Crossing borehole is an effective means to eliminate the outburst risk of coal and gas. The influence of borehole inclination angle, borehole diameter, and drainage time on the effective radius of gas drainage are studied by numerical simulation and engineering example verification. The study shows that the effective radius changes in a “U” shape with the increase in borehole inclination angle. When the angle α of the borehole and coal seam plane decreases, the effective radius increases. Furthermore, the coal mass around the borehole is broken by shear deformation, which is consistent with the results of the inner peephole. The effective radiuses are different in coal seam dip X and strike Y. When α is small, the effective radius in the X direction is generally wider. When α is large (α ≤ 90°), the effective radiuses in the X and Y directions are close. The effective radius is positively correlated with the borehole diameter and is more significantly influenced by increasing borehole diameter when α is small. The effective radius increases as a negative exponential function with time and eventually converges to a constant. The study has practical implications for the design of crossing borehole in the coal seam floor.

/pdf/numerical-investigation-on-potential-influencing-factors-1mtw4km3.pdf

Numerical Investigation on Potential Influencing Factors Affecting Drainage Effective Radius of Crossing Borehole

Geological CO2 sequestration (GCS) can help mitigate global warming and enhance methane recovery from coal beds. However, few studies have linked the effects of CO2 to surface chemistry changes controlling wetting behavior in deep coal beds. Contact angles (CAs) of CO2/N2-high volatile bituminous coal-water systems were measured under different temperatures and pressures. The surface chemistry and physical structure of coals were characterized to investigate changes in physicochemical properties and their relations with wettability after reactions. For N2 treatment, the time-dependence of static and dynamic CAs were insignificant, ranging within 4°. For gaseous CO2 treatment, the static CAs and the average advancing angles increased slightly. With supercritical (sc) CO2, both the static and dynamic CAs increased significantly, and adv changed to intermediate-wet (92°). Reactions with minerals exposed to scCO2 resulted in greater surface roughness and heterogeneity, greater contact angle hysteresis and more surface sites occupied by scCO2 rather than H2O. Increases in hydrophobic functional groups and decreases in hydrophilicity were shown by FTIR spectra, reflecting the shedding of polar oxygen-containing functional groups, reduction of hydrogen bonds, and increasing percentage of hydrocarbons. XRD patterns obtained following scCO2-treatment showed that crystallite growth and molecular polymerization were higher toward graphite-like. The calculated structural parameters of functional groups and crystallites both showed elevated coal rank. Changes in crystallite structure, notably higher carbon content and decreased negative surface charge, are unfavorable for water-wetting. This study contributes to understanding surface chemistry changes responsible for decreased wettability during CO2-enhanced coal bed methane recovery and GCS in coal reservoirs. 

Experimental investigation into coal wettability changes caused by reactions with scCO2-H2O

The No. 3 coal seam in the Yuxi Coal Mine has a measured maximum gas content of 25.59 m3/t, along with a maximum gas pressure of 2.9 MPa, indicating its high risk to gas and outbursts. To mitigate outburst risks of the coal seam, the 1301 working face has been implemented gas pre-drainage measures by grid boreholes from underlying roadways. After one year of extraction, it was confirmed that the gas content at all 33 test sites was below 8 m3/t, meeting the outburst prevention standards. However, during subsequent coal tunnel excavation, the gas desorption index K1 value frequently exceeded the standard, resulting in numerous occurrences of abnormal gas emission or small-scale outbursts. To tackle the challenges associated with safe excavation following the first round regional outburst prevention measures, a research and industrial trial of CO2 gas fracturing(CO2-Frac) technology for secondary outburst prevention and rapid excavation was completed. Results show that the dual-hole and high-pressure (185MPa) CO2-Frac considerably contribute to outburst prevention. K1 exceedances per hundred meters of tunnel excavations were from average 2.54 without CO2-Frac to average 0.28 after the new technology being implemented, leading to a eight fold reduction. Additionally, the monthly excavation footage increased from average 81.64 m without CO2-Frac to average 162.42 m with CO2-Frac, resulting in a 2-fold improvement. The dual-hole and high-pressure CO2-Frac is an advanced technology for safe and efficient excavation for secondary outburst elimination in highly outburst-prone coal seams in the Yuxi Coal Mine, with potential for widespread application in similar coal seam conditions. 

Study on Safety Tunneling Technology of Secondary Outburst Elimination by CO2 Gas Fracturing in High Outburst Coal Seam

Pore structure reconfiguration and microscale fluid response in bituminous coal under CO2 and flue gas-enhanced coalbed methane recovery

Nifemnbapr Perovskite Pre-Catalysts for Stable Steam Reforming of Heavy Crude Oil

Mo-based catalysts for CH4/H2S reforming to hydrogen production: effect of hydroxyl concentration of the support

To solve the problems of high ground stress, low permeability and low coalbed methane recovery, this paper proposes an optimization technique for improving coal seam permeability, i.e., water-gas alternating displacement enhances coalbed methane recovery (WGA-ECBM). The seepage field of gas-water two-phase flow is derived and a multiphase fluid-solid coupling model between physical parameters is established. This paper studies the effects of three displacement methods (water injection, air injection and water-air alternating injection) on methane recovery. The influence of injection pressure and injection duration on the WGA-ECBM technique is also studied. The
 
  
 
 
 V
 
 
 C
 
 
 H
 
 
 4
 
 
 
 
 
 (methane volume fraction) of the three methods decrease by 17.84%, 12.72% and 44.62% in the borehole, respectively. The influence ranges (diameter) are 24.68 m, 149.73 m and 135.21 m, respectively. Pore pressures and displacement area also increase as injection pressure increases in a 5 MPa gradient. And 
 
 
 
 V
 
 
 C
 
 
 H
 
 
 4
 
 
 
 
 
 in the borehole decrease by 35.57%, 43.82%, 46.78% and 51.72% from 10 MPa to 25 MPa, respectively. With the increase of injection duration, the influence range expands significantly but the pore pressure distribution changes little. And 
 
 
 
 V
 
 
 C
 
 
 H
 
 
 4
 
 
 
 
 
 decreases slightly in the displaced area and the displacement velocity gradually slows down. This technique promotes multiphase flow migration in coal seam, and increases the displacement range and methane recovery efficiency.

/pdf/effects-of-injection-pressure-and-duration-on-alternate-high-kh3x8izd.pdf

Effects of Injection Pressure and Duration on Alternate High-Pressure Water-Gas Sequestration of Coalbed Methane

Wettability in CO2–H2O–coal directly determines pore-scale configuration and further affects practice projects on the hectometer scale; however, coal rank and thermophysical condition dependences of wettability and their underlying mechanisms are inadequate. Potential candidate coal beds (hvBb, hvAb, mvb, sa) were considered, and wettability reflected by static/dynamic contact angles was evaluated under in situ reservoir conditions (temperature and pressure) by the captive bubble method. Physicochemical properties with potential effects on wetting behavior were characterized via SEM-EDS, FTIR, and XRD. Static/dynamic contact angles increased with pressure and coal rank, while they decreased with temperature, typically showing intermediate- or CO2-wetting in deep reservoirs. Advancing contact angles depended less on temperature and coal rank than static/receding contact angles. At a low temperature (T = 25 °C), CO2-wetting was stronger and was more significantly affected by coal rank and pressure. The curves of contact angles and CO2 density as a function of pressure almost coincided and increased steeply at pressures between 60 and 90 bar (∼64.3 bar). With increasing metamorphic degrees, defects, oxygen content, and polar oxygen-containing functional groups (hydroxyl, etc.) on the surface decreased, while C/O (the ratio of carbon to oxygen), C, and aromatic hydrocarbons increased. The results of the gray relational model indicate that C/O and hydroxyl are the primary factors affecting wettability in chemical properties. With coal rank, the degree of aromatic ring condensation increased and inorganic minerals (e.g., clay minerals and calcite) reduced; the crystallite structure tended to grow and become graphite-like. Measurement data and an understanding of wetting behavior have implications for project site selection and provide input parameters for field-scale reservoir modeling. 

Wetting Behavior of CO2–H2O–coal with Different Metamorphic Degrees under In Situ Thermophysical Conditions

Pore compaction and crack evolution of shale rock under high-speed impact loading and different confining pressures

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