These characteristics include primarily (1) pronounced structure, (2) low oil viscosity, (3) high permeability, and (4) low oil velocities.
Water drive and hydraulic control are terms used in designating a mechanism that involves the movement of water into the reservoir as gas and oil are produced. Water influx into a reservoir may be edgewater or bottomwater, the latter indicating that the oil is underlain by a water zone of sufficient thickness so that the water movement is essentially vertical. The most common source of water drive is a result of expansion of the water and the compressibility of the rock in the aquifer; however, it may result from artesian flow. The important characteristics of a water-drive recovery process are the following:
1. The volume of the reservoir is constantly reduced by the water influx. This influx is a source of energy in addition to the energy of liquid expansion above the bubble point and the energy stored in the solution gas and in the free, or cap, gas.
2. The bottom-hole pressure is related to the ratio of water influx to voidage. When the voidage only slightly exceeds the influx, there is only a slight pressure decline. When the voidage considerably exceeds the influx, the pressure decline is pronounced and approaches that for gas-cap or dissolved gas-drive reservoirs, as the case may be.
3. For edgewater drives, regional migration is pronounced in the direction of the higher structural areas.
4. As the water encroaches in both edgewater and bottomwater drives, there is an increasing volume of water produced, and eventually water is produced by all wells.
5. Under favorable conditions, the oil recoveries can be quite high.
Leave a Reply