acquisition and automatic correction, reduce human error, and improve detection efficiency and standardization. Attached Figure Description

[0054] Figure 1 This is a schematic flowchart of a high-precision rock porosity measurement method based on the lost-wax method provided in an embodiment of the present invention.

[0055] Figure 2 This is a schematic diagram of the on-site fractured rock sampling area provided in an embodiment of the present invention.

[0056] Figure 3 This is a schematic diagram of a thermal desorption device provided in an embodiment of the present invention.

[0057] In the picture:

[0058] 1-Pre-drilled hole, 2-Core sample, 3-Rock fissure, 4-Collection device, 5-Rotary sample consolidator, 6-Rotary handle, 7-Sealing cap, 8-Heating unit, 9-Filter screen, 10-Collection hole. Detailed Implementation

[0059] The present invention will now be described in detail with reference to embodiments and accompanying drawings. However, it should be understood that the embodiments and drawings are for illustrative purposes only and do not constitute any limitation on the scope of protection of the present invention. All reasonable modifications and combinations included within the inventive spirit of the present invention fall within the scope of protection of the present invention.

[0060] Example 1:

[0061] like Figure 1 This embodiment provides a high-precision rock porosity measurement method based on the lost-wax method, with measurements completed collaboratively in the field and laboratory. The experimental equipment includes a wax injection device, a cryogenic core drilling device, a thermal desorption device, and an intelligent computing device, enabling end-to-end control from on-site wax injection to laboratory data analysis.

[0062] In this embodiment, the equipment and steps required to implement each step of the present invention are described as follows:

[0063] Wax injection stage:

[0064] The wax injection equipment includes a wax storage tank, an electric heater, a surfactant addition device, a multi-stage pressure pump, and wax injection pipelines. It is used to inject molten wax with added surfactants into the pre-drilled hole 1 under pressure according to a multi-stage pressure program. Figure 2 This diagram illustrates the wax injection process. Multiple rock fissures 3 are visible within the rock. Several pre-drilled holes 1 are made on the rock sampling plane. Wax is injected into these holes through the wax injection pipeline to fully fill the rock fissures 3. After the wax solidifies, the next stage, core sample extraction, begins.

[0065] Cryogenic core drilling stage:

[0066] The cryogenic core drilling equipment includes a core drill, a coolant thermostat, a miniature thermocouple (or fiber optic sensor) integrated near the drill bit, a hollow drill rod, and an insulating casing. Temperature control and cooling at this stage prevent the wax sample from melting and leaking due to frictional heat during drilling. The temperature sensor is used to monitor the interface temperature between the drill bit and the sample in real time. The insulating casing is a vacuum-insulated structure used to maintain the sample's low temperature during transport, ensuring the stability of the wax filling material's morphology.

[0067] Thermal desorption stage:

[0068] The thermal desorption equipment includes a collection device 4, a rotary solidification instrument 5, and a high-precision electronic balance, which are used for the complete melting and accurate weighing of the wax liquid. The collection device 4 is equipped with a programmable temperature-controlled heating unit 8, with temperature uniformity better than ±2℃. The high-precision electronic balance is connected to a data processing module to automatically record the quality of the melted wax.

[0069] See Figure 3 The thermal desorption equipment includes a collection device 4, with a sealing cap 7 at its top opening. A rotary sample concentrator 5 is horizontally mounted inside the device; the sample is driven to rotate within the chamber by rotating a handle 6, ensuring uniform heating of the molten wax. A programmable temperature-controlled heating unit 8 is arranged on the inner wall of the collection device 4, and a filter screen 9 is located at the bottom. A collection hole 10 is located below the filter screen 9; the molten wax, after being filtered through the filter screen, drips through the collection hole 10 onto a high-precision electronic balance below for weighing.

[0070] Data processing stage:

[0071] The system uses a computer or other intelligent device to receive weighing data, store wax physical property parameters, perform shrinkage correction, and calculate effective porosity and total porosity. The computer has a pre-stored database of solidification shrinkage rates for different wax materials, which can automatically select the appropriate shrinkage correction coefficient based on the type of wax used. It also stores empirical values ​​for rock skeleton density, supporting user-defined input. The system automatically generates a test report including effective porosity, total porosity, and measurement uncertainty.

[0072] Once the equipment was ready, the experiment began.

[0073] After selecting a test point at a granite roadbed site, a pre-drilled hole 1 with a diameter of 100mm and a depth of 200mm was drilled on the surface of the target rock mass using a hollow drill bit. Rock debris inside the hole was thoroughly cleaned to ensure that the hole wall was clean, providing good conditions for the subsequent full penetration of wax liquid.

[0074] Low-melting-point microcrystalline wax (62℃ melting point, viscosity 8.5 cP at 60℃, solidification shrinkage 5.2%) was placed in a wax storage tank and heated to 75℃ using an electric heater to completely melt it, while adding 1.2 w t A % nonionic fluorocarbon surfactant was added and thoroughly stirred to ensure uniform dispersion. A multi-stage pressure pump was started, employing a two-stage wax injection process: first, a low pressure of 0.08 MPa was injected for 8 minutes to allow the wax to preferentially fill macroscopic fissures; then, the pressure was gradually increased to 0.35 MPa and maintained for 25 minutes to allow the modified wax to overcome capillary resistance and enter micropores. A 30-minute interval was maintained after each injection, allowing the previously injected wax to completely solidify before re-injecting, until no more wax could be injected. During the wax injection process, the total volume V of the injected wax was recorded using a flow meter. inj =156mL. After wax injection, stop pressurizing and allow the wax to cool and solidify naturally in the pores, forming a paraffin filling material consistent with the pore structure of the rock.

[0075] After the wax-injected area has completely cooled and solidified, cryogenic core sampling is carried out. Ethylene glycol antifreeze with a freezing point of -25℃ is injected into the coolant constant temperature chamber, and the circulation pump is started to deliver the coolant through the hollow drill pipe cavity to the drill bit cutting edge, achieving dynamic cooling during drilling. During drilling, the drilling pressure is controlled at 10kN, the rotation speed at 200r / min, and the drilling rate at 2cm / min. An intermittent "drill-pause-cooling" mode is adopted, pausing for 1.5 minutes after every 8cm of drilling to effectively control the drill bit temperature. A miniature thermocouple integrated at the drill bit tip monitors the temperature at the drill bit-core interface in real time, ensuring it remains below 48℃, below the paraffin melting point (62℃), to prevent the paraffin from melting away from the pores. Finally, a columnar core sample with a diameter of 50mm is drilled out, immediately placed in a vacuum-insulated casing, and sealed for preservation.

[0076] The sealed core sample was transferred to the laboratory for thermal desorption and weighing. First, the core was removed, and the total mass of the core and the paraffin wax adhering to it and in the pores was accurately weighed (M1 = 345.6 g). Then, the core was placed on the rotary sample collector 5 of the collection device 4, and the heating unit 8 was activated to raise the temperature to 70°C (8°C above the melting point of paraffin wax) at a rate of 3°C / min, and held at this temperature for 20 minutes to allow the paraffin wax in the pores to fully melt. The molten wax was collected in a weighing dish through a filter screen 9 and a collection hole 10, and the recovered paraffin wax mass m was measured. wax =8.42g. The dewaxed core was removed, and the surface residual wax was cleaned with xylene organic solvent. It was then dried to constant weight, and the dry weight of the rock skeleton was measured as M2 = 336.5g. Simultaneously, the total volume V of the cleaned and dried core was measured using the water displacement method. total =128.4cm³.

[0077] Based on the above data, the following calculations were performed: Paraffin density ρ wax =0.91g / cm³, melted wax volume V wax =8.42 / 0.91=9.25cm³. Considering a solidification shrinkage rate of 5.2%, the corrected effective pore volume V pore_effective =9.25×(1+0.052)=9.73cm³. Effective porosity φ effective =9.73 / 128.4×100%=7.58%.

[0078] Based on the total volume V of injected wax recorded during the wax injection phase inj =156mL, after deducting the residue in the wax injection tubing and the surface adhesion at the orifice (the correction factor k=0.12 was determined through previous calibration tests, and the surface adhesion is estimated to be about 5mL), the total pore volume V is calculated. pore_total =156×0.12-5=13.72cm³. Total porosity φ total =13.72 / 128.4×100%=10.69%.

[0079] To verify the measurement accuracy, the effective porosity result of this method was compared with the result measured by the helium specific gravity method (7.62%), with a relative error of 0.5%; the total porosity result was compared with the result analyzed by high-resolution CT scan images (10.58%), with a relative error of 1.0%. The results show that the method of this invention effectively overcomes the problems of wettability, viscosity, and shrinkage of the traditional lost-wax method, and achieves high-precision porosity measurement.

[0080] Example 2:

[0081] Example 2 demonstrates the technical effects of the present invention for complex fractured rock masses. At a selected testing point in a sandstone outcrop, the rock exhibits well-developed natural fractures, including macroscopic fractures (width 0.1-2 mm), micro-fractures (width 10-100 μm), and intergranular pores (pore diameter 1-50 μm), among other multi-scale pore structures.

[0082] The measurement procedure was similar to that in Example 1, but in the wax injection filling step, considering the strong hydrophilicity and well-developed micropores of sandstone, the amount of surfactant added was increased to 1.5w. t The process employs a three-stage pressure wax injection technique: low-pressure injection at 0.05 MPa for 5 minutes (filling macroscopic fissures), medium-pressure injection at 0.15 MPa for 15 minutes (filling microscopic fissures), and high-pressure injection at 0.4 MPa for 30 minutes (filling intergranular pores). During the wax injection process, a miniature pressure sensor integrated into the injection pipeline monitors the pressure changes within the pores. When the pressure stabilizes, it indicates that the filling is essentially complete.

[0083] The steps of cryogenic core sampling and thermal desorption are basically the same as in Example 1. The effective porosity φ is calculated. effective =12.8%, total porosity φ total =18.3%. The core was cut open along the axial direction and observed by scanning electron microscopy. The results showed that traces of wax filling were visible in the micro-fractures and intergranular pores, indicating that the modified wax liquid successfully entered the multi-scale pore system; the interface between the pore edge and the wax body was dense and there were no obvious shrinkage gaps, proving that the selection of the shrinkage correction coefficient was reasonable.

[0084] To verify the reliability of the total porosity calculation results, the volume of the skeleton was measured by the helium true density method after crushing the same batch of rock blocks. The total porosity was then calculated by combining the total volume from the drainage method, and the result was 18.1%, which is highly consistent with the result of the method of this invention (18.3%), with a relative error of only 1.1%. This example fully demonstrates the effectiveness and reliability of the present invention in the porosity measurement of complex fractured rock masses.

[0085] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A high-precision method for measuring the porosity of rocks based on the lost-wax casting method, characterized in that, Includes the following steps: S1. Drilling pre-drilled holes Multiple pre-drilled holes (1) were drilled on the fractured rock surface in the area to be cored and the hole walls were cleaned. S2, Wax Injection Filling Low-melting-point wax liquid with added surfactant was heated and melted. The wax liquid was then injected into the pre-drilled hole (1) using a multi-stage pressure wax injection process. First, the wax liquid filled the macroscopic pores under low pressure. Then, the pressure was gradually increased to the set pressure and maintained for a certain period of time to allow the wax liquid to overcome capillary resistance and fully penetrate and fill the micropores inside the rock. The total volume V of the injected wax liquid was recorded. inj The wax is then corrected for by the solidification shrinkage rate and subsequently cooled and solidified to form a paraffin filler. S3. Core Sampling and Core Preservation A core drilling machine equipped with a coolant circulation system was used to drill core samples from the wax injection area. The working temperature of the drill bit was controlled to be lower than the melting point of the wax to prevent the wax from melting out and to obtain a core sample containing paraffin filler (2). After the sample was taken out, it was immediately put into a vacuum insulated protective sleeve for storage to prevent external heat from entering. S4, Thermal Desorption and Correction The core sample (2) was removed and its pores and surface paraffin were accurately weighed and recorded as the total mass M1. Then, the core sample (2) was placed on the rotary solidification apparatus (5) in the collection device (4), and the heating unit (8) was used to make the temperature inside the instrument higher than the melting point of the wax. The wax inside the sample was completely melted and flowed out through the filter screen (9) and the collection hole (10). The mass m of the outflowing paraffin was completely collected and weighed. wax ; S6. Cleaning and Dry Weight Measurement The rock core was cleaned using an organic solvent that has no dissolving effect on the rock to remove trace amounts of paraffin residue; the cleaned rock core was then completely dried to constant weight, and its dry weight M2 was obtained. S5. Porosity Calculation Based on the pre-determined wax solidification volume shrinkage rate η, η was calibrated through solidification experiments of the same batch of wax under simulated pore conditions. The value of η ranged from 5-8%. The melted wax volume was corrected to obtain the effective pore volume V. pore_effective : V pore_effective = (m wax / r wax ) × (1+n) (1) In the above formula, ρ wax The density of the wax; Total volume V of the rock core total The volume of the core after washing and drying is obtained by directly measuring the volume using the drainage method, or by measuring the dry weight M2 and the rock skeleton density ρ. matrix calculate: In total = V skeleton + In pore_effective = M2 / ρ matrix + In pore_effective (2) In the above formula, V skeleton V represents the volume of the core skeleton. pore_effectivew Effective pore volume; Effective porosity φ effective for: φ effective = V pore_effective / V total × 100% (3) Based on the total volume V of injected wax recorded during the pressure wax injection phase inj Considering potential surface leakage losses during the wax injection process, and correcting for the quality difference in the wax storage tank before and after wax injection, the total pore volume V of the rock can be calculated. pore_total : V pore_total = V inj × k - V surface (4) In the above formula, k is a correction coefficient that takes into account the diffusion of wax liquid to the periphery of the sampling area during the wax injection process, which was calibrated through previous experiments; V surface The total porosity φ is the volume of wax adhering to the sample surface, estimated by observation or weighing. total for: φ total = V pore_total / V total × 100% (5) The effective porosity φ of the target material is obtained. effective and total porosity φ total .

2. The method for high-precision measurement of rock porosity based on the lost-wax method according to claim 1, characterized in that, The multi-stage pressure wax injection process described in step S2 specifically includes: S2.1 Heating low-melting-point wax to a molten state and adding 1.0-1.5 wt% of nonionic fluorocarbon surfactant can reduce the contact angle between the wax liquid and the quartz surface from 80-90° to 20-30°, significantly improving wettability and reducing the interfacial tension between the wax liquid and the rock mineral surface. S2.2 A multi-stage pressure wax injection process is adopted using a pressure injection device. First, according to the rock mechanical properties, the wax is injected at a low pressure of 0.05-0.1MPa for 5-10 minutes to fill macroscopic fractures and large pores. Then, the pressure is gradually increased to 0.2-0.5MPa and maintained for 20-40 minutes to allow the wax to overcome capillary resistance and enter the micropores. The multi-stage pressure wax injection process is used to inject multiple times until the wax can no longer be injected, to prevent the wax from flowing away through the dominant channels and to ensure that all pores are fully filled. S2.3 After wax injection is completed, allow the paraffin wax to solidify naturally or with assisted cooling, and record the total volume V of the injected wax. inj .

3. The method for high-precision measurement of rock porosity based on the lost-wax method according to claim 2, characterized in that, The low-melting-point wax is a paraffin or modified wax with a melting point range of 55-70℃, and its viscosity at 60℃ is less than 10 cP and its solidification shrinkage rate is less than 6%, in order to ensure good fluidity and dimensional stability.

4. The method for high-precision measurement of rock porosity based on the lost-wax method according to claim 1, characterized in that, Step S3 specifically includes the following steps: S3.1 uses antifreeze with a freezing point below -20℃ as coolant, which circulates through the inner cavity of the hollow drill pipe to cool the cutting edge of the drill bit; S3.2 adopts an alternating operation mode of "drilling-pause-cooling", pausing drilling for 1-2 minutes every 5-10cm of drilling; S3.3 Control the drilling pressure of the core drill to be 5-15kN, the rotation speed to be 100-300r / min, and the advance rate to be no more than 3cm / min; S3.4 integrates a miniature temperature sensor near the drill bit to monitor the temperature at the interface between the drill bit and the sample in real time. When the temperature approaches the melting point of the wax, it will automatically alarm or stop drilling. After drilling S3.5 is completed, the sample is immediately placed into a vacuum-insulated casing to prevent external heat from entering and causing the wax to melt prematurely.

5. The method for high-precision measurement of rock porosity based on the lost-wax method according to claim 1, characterized in that, The pre-drilled hole (1) has a diameter of Φ80-120 mm and a depth of 150-300 mm.

6. The method for high-precision measurement of rock porosity based on the lost-wax method according to claim 1, characterized in that, In step S4, the heating unit (8) is a programmable temperature-controlled electric furnace or an oil bath device, with a controllable heating rate and temperature uniformity better than ±2℃.

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