Multi-dimensional comprehensive evaluation method for hydraulic slotting coal body effect under true triaxial condition

Abstract

The invention belongs to the technical field of coal mine gas disaster prevention and control, and relates to a multi-dimensional comprehensive evaluation method for a hydraulic slotting coal body effect under a true triaxial condition, and the method comprises the following steps: S1, sample preparation and true triaxial stress loading; s2, implementing multi-source information dynamic synchronous monitoring in the slotting process; s3, performing multi-dimensional quantitative inspection on the end office effect; and S4, establishing a comprehensive evaluation index system and effect grading. According to the method, the real states (sigma1gt, sigma2gt and sigma3) of the underground coal body are simulated in a true triaxial stress environment, so that the hydraulic slotting effect is accurately evaluated. Firstly, compared with a traditional conventional triaxial test, the method adopts an independent three-dimensional stress loading system and is combined with a raw coal sample of a prefabricated drill hole, a complex stress field under a raw coal mine is reduced more accurately, and deviation between an evaluation result and an actual working condition is avoided. Meanwhile, the method integrates dynamic monitoring and static inspection, comprehensively covers from mechanical behaviors to seepage characteristics, and makes up for the deficiency of existing single index evaluation.

Description

Technical Field

[0001] This invention belongs to the field of coal mine gas disaster prevention and control technology, and relates to a multi-dimensional comprehensive evaluation method for the effect of hydraulically cut coal body under true triaxial conditions. Background Technology

[0002] Coal has long dominated my country's energy consumption structure. However, with increasing mining depth, the difficulty of controlling gas in high-gas, low-permeability coal seams has become increasingly prominent. Hydraulic slotting technology, as a key measure to improve the efficiency of gas extraction in low-permeability coal seams, uses high-pressure water jets to cut slots in the coal seam, promoting pressure relief and permeability enhancement, and expanding gas flow channels. It has been widely applied in underground coal mine drilling, including cross-seam drilling, in-seam drilling, and coal seam exposure in rock passages. This technology is particularly suitable for coal seam conditions with high ground stress, high gas content, and low permeability, effectively improving gas extraction and preventing rockbursts.

[0003] However, current methods for evaluating the effectiveness of hydraulic fracturing still have significant limitations. On the one hand, commonly used evaluation criteria in the field rely heavily on macroscopic data such as gas extraction concentration and extraction volume, or on single permeability tests under conventional triaxial stress conditions in the laboratory. These methods cannot accurately reproduce the actual three-dimensional stress state underground (i.e., σ1>σ2>σ3), resulting in poor consistency between evaluation results and actual working conditions. For example, laboratory conditions often cannot simulate comprehensive geological factors such as coal seam depth, geostress distribution, and heterogeneity of coal structure, making it difficult to reflect the permeability evolution of the coal body in the actual stress field after fracturing. On the other hand, existing evaluations are mostly "post-hoc" analyses, unable to achieve real-time monitoring and capture of dynamic behaviors such as fracturing network expansion, coal slag migration, and stress redistribution during the fracturing process. Therefore, it is difficult to optimize fracturing process parameters and prevent problems such as hole blockage, blowouts, and drill bit sticking during construction.

[0004] Furthermore, existing evaluation systems employ relatively singular indicators, often focusing on coal output or local permeability changes, lacking a comprehensive consideration of fracture morphology, pressure relief range, permeability anisotropy, and gas extraction potential. For example, while some technical solutions attempt to evaluate effectiveness by statistically analyzing coal output, measuring fracture width, and calculating expansion deformation rate, or by using radar chart matching and graphic similarity algorithms to evaluate the adaptability of fracture models under different coal seam conditions, and even attempt to use transient electromagnetic methods to detect the roof moisture content after fracturing to indirectly assess fracture development, these methods either fail to consider the influence of the actual three-dimensional stress path or fail to form a systematic, dynamic, and multi-indicator collaborative evaluation model, thus failing to achieve a scientific, quantitative, and comprehensive evaluation of the pressure relief and permeability enhancement effect of fractures.

[0005] Therefore, there is an urgent need in this field for a method that can dynamically monitor and simulate the entire process of hydraulic fracturing under realistic downhole three-dimensional stress conditions, and comprehensively and quantitatively evaluate the fracturing effect from multiple perspectives, including fracture morphology, pressure relief range, permeability anisotropy, and extraction potential. Such a method will provide a scientific basis for optimizing fracturing process parameters, further improving the efficiency and safety of gas extraction from low-permeability coal seams. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a multi-dimensional comprehensive evaluation method for the effect of hydraulically cut coal seams under true triaxial conditions. By integrating true triaxial loading, multi-source information synchronous monitoring, and final destructive testing, a comprehensive evaluation system covering "process-morphology-attribute-function" is constructed, which realizes accurate, quantitative, and forward-looking evaluation of the cut effect, thereby overcoming the shortcomings of the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A multi-dimensional comprehensive evaluation method for the effect of hydraulically slotted coal seams under true triaxial conditions includes the following steps: S1 Specimen Preparation and True Triaxial Stress Loading; S2 kerfing process implementation and dynamic synchronous monitoring of multi-source information; S3 Endgame Effect Multidimensional Quantitative Examination; S4 establishes a comprehensive evaluation index system and effect classification.

[0008] Furthermore, in step S1, a standard raw coal sample containing pre-drilled holes is prepared and placed in a true triaxial pressure chamber, where three-dimensional stresses σ1, σ2, and σ3 are applied independently, wherein σ1>σ2>σ3, to simulate the in-situ stress state of the target coal seam.

[0009] Furthermore, in step S2, hydraulic cutting is performed while maintaining true triaxial stress, and spatiotemporal information of crack propagation, stress-strain response information of coal body, evolution information of pore pressure field, and dynamic evolution information of permeability are collected simultaneously.

[0010] Furthermore, the spatiotemporal information of crack propagation is monitored and the spatiotemporal evolution of micro-fracture events is located by an acoustic emission sensor array arranged in the true triaxial pressure chamber, and the volume of the crack-affected area and the crack network complexity index are calculated.

[0011] Furthermore, the stress-strain response information of the coal body is monitored by strain gauges or digital image correlation systems attached to the surface of the standard raw coal sample during the cutting process to detect the three-dimensional strain field changes on the surface of the standard raw coal sample, and the strain change curves and maximum stress relief strain values ​​in each stress direction are output.

[0012] Furthermore, the pore pressure field evolution information is monitored by pore pressure sensors embedded in different locations inside the standard raw coal sample to observe the propagation and attenuation of the slotted water pressure.

[0013] Furthermore, the permeability dynamic evolution information is obtained by injecting gas into the borehole of the standard raw coal sample and measuring the permeability changes in different directions using the transient pressure pulse method or the steady-state method, and outputting the permeability anisotropic evolution curve and the maximum amplification factor.

[0014] Furthermore, in step S3, multi-dimensional quantitative verification is performed by fine characterizing the morphology of the seam mesh, strengthening the test of permeability characteristics, and simulating the equivalent extraction efficiency.

[0015] Furthermore, the fine characterization of the fracture network morphology is achieved by CT scanning, three-dimensional optical scanning, or by photographing the standard raw coal sample after cutting it, to obtain the true three-dimensional geometric morphology of the fracture groove and derived cracks, and to quantitatively evaluate the fracture length, width, depth, and complexity of the fracture network.

[0016] Furthermore, the permeability enhancement test is conducted by measuring the permeability of the standard raw coal sample after slotting and the standard raw coal sample without slotting under different gas pressures under different confining pressures, and calculating the permeability enhancement factor and anisotropy ratio of the slotted area.

[0017] Furthermore, the penetration enhancement factor K improvement = K after / K before The K after K represents the permeability of the standard raw coal sample after slotting. before The permeability of the uncut standard raw coal sample is given.

[0018] Furthermore, the equivalent extraction efficiency simulation involves repositioning the standard raw coal sample after slotting into a true triaxial pressure chamber to restore the three-dimensional stresses σ1, σ2, and σ3, connecting the gas source to the extraction system, simulating the underground gas extraction process, recording the gas pressure decay curve and cumulative extraction volume in the borehole of the standard raw coal sample, and calculating the extraction efficiency improvement rate.

[0019] Further, in step S4, a weighted comprehensive evaluation index S is constructed, S=α*F1+β*F2+γ*F3+δ*F4, where α, β, γ, and δ are weighting coefficients, and F1~F4 are normalized functions of the permeability enhancement factor, pressure relief strain value, mesh complexity index, and extraction efficiency improvement rate, respectively, and the effect is graded according to the S value.

[0020] Furthermore, an S value of 0.4 to 0.45 is considered excellent, 0.45 to 0.50 is considered good, 0.50 to 0.55 is considered average, and above 0.55 is considered poor.

[0021] The beneficial effects of this invention are as follows: The technical solution of this invention achieves accurate evaluation of the hydraulic fracture effect by simulating the real state of underground coal seams (σ1>σ2>σ3) under true triaxial stress environment. Firstly, its environmental realism is significantly improved: compared to traditional conventional triaxial testing, this method uses an independent three-dimensional stress loading system combined with pre-drilled raw coal samples to more accurately reproduce the complex stress field underground in coal mines, avoiding deviations between evaluation results and actual working conditions. Simultaneously, comprehensive dimensions are a core advantage: the method integrates dynamic monitoring (step S2, including AE sensor array capturing fracture propagation, strain gauge monitoring of pressure relief strain, and pore pressure sensor tracking of water pressure attenuation and real-time changes in permeability) and static inspection (step S3, such as CT scan of fracture network morphology and permeability enhancement calculation), comprehensively covering everything from mechanical behavior to seepage characteristics, thus overcoming the shortcomings of existing single-index evaluations.

[0022] Secondly, the quantification of indicators and the forward-looking nature of functions further enhance the practical value of the scheme. Through simultaneous collection and calculation of multi-source data, all indicators are objectively quantified, avoiding subjective judgment and facilitating scheme comparison and database construction. More importantly, the equivalent extraction efficiency simulation stage predicts the on-site gas extraction potential in the laboratory (such as pressure decay curves and cumulative extraction volume), extending the evaluation from morphological attributes to engineering functions, significantly improving the accuracy of prediction.

[0023] Finally, the scheme provides quantitative guidance for coal mine gas control: based on the weighted comprehensive index S and combined with effect grading, it directly outputs suggestions for optimizing process parameters, thereby guiding the "one policy per mine" practice and improving overall safety and efficiency.

[0024] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a flowchart illustrating a multi-dimensional comprehensive evaluation method for the effect of hydraulically cut coal seams under true triaxial conditions, as described in this embodiment. Detailed Implementation

[0026] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0027] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0028] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0029] Example 1 like Figure 1 As shown in this embodiment, a multi-dimensional comprehensive evaluation method for the effect of hydraulically cut coal seams under true triaxial conditions is provided. The specific steps are as follows: S1 Sample Preparation and True Triaxial Stress Loading: A standard raw coal sample with dimensions of 100 mm (or 150 mm) was selected (in another embodiment, a standard raw coal sample with dimensions of 150 mm can also be used). This sample was taken from a coal seam in a coal mine in Shanxi Province and has typical low permeability characteristics. A borehole with a diameter of 10 mm and a depth of 50 mm was pre-drilled at the center of the sample to simulate underground drilling. The standard raw coal sample was placed in a true triaxial pressure chamber equipped with independent hydraulic loading devices in the X, Y, and Z directions, which can precisely control the stress in each direction. Three-dimensional stresses σ1 = 10 MPa (maximum principal stress in the vertical direction), σ2 = 8 MPa (intermediate principal stress in the horizontal direction), and σ3 = 5 MPa (minimum principal stress in the horizontal direction, where σ1 > σ2 > σ3) were independently applied to simulate the in-situ stress state of the target coal seam.

[0030] The loading process is carried out step by step. First, σ3 to 5MPa is applied and stabilized for 5 minutes; then σ2 to 8MPa is applied and stabilized for 5 minutes; finally, σ1 to 10MPa is applied and stabilized for 10 minutes to ensure that the stress field is uniformly distributed and there is no slippage.

[0031] S2 kerfing process implementation and multi-source information dynamic synchronous monitoring: Under the aforementioned true triaxial stress state, hydraulic kerfing is performed according to the set kerfing process parameters. Specific parameters are: water pressure 20MPa, flow rate 10L / min, action time 5min, and nozzle movement speed 2mm / s. The drill rod is inserted into the pre-drilled hole, and the nozzle is driven by a high-pressure water pump to move along the borehole axis to achieve hydraulic kerfing. Simultaneously, the following information is collected: a. Spatiotemporal information of crack propagation: An acoustic emission (AE) sensor array (eight sensors, evenly distributed around the sample) deployed in a true triaxial pressure chamber was used to monitor and locate the spatiotemporal evolution of micro-fracture events within the coal mass during the cracking process. The collected AE event data was processed by software to generate a three-dimensional point cloud map, and the volume of the crack-affected area was calculated to be 200 cm³. 3 The complexity index (fractal dimension) of the stitch mesh is 1.8.

[0032] b. Coal body stress-strain response information: The three-dimensional strain field changes on the sample surface during the cutting process were monitored using strain gauges (12 in total, distributed across the three principal stress directions) attached to the surface of the standard raw coal sample. Data showed that the maximum strain value of the pressure relief expansion strain perpendicular to the cut surface (σ3 direction) was 0.5%, while strain fluctuations in other directions were less than 0.2%. Strain change curves for each stress direction were output, showing significant strain recovery within 1 minute after the cutting began.

[0033] c. Pore pressure field evolution information: The propagation and attenuation of water pressure in the borehole were monitored using four pore pressure sensors (located at 5mm, 10mm, 20mm, and 30mm around the borehole perimeter) embedded in the standard raw coal sample. The results showed that the water pressure propagated outward from the borehole center, with a peak pressure of 18MPa at 5mm and attenuating to 2MPa at 30mm, indicating that the hydraulic energy effect range was approximately 25mm.

[0034] d. Dynamic evolution of permeability: After the intervals during the slotting process (paused every 1 minute), nitrogen (N2) was injected into the borehole of the standard raw coal sample. The permeability changes in the directions parallel and perpendicular to the slot surface were measured using the transient pressure pulse method. The initial permeability was 0.1 mD. After slotting, the permeability in the parallel direction increased to 0.4 mD, and in the perpendicular direction it increased to 0.3 mD, with a maximum increase of 4 times. The output permeability anisotropy evolution curve showed an anisotropy ratio (parallel / perpendicular) of 1.33.

[0035] S3 Final Effect Multi-Dimensional Quantitative Verification: After the slit and dynamic tests are completed, the confining pressure is removed, and standard raw coal samples are taken out for the following tests: e. Detailed characterization of the suture network morphology: The specimen was non-destructively scanned using a CT scanner to obtain the true three-dimensional geometric morphology of the suture groove and derived cracks. The main suture length was 80 mm, the suture width was 2 mm, the suture depth was 40 mm, the suture network complexity (number of branch sutures) was 5, and the fractal dimension was 1.8.

[0036] f. Permeability Enhancement Test: Under different confining pressures (5 MPa, 8 MPa, and 10 MPa), the permeability of the standard raw coal sample after slotting and the control sample without slotting were measured at a gas pressure of 1 MPa. The permeability after slotting was 0.4 mD, and the permeability without slotting was 0.1 mD. The permeability enhancement factor K was calculated. improvement = K after / K before = 4, and the anisotropy ratio (parallel / perpendicular) is 1.33.

[0037] g. Simulation of equivalent extraction efficiency: The standard raw coal sample after slotting was placed back into a true triaxial pressure chamber, restoring the three-dimensional stresses σ1=10MPa, σ2=8MPa, and σ3=5MPa. A gas source (initial gas pressure 2MPa) was connected to the extraction system (extraction negative pressure -0.05MPa) to simulate the underground gas extraction process. The gas pressure decay curve in the borehole (it takes 10 minutes to decay from 2MPa to 0.5MPa) and the cumulative extraction volume (total 50mL) were recorded. Compared with the unslotted control group, the extraction efficiency improvement rate was calculated to be 30%.

[0038] S4 Establish a comprehensive evaluation index system and effect classification: Based on the above data, construct a weighted comprehensive evaluation index S. The weight coefficients are determined by the analytic hierarchy process: α=0.4 (permeability enhancement factor), β=0.2 (pressure relief strain value), γ=0.2 (mesh complexity index), δ=0.2 (extraction efficiency improvement rate). The normalization functions F1~F4 adopt the maximum value normalization method to map each index to the [0,1] interval. Specific calculation: F1(4)=1, F2(0.5%)=0.00125, F3(1.8)=0.45, F4(30%)=0.075, S=0.4*1 + 0.2*0.00125 + 0.2*0.45 + 0.2*0.075=0.505≈0.51. Based on the S-value range (0.4~0.45 is excellent, 0.45~0.50 is good, 0.50~0.55 is medium, and above 0.55 is poor), the effect is graded as medium, and the following optimization directions for process parameters are provided: Under the current parameters, the complexity of the seam mesh is relatively high, but the permeability increase can be further improved. It is recommended to optimize the water pressure to 25MPa to increase the number of branch seams.

[0039] This embodiment achieves a multi-dimensional comprehensive evaluation of the hydraulic kerfing effect through the above steps. The evaluation results are reliable and easy to guide field applications.

[0040] Example 2 This embodiment provides another multi-dimensional comprehensive evaluation method for the effect of hydraulic slotting in coal seams under true triaxial conditions. The difference from Embodiment 1 is that in step S1, three-dimensional stresses σ1=15MPa, σ2=10MPa, and σ3=6MPa are applied; in step S2, the slotting process parameters are adjusted to water pressure 25MPa, flow rate 12L / min, action time 6min, and nozzle moving speed 3mm / s.

[0041] Specific monitoring results: The volume of the area affected by the crack is 250 cm³. 3 The complexity index of the seam mesh is 2.0; the maximum value of the pressure relief expansion strain is 0.6%; the pore pressure attenuation range is approximately 30 mm; the maximum permeability increase factor is 5 times; and the anisotropy ratio is 1.5. In step S3, the seam length is 90 mm, the seam width is 2.5 mm, and the seam depth is 45 mm; the permeability enhancement factor is 5; and the extraction efficiency improvement rate is 40%. In step S4, the calculation is S = 0.4*1 + 0.2*0.0012 + 0.2*0.4 + 0.2*0.08 = 0.496 ≈ 0.50, and the effect is rated as good. The nozzle speed can be further adjusted to 4 mm / s to optimize anisotropy.

[0042] This embodiment verifies the applicability of the evaluation method under different stresses and process parameters, and the results show that the kerfing effect is more significant under high stress.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A multi-dimensional comprehensive evaluation method for the effect of hydraulically cut coal seams under true triaxial conditions, characterized in that, Includes the following steps: S1 specimen preparation and true triaxial stress loading; S2 kerfing process implementation and dynamic synchronous monitoring of multi-source information; S3 Endgame Effect Multidimensional Quantitative Examination; S4 establishes a comprehensive evaluation index system and effect classification.

2. The method according to claim 1, characterized in that, In step S1, a standard raw coal sample containing pre-drilled holes is prepared and placed in a true triaxial pressure chamber. Three-dimensional stresses σ1, σ2, and σ3 are applied independently, where σ1>σ2>σ3, to simulate the in-situ stress state of the target coal seam.

3. The method according to claim 2, characterized in that, In step S2, hydraulic cutting is performed while maintaining true triaxial stress, and spatiotemporal information of crack propagation, stress-strain response information of coal body, pore pressure field evolution information, and dynamic evolution information of permeability are collected simultaneously.

4. The method according to claim 3, characterized in that, The spatiotemporal information of crack propagation is monitored and located by an acoustic emission sensor array arranged in the true triaxial pressure chamber to determine the spatiotemporal evolution of micro-fracture events, and the volume of the crack-affected area and the crack network complexity index are calculated.

5. The method according to claim 4, characterized in that, The stress-strain response information of the coal body is monitored by strain gauges or digital image correlation systems attached to the surface of the standard raw coal sample during the cutting process to detect the three-dimensional strain field changes on the surface of the standard raw coal sample, and the strain change curves and maximum stress relief strain values ​​in each stress direction are output.

6. The method according to claim 5, characterized in that, The evolution information of the pore pressure field is monitored by pore pressure sensors embedded in different locations inside the standard raw coal sample to observe the propagation and attenuation of the water pressure in the cut.

7. The method according to claim 6, characterized in that, The permeability dynamic evolution information is obtained by injecting gas into the borehole of the standard raw coal sample and measuring the permeability changes in different directions using the transient pressure pulse method or the steady-state method, and outputting the permeability anisotropic evolution curve and the maximum amplification factor.

8. The method according to claim 7, characterized in that, In step S3, multi-dimensional quantitative verification is performed by fine characterizing the morphology of the seam mesh, strengthening the test of permeability characteristics, and simulating the equivalent extraction efficiency.

9. The method according to claim 8, characterized in that, The fine characterization of the fracture network morphology is achieved by CT scanning, three-dimensional optical scanning, or by photographing the standard raw coal sample after cutting it, to obtain the true three-dimensional geometric morphology of the fracture groove and derived cracks, and to quantitatively evaluate the fracture length, width, depth and complexity of the fracture network.

10. The method according to claim 9, characterized in that, The enhanced permeability test involves measuring the permeability of the standard raw coal sample after cutting and the standard raw coal sample without cutting under different confining pressures, and calculating the permeability enhancement factor and anisotropy ratio of the cut area.

11. The method according to claim 10, characterized in that, The permeability enhancement factor K improvement = K after / K before The K after K represents the permeability of the standard raw coal sample after slotting. before The permeability of the uncut standard raw coal sample is given.

12. The method according to claim 10, characterized in that, The equivalent extraction efficiency simulation involves repositioning the standard raw coal sample after slotting into a true triaxial pressure chamber to restore the three-dimensional stresses σ1, σ2, and σ3, connecting the gas source to the extraction system, simulating the underground gas extraction process, recording the gas pressure decay curve and cumulative extraction volume in the borehole of the standard raw coal sample, and calculating the extraction efficiency improvement rate.

13. The method according to claim 12, characterized in that, In step S4, a weighted comprehensive evaluation index S is constructed, S=α*F1+β*F2+γ*F3+δ*F4, where α, β, γ, and δ are weighting coefficients, and F1~F4 are normalized functions of the permeability enhancement factor, pressure relief strain value, mesh complexity index, and extraction efficiency improvement rate, respectively. The effect is graded according to the S value.

14. The method according to claim 12, characterized in that, The S value is 0.4~0.45 for excellent grade, 0.45~0.50 for good grade, 0.50~0.55 for medium grade, and above 0.55 for poor grade.

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