E206

The Evolution Law of Parameters and Inversion Model in Coalbed Methane Wellbore PreExtraction Process: Research and Applications〇Li Yan, Hu Wen, Ana Maria CRUZ.
This study investigated the key parameters of coalbed methane (CBM) pre-extraction processes, using amathematical model of coupled fluid-solid behavior. It developed a backward calculated model to estimate theresidual CBM pressure and content in the coal seam from borehole extraction data. It revealed the spatiotemporalmigration rule and distribution patterns of CBM and identified outburst-prone areas. The study improves theunderstanding of CBM behavior and suggestions for improved safety and disaster prevention in coal production.
1. INTRODUCTION
Coalbed methane (CBM), a significant hazard in
Chinese coal mines, accounts for 80% of major
accidents. Despite its risks, CBM offers potential as aclean energy source. For mines with high CBM content,
simultaneous extraction of coal and CBM is crucial forresource utilization and disaster management. Preextraction reduces CBM risks, but existing methods
have limitations. In this study, a novel approachleveraging pre-draining boreholes and smart mining
technologies is proposed. By integrating monitoringdata and fluid-solid coupling models, we backward
calculate key CBM parameters, enhancing precisionand real-time monitoring during extraction, allowing
the monitoring and mitigation of hazards.
2. METHODOLOGY
2.1 Fluid-solid coupling model of CBM-containing
coal
By integrating principles from poroelasticity,hydrodynamics, and rock mechanics, stress-strain
equations (Equation 1) for CBM-containing coal,
dynamic models for porosity and permeability
variation (Equations 2, 3), models for fracture ratio andpermeability variation (Equations 4, 5), CBM diffusionand flow (Equations 6, 7), and extraction negative
pressure decay (Equation 8) were formulated. Theseequations constitute the basis for investigating dynamic
aspects of CBM in coal seams, serving as theoreticalfoundations for CBM exploration and development, asshown in Fig.1. Changes in porosity and fracture ratio
are influenced by adsorption-desorption strain, wetstrain, and CBM stress deformation. Throughnumerical simulations, this approach enhances theaccuracy of predicting coal seam mechanics and CBM
dynamics, providing reliable support for dynamicparameter backward calculation in CBM extraction
from coal seams.
G1.Gui ,ij + 1 − 2 ui ,ij −  p (p ),i +  M ( sw − sw 0 ),i −  s (  s ),i −  p,i + Fi = 0
 ( p − p0 )
1 − m 0+  M ( sw − sw 0 ) +  sgVsg 2.m = 1 − exp − K  eff 1 − K

()m


Y

3
 1
 ( p − p0 )  

1 − m 0−1−+  M ( sw − sw 0 ) +  sgVsg   
3.km = km 0 eff Km  m 0 m 0  exp ( − KY  ) 
4. =  exp  − 1  e ff + 1  e ff −  V −  ( s − s ) 
ff0fmsg
sg
Mww
0

KfKm










5.k f = k f 0 exp  − 3  ef ff + 3  me ff − 3 sgVsg − 3 M ( sw − sw 0 ) Km K f
 VL pm Mg
c2exp−(T−T)
p+
t  sn 1 + c1 pm
 RT
 pm + p L

1 − m 0 1−    exp ( − KY  eff )Mg 6.  =−(p−p)
mfg

 RT
 Mg  t   ( p − p0 )


1−+
(s−s)+p

Mww0m



Km
 RT  VL pmc2
exp  −(T − Tt )      sg
p+p1+cpmL1 m   ( pm − p fg ) ( pf f )kf−  pf
p f  =
 7.



tg







− s2 x8. p ( x) = s1eFig.1 Fluid-solid coupling model
2.2 Simulation experiment of CBM extraction
Dimensional analysis and physical similarity
criterion were used to relate CBM flow rate to various
parameters in pre-drainage. Dimensionless parameters,π1 and π2, describing CBM extraction patterns, werederived. A formula was developed using theseparameters to retro-calculate CBM pressure based on
observed extraction rates, facilitating analysis ofextraction effectiveness in coal seams. An experimental
setup ensured similarity between the prototype and themodel, with a schematic diagram and image

highlighting conditions for maintaining similarity, as
shown in Fig.2.

Fig. 2 Experimental system structure diagram
2.3 Backward calculation model for CBMparameters

Fig.3 CBM parameters backward calculation workflow
Based on the previous discussion, an inversionmodel for calculating key parameters of CBM in thecoal seam from monitoring data can be established asshown in Fig.3. We divide the process of backward
calculation of key parameters during the CBM
extraction process in the coal seam from monitoringdata into five parts: basic parameter collection, forward
calculation, model fitting, data acquisition, andbackward calculation.
3. RESULTS AND DISCUSSION（a）2D View（b）3D View
Fig. 4 CBM pressure distribution cloud map

The method of coalbed gas pressure inversion
based on drilling and extraction flow was applied to theexperimental working face. A sophisticated monitoring
system was installed to collect coalbed gas data frompre-extraction boreholes, which were then input into
the reverse calculation model to obtain distribution
maps of coalbed gas pressure and content. The
graphical representation illustrates the spatial
distribution pattern of gas pressure, highlighting theheterogeneity within the coal seam. Additionally, ananalysis of potential hazards associated with the spatial
variability of the coal seam was conducted, as shown in
Fig.4.
4. CONCLUSION
This study provides a novel approach for
capturing residual CBM parameters during the CBM
extraction process. Prevention and control workers incoal mines can use this approach to calculate and tailor
strategies for different working faces based on practicalconsiderations. Additionally, the calculated results canbe leveraged for further research on adjusting CBM
extraction strategies to enhance efficiency.
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