P/Z Plots

In the last volume of "Subsurface News" we carried an article on a P/Z Plot for a partial water drive gas reservoir. In that article we showed that the P/Z plot for a partial water drive reservoir was sensitive to the rate of production (Figure 4) and actually plotted as two straight lines. A number of readers have asked how a partial water drive reservoir could exhibit a straight line plot of the P/Z data, and more importantly how an evaluator can distinguish a partial water drive reservoir from a truly volumetric reservoir.

Analysis of the field performance data indicates that the gas production rate from the reservoir was relatively constant during the period that the P/Z data plotted as a straight line. During the same period, the rate of water influx was also constant in barrels of water per day. The water influx rate had reached its maximum or critical flow rate for the reservoir and could not respond to the rapid decline in the reservoir pressure by supplying increased volumes of water for the increased pressure differential.

Different techniques have been proposed for recognizing and categorizing water drive reservoirs including:

1. A convex shaped P/Z plot.
2. Deviation of a Log-Log plot of pressure loss versus cumulative gas production from a 45 degree line. 
3. Curved Havlena-Odeh plot.

None of these techniques work for this reservoir, and did not replace sound and thorough reservoir evaluation. (Figures 5, 6 and 7)

When the performance data is at variance with the valumetric data, the evaluation team needs to double check all the data. For this particular reservoir, a water contact had been seen on the initial well logs, the sand was thick and covered a large area of the Gulf of Mexico, and cased-hole logs indicated a change in the gas-water contact in a portion of the reservoir after production was initiated.

A partial water drive was active in the reservoir and the P/Z plot could not be used to determine the reserves and the original gas in place.

ABNORMALLY PRESSURED GAS RESERVOIRS

Abnormally pressured gas reservoirs frequently project out on a P/Z plot to a greater volume than the actual volume of gas in place. An accepted rule of thumb is 2 to 1, two times projected to actual gas in place. The cause of this phenomena is the compaction of the reservoir rock, as the reservoir pressure is reduced with production of the gas from the reservoir.

In the Gulf Coast region, abnormally pressured reservoirs are frquently the result of the trapping conditions preventing the dewatering of the formation as it is buried. If the reservoir is an unconsolidated sand, that sand will compact with production as if it were being buried and dewatered.

The effect of this compaction is a decrease in porosity and permeability, and an increase in the water saturation. The change in porosity is given by the equation.

The abnormally pressured reservoir will exhibit a formation compressibility factor (Cf) approximately that it would have had at the depth at which dewatering ceased. The factor Cf is reservoir specific and varies from 200x10-6 at depths of 10,000 to 15,000 feet. With the decrease in porosity there is an associated decrease in the absolute permeability of the reservoir rock. The magnitude of this decrease requires special rock studies or knowledge of the relationship between porosity and permeability for increasing overburden pressures.

As the reservoir rock is compacted, the water saturation in the reservoir rock increases because the reservoir water expands and the porous volume of the reservoir decreases. As a rsult, there is a larger volume of water filling a decreased reservoir volume. The following equation raltes this change in water saturation.

This increase in awater saturation will further reduce the permeability of the reservoir rock to gas.

The combined impact of decreasing permeability and increasing water saturation explains why many abnormally presured reservoir experience decreasing deliverability, increasing water production and eventually sand up.