Hydro-thermal-solid modeling of artificial ground freezing through cold gas convection

Abstract

The inrush of groundwater impairs heating efficiency during oil shale in situ pyrolysis mining, and it also increases oily waste-water production. Artificial ground freezing (AGF) is an ideal method to block the adverse groundwater infiltration, while the poor freezing performance and high energy consumption of the current AGF pattern are the biggest obstacles, even failing at high-velocity seepage flow. This study proposed and investigated a novel pattern of AGF through cold gas convection (GC) for oil shale in situ reservoir, this pattern have the potential to enhance the freezing performances, especially when encounters the high-velocity seepage flow. In this study, a coupled hydro-thermal-solid model was developed and numerical simulations were performed, the enhancement mechanism of freezing performances and energy conversion have been analyzed to reveal the superiority of the GC-AGF. The results indicated that the maximum freezing rates were twice the current AGF. With seepage flow, the shape of the frozen zone developed elliptically perpendicular to the seepage flow direction, and the effective radius of the frozen zone formed by GC-AGF decreased only by 10%–20 %, even at a higher-velocity seepage flow. The analysis showed that the gas-water two-phase zone was a barrier to the direct heat exchange between the infiltrated groundwater and the frozen zone, which was the enhancement mechanism of GC-AGF. Correspondingly, the specific energy consumption reduced from 8.0 × 107 to 3.8 × 107 J/m at a higher-velocity seepage flow. The results showed that the proposed GC-AGF method significantly enhanced the freezing performance with low energy consumption.