Gas Migration Patterns with Different Borehole Sizes in Underground Coal Seams: Numerical Simulations and Field Observations

Gas flow in a coal seam is a complex process due to the complicated coal structure and the sorption characteristics of coal to adsorbable gas (such as carbon dioxide and methane). It is essential to understand the gas migration patterns for different fields of engineering, such as CBM exploitation, underground coal mine gas drainage, and CO2 geo-sequestration. Many factors influence gas migration patterns. From the surface production wells, the in-seam patterns of gas content cannot be quantified, and it is difficult to predict the total gas production time. In order to understand the gas flow patterns during gas recovery and the gas content variations with respect to production time, a solid-fluid coupled gas migration model is proposed to illustrate the gas flow in a coal seam. Field data was collected and simulation parameters were obtained. Based on this model, different scenarios with different borehole sizes were simulated for both directional boreholes and normal parallel boreholes in coal seams. Specifically, the borehole sizes for the directional boreholes were 10 m, 15 m, and 20 m. The borehole sizes for the normal parallel boreholes were 2 m, 4 m, and 6 m. Under different gas drainage leading times, the total gas recovery and residual gas contents were quantified. In Longwall Panel 909 of the Wuhushan coal mine, one gas drainage borehole and five 4 m monitoring boreholes were drilled. After six months of monitoring, the residual gas content was obtained and compared with the simulation results. Of the total gas, 61.36% was drained out from the first 4 m borehole. In this field study, the effective drainage diameter of the drainage borehole was less than 8 m after six months of drainage. The gas drainage performance was tightly affected by the borehole size and the gas drainage time. It was determined that the field observations were in line with the simulation results. The findings of this study can provide field data for similar conditions.

Improvement of Gas Drainage Efficiency via Optimization of Sealing Depth of Cross-Measure Boreholes

The sealing depth of a gas-drainage borehole is critically important as it directly affects the efficiency of the whole drainage system. In order to determine the shortest reasonable sealing depth, in this paper, a theoretical drainage model using different sealing depths was proposed. Based on theoretical analysis presented, two parts of the fractures system surrounding the drainage borehole were proposed, i.e. the fractures induced by roadway excavation and the fractures induced by borehole drilling. A series of geological in-situ tests and simulations research were conducted to determine the stress and fracture distributions in the surrounding rock of the borehole. The depths of crushing zones, plastic zones and stress concentration zones were determined as 5 m, 2 m and 12 m, respectively. Meanwhile, stress simulation shows that the depth of the stress concentration zone was 12 m from the roadway wall and the stress peak was located at the depth of 8 m, which can be verified by the results of drilling penetration velocity analysis. To determine the optimum sealing depth, gas drainage holes with different sealing depths were drilled in the field. The field results revealed that the crushing zones were the main area for air leakage, and the stress concentration induced by roadway excavation assisted in the reduction of air leakage. Therefore, the optimized sealing depth should both cover the plastic zone and the stress concentration zone. The research achievements can provide a quantitative method for the determination of optimum sealing depth in cross-measure drainage boreholes.

Speculum Observation and Trajectory Measurement in Gas Extraction Drilling: A Case Study of Changling Coal Mine

Coal will still be China’s basic energy for quite a long time. With the increase of mining depth, gas content and pressure also increase. The problems of gas emission and overrun affect the safety and efficient production of coal resource to a certain extent. In this work, the field test of gas drainage borehole peeping and trajectory measurement in coal seam of Changling coal mine are carried out. These coal seams include C5b coal seam, upper adjacent C5a coal seam, C6a coal seams, C6c in lower adjacent strata, and C5b coal seam in high-level borehole. The view of gas drainage borehole peeping and trajectory measurement in the working seam, upper adjacent layer, lower adjacent layer, and high position are obtained. It is found that the hole collapses at the position of about 20 m in both adjacent strata and high-level boreholes, and there are a lot of cracks in the high-level boreholes before 12 m. The deviation distance of high-level borehole is large, and the actual vertical deviation of upper adjacent layer is small. Finally, the strategies to prevent the deviation of drilling construction are put forward. It includes four aspects: ensuring the reliability of drilling equipment, reasonably controlling the drilling length, standardizing the drilling, and reasonably selecting the drilling process parameters.

Research on Optimization of Key Areas of Drainage Borehole Sealing in Ultrathick Coal Seam

Gas drainage is an important means of gas control. The influence of the key position of the sealing hole on gas drainage was studied by theoretical and numerical simulation combined with field measurement to solve low gas concentration in gas predrainage boreholes in coal mines of China. By analyzing the distribution of cracks around the boreholes and the law of air leakage and simulating the drainage effect of different sealing areas (8 m, 12 m, and 16 m), it was proposed that the key position of the sealing hole should be in the prepeak stress concentration area. According to the actual situation of Baode Mine, the sealing test scheme of different sealing areas was put forward, and the field test was carried out to obtain the key sealing area of gas predrainage boreholes in Baode Mine. Research shows that when the sealing area is 8–16 m, the average gas concentration is 63.57%, and the average pure gas flux is 0.408 m3/min. The sealing effect of this area is better, with fewer cracks, than that of the existing sealing area, effectively preventing gas leakage and increasing the gas concentration and gas scalar.

DETERMINATION OF METHANE DESORPTION ZONE FOR THE DESIGN OF A DRAINAGE BOREHOLE PATTERN (CASE STUDY: E4 PANEL OF THE TABAS MECHANIZED COAL MINE, IRAN)

Underground coal mining is known as one of the major sources of methane emissions which mainly occurs after underground coal extraction. Rock strata in-situ methane can potentially be the most significant hazard in coal mining operations. To prevent or minimize the risks of methane emissions, methane drainage approaches have been adopted by coal mines. Rock mass methane drainage is the most efficient and effective approach toward controlling methane hazards as it prevents and reduces the frequency of methane emissions, outflows into the working area and sudden outbursts of methane and rocks. The method includes drilling boreholes from the tailgate side to the unstressed zone in the roof and floor strata above and below a working coal seam. The coal seam gas content in Tabas Parvadeh I is estimated to be about 16 m3 /t, which is relatively high. Based on exploration data, five distinct coal seams have been identified (B1, B2, C1, C2 and D) at the coal deposit and currently C1 is being worked. Considering the high value of C1 gas content and surrounding rocks, the Methane Drainage System (MDS) has been utilized for gas drainage. This paper tries to determine the desorption area which is essential and helpful for the selection of an effective drilling pattern into the adjacent coal seams. In this study, the methane drainage zone in the E4 panel of the Tabas coal mine was calculated using experimental equations and a drainage borehole pattern was determined.

Effect of packer design on hydraulic fracturing of coal seam

The article addresses the issue of in-situ methane drainage from coal seams using multiple hydraulic fracturing. The authors undertake the stress–strain analysis of rock mass in the vicinity of a drainage borehole subjected to loading by sealing packers using the finite element method. The pressures at the packer and borehole wall contacts are calculated. It is shown how the length and spacing of the packers influence the values of the maximum axial tensile stresses on the borehole wall.

Instability-negative pressure loss model of gas drainage borehole and prevention technique: A case study

The internal collapse of deep seam drainage borehole and negative pressure loss represents a serious technical problem affecting gas drainage. To address this problem a creep model of coal around borehole was established based on the plastic softening characteristics of coal. The final collapse time of the borehole was determined and used to derive the three stages of the borehole collapse process. The model of negative pressure loss in drainage borehole was established according to the theory of fluid dynamics, the model of methane gas flow and the creep model of the coal around the borehole. The relationship between the negative pressure loss of drainage and the change of borehole aperture was derived, thereby revealing the main influencing factors of the negative pressure loss in the borehole. A drainage technique named “Full-hole deep screen mesh pipe” was introduced and tested to prevent the collapse of borehole and reduce the negative pressure loss. The result shows that after the borehole was drilled, the borehole wall was affected by the complex stress of the deep coal seam, the coal surrounding the borehole collapsed or presented the characteristics of creep extrusion towards the borehole. The “Full-hole deep screen mesh pipe drainage technology” could effectively control the collapse as well as the deformation of the borehole and reduced the negative pressure loss. Compared with the traditional drainage technology, the methane gas drainage concentration was increased by 101% and the gas flow was increased by 97% when the methane gas was drained for 90 days, the gas drainage efficiency increased significantly.