# Stress and Damage Induced Gas Flow Pattern and Permeability Variation of Coal from Songzao Coalfield in Southwest China

^{1}

^{2}

^{3}

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Results and Analysis

#### 2.1. Characteristics of Gas Flow Rate and Pattern Change in the Intact Coal and Damaged Coal

^{2}of fitting curves. The fitting equation is shown as below,

^{3}/s), σ

_{e}is the effective stress (MPa), a and b are the fitting parameters related to material properties. The “a” decreased and the “b” increased with increasing injection pressure according to the experimental results. The fitting parameters and coefficients of determination under different injection pressures are shown in Table 1.

^{2}), L is the length of the coal specimens (m), A is the cross-sectional area of the coal specimens (m

^{2}), P

_{1}is the gas pressure at the upper stream or inlet of specimens (Pa), and P

_{2}is the gas pressure at the downstream or outlet of the specimens (Pa), and q

_{pi}is the pseudo-initial flow rate.

#### 2.2. Characteristics of Permeability Change in the Intact Coal and Damaged Coal

_{pi})/μ represents the instability of gas flow induced permeability increase. Equation (6) can be transformed into:

_{real}= Kμ/[μ + Kα(q − q

_{pi})] is defined to be the permeability in the damaged coal under low effective stress, which increases with increasing q as the result of instability of gas flow induced by high ratio of injection gas pressure over effective stress. Therefore, Equation (7) can be transformed into:

## 3. Experiment Setup for Tests

#### 3.1. Experiment Apparatus

#### 3.2. General Geology of the Sampling Area

_{2}b of Songzao coalfield in Chongqing, southwest China. The geological map of Songzao coalfield highlighting the sampling area in the field is shown in Figure 11. Table 2 lists the geological profiles of the coal seam. The elevation in the area ranges from 360 to 1300 masl. The oldest outcropping strata in the area is the Middle Silurian Hanjiadian formation, and the newest outcropping strata in the area is the Upper Triassic Xujiahe formation. The coal-bearing strata is the Permian Longtan formation. The coal-bearing strata shows marine-continental transitional facies. The overburden depth ranges between 500 and 540 m with a 460–500 m thick alluvium layer at the top. It has a dip angle ranging from 35° to 37° and an average thickness of 0.75 m, ranging from 0.22 m to 1.20 m. The coal seam K

_{2}b is a gas infiltrated layer, with a gas content of 10.14 m

^{3}/t. The temperature in the coal seam K

_{2}b is in the range of 28.5 °C to 29.3 °C. The average vitrinite content of the coal is 68.4%, inertinite content is 18.7%, clay mineral content is 8.4%, oxide content is 2.0%, carbonate content is 0.4%, and sulfide content is 2.1%.

#### 3.3. Characterisation of Coal

#### 3.4. Specimen Preparation

#### 3.5. Experiment Procedure and Data Treatment

_{c}is the confining pressure (MPa), P

_{i}is the gas injection pressure (MPa) and P

_{0}is the gas outlet pressure (atmospheric pressure for the experiment).

_{e}= 1, 2, 3, 4, 5, 6 and 7 MPa were applied to the specimen, respectively. Then, the high-pressure helium tank was connected to the specimen to inject the inert gas helium into the specimen up to the specified injection pressure (from 0.3 to 4.5 MPa). The gas flow rate at each pressure point was measured.

_{e}= 7 MPa (injection pressure = 4.5 MPa) was applied to the specimen that was used in the experiment of Condition 1. The deviatoric stress was then applied to the specimen with a speed of 0.1 mm/min till its post-peak stage. Then, the stress was released. The isotropic in-situ stress states of σ

_{e}= 1, 2, 3, 4, 5, 6 and 7 MPa were applied again to the specimen, respectively. The corresponding injection pressure was injected and the flow rate of gas at each pressure point was measured.

## 4. Conclusions

- (1)
- Both the gas flow rates in the intact coal and damaged coal increased with increasing injection pressure. The gas flow rate in the damaged coal was higher than that in the intact coal by two to three orders of magnitude. The logarithmic relation was found to be available for providing the highest coefficients of determination R
^{2}of fitting curves to describe the relations between the gas flow rates and effective stresses of both the intact coal and damaged coal. - (2)
- Pseudo-initial flow rates were observed in the intact coal under low effective stress. Klinkenberg effect may account for this phenomenon. Pseudo-initial flow rate represents the level of slippage force. Based on the pseudo-initial flow rate, the equation to describe relations between gas flow rate and square of pressure gradient in the intact coal under low effective stress has been proposed. As the effective stress increased, the start-up pressure gradients were observed. The start-up pressure gradient increased as the effective stress increased, which indicated that the mobility of gas in the coal was weakened. The start-up pressure gradient represents the level of frictional resistance. Based on the start-up pressure gradient, the equation to describe the relations between gas flow rate and square of pressure gradient in the intact coal has been proposed.
- (3)
- The gas flow rate in the damaged coal increased nonlinearly as the square of pressure gradient increased under low effective stress. As the effective stress increased, the increase of gas flow rate in the coal turned to be linear. Based on the quadratic function relation and pseudo-initial flow rate, the equation to describe the relations between gas flow rate and square of pressure gradient in the damaged coal under low effective stress has been proposed. In the damaged coal, the original skeleton was damaged and recombined. The increasing injection gas pressure under low effective stress promoted the damage and recombination of coal, which led to the erosion of coal particles. The phenomena would enhance the permeability of coal. Therefore, the coefficient of additional acceleration or resistance α was negative.
- (4)
- The apparent permeability in the intact coal calculated by Darcy’s law decreased or increased with increasing injection pressure under the same effective stresses. Whereas, the absolute permeability corrected by the analysis above stayed constant with increasing injection pressure. The deviation of the apparent permeability calculated by Darcy’s law is the result of ignoring the Klinkenberg effect and frictional resistance. The corrected absolute permeability of the intact coal showed advantages in accurately estimating the performance of coal reservoirs.
- (5)
- The permeability in the damaged coal under low effective stress increased with increasing injection pressure. As the effective stress increased, the permeability in coal turned to be constant with increasing injection pressure. Compared with the intact coal, more connecting and obvious fractures are observed in the damaged coal. Thus, the permeability in the damaged coal was higher than that in the intact coal by two to three orders of magnitude.

## Supplementary Materials

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 6.**The apparent permeability and corrected absolute permeability in the intact coal under different injection pressures and effective stresses: (

**a**) the difference between the apparent permeability and corrected permeability caused by Klinkenberg effect; and (

**b**) the difference between the apparent permeability and corrected permeability caused by frictional resistance.

**Figure 7.**The permeability in the damaged coal under different injection pressures and effective stresses.

**Figure 8.**The permeability against confining stress at the same injection pressure: (

**a**) permeability against confining stress in intact coal; and (

**b**) permeability against confining stress in damaged coal.

**Figure 10.**Thermo-Hydro-Mechanical (THM) coupling with triaxial servo-controlled seepage apparatus for coal and rock: 1—Lifter; 2—Pressure vessel; 3—Sensor of axial stress; 4—Oil tank; 5—Movable work platform; 6—Heater tubes; 7—Oil inlet valve; 8—Sensor of axial displacement; 9—Hydraulic cylinder of axial stress; 10—Force plate; 11—Gas inlet valve; 12—Gas outlet valve; 13—Air drain valve; 14—Hydraulic oil inlet and drain valve; 15—Circumferential extensometers; 16—Specimens.

**Figure 13.**The experimental stress and injection pressure conditions: (

**a**) Condition 1; and (

**b**) Condition 2. Red circles are the measured points.

Injection Pressure (Intact Coal) | a | b | R^{2} | Injection Pressure (Damaged Coal) | a | b | R^{2} |
---|---|---|---|---|---|---|---|

1 MPa | −0.006 | 0.009 | 0.993 | 1 MPa | −0.664 | 1.264 | 0.966 |

2 MPa | −0.017 | 0.030 | 0.997 | 2 MPa | −4.806 | 8.5659 | 0.904 |

3 MPa | −0.032 | 0.061 | 0.993 | 3 MPa | −9.871 | 18.453 | 0.900 |

4 MPa | −0.054 | 0.105 | 0.994 | 4 MPa | −12.580 | 24.977 | 0.944 |

Layer Number | Lithology | Thickness (m) |
---|---|---|

1 | Alluvium | 460 |

2 | Sandstone | 5.45 |

3 | Sandy mudstone | 2.65 |

4 | Coal seam | 1.83 |

5 | Sandy mudstone | 7.43 |

6 | Limestone | 1.25 |

7 | Sandstone | 3.96 |

8 | Limestone | 1.01 |

9 | Sandy mudstone | 3.18 |

10 | Argillaceous limestone | 4.90 |

11 | Coal seam | 0.75 |

12 | Sandy mudstone | 3.72 |

13 | Coal seam | 0.24 |

14 | Sandy mudstone | 3.41 |

15 | Siliceous limestone | 1.56 |

16 | Calcareous mudstone | 1.19 |

BET Surface Area (m^{2}/g) | Langmuir Surface Area (m^{2}/g) | Total Pore Volume (cm^{3}/g) | Average Pore Width (Å) | Mad (%) | Aad (%) | Vad (%) | Fcad (%) |
---|---|---|---|---|---|---|---|

0.2997 | 0.4744 | 0.0014 | 189.5302 | 1.17 | 13.16 | 21.03 | 64.64 |

UCS (MPa) | Density (kg/m^{3}) | Young’s Modulus (GPa) | Poisson’s ratio | ||||

14.72 | 1460 | 4.77 | 0.21 |

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**MDPI and ACS Style**

Li, M.; Cao, J.; Li, W. Stress and Damage Induced Gas Flow Pattern and Permeability Variation of Coal from Songzao Coalfield in Southwest China. *Energies* **2016**, *9*, 351.
https://doi.org/10.3390/en9050351

**AMA Style**

Li M, Cao J, Li W. Stress and Damage Induced Gas Flow Pattern and Permeability Variation of Coal from Songzao Coalfield in Southwest China. *Energies*. 2016; 9(5):351.
https://doi.org/10.3390/en9050351

**Chicago/Turabian Style**

Li, Minghui, Jie Cao, and Wenpu Li. 2016. "Stress and Damage Induced Gas Flow Pattern and Permeability Variation of Coal from Songzao Coalfield in Southwest China" *Energies* 9, no. 5: 351.
https://doi.org/10.3390/en9050351