Many studies indicate that the recovery from true solution gas-drive reservoirs by primary depletion is essentially independent of both individual well rates and total or reservoir production rates. Keller, Tracy, and Roe showed that this is true even for reservoirs with severe permeability stratification where the strata are separated by impermeable barriers and are hydraulically connected only at the wells.12 The Gloyd-Mitchell zone of the Rodessa Field is an example of a solution gas-drive reservoir that is essentially not rate sensitive (i.e., the recovery is unrelated to the rate at which the reservoir is produced). The recovery from very permeable, uniform reservoirs under very active water drives may also be essentially independent of the rates at which they are produced.
Many reservoirs are clearly rate sensitive and, for this reason, many governing bodies have imposed allowables that limit the production to a specified rate in order to ensure a maximum overall recovery of the well. These allowables are not typically reservoir specific and in conventional fields may be restrictive. Reservoir engineers can calculate a maximum efficient rate (MER) for a specific reservoir. Production at or below this rate will yield a maximum ultimate recovery, while production above this rate will result in a significant reduction in the practical ultimate oil recovery.13,14 Governing bodies have been known to adjust allowables for a reservoir when an MER has been proven.
Rate-sensitive reservoirs imply that there is some mechanism(s) at work in the reservoir that, in a practical period, can substantially improve the recovery of the oil in place. These mechanisms include (1) partial water drive, (2) gravitational segregation, and (3) those effective in reservoirs of heterogeneous permeability.
When initially undersaturated reservoirs are produced under partial water drive at voidage rates (gas, oil, and water) considerably in excess of the natural influx rate, they are produced essentially as solution gas-drive reservoirs modified by a small water influx. Assuming that recovery by water displacement is considerably larger than recovery by solution gas drive, there will be a considerable loss in recoverable oil by the high production rate, even when the oil zone is eventually entirely invaded by water. The loss is caused by the increase in the viscosity of the oil, the decrease in the volume factor of the oil at lower pressures, and the earlier abandonment of the wells that must be produced by artificial lift. Because of the higher oil viscosity at the lower pressure, producing water-oil ratios will be higher, and the economic limit of production rate will be reached at lower oil recoveries. Because of the lower oil volume factor at the lower pressure, at the same residual oil saturation in the invaded area, more stock-tank oil will be left at low pressure. There are, of course, additional benefits to be realized by producing at such a rate so as to maintain high reservoir pressure. If there is no appreciable gravitational segregation and the effects of reservoir heterogeneity are small, then the MER for a partial water-drive reservoir can be inferred from a study of the effect of the net reservoir voidage rate on reservoir pressure and the consequent effect of pressure on the gas saturation relative to the critical gas saturation (i.e., on gas-oil ratios). The MER may also be inferred from studies of the drive indices (Eq. [7.3]). The presence of a gas cap in a partial water-drive field introduces complications in determining the MER, which is affected by the relative size of the gas cap and the relative efficiencies of oil displacement by the expanding gas cap and by the encroaching water.
The Gloyd-Mitchell zone of the Rodessa Field was not rate sensitive because there was no water influx and because there was essentially no gravitational segregation of the free gas released from solution and the oil. If there had been substantial segregation, the well completion and well workover measures, which were taken in an effort to reduce gas-oil ratios, would have been effective, as they are in many solution gas-drive reservoirs. In some cases, a gas cap forms in the higher portions of the reservoir, and when high gas-oil ratio wells are penalized or shut in, there may be a substantial improvement in recovery, as indicated by Eq. (7.11), for a reduction in the value of the produced gas-oil ratio Rp. Under these conditions, the MER is that rate at which gravitational segregation is substantial for practical producing rates.
Gravitational segregation is also important in many gas-cap reservoirs. The effect of displacement rate on recovery by gas-cap expansion when there is substantial segregation of the oil and gas. The studies presented on the Mile Six Pool show that, at the adopted displacement rate, the recovery will be approximately 52.4%. If the displacement rate is doubled, the recovery will be reduced to about 36.0%, and at very high rates, it will drop to 14.4% for negligible gravity segregation.
Gravitational segregation also occurs in the displacement of oil by water, and like the gas-oil segregation, it is also dependent on the time factor. Gravity segregation is generally of less relative importance in water drive than in gas-cap drive because of the much higher recoveries usually obtained by water drive. The MER for water-drive reservoirs is that rate above which there will be insufficient time for effective segregation and, therefore, a substantial loss of recoverable oil. The rate may be inferred from calculations similar to those used for gas displacement from laboratory studies. It is interesting that, in the case of gravitational segregation, the reservoir pressure is not the index of the MER. In an active water-drive field, for example, there may be no appreciable difference in the reservoir pressure decline for a severalfold change in the production rate, and yet recovery at the lower rate may be substantially higher if gravity segregation is effective at the lower rate but not at the higher.
As water invades a reservoir of heterogeneous permeability, the displacement is more rapid in the more permeable portions, and considerable quantities of oil may be bypassed if the displacement rate is too high. At lower rates, there is time for water to enter the less permeable portions of the rock and recover a larger portion of the oil. As the water level rises, water is sometimes imbibed or drawn into the less permeable portions by capillary action, and this may also help recover oil from the less permeable areas. Because water imbibitions and the consequent capillary expulsion of oil are far from instantaneous, if appreciable additional oil can be recovered by this mechanism, the displacement rate should be lowered if possible. Although the MER under these circumstances is more difficult to establish, it may be inferred from the degree of the reservoir heterogeneity and the capillary pressure characteristics of the reservoir rocks.
In the present discussion of MER, it is realized that the recovery of oil is also affected by the reservoir mechanisms, fluid injection, gas-oil and water-oil ratio control, and other factors and that it is difficult to speak of rate-sensitive mechanisms entirely independently of these other factors, which in many cases are far more important.
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