**5. Identification and Repair of Failed Gas Extraction Boreholes**

## *5.1. Extraction Data Correction*

Extraction data are generally measured by extraction monitoring system on-line or measured manually. Although advanced extraction monitoring system and measuring instruments are adopted, there are still technical difficulties to maintain the accuracy of the system and instruments in the long process of extraction, which inevitably results in the deviation between the extraction data and the real data. For this reason, the difference between gas extraction monitoring data and manual data is investigated, the deviation law of gas-extraction monitoring data is analyzed, the correction model of gas extraction data is established, and the automatic calibration of gas-extraction monitoring data is achieved using computer technology to improve the accuracy of mine gas-extraction monitoring data. The gas-extraction data correction process is shown in Figure 6.

**Figure 6.** Flow chart of gas-extraction data correction.

In addition, based on the different characteristics of gas conditions in the gas pumping station and the extraction pipeline, the extraction gas quantity needs to be converted into the gas quantity in the standard state to be calculated. For the monitoring device which can directly display the gas quantity in the standard condition, the default parameter values (temperature, pressure, etc.) in the device should be updated in time according to the actual gas conditions of the equipment installation site. Finally, the accurate measurement of gas-extraction data is based on the accuracy of the sensor, and the sensor needs to be adjusted regularly and tested on the spot. If the test result deviates greatly, the sensor should be replaced or repaired in time.

#### *5.2. Identification of Failed Gas-Extraction Boreholes*

After the formation of gas-extraction boreholes, under the action of the stress field around the boreholes, the boreholes are often unstable, the cracks between the boreholes are connected, and the cracks between the boreholes and the surrounding rock are connected, resulting in the decrease or even interruption of gas drainage concentration and flow

rate [49,50]. Therefore, it is necessary to identify the failure gas-extraction boreholes so as to guide the restoration of the failed boreholes.

In the extraction pipe network, the gas confluence of each extraction hole enters the branch pipe of the extraction unit, and a flow sensor is installed on the branch pipe to measure the extraction flow of the whole extraction unit. The schematic diagram of the extraction system is shown in Figure 7. The pumping system only depends on the data on the branch pipe and the main pipe, which makes it difficult to grasp the extraction data and state of a single borehole in real time and accurately. Therefore, this paper adopts the data source processing method. When all the extraction boreholes of the extraction unit are connected into the initial stage of the extraction system, the gas-extraction parameter measuring instrument is used to measure the gas flow of each borehole. At the same time, the total flow of the extraction unit is read by the branch pipeline flowmeter, and the flow ratio of each drill hole is calculated by Formula (12).

$$
\phi\_k = \frac{Q\_k}{Q\_c} \tag{12}
$$

In the formula, *φ<sup>k</sup>* is the *k* borehole flow ratio, *Qk* is the *k* borehole flow, and *Qc* is the total branch flow.

In the initial stage of extraction, due to the pressure relief and permeability enhancement of boreholes and the negative pressure of extraction, the gas flow rate of each borehole is larger and the difference of flow ratio is small. With the extraction time lengthens, there are some unstable and air leakage boreholes one after another, and a large amount of air will be mixed in the gas extracted from these boreholes, which will be merged into the extraction pipeline to reduce the extraction flow [20]. At this point, according to the sub-source data processing method, the flow ratio of this kind of boreholes decreases significantly, while the flow ratio of normal borehole increases significantly. According to the extraction requirements of each mine, when the borehole flow ratio is lower than the critical value *φL*, it can be judged as failed gas-extraction boreholes, and technical measures need to be taken to repair or close the extraction boreholes in order to achieve efficient gas extraction.

**Figure 7.** Schematic diagram of pumping pipe network.
