PlZ PLOTS

The P/Z plot is a visual solution to the gas material balance, where the original gas volume equals the remaining gas volume plus the volume of gas produced. In mathematical terms this is expressed as:

If the volume of a reservoir does not change with production, this equation will plot as a straight line. If water enters into and reduces the size of the reservoir, then the plot will form a curved line. Whereas the straight line can be extrapolated to determine the original gas in place, the curved line can only serve to identify that there is a full or partial water drive affecting the performance of the reservoir.

All gas reservoirs that are volumetric will generate a P/Z that plots as a straight line. (Volumetric reservoirs are reservoirs where the physical volume does not change with production of the oil and gas reserves.) The reverse statement that all P/Z's that plot as a straight line indicate a volumetric reservoir is not accurate.

Partial water drive reservoirs, where the rate of production greatly exceeds the rate of water influx will plot as a straight line and cannot be distinguished from a volumetric reservoir. Figure 1 is a P/Z plot for a large gas reservoir in the Gulf of Mexico. Up to the recovery of about 300 BCF the P/Z plot formed a very creditable straight line that extrapolated out to an original gas in place volume of 710 BCF, a volume much greater than the mapped volume of 500 BCF. After the production of about 300 BCF the P/Z plot became concave and formed a new straight line that extrapolated out to an original gas in place volume of about 464 BCF.

The P/Z plot for a partial water drive reservoir is sensitive to the rate of production and projects out to a greater than actual gas in place volume. Notice in Figure 2 that the rate of production from the reservoir doubled during the period of time that the _P/Z _plot changed slope. A Havlena and Odeh plot would be required to determine the original gas in place.

When evaluating a reservoir using data, la wa,ys look for the possible efi partial water drive.

Q L T - SCREW FAULTS (QUICK LOOK TECHNIQUE)

A screw fault is a fault that laterally reverses its direction of dip. This is a mapping contrariety that cannot exist in extensional areas and has only a questionable possibility in compressional areas.

Consider Fault B in Figure 3 which is a completed structure map on the AA Sand. Follow the fault trace from West to East. Notice to the West that the contours indicate a fault downthrown to the South. Consider the area between the -8,400-ft and -8,350-ft contours downthrown to the fault in the eastern region. At this position the map indicates that the fault changed its direction of dip from South to North. Observe all the contours placed downthrown to the fault from Location A to B as in contrast to only one contour (-8,400-ft) upthrown. Fault B is a screw fault.

Also, consider Location D (Figure 3) on Fault A. Notice that Fault A has reduced in size e--'(Vertical Separation) from 300 ft at Location C to approximately 50 ft at Location D. Fault A is working toward becoming a screw fault.

A screw fault is indicative of an incorrect fault interpretation. Figure 4 is the correct interpretation for the area mapped and is significantly different. The prospective areas are totally different.

A map with a recognized screw fault in an area of interest should be rejected until the problem can be resolved.