Sealing effects of marginal gas injection on oil shale in situ pyrolysis exploitation

Abstract

Groundwater inrush and outward diffusion of pyrolysis products are two disadvantageous factors for oil shale in situ pyrolysis, leading to the loss of heat injected and groundwater pollution. Therefore, forming a closed in situ pyrolysis system is extremely important. The sealing effects of marginal gas injection were studied by numerical simulation. A two-dimensional model was established according to the engineering geology conditions of the National Pilot Project for Oil Shale in situ Exploitation of China (NPOSE), in which methane was regarded as the representative of pyrolysis products. The migration state of methane in dissolved and gas phases during in situ pyrolysis was revealed. The results showed that proper marginal gas injection pressure can effectively inhibit the outward diffusion of methane. Meanwhile, a marginal gas injection pressure of 0.60 or 0.61 MPa can effectively reduce water yield to ~0.5 m3/h. Consequently, a closed in situ pyrolysis system can be effectively formed by marginal gas injection, and the mass exchange inside and outside the pyrolysis zone is insulated, ensuring a clean and efficient pyrolysis process.