Abstract: The unsteady thermal flow is analyzed based on energy conservation equation in which heat exchange, convection and dissipation are considered. Two concepts of formation thermal length and formation thermal effect period are introduced. The former indicates the effectiveness of heat injected; the latter means the heat affected length under one logarithmic-period reduction of formation temperature. Also, three dimensionless parameters are introduced, including convection velocity, injection rate and dimensionless time, which can be used for analyzing the underground thermal distribution and variation. One-dimension numerical calculation shows that there exists a thermal effect period under the condition of no convection; on the other hand, the convection will effectively enhance heat spreading distance and heat effectiveness.

Key words: thermal convection, thermal recovery, energy conservation, formation