_{2}to Displace Coalbed Methane: A Case Study

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Injecting N_{2} to displace methane is an effective way to enhance coalbed methane drainage, and the influence radius of this process is an important factor in borehole arrangement. To determine reasonable spacing between injection boreholes and discharge boreholes, experimental and theoretical studies were carried out. The change of rule for the influence radius was determined based on the flow rate changes at the discharge boreholes when injecting gas into a coal seam in the field. Based on gas seepage-diffusion theory, a model for injecting N_{2} to displace coalbed methane was established. Through numerical simulation, the time characteristics of the influence radius were analyzed. The results show the following: Under different gas injection pressure conditions, the influence radius increases exponentially as injection time increases, but the rate of increase of the influence radius decreases gradually. For the same injection time, the higher the injection pressure, the wider the influence radius will be. After obtaining field results, regression analysis was applied to analyze the numerical results of gas injection at different pressures, and then, the quantitative relationship between the injection influence radius _{2} to displace methane.

As the main greenhouse gas, efforts to reduce CO_{2} emissions from the combustion of fossil fuels have received increasing attention due to concerns over global climate change in recent years [_{2} into the deep coal seams to enhance coalbed methane (ECBM) has been an efficient method for the CO_{2} capture, utilization, and sequestration (CCUS) engineering [_{2} into coal seams, which can efficiently reduce the partial pressure of CH_{4} in the adsorption equilibrium conditions and ultimately enhance the CH_{4} desorption from the surfaces of the micro- and mesopores in coal.

At the end of the 20th century, CO_{2} was injected for increasing coalbed methane (CO_{2}-ECBM) in the United States San Juan Basin; this approach was the prelude to CBM coalbed gas injection in the field driving the technology for methane [_{2}-ECBM field test in San Juan Basin, Black Warrior Basin, Illinois Basin, and Central Appalachian Basin. In Hokkaido, Japan, Poland, and Alberta, Canada, field trials of different sizes were also carried out. China United Coalbed Methane Co. Ltd. was the first mine that conducted CO_{2} injection into the ground in Jincheng, China. Next, China carried out the low-pressure (<0.6 MPa) N_{2}-ECBM test in Pingdingshan Coal Mine and Yangquan Coal Mine.

With the development of coalbed gas injection technologies to promote gas drainage, scholars conducted many experimental and theoretical studies, and their studies played an active part in the popularization and application of the technology [

Assuming that the movement of adsorption gas desorbing from the coal micropore and moving to the fissure system follows Fick diffusion law, then the equation for CH_{4} and N_{2} diffusing in the pore system is

In the equation, _{4} and _{2}. ^{3}. ^{2}/s. ^{3}·s), reflecting the mass exchange of the matrix between the adsorption state in the pore system and the free state in the fissure system.

Assuming that the migration of free gas in the coal seam can be treated as a fluid filtration process, then the mass conservation equation [

In the equation, ^{3} and

In the equation, ^{2}, ^{2}, and

^{3};

The components of the adsorption state under the hypothetical equilibrium pressure

In this formula, ^{3}, ^{3}, ^{3}/kg, and ^{-1}. Finally,

The mass exchange between an adsorption state on the coal surface and a free state in the fissure system can be defined as

In the equation,

Because the injection pressure is usually not large, the compressibility of the gas is ignored. Considering the gas component as an ideal gas, the ideal gas state equation can be represented as

In the equation,

Plugging equations (

Equations (

In this paper, Shigang Coal Mine is selected to carry out the test. The Shigang Coal Mine is located in the northeast of the Qinshui Basin in China; the geological structure of the coal seam is complex. The thickness of the no. 15 coal seam, respectively, is 5.93-8.22 m, and its dip angle is approximately 8°. The gas content of the no. 15 coal seam is 10.71-15.21 m^{3}/t, and the maximum methane pressure is 0.76 MPa.

To build a numerical model for injecting N_{2} displacement of coalbed methane, the following basic information needs to be assumed: (1) The in situ stress condition and the anisotropy of the coal seam is neglected. (2) The original gas pressure and gas content in coal seam are the same everywhere. (3) Gas flow in the coal seam is an isothermal process. Adsorption and desorption conform to the generalized Langmuir isothermal adsorption equation. (4) In the process of gas injection, the injection pressure will not decrease with the increase in borehole depth. The gas pressure in the borehole stays constant. (5) Gas in the coal seam has a constant composition.

Although boring holes and injecting N_{2} into coal seam to displace methane are three-dimensional processes in spacing, considering the feasibility and effectiveness of numerical calculations, we simplify it to a two-dimensional problem, selecting the vertical direction of the borehole as the research direction. The numerical model is shown in Figure

Model for the numerical simulation.

The initial and boundary conditions of the model are as follows:

The original CH_{4} pressure of coalbed is _{2} pressure is 0

The model boundary flow is 0, the atmospheric pressure of the mine is 0.1 MPa, and N_{2} injection pressure is set to 0.5 MPa

The engineering conditions need to be simulated as follows. The excavation roadway is 4 m high. The borehole radius is set to 90 mm in the model. There is a no flow rate boundary around the model. The injection borehole pressures are 0.5 MPa, 0.8 MPa, 1.2 MPa, and 2.0 MPa. Parameters of physical properties in numerical simulation are obtained from the laboratory data of coal samples in Shigang Coal Mine, as shown in Table

Parameters of physical properties in numerical simulation.

Symbol | Parameter | Number | Unit | Symbol | Parameter | Number | Unit |
---|---|---|---|---|---|---|---|

Coal density | kg/m^{3} |
Coal porosity | 0.05 | ||||

CH_{4} density |
0.717 | kg/m^{3} |
N_{2} density |
1.25 | kg/m^{3} | ||

CH_{4} dynamic viscosity coefficient |
Pa·s | N_{2} dynamic viscosity coefficient |
Pa·s | ||||

CH_{4} Langmuir constant |
0.03832 | m^{3}/kg |
N_{2} Langmuir constant |
0.01658 | m^{3}/kg | ||

CH_{4} Langmuir constant |
0.51 | 1/MPa | N_{2} Langmuir constant |
0.46 | 1/Mpa | ||

Permeability of coal | m^{2} |
Gas pressure under standard conditions | 0.1 | MPa |

Usually, before injecting N_{2} into coal seam, N_{2} content in coal seam was very low. Therefore, to simplify numerical simulation, N_{2} content in coal seam was ignored. Figure _{2} cloud. It can be seen from Figure _{2} pressure range gradually increased; therefore, the range in which the N_{2} pressure rises from 0 to 0.05 MPa is defined as the influence radius of the gas injection.

N_{2} pressure in coalbed around the borehole at different times under 0.5 MPa gas injection pressure.

1 h

4 h

10 h

At an injection pressure of 0.5 MPa, the trend of the N_{2} pressure in the direction along the _{2} pressure around the borehole is 0 Pa. As the injection time increases, the influence area of N_{2} near the coal wall increases too. After 1 h gas injection, the influence radius of N_{2} reaches 0.75 m. After 4 h, the influence radius of N_{2} reaches 1.40 m. After 10 h, the influence radius of N_{2} reaches 2.10 m. The trend of the N_{2} pressure in the direction perpendicular to the _{2} pressure around the borehole is 0 Pa. As the injection time increases, the influence area of N_{2} near the coal wall increases too. After 1 h gas injection, the influence radius of N_{2} reaches 0.63 m. After 4 h, the influence radius of N_{2} reaches 1.23 m. After 10 h, the influence radius of N_{2} reaches 1.85 m.

N_{2} pressure along the

N_{2} pressure perpendicular to the

From the numerical simulation results, the difference of the influence radius of the gas injection in horizontal direction (bedding direction) and in vertical direction (vertical bedding direction) existed. The radius of the gas injection in the vertical bedding direction is smaller than that in the bedding direction, and the difference is related to the existence of the power of the gas injection time,

The comparison table of gas injection influence radius in vertical and horizontal directions.

Classification | Injection time (h) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |

Radius in horizontal direction (m) | 0.75 | 1.00 | 1.20 | 1.40 | 1.50 | 1.70 | 1.80 | 1.90 | 1.90 | 2.10 |

Radius in vertical direction (m) | 0.63 | 0.84 | 1.05 | 1.23 | 1.34 | 1.49 | 1.60 | 1.68 | 1.70 | 1.85 |

Difference (m) | 0.12 | 0.16 | 0.15 | 0.17 | 0.16 | 0.21 | 0.20 | 0.22 | 0.20 | 0.25 |

Difference between the horizontal and vertical distances with injection time.

In this paper, Shigang Coal Mine is selected to carry out the test. The Shigang Coal Mine is located in the northeast of the Qinshui Basin in China. The basic situation is shown in Section

An injecting N_{2} system consists of the connecting pipe, quick joint, borehole sealing device, high pressure switch, pressure gauge, etc., as shown in Figure

Diagram of gas injection.

Plan

The borehole arrangement is shown in Figure

Borehole arrangement in the effective gas injection radius test.

Borehole parameters in the injecting N_{2} influence radius test.

Borehole type | Specifications and sealing requirements of borehole | |||
---|---|---|---|---|

Depth (m) | Diameter (mm) | Sealing depth (m) | Sealing method | |

N_{2} injection borehole |
60 | 94 | 10 | |

Observation borehole | 60 | 94 | 10 | |

Borehole spacing (m) |

After discharging the borehole continuously for approximately 12 h, the field gas injection started. The flow rate before gas injection was balanced. In the process of gas injection, the gas injected in a coal seam accumulated in the coal, and then it formed a large pressure difference between injection borehole and discharge borehole, so the coalbed methane moved from the high-pressure side (injection borehole) to the low-pressure side (discharge borehole). At an early stage of gas injection, the flow rate of discharge borehole increased obviously. As the injection time increased, the flow rate of discharge borehole tended to rise steadily in spite of some local instability until the maximum mix flow rate of discharge borehole appeared. Then, the data were measured continuously with the variation of not more than 5%. The data variation condition of each group is shown in Figure

Flow change of releasing borehole at different spacing.

Mixture flow

Pure flow

It can be seen from Figure

Test results for the influence radius.

N_{2} injection time (h) |
N_{2} injection influence radius (m) |
Increase radius before and after N_{2} injection flow rate |
---|---|---|

1 | 0.8 | 10.9 |

2 | 1.0 | 6.7 |

4 | 1.5 | 6.6 |

7 | 2.0 | 7.2 |

Table

The results of the numerical simulation and the field test are shown in Figure

Influence radius under 0.5 MPa N_{2} injection pressure.

Adopting the numerical model above, by changing the injection pressure, the impact of the injection pressure on the influence radius of the gas injection is studied, as we can see in Figure _{2} pressure around the boreholes has the same properties as when the injection pressure is 0.5 MPa. The influence radius increases with increasing time. When the injection pressure is 0.8 MPa, the N_{2} influence radius reaches 1 m after 1 h of injection. The N_{2} influence radius reaches 1.8 m after 4 h of injection. The N_{2} influence radius reaches 2.7 m after 10 h of injection. When the injection pressure is 1.2 MPa, the N_{2} influence radius reaches 1.2 m after 1 h of injection. The N_{2} influence radius reaches 2.2 m after 4 h of injection. The N_{2} influence radius reaches 3.5 m after 10 h of injection. When the injection pressure is 1.6 MPa, the N_{2} influence radius reaches 1.3 m after 1 h of injection. The influence radius of N_{2} reaches 2.6 m after 4 h of injection. The influence radius of N_{2} reaches 3.8 m after 10 h of injection. When the injection pressure is 2.0 MPa, the influence radius of N_{2} reaches 1.5 m after 1 h of injection. The influence radius of N_{2} reaches 3.0 m after 4 h of injection. The influence radius of N_{2} reaches 4.6 m after 10 h of injection.

N_{2} pressure along

0.8 MPa

1.2 MPa

1.6 MPa

2.0 MPa

Meanwhile, from Figure

Influence radius under different gas injection pressures.

The fitting formula and fitting coefficient.

Gas injection pressure (MPa) | ||||
---|---|---|---|---|

0.5 | 0.7495 | 0.446 | 0.99803 | |

0.8 | 0.9947 | 0.4491 | 0.99641 | |

1.2 | 1.2072 | 0.4544 | 0.99729 | |

1.6 | 1.3294 | 0.4605 | 0.99522 | |

2.0 | 1.4878 | 0.4704 | 0.98169 |

Injection pressure is an important factor in the influence radius.

Coefficients of

In order to verify the calculation formula (

Comparison of the calculation results and the test results.

Gas injection time (h) | Radius of field test (m) | Radius of numerical simulation (m) | Error | Formula ( |
Relative error |
---|---|---|---|---|---|

1 | 0.8 | 0.75 | 6.25% | 0.76 | 4.80% |

2 | 1.0 | 1.0 | 0 | 1.04 | 3.65% |

4 | 1.5 | 1.4 | 6.67% | 1.41 | 5.96% |

7 | 2.0 | 1.8 | 10% | 1.81 | 9.54% |

To determine reasonable parameters for injection borehole, the increasing rate of influence radius under different injection pressures is calculated using the quantitative relation formula (

Increment rate of Influence radius under different gas injection pressures.

In the formula,

From Figures

Influence radius under different gas injection pressures.

Reasonable spacing between N_{2} injection borehole and N_{2} drainage borehole under different N_{2} injection pressures.

Based on numerical simulation and field test analysis of the time characteristics of the injecting N_{2} influence radius, we conclude the following:

The influence radius and injection time can both be expressed as exponents, and their fitting degrees are linear with injection pressure. Based on this and the results obtained from the relation between the influence radius and the injection time, the pressure tends to agree with the results obtained from the field test. The difference is no more than 10%

Reasonable spacing between injection boreholes and discharge boreholes can be determined from the increasing rate of the influence radius. When the increasing rate of the influence radius is less than 10%, the influence radius is the reasonable spacing of the boreholes. Using this method, the reasonable spacing between the injection borehole and the discharge borehole at Shigang Coal Mine was determined to be 1.56 m, and 1.5 m was used for the field test

At different injection pressures, the reasonable spacing between injection boreholes and discharge boreholes tend to be linearly related to the injection pressure

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

This work was carried out with funding from the National Natural Science Foundation of China (Grant nos. 51174081 and 51404091).

Original record of the field experiment that will support the conclusion of the article.

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