Reservoir Fluid Flow and Natural Drive Mechanisms

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Solution Gas Drive Reservoir Performance

Pressure, GOR, Water Production Profiles

Once free gas develops and the saturation exceeds a critical value, the gas will start to be produced in disproportionate quantities. Figure 1 illustrates a typical GOR history.

Illustration a typical GOR history — **FIGURE 1** Illustration a typical *GOR* history

Above the bubble-point the GOR is constant, since all the gas production comes solely from the dissolved gas. As the pressure declines below the bubble-point a free gas phase develops and the gas in solution declines slightly. The produced gas continues to be supplied solely from the dissolved gas until the gas saturation exceeds the critical value. During this period the GOR declines. Subsequently, the free gas begins to flow and constitutes part of the produced GOR. Its contribution increases as the gas saturation increases and far exceeds the decline in the amount of gas in solution. This is why the GOR rises sharply. The subsequent decline in GOR sets in when the reservoir pressure declines significantly and much of the original dissolved gas has been produced. At that point, the contribution of the gas in solution to GOR becomes negligible, and the GOR may be approximated by

$GOR=\dfrac{k_{rg}\ \mu _{o}\ B_{0}}{k_{ro}\ \mu _{g}\ B_{g}}$

Since B_g is proportional to its value increases rapidly as the pressure declines Figure 2. On the other hand, the change in the values of the other parameters is relatively small. This explains the rapid decline of the GOR shown in Figure 1.

Bg versus pressure — **FIGURE 2** Bg Vs pressure

Initially, the water production may be zero or close to it, especially if no aquifer exists. This is so because the water saturation in the absence of an aquifer is usually equal to or very close to the critical saturation required for flow. As the pressure declines, the rock and water expand. Thus the relative water saturation increases slightly and water produc tion may also increase slightly as shown in Figure 1.

Pressure Maintenance

In many solution gas drive reservoirs, pressure maintenance by water injection is employed to enhance ultimate recovery. There are several advantages of initiating water injection at or near the bubble-point pressure and prior to the development of a significant gas phase. One advantage is linked to the nature of relative permeabilities. The oil and water relative permeabilities are higher when a gas phase is not present than when an appreciable gas saturation is established. Thus, for the same pressure drawdown at the wells, the oil production rate is higher. Another important advantage is that the rate of water injection required to maintain a certain level of production is lower. To maintain pressure, the injection rate must be equal to the total fluid withdrawal in reservoir volumes. The total fluid withdrawal associated with one surface volume is

$B_{t} + (R_{p} - R_{si})\, B_{g}$

where:

B_t= two phase formation-volume factor, $\tfrac{RB}{STB}$
R_p= cumulative produced GOR, $\tfrac{SCF}{STB}$
R_si= initial gas in solution, $\tfrac{SCF}{STB}$
B_g= gas formation-volume factor, $\tfrac{RB}{scf}$

At or close to the bubble-point, R_p=R_si and the withdrawal is approximately B_ti=B_oi, conversely, if pressure maintenance is started at a pressure level significantly below the bubble-point pressure, the value of (R_p−R_si)B_g can be significant and the total withdrawal associated with one surface volume is considerably larger than B_oi. Thus more water injection is required to maintain the same level of oil production. From an economic point of view it is advisable to initiate pressure maintenance at or close to the bubble-point pressure.