Alternate Producing and Gas Repressuring for Greater Oil Recovery

Considerations are presented to support a theory that alternately producing and repressuring a reservoir with gas will flush oil out of zones of low permeability in a reservoir under conditions in which conventional gas cycling would be relatively ineffective. Gas repressuring and batch gas cycling (alternately producing and repressuring) in semidepleted oil reservoirs may be a practical secondary recovery practice in many fields. Although this practice may not be as successful as water flooding in most shallow oil reservoirs, it holds considerable promise as a means of increasing recovery from deep reservoirs where water flooding would not be economically practicable. Batch-type gas cycling will sweep a greater part of a non-homogeneous reservoir than conventional gas cycling and therefore should result in a greater oil recovery. It can be used successfully where two or more reservoirs can be operated as a unit. Also, it can be used in a single reservoir if the reservoir is also used for gas-storage operations. This cycling procedure can be used successfully in reservoirs whose states of depletion range from newly developed oil fields to fields depleted by primary production. Introduction From a study of methods of predicting the performance of semidepleted oil reservoirs used for gas storage made recently by Bureau of Mines engineers, it was concluded that the amount of oil recoverable by repressuring and cycling a reservoir may be as great as that produced during primary recovery operations. It appeared also that the normal operations of a gas-storage project would generally gas-sweep a larger part of a reservoir than gas cycling at essentially constant pressure, especially in unfractured reservoirs with large variations in permeability. The flow of fluids in a reservoir used for gas storage differs from that in reservoirs subjected to gas cycling. In normal gas-cycling operations in non-homogeneous reservoirs the reservoir pressure is essentially constant, and the major flow of gas is through the more permeable zones. When a reservoir is used for gas storage, the oil-recovery operation may be in effect an alternate repressuring and partial depletion process.