Numerical simulation of solid-fluid-thermal coupling in the heating stage of in-situ injection of supercritical water for hydrogen production from coal

ABSTRACT

Hydrogen production via in-situ injection of supercritical water into underground coal seams is a new coal conversion technology. This study proposes a solid-fluid-heat coupling mathematical model for the heating stage of the injection of supercritical water into the coal seam and studies the evolution law of the temperature distribution, pore pressure, solid deformation, and other aspects of the coal seam, the roof strata, and the floor strata using numerical simulation. The results showed that after the injection of supercritical water, the temperature and flow rate near the injection well increased rapidly, gradually decreased, and expanded outwards. Subsequently, the temperature rose to about 1050°C and the flow rate decreased slightly near the injection well. The flow rate near the production well showed a negative exponential growth pattern six months prior; however, the temperature and flow rate remained constant after six months. The temperature around 700 m from the injection well changed little over one year. A large compression deformation zone may form above the injection well and the maximum uplift displacement of the deformation zone within one year can reach 8 mm. The extreme value of vertical stress was located in the surrounding rock layer closest to the coal seam and its maximum value can be 17 MPa.

KEYWORDS:

• coal supercritical water gasification
• in-situ
• solid-fluid-thermal coupling model
• numerical simulation
• seepage

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 52122405). The authors greatly appreciate the suggestions from the anonymous reviewers for improving the paper’s quality.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Nomenclature

$\lambda \left(T\right)$	=	One of the Lamé constants, as a function of temperature [$\mathrm{M}{\mathrm{L}}^{-1}{\mathrm{T}}^{-2}$]
$\mu \left(T\right)$	=	One of the Lamé constants, as a function of temperature [$\mathrm{M}{\mathrm{L}}^{-1}{\mathrm{T}}^{-2}$]
$k_i$	=	Permeability coefficient [$\mathrm{L}{\mathrm{T}}^{-1}$]
$u_i$	=	i-directional displacement [L]
$u_j$	=	j-directional displacement [L]
$F_i$	=	i-directional physical component [$\mathrm{M}{\mathrm{L}}^{-2}{\mathrm{T}}^{-2}$]
$\beta$	=	Coefficient of thermal expansion of the rock mass obtained from experiments
$T_r$	=	Rock temperature [℃]
$T_{,i}$	=	Temperature gradient in direction i [℃${\mathrm{L}}^{-1}$]
${\rho }_r$	=	Rock density [$\mathrm{M}{\mathrm{L}}^{-3}$]
$C_{pr}$	=	Rock specific heat capacity at constant pressure [$\mathrm{k}\mathrm{g}{\mathrm{L}}^{-1}{\mathrm{T}}^{-2}{ }^{-1}$]
$n$	=	Coal seam porosity
$W_r$	=	Source sink term for solid deformation
$t$	=	Time [T]
$W_s$	=	Source sink term for heat transfer from rock
${\lambda }_r$	=	Rock thermal conductivity [$\mathrm{M}\mathrm{L}{\mathrm{T}}^{-3}{ }^{-1}$]
$C_{vw}$	=	Supercritical water constant specific heat capacity [${\mathrm{L}}^2{\mathrm{T}}^{-2}{ }^{-1}$]
$C_{pw}$	=	Supercritical water constant pressure specific heat capacity [${\mathrm{L}}^2{\mathrm{T}}^{-2}{ }^{-1}$]
$h_{,i}$	=	Partial derivative of gross head
${\rho }_w$	=	Supercritical water density [$\mathrm{M}{\mathrm{L}}^{-3}$]
$T_w$	=	Supercritical water temperature [℃]
${\lambda }_w$	=	Supercritical water thermal conductivity [$\mathrm{M}\mathrm{L}{\mathrm{T}}^{-3}{ }^{-1}$]
${\beta }_w$	=	Coefficient of compression of water [${\mathrm{M}}^{-1}\mathrm{L}{\mathrm{T}}^2$]
$q_i$	=	Flow Rate [$\mathrm{L}{\mathrm{T}}^{-1}$]
$W_w$	=	Source sink term for convective heat transfer
$\Theta$	=	Volumetric stress [$\mathrm{M}{\mathrm{L}}^{-1}{\mathrm{T}}^{-2}$]
${\sigma }_{ij}$	=	Stress tensor [$\mathrm{M}{\mathrm{L}}^{-1}{\mathrm{T}}^{-2}$]
${\epsilon }_{ij}$	=	Strain tensor
${\delta }_{ij}$	=	Kronecker symbol

Additional information

Notes on contributors

Zixiang Zhang

Zixiang Zhang, MSc, engaged in research on in-situ modified fluidised mining.

Yangsheng Zhao

Yangsheng Zhao, PhD, Professor, Member of the Chinese Academy of Sciences, engaged in research on in-situ modified fluidised mining.

Zijun Feng

Zijun Feng, PhD, Professor, received the National Excellent Young Scientists Fund, engaged in research on geothermal development.