World-wide experience indicates that thermal methods are the only effective methods for the in-situ recovery of heavy oils. Furthermore, the steady progress within horizontal well technology increases the applicability of thermal methods in conjunction with horizontal wells. The steam-assisted gravity drainage (SAGD) process is the prominent example of the synergism of horizontal wells with steam injection.

The major portion of the dollar per barrel operational cost for steam injection is currently attributed to the generation of steam and water handling requirements. In the future, more stringent environmental regulation might further prohibit the use of water for the recovery of heavy oils.

Naptha is present at the site as a diluent for pumping and pipe line transport of the produced heavy oil. The injection of naptha vapour instead of water vapour combines an effective thermal process with the diluent mechanism of the naptha. We present the results of simple analytical calculations, demonstrating the applicability of naptha-assisted gravity drainage (NAGD) compared to the SAGD process.

Numerical simulations have been performed on a two-dimensional cross section perpendicular to the horizontal well pair. We discuss in detail the reservoir mechanisms present during NAGD. For the conditions studied naptha circulation rates are some 14 times the bitumen production rate. Evaluation of the process shows that it will be economic with naptha recycling and by minimising the naptha remaining in the reservoir.

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