Alternate Injection of HPG and Water - A Two Well Pilot

Abstract Many areas of the Hassi Messaoud oil reservoir in Algeria are undergoing miscible displacement by high pressure gas injection. Because of extreme reservoir heterogeneity and the unfavorable mobility ratio of miscible displacement by gas injection, the sweep efficiency, particularly the vertical sweep, is poor. As a result a two well pilot operation was started in the field in late 1968 to: study the improvement in the sweep which could be achieved by following a miscible high pressure gas front with the alternate injection of gas and water, and study the injectivities and gas-oil ratio control of the process. Ways to improve the injectivities and any problems which arise after water breakthrough are also to be studied. The pilot is still in progress but results are now available on the sweep, injectivities, and gas-oil ratio control Breakthrough of the gas front into the producer occurred after 6 months of producer occurred after 6 months of injection but the water-gas front has not yet broken through after 32 months of injection. The gas front achieved a 22% volumetric sweep at breakthrough. The water-gas front has not yet broken through and has achieved a volumetric sweep of at least 85%. These are to be compared to a volumetric sweep of 10% to 12% at gas breakthrough for-gas injection alone in other areas of the field. Miscible displacement ahead of the water-gas front has been continued by the maintenance of a free gas band. The gas-oil ratio production after gas breakthrough has been held to only 10% above solution by controlling the ratio of gas to water injected per cycle. A cumulative injection ratio of gas to water within the reservoir of 1.1 to 1.2 gave the desired results. This was in agreement with the design based on relative permeability curves.