DST Analysis

DST data was available for 6 wells in the unitized area. Wells that produced liquid during the DST were analyzed with the help of standard Horner plots. Gas wells were analyzed by two methods, i.e., by the SPE Monograph method suggested by Mathews and Russel³ and by either of the high pressure gas test analysis or the low pressure gas test analysis method⁴. The high pressure gas test analysis was employed when the pressures in the Horner straight line portion exceeded 1500 psi while the low pressure test was used when these pressures were less than 1500 psi. For gas wells with shut-in pressures were close to 1500 psi, both the high and low pressure methods were used. The results of the DST analysis are summarized in Table 2. The DST build-up plots and calculations are included in Appendix B.

Table 2 shows the intervals of the sands that are present in each of the wells along with the intervals that were tested by DST and are open for production. It also includes the completion date and the initial reservoir pressure (Pi, psi) calculated from the DST. The decline of the reservoir pressure with time is plotted in Figure 20 and it shows a rapid decline of reservoir pressure from 1600 psi to 800 psi in a period of just more than two years. By the beginning of 1995, the fluid levels above the bottom-hole pump ranged from less than 50 ft to a maximum of 200 ft across the unit. Such a rapid fall in the reservoir pressure is characteristic of a solution-gas drive reservoir with no pressure support.

The average effective permeability as calculated by one or more of the relevant methods of DST-analysis stated before, is shown in Table 2. Table 3 compares the permeability, of each of the sands, derived from the porosity-permeability correlation (Figure 9) with that calculated from the DST. The DST interval in every case is bigger than the sand thickness in the well. During permeability calculations, the effective pay thickness of the reservoir was taken to be the total vertical thickness of effective-porosity present within the DST interval. Only the sand units (#1, #2, and #3) were considered to contain effective-porosity. This significant difference between permeability calculated from DST and that obtained from the correlation, developed on core data, indicates that either a high degree of flow-heterogeneity exists around the wells or that the limitations inherent to DST measurements are coming into display. If the cause behind this difference in permeability values is indeed the flow-heterogeneity then a more detailed characterization needs to be carried out within each of the identified sand bodies.

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February 2000
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