Preparation method of natural gas hydrate large physical model fracturing experimental heterogeneous rock sample

Abstract

The present application belongs to the technical field of natural gas hydrate development, and relates to a preparation method of a natural gas hydrate large physical model fracturing experiment heterogeneous rock sample, physical and mechanical parameters obtained by the method are checked through field pressure cone penetration test and pressure maintaining core physical property test data, so that the accuracy of rock sample preparation data is ensured; the prepared rock sample selects minerals according to target reservoir mineral components, mineral proportions and particle size compositions, and the physical and mechanical parameters of the rock sample skeleton are consistent with the target reservoir, so that the reliability of the rock sample preparation result is ensured; on the basis of the rock sample skeleton, the top and bottom of the rock sample are treated in layers by using the capillary force seepage and absorption of the rock sample pores, so that the heterogeneous hydrate reservoir rock sample is prepared, and a foundation is laid for experimental research and analysis of fracture propagation behavior in the heterogeneous natural gas hydrate reservoir.

Description

Technical Field

[0001] This invention belongs to the field of natural gas hydrate development technology, and relates to a method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates. Background Technology

[0002] Natural gas hydrate is a recognized potential clean alternative energy source. Researchers have conducted extraction trials of natural gas hydrate around the world. However, the current single-well production and stable production cycles of these trials are insufficient to meet the demands of industrial-scale development of natural gas hydrate. Low single-well production capacity and short stable production cycles are key constraints on the industrial-scale development of natural gas hydrate.

[0003] Hydraulic fracturing can effectively expand the hydrate decomposition front, increase the hydrate decomposition rate, and improve reservoir seepage conditions, thereby significantly improving single-well productivity and production stability. Therefore, conducting large-scale physical simulation experiments of hydraulic fracturing in natural gas hydrate reservoirs to reveal the propagation law of fracturing fractures in hydrate reservoirs is crucial for clarifying the fracture geometry and control mechanism in hydrate reservoirs. However, due to the consolidation and strengthening effect of hydrates, there are significant mechanical heterogeneities between natural gas hydrate reservoirs and the surrounding layers. When hydraulic fracturing fractures extend to the reservoir-layer interface, the fracture propagation law is complex and directly affects the spatial geometry of the fractures. Therefore, conducting hydraulic fracturing experiments on heterogeneous rock samples from hydrate reservoirs is particularly important.

[0004] Due to the difficulty in maintaining temperature and pressure for core sampling in natural gas hydrate reservoirs, current technologies cannot yet meet the requirement for large-size rock samples from hydraulic fracturing of natural gas hydrate reservoirs. CN117451526A discloses an experimental method for simulating fracturing of marine hydrate sediments, mainly including: preparing an artificial core framework based on the mineral composition and physical and mechanical properties of the target hydrate reservoir to simulate the geological conditions of the hydrate reservoir; quantitatively controlling the saturation of hydrate-like substances generated within the artificial core framework; performing fracturing simulation experiments in a low-temperature environment after antifreezing treatment of the hydraulic system and fracturing fluid; observing the fracture morphology and distribution after the experiment, using MATLAB to reduce noise in the experimental data, plotting fracturing curves, and analyzing the experimental results. CN112525661A discloses a method for preparing simulated rock samples of natural gas hydrates, which uses a pressed framework, saturated solution, and refrigeration to form simulated rock samples to replace in-situ natural gas hydrate rock samples, providing experimental rock samples for simulating the rock mechanical properties of natural gas hydrate reservoirs and related simulation experiments. The aforementioned existing technologies can all meet the requirements for preparing homogeneous large-sized rock samples, but cannot meet the experimental testing requirements for heterogeneous rock samples.

[0005] In summary, there is an urgent need to develop a method for preparing large-size heterogeneous rock samples from natural gas hydrate reservoirs to meet the testing requirements of large-scale fracturing experiments in hydrate reservoirs. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates. This method enables the preparation of large-size heterogeneous rock samples, meeting the testing requirements of large-scale fracturing experiments in hydrate reservoirs.

[0007] To achieve this objective, the present invention adopts the following technical solution: This invention provides a method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates, the method comprising the following steps: (1) Determine the required material components for the standard core based on the material composition parameters and physical and mechanical parameters of the target reservoir, then weigh the mineral materials, mix them with water, and press them into a standard core. (2) Based on the test results of the physical and mechanical parameters of the standard rock core obtained in step (1), weigh the mineral material, add water and mix it, then fill it into the mold containing the fracturing simulation pipe column, and press it into shape to obtain a large-size artificial rock sample skeleton. (3) The large-sized artificial rock sample skeleton obtained in step (2) is saturated with aqueous solutions containing hydrate inhibitors and those without hydrate inhibitors, and then methane hydrate is synthesized in situ using the saturated methane gas method to obtain the heterogeneous rock sample of the large model fracturing experiment of the natural gas hydrate.

[0008] The method for preparing heterogeneous rock samples for large-scale hydraulic fracturing experiments of natural gas hydrates described in this invention is based on the study of the morphological characteristics of hydraulic fracturing fractures in natural gas hydrate sediments, especially the fracture propagation law under heterogeneous reservoir conditions. It involves preparing a standard core with material composition and physical-mechanical parameters consistent with the target reservoir, thereby obtaining a large-size artificial rock sample skeleton. Based on this skeleton, the rock sample is layered using capillary force permeation through the pores, thus preparing heterogeneous hydrate reservoir rock samples. This lays the foundation for experimental research and analysis of fracture propagation behavior in heterogeneous natural gas hydrate reservoirs.

[0009] Preferably, the material composition parameters of the target reservoir in step (1) include mineral components, mineral proportions, and particle size distribution.

[0010] The mineral composition, mineral ratio, and grain size distribution provide a basis for the selection of mineral raw materials used in rock sample preparation.

[0011] Preferably, the material composition parameters of the target reservoir in step (1) are determined using an X-ray diffractometer and a laser particle size analyzer.

[0012] Preferably, the physical and mechanical parameters of the target reservoir in step (1) include physical properties and uniaxial compressive strength.

[0013] The physical properties include porosity, permeability, etc., but are not limited to these. Those skilled in the art can make adaptive adjustments according to the actual application scenario.

[0014] Preferably, the physical and mechanical parameters of the target reservoir in step (1) are obtained by well logging data and core analysis of the target reservoir, and are verified by field pressure cone penetration test and pressure-holding core physical property test data.

[0015] Preferably, in step (1), after weighing the mineral materials, cement is added for bonding, and then water is added for mixing.

[0016] Preferably, after pressing in step (1), the material is sequentially demolded, hydrated, and dried.

[0017] In order to ensure complete cement hydration, the rock sample is compacted, demolded and cored, then hydrated in a steam environment at 40, 60 and 70°C in sequence, and finally dried in a ventilated environment at room temperature and pressure.

[0018] Preferably, based on the physical and mechanical parameters of the target reservoir described in step (1), the amount of water added, the amount of cement, and the pressing process parameters are adjusted until the physical and mechanical parameters of the standard core match those of the target reservoir.

[0019] The physical and mechanical properties of the target reservoir are used as key simulation parameters. By adjusting the amount of water added, the amount of cement, and the pressing process parameters, the physical and mechanical properties of the standard core are made to match those of the target reservoir.

[0020] It should be noted that this invention does not specifically limit the composition of the mineral materials, the amount of cement, the amount of water added, or the pressing process parameters. Those skilled in the art can select appropriate mineral material composition, cement amount, water added, and pressing process parameters according to actual application conditions. The pressing process parameters may include pressing pressure, pressing time, etc., but are not limited to the contents listed above.

[0021] Preferably, in step (2), after weighing the mineral materials, cement is added for bonding, and then water is added for mixing.

[0022] Preferably, the fracturing simulation string in step (2) is designed with an outer diameter based on the principle of geometric similarity and the actual well completion string parameters. The bottom of the fracturing simulation string has a pre-set fracturing hole, and the fracturing simulation string is pre-embedded in the center of a large-size artificial rock sample skeleton.

[0023] In step (2), the large-sized artificial rock sample skeleton is prepared by filling it in small amounts multiple times, and after each filling, it is shaken thoroughly.

[0024] Preferably, after pressing and molding in step (2), the material is sequentially demolded, hydrated, and dried.

[0025] In order to ensure complete cement hydration, the rock sample is first demolded and sampled after being loaded and pressed into shape. Then, it is hydrated in a water vapor environment at 40, 60 and 70°C in sequence, and finally dried in a ventilated environment at room temperature and pressure.

[0026] Preferably, the size of the large-sized artificial rock sample skeleton in step (2) is 300mm×300mm×300mm.

[0027] Preferably, the specific steps of the layered saturation in step (3) include: uniformly saturating the top of the large-sized artificial rock sample skeleton with formation water containing hydrate inhibitors, and uniformly saturating the bottom of the large-sized artificial rock sample skeleton with formation water without hydrate inhibitors.

[0028] The step of layered saturation utilizes the capillary force of the rock sample pore structure to saturate the top and bottom of the large-sized artificial rock sample skeleton with formation water containing hydrate inhibitors and formation water without hydrate inhibitors, respectively.

[0029] It should be noted that the present invention does not specifically limit the type of hydrate inhibitor, such as ethylene glycol, and those skilled in the art can make an adaptive selection according to the actual application.

[0030] Preferably, the amount of formation water containing hydrate inhibitors at the top of the large-sized artificial rock sample skeleton is calculated based on the water saturation of the overlying layer, one-third of the volume of the large-sized artificial rock sample skeleton, and the porosity.

[0031] Preferably, the amount of formation water without hydrate inhibitors at the bottom of the large-size artificial rock sample skeleton is calculated based on the target reservoir hydrate saturation, two-thirds of the volume of the large-size artificial rock sample skeleton, and porosity.

[0032] Preferably, the in-situ synthesis of methane hydrate in step (3) is carried out at 1-5℃ and 3-10MPa.

[0033] The temperature for the in-situ synthesis of methane hydrate is 1-5℃, for example, it can be 1℃, 2℃, 3℃, 4℃ or 5℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] The pressure for the in-situ synthesis of methane hydrate is 3-10 MPa, for example, it can be 3 MPa, 5 MPa, 6 MPa, 8 MPa or 10 MPa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the top of the heterogeneous rock sample in the large-scale fracturing experiment of natural gas hydrate in step (3) does not contain natural gas hydrate, while the bottom contains natural gas hydrate.

[0036] In situ synthesis of methane hydrate using saturated methane gas method was unsuccessful due to the presence of hydrate inhibitors in the pores at the top of the rock sample. However, the formation water in the pores at the bottom of the rock sample synthesized natural gas hydrate in the pores under the action of methane gas in a low-temperature environment, ultimately forming a heterogeneous rock sample with no natural gas hydrate at the top and natural gas hydrate at the bottom.

[0037] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0038] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates. The obtained physical and mechanical parameters are verified by in-situ cone penetration tests and core property tests, ensuring the accuracy of the rock sample preparation data. The rock samples prepared by the present invention select minerals according to the target reservoir mineral composition, mineral ratio, and grain size composition, and the physical and mechanical parameters of the rock sample skeleton are consistent with those of the target reservoir, ensuring the reliability of the rock sample preparation results. Based on the rock sample skeleton, the present invention utilizes the capillary force of rock sample pores to process the top and bottom of the rock sample in layers, thereby preparing heterogeneous hydrate reservoir rock samples, laying the foundation for experimental research and analysis of fracture propagation behavior in heterogeneous natural gas hydrate reservoirs. Detailed Implementation

[0039] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0040] This invention provides a method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates, the method comprising the following steps: (1) The mineral composition, mineral ratio and particle size composition of the target reservoir were determined by X-ray diffractometer and laser particle size analyzer. The physical properties and uniaxial compressive strength of the target reservoir were obtained by well logging data and core analysis. The results were verified by field pressure cone penetration test and pressure-holding core physical property test data.

[0041] Based on the mineral composition, mineral ratio, grain size distribution, physical properties, and uniaxial compressive strength of the target reservoir, the required material composition for the standard core is determined. Then, the mineral materials are weighed, cement is added for bonding, water is added for mixing, and the standard core is pressed. After demolding and core extraction, the core is sequentially hydrated in a steam environment at 40, 60, and 70°C, and finally dried under ventilation at normal temperature and pressure. Based on the physical properties and uniaxial compressive strength of the target reservoir, the amount of water added, the amount of cement, and the pressing process parameters are adjusted until the physical properties and uniaxial compressive strength of the standard core match those of the target reservoir.

[0042] (2) Based on the test results of the physical and mechanical parameters of the standard core obtained in step (1), mineral materials were weighed, cement was added for bonding, and then water was added and mixed before filling into the mold containing the fracturing simulation string. The filling was done in small amounts and multiple times, and the fracturing simulation string was fully vibrated after each filling. The outer diameter of the fracturing simulation string was designed according to the principle of geometric similarity and combined with the actual well completion string parameters. The bottom of the fracturing simulation string was pre-set with fracturing holes. The fracturing simulation string was pre-embedded in the center of the large-size artificial rock sample skeleton. After loading and pressing to form the shape, the sample was demolded and sampled. Then, it was hydrated in a water vapor environment at 40, 60 and 70℃ in sequence. Finally, it was dried under normal temperature and pressure to obtain a large-size artificial rock sample skeleton of 300mm×300mm×300mm.

[0043] (3) The top of the large-size artificial rock sample skeleton obtained in step (2) is uniformly saturated with formation water containing hydrate inhibitors. The amount of formation water containing hydrate inhibitors is calculated based on the water saturation of the overlying layer, one-third of the volume of the large-size artificial rock sample skeleton, and the porosity. The bottom of the large-size artificial rock sample skeleton is uniformly saturated with formation water without hydrate inhibitors. The amount of formation water without hydrate inhibitors is calculated based on the hydrate saturation of the target reservoir, two-thirds of the volume of the large-size artificial rock sample skeleton, and the porosity. Then, methane hydrate is synthesized in situ using the saturated methane gas method at 1-5℃ and 3-10MPa to obtain a heterogeneous rock sample for fracturing experiment of natural gas hydrate large model without natural gas hydrate at the top and containing natural gas hydrate at the bottom.

[0044] Example 1 This embodiment uses the natural gas hydrate reservoir in the Shenhu area of ​​the South China Sea as an example to further illustrate the present invention. The method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates includes the following steps: (1) Based on the logging data and core data of the pilot borehole for natural gas hydrate drilling in the Shenhu area of ​​the South China Sea, rock mechanics numerical simulation software was first used in combination with logging resistivity, sonic transit time and density data to conduct rock mechanics parameter analysis of the target reservoir, and the data were verified by the field pressure cone penetration test and pressure-holding core physical property test data in the Shenhu area; secondly, X-ray diffractometer and laser particle size analyzer were used to determine the mineral content and particle size composition of the core.

[0045] Analysis and testing revealed that the natural gas hydrate reservoir in the target area is mainly composed of silty clay, with clay minerals accounting for 30%, quartz 35%, and the remainder being calcite, etc., with a grain size of 4-63 μm. The uniaxial compressive strength of the reservoir framework is approximately 1.5 MPa, the porosity is 38%, and the permeability is around 10 mD.

[0046] The standard core material formulation was determined based on the mineral composition, mineral ratio, and particle size distribution of the target reservoir: 36.7% 400-mesh calcite, 33.3% 325-mesh quartz sand, 30% clay minerals (1000-mesh montmorillonite, 3000-mesh kaolinite, 200-mesh chlorite, and 200-mesh illite), with 1% cementitious binder added. The amount of water was 50% of the sum of the mass of the clay minerals and cement. After filling the core, it was compacted at 3 MPa for 24 hours. After compaction, the core was demolded and cored, then hydrated in a steam environment at 40, 60, and 70℃ in sequence, and finally dried under normal temperature and pressure.

[0047] The prepared silty mudstone core has a porosity of 37-39%, a permeability of 8-15 mD, a uniaxial compressive strength of 1.45 MPa, and 90% of the mineral particles are smaller than 47.365 μm with a median particle size of 15.893 μm, which is consistent with the porosity, permeability, particle size, and mechanical characteristics of silty mudstone-type natural gas hydrate reservoirs in the South China Sea.

[0048] (2) Based on the actual fracturing string using a 9-5 / 8″ casing size, and according to the principle of geometric similarity, design a fracturing simulation string with an outer diameter of 1cm, and pre-set a fracturing hole with a diameter of 2mm at the bottom of the fracturing simulation string, and fix the fracturing simulation string at the center of a large-size artificial rock sample skeleton.

[0049] Weigh the materials and 1% cement according to the above proportions, add water and stir to prepare a slurry. The mass of water added should be 50% of the sum of the mass of clay minerals and cement. Fill the slurry into a mold containing a fracturing simulation tubing string. Fill the mold in small amounts multiple times, and shake it after each filling to remove air bubbles. After filling, compact it with a pressure of 3 MPa for 24 hours. After compaction, demold and take samples first, then hydrate them in a water vapor environment at 40, 60, and 70℃ in sequence, and finally dry them in ventilation at room temperature and pressure to obtain a large-sized artificial rock sample skeleton of 300mm×300mm×300mm.

[0050] (3) Based on the prepared large-size artificial rock sample skeleton, firstly, by utilizing the capillary force of the pore structure in the rock sample, combined with the water saturation of the overlying layer of the hydrate reservoir of 40%, the volume of one-third rock sample of 3420 mL and the porosity of 38%, the top of the large-size artificial rock sample skeleton was saturated with 1368 mL of 25% ethylene glycol aqueous solution; secondly, based on the actual natural gas hydrate reservoir saturation of about 30%, the volume of two-thirds rock sample of 6840 mL and the porosity of 38%, the bottom of the large-size artificial rock sample skeleton was uniformly saturated with 2052 mL of formation water.

[0051] In a low-temperature and high-pressure environment of 2℃ and 10MPa, methane hydrate was synthesized in situ using the saturated methane gas method. Due to the presence of 25% ethylene glycol solution in the pores at the top of the rock sample, natural gas hydrate could not be synthesized in the top third of the pores. However, the formation water contained in the pores at the bottom of the rock sample synthesized natural gas hydrate in the pores under the action of methane gas. Finally, a heterogeneous rock sample with a water saturation of 40% at the top and a hydrate saturation of 30% at the bottom was obtained for the large model fracturing experiment.

[0052] In summary, the method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates provided by this invention ensures the accuracy of the rock sample preparation data by verifying the obtained physical and mechanical parameters through in-situ cone penetration tests and core property tests. The rock samples prepared by this invention select minerals based on the target reservoir's mineral composition, proportions, and grain size distribution, and the physical and mechanical parameters of the rock sample skeleton are consistent with those of the target reservoir, guaranteeing the reliability of the rock sample preparation results. Furthermore, based on the rock sample skeleton, this invention utilizes the capillary action of the rock sample pores to process the top and bottom of the rock sample in layers, thereby preparing heterogeneous hydrate reservoir rock samples. This lays the foundation for experimental research and analysis of fracture propagation behavior in heterogeneous natural gas hydrate reservoirs.

[0053] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing heterogeneous rock samples for large-scale fracturing experiments of natural gas hydrates, characterized in that, The preparation method includes the following steps: (1) Determine the required material components for the standard core based on the material composition parameters and physical and mechanical parameters of the target reservoir, then weigh the mineral materials, mix them with water, and press them into a standard core. (2) Based on the test results of the physical and mechanical parameters of the standard rock core obtained in step (1), weigh the mineral material, add water and mix it, then fill it into the mold containing the fracturing simulation pipe column, and press it into shape to obtain a large-size artificial rock sample skeleton. (3) The large-sized artificial rock sample skeleton obtained in step (2) is saturated with aqueous solutions containing hydrate inhibitors and those without hydrate inhibitors, and then methane hydrate is synthesized in situ using the saturated methane gas method to obtain the heterogeneous rock sample of the large model fracturing experiment of the natural gas hydrate.

2. The preparation method according to claim 1, characterized in that, The material composition parameters of the target reservoir in step (1) include mineral composition, mineral proportion and particle size distribution; Preferably, the material composition parameters of the target reservoir in step (1) are determined using an X-ray diffractometer and a laser particle size analyzer.

3. The preparation method according to claim 1 or 2, characterized in that, The physical and mechanical parameters of the target reservoir in step (1) include physical properties and uniaxial compressive strength; Preferably, the physical and mechanical parameters of the target reservoir in step (1) are obtained by well logging data and core analysis of the target reservoir, and are verified by field pressure cone penetration test and pressure-holding core physical property test data.

4. The preparation method according to any one of claims 1-3, characterized in that, After weighing the mineral materials in step (1), cement is added for bonding, and then water is added for mixing; Preferably, after pressing in step (1), the material is sequentially demolded, hydrated, and dried.

5. The preparation method according to claim 4, characterized in that, Based on the physical and mechanical parameters of the target reservoir described in step (1), adjust the amount of water added, the amount of cement, and the pressing process parameters until the physical and mechanical parameters of the standard core match those of the target reservoir.

6. The preparation method according to any one of claims 1-5, characterized in that, In step (2), the fracturing simulation string is designed with an outer diameter based on the principle of geometric similarity and the actual well completion string parameters. The bottom of the fracturing simulation string has a pre-set fracturing hole, and the fracturing simulation string is pre-embedded in the center of a large-size artificial rock sample skeleton. Preferably, after pressing and molding in step (2), the material is sequentially demolded, hydrated, and dried.

7. The preparation method according to any one of claims 1-6, characterized in that, The dimensions of the large-sized artificial rock sample skeleton mentioned in step (2) are 300mm × 300mm × 300mm; Preferably, the specific steps of the layered saturation in step (3) include: uniformly saturating the top of the large-sized artificial rock sample skeleton with formation water containing hydrate inhibitors, and uniformly saturating the bottom of the large-sized artificial rock sample skeleton with formation water without hydrate inhibitors.

8. The preparation method according to claim 7, characterized in that, The amount of formation water containing hydrate inhibitors at the top of the large-size artificial rock sample skeleton is calculated based on the water saturation of the overlying layer, one-third of the volume of the large-size artificial rock sample skeleton, and the porosity. Preferably, the amount of formation water without hydrate inhibitors at the bottom of the large-size artificial rock sample skeleton is calculated based on the target reservoir hydrate saturation, two-thirds of the volume of the large-size artificial rock sample skeleton, and porosity.

9. The preparation method according to any one of claims 1-8, characterized in that, The in-situ synthesis of methane hydrate in step (3) is carried out at 1-5℃ and 3-10MPa.

10. The preparation method according to any one of claims 1-9, characterized in that, The top of the heterogeneous rock sample in the large-scale fracturing experiment of natural gas hydrate in step (3) does not contain natural gas hydrate, while the bottom contains natural gas hydrate.