Anomalies are observable in oil and gas well production data that are attributable to one or more of the following factors:
- Time well is on production
- Reservoir pressure
- Completion method and efficiency
- Reservoir quality and tank size
With proper analysis methods, the effect of the first two factors can be normalized out so that assessment of completion efficiency and/or reservoir quality becomes possible. This assessment is accomplished with heterogeneity indexing which may be generally defined as follows:
where HIFliud is the heterogeneity index for any type of fluid production ratio. Fluid may be oil, gas, barrels of oil equivalent, total liquid, gas-oil ratio or water cut and may consist of either “rate” or “cumulative” numbers.
A well with no heterogeneity (an average well for the field or formation) has an HI equal to 1.0. When HI is examined over n periods of time, a heterogeneity signature is obtained. An example is shown in Figure 1. The shape of this signature, relative to the benchmark of HI = 1.0, can be indicative of completion or reservoir anomalies.
Different variations and combinations of HI can be used in analysis work depending on whether well completion efficiency or reservoir characterization is being studied. For example, the early time portion of an HI signature is useful for evaluating completion efficiency, and the late time portion for reservoir characterization. The heterogeneity signature shown in Figure 1 is an example. This well is indicated to have a good completion efficiency (from the early time portion of the signature which is greater than 1.0) and limited reservoir tank size (from the late time portion which is declining). From 3-D seismic, the well was later shown to be located in a small fault block which was consistent with its heterogeneity signature, which indicated a small tank size or drainage area.
Equation 1 is generally based on a ratio of cumulative production data to normalize out discontinuities from temporary well shut-ins. The factors which influence it in a typical well under primary recovery are flow rate (q) and the decline rate (dt). Thus, HI becomes a function of the well’s initial flow rates (qi ) and decline rate (dt) compared with the same factors for the average well in the field or formation. The first variable, qi, is influenced by a) how efficiently fluids are able to move from the reservoir into the wellbore (referred to in this paper as completion efficiency) and b) the near wellbore rock quality and reservoir conditions. The second variable, dt , is influenced by a) the reservoir geometry and rock quality in the extended drainage area around the well and b) the existing reservoir drive mechanism. These factors cause the decline rate, dt , to be sensitive to various types of reservoir heterogeneities. The shape of HI signatures is influenced by these heterogeneities and shows information not normally apparent from the standard production plots typically used by industry.
Type curves for various families of heterogeneity signature phenomena observable in producing wells were developed from simulation studies. Various reservoir heterogeneities and completion efficiencies were incorporated in a standard 3-D fluid simulation model to develop the basic type curves shown in Figures 2 and 3. They include the following:
- Type 1 Good completion, larger than average reservoir tank size
- Type 2 Poor completion, larger than average reservoir tank size
- Type 3 Good completion, smaller than average reservoir tank size
- Type 4 Poor completion, smaller than average reservoir tank size
These type curves approximate actual heterogeneity signatures observed in thousands of wells from a number of fields and formations. They are designed to be used as a comparator with actual well data to assess performance. For best results, they should be calibrated to specific formations and geographical regions using completion and rock data from that area.
An application of Equation 1 includes the evaluation of well completions for behind pipe water channeling and/or the long term effects of a water drive in the reservoir. This can be analyzed using a delta HI relationship such as that shown in Equation 2.
The influx of abnormal water production will be seen as a positive value in Equation 2 with an increasing slope on the signature plot.