Reservoir Fluid Flow and Natural Drive Mechanisms

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Water Drive Reservoir Performance

Typical Producing Performance

Pressure, GOR, Water Cut

Whether water influx materially influences the behavior of the reservoir depends on its magnitude. For reservoirs predominantly producing under water drive (i.e., the water influx approximately balances the total withdrawal), the reservoir pressure is maintained (Figure 1).

the reservoir pressure is maintained — **FIGURE 1** The reservoir pressure is maintained

The GOR stays approximately constant at the solution gas level because there will not be any free gas flowing. Water cut, which is defined as percent water in the total fluid produced, will increase in stepwise fashion. Until water breaks through into a well, the water cut will probably be negligible. However, as soon as water breaks through, a jump in water cut occurs due to the sudden rise in water production. It continues at about the same level until water breaks through in another well. How quickly an individual well “waters out” after water breaks through depends on the ratio of the viscosity of the oil to water, the relative permeability characteristics, and the degree of reservoir heterogeneity. When the mobility ratio of water to oil is favorable, that is, when it is less than or equal to 1.0, the well will normally water out shortly after water breakthrough, although the time to breakthrough will be longer. The mobility ratio is defined by

$M=\dfrac{\dfrac{k_{rw}}{\mu _{w}}}{\dfrac{k_{ro}}{\mu _{o}}}$ …………(48)

where $k_{rw}$ and $k_{ro}$ are measured at the residual oil and water saturations, respectively.

For reservoirs that are not under strong water drive, the performance may be only partially influenced by water influx.

Selective Water Cut Control

This resume of the history of the Coldwater field in Isabella County, Michigan (Criss and McCormick 1962, McCormick 1975), is a good example of a reservoir under an effective natural water drive. This field was discovered in 1944 and its development completed in 1946, with 81 producing wells. Oil production was from a vugular dolomite, had a 48.6^º API gravity, and had been regulated since the discovery of the field, ranging from 4600 $\tfrac{B}{D}$ (731 $\tfrac{m^3}{D}$ ) to 6700 $\tfrac{B}{D}$ (1065 $\tfrac{m^3}{D}$ ). By the end of 1952, oil declined slowly to 3600 $\tfrac{B}{D}$ (572 $\tfrac{m^3}{D}$ ) and water production increased from 1800 $\tfrac{B}{D}$ to 21,000 $\tfrac{B}{D}$ (3340 $\tfrac{m^3}{D}$ ). The cumulative oil and water production was 12.763 million barrels (2.03 million $m^{3}$ ) and 25.8 million barrels (4.1 million $m^{3}$ ) respectively. The bottomhole pressure from its original value of 1453 psi (10.0 MPa) dropped to 1378 psi (9.5 MPa) by the end of 1952, but it still was above the bubble-point (1190 psi; 9.5 MPa). This pressure drop of 75 psi (0.52 MPa) over several years of production indicates a strong water drive. By the end of the same year 7.1 million barrels, or 56% of the field recovery, was recovered by flowing wells and there were only 13 water-free flowing wells remaining. All other wells flowed until a water cut of 5% to 10% was reached.

By 1961, the Coldwater field had reached an advanced rate of depletion with all wells producing with a water cut in excess of 80%. The field production continued under proration at 85 $\tfrac{B}{D}$ (13.5 $\tfrac{m^3}{D}$ ) per well until the end of 1961 when allowable restrictions were removed. By this time, a cumulative recovery of 20 million barrels (3.2 million $m^{3}$ ) of oil had been reached. Eventually, all wells were equipped to pump and fluid volumes lifted reached 600 (95.4 $\tfrac{m^3}{D}$ ) to 800 $\tfrac{B}{D}$ (127 $\tfrac{m^3}{D}$ ) per well. By 1973, the oil production further declined to about 150 $\tfrac{B}{D}$ (24. $\tfrac{m^3}{D}$ ) from 20 active wells. Up to 1974 cumulative recovery reached 21.94 million barrels (3.5 million $m^{3}$ ) and water continued to encroach into all wells, even those located at the crest of the structure. Fluid levels indicated that the reservoir was under active water drive. By returning the produced water into the aquifer, the reservoir pressure was kept at or near the initial saturation pressure and GOR remained constant until the end of the producing life of the field.