The ratio of the rate of production, expressed in STB/day for liquid flow, to the pressure drawdown at the midpoint of the producing interval, is called the productivity index, symbol J.
The productivity index (PI) is a measure of the well potential, or the ability of the well to produce, and is a commonly measured well property. To calculate J from a production test, it is necessary to flow the well a sufficiently long time to reach pseudosteady-state flow. Only during this flow regime will the difference between 
and pwf be constant. It was pointed out in section 8.3 that once the pseudosteady-state period had been reached, the pressure changes at every point in the reservoir at the same rate. This is not true for the other periods, and a calculation of productivity index during other periods would not be accurate.
In some wells, the PI remains constant over a wide variation in flow rate such that the flow rate is directly proportional to the bottom-hole pressure drawdown. In other wells, at higher flow rates the linearity fails, and the PI index declines, as shown in Fig. 8.15. The cause of this decline may be (1) turbulence at increased rates of flow, (2) decrease in the permeability to oil due to presence of free gas caused by the drop in pressure at the wellbore, (3) increase in oil viscosity with pressure drop below bubble point, and/or (4) reduction in permeability due to formation compressibility.
Figure 8.15 Decline in productivity index at higher flow rates.
In depletion reservoirs, the productivity indices of the wells decline as depletion proceeds, owing to the increase in oil viscosity as gas is released from solution and to the decrease in the permeability of the rock to oil as the oil saturation decreases. Since each of these factors may change from a few to severalfold during depletion, the PI may decline to a small fraction of the initial value. Also, as the permeability to oil decreases, there is a corresponding increase in the permeability to gas, which results in rising gas-oil ratios. The maximum rate at which a well can produce depends on the productivity index at prevailing reservoir conditions and on the available pressure drawdown. If the producing bottom-hole pressure is maintained near zero by keeping the well “pumped off,” then the available drawdown is the prevailing reservoir pressure and the maximum rate is 
.
In wells producing water, the PI, which is based on dry oil production, declines as the watercut increases because of the decrease in oil permeability, even though there is no substantial drop in reservoir pressure. In the study of these “water wells,” it is sometimes useful to place the PI on the basis of total flow, including both oil and water, where in some cases the watercut may rise to 99% or more.
The injectivity index is used with saltwater disposal wells and with injection wells for secondary recovery or pressure maintenance. It is the ratio of the injection rate in STB per day to the excess pressure above reservoir pressure that causes that injection rate, or
With both productivity index and injectivity index, the pressures referred to are sandface pressures, so that frictional pressure drops in the tubing or casing are not included. In the case of injecting or producing at high rates, these pressure losses may be appreciable.
In comparing one well with another in a given field, particularly when there is a variation in net productive thickness but when the other factors affecting the productivity index are essentially the same, the specific productivity index Js is sometimes used, which is the productivity index divided by the net feet of pay, or
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