Numerical models of thermal recovery processes arising in petroleum engineering require accurate estimates of the interaction between a reservoir and the surrounding cap and base rock. It is important to obtain an accurate approximation of the energy flux at the interface between the reservoir and the surrounding rock, and the distribution of the temperature in the surrounding rock is of only secondary importance. A flux calculation procedure implemented as a post processing algorithm based upon a finite element method is used to calculate an accurate approximation to the energy losses. Oscillatory behavior is observed in the computed temperature distribution using a standard computational scheme. If the time derivatives are lumped, the oscillatory behavior is avoided. Computational experiments performed on sample test cases possessing analytical solutions indicate an excellent agreement between the approximations obtained using the lumped procedure and the exact solution.