Pyrolysis patterns of in-situ steam injection for oil shale recovery with three-dimensional discrete fracture network

In-situ steam injection for oil shale exploitation (MTI) could significantly improve the in-situ pyrolysis efficiency of oil shale and enhance crude oil mobility. With Fushun oil shale as subject for analysis, this study proposes a thermal-hydraulic-mechanical (THM) coupling model containing a three-dimensional discrete fracture network via numerical simulation based on the Baecher disk model and the Fisher distribution to evaluate the actual pyrolysis effect through variations of the average reservoir temperature and the effective pyrolysis volume. Results show that, this MTI technology allowed 82.73 % of the oil shale reservoir to be pyrolyzed after 1.000 a of heat injection, with 13.33 % of energy loss reduction compared with that under the single heat-injection well mode. During heat injection, reservoir stress concentrated around the production wells, fracture intersections, and the top and bottom of the oil shale layer, with the maximum reservoir stress reaching 34.57 MPa. During pyrolysis, substantial settlement occurred around the heat injection wells, reaching 1.02 m after 1.000 a of continuous heat injection. As heat injection ceased, the elastic rebound of the formation recovered 16.0 % of the settlement loss. This study provides references for accurately evaluating the pyrolysis effect, ensuring the safety of mining engineering, and optimizing the heat-injection exploitation scheme in the large-scale implementation of MTI technology.