PfEFFER Concepts Pay

Pay Determination

Reservoir intervals that will contribute to reservoir production are known as "pay". Intervals that are accepted or eliminated from consideration as pay are done so on the basis of their fluid saturation content, porosity, permeability, and shaliness. The recognition of pay zones is an essential part of reservoir evaluation both as a guide to perforation depths and in the computation of field reserves. The terminology of pay determination is rather loose, but the criteria defined below are consistent with common usage. In the example shown, a sandstone-shale reservoir interval is subdivided into a hierarchy of sub-intervals according to cut-offs applied to logs and curves calculated from logs. The definitions are:

1. Gross reservoir interval: the unit between the top and base of the reservoir that includes both reservoir and non-reservoir intervals;
2. Gross sandstone (or limestone, dolomite, carbonate): the summed thickness of intervals that are determined to be sandstone, usually determined by a Vsh cut-off;
3. Net sandstone (or limestone, dolomite, carbonate): the summed thickness of gross sandstone zones that have effective porosity and permeability, usually determined by a porosity cut-off;
4. Gross pay: the summed thickness of net sandstone zones that has hydrocarbon saturation considered sufficient for economic production, usually determined by a water-saturation cut-off;
5. Net pay: the summed thickness of gross pay zones that should yield water-free production, usually determined by an irreducible bulk volume water cut-off.

Figure 16: Pay determination hierarchy

The hierarchy of pay cut-off criteria are implemented on the Home Area of a PfEFFER worksheet as a method that colors the cells of a zone according to whether the log values are greater than or less than CUT-OFF values set by the user in the PARAMETERS column. The cut-off criteria are: fraction of shale (Vsh), porosity ( f ), water saturation (Sw), and bulk volume water (BVW). Cells in the VSH, PHI, SW, and BVW columns are colored by the criteria applied individually, and cells in a PAY column are colored if the log values of the zone satisfy all the cut-off criteria. The method may be applied to the evaluation of a reservoir when the cut-offs are known from field experience. Alternatively, the option can be used in the reverse sense to establish appropriate cut-off criteria based on the reservoir profile implicit on the worksheet and interpretation and/or knowledge of production characteristics.

The cut-offs of porosity, water saturation, and "critical" bulk volume water can all be plotted on a Pickett plot as straight lines. An example is shown for the zones in the hypothetical Rottweiler Sandstone, where the cut-off boundaries collectively isolate zones that should produce water-free hydrocarbon. Pay cut-offs used throughout industry are typically "rules-of-thumb" based either on generalized experience or (preferably) drawn from the production history of the field or the play. Notice that when the cut-offs are already known for the reservoir prior to logging, the cut-off boundaries can be drawn on the Pickett plot and used immediately to draw conclusions of the productivity of logged zones (Figure 17). In cases where cut-offs are unknown or rather vague, then the Pickett plot can be used as a tool to determine appropriate values that differentiate pay from non-pay zones.

Figure 17: Pay cut-off criteria for porosity, water saturation, and bulk volume water located as boundary lines on a Pickett plot.

Z-Plot: The Third Dimension

Z-plots have been used for many years in log analysis as a means to plot a third log variable ("Z") on a conventional log crossplot (log axes "X" and "Y"). The most common example is the use of the gamma ray log on a density-neutron porosity crossplot. The density and neutron porosity values of a zone determine its coordinate location on the crossplot, and its position is marked with the gamma ray value rather than a geometrical symbol. The value can be considered as a "distance" in the vertical dimension. As an additional improvement, a color symbol is often substituted for the number scaling. The use of color provides an immediate perception of third axis location as opposed to the confusion of a mass of numbers in a conventional Z-plot.

The color Z-plot convention is used in PfEFFER to colorize points on the Pickett plot according to values selected from one of ten ATTRIBUTE columns. A common choice for the colored Z-variable is depth, so that the reservoir structure can be seen more readily, particularly in complex successions where the depth-link lines become entangled. However, shale proportion, photoelectric factor, and other log or core measurements have also proved extremely useful in the discrimination of geological or engineering characteristics.