By David Wheeler, Jerry A. Pope, and William A. Miller, Ensign Operating Company

Eva South Unit in Texas County, Oklahoma is a new project funded under the DOE Reservoir Class Field Demonstration Program (Class Revisit). Eva South Unit produces from transgressive, fluvial valley-fill sandstones of the Pennsylvanian age Morrow Formation. The reservoir displays both lateral and vertical heterogeneity due to episodic channel abandonment during deposition and subsequent faulting. This has resulted in reservoir compartmentalization that interferes with waterflood sweep efficiency. Ensign Operating Company has acquired high resolution, P-wave and P-S wave three-dimensional (3C3D) data to enhance reservoir characterization of Eva South Unit. These data are being used to reduce drilling risks and determine the optimal placement of horizontal wells.

Field History

Ensign acquired the 30-year old Eva South field in 1992 and began waterflooding in 1993. Incremental secondary recovery as of September 1999 was 601 MBO. Ultimate recovery based on the waterflood was projected at 1,276 MBO or 17.7% of original-oil-in-place (OOIP). Ensign has determined that an additional 450 MBO could be recovered through improved sweep efficiency.

Figure 4 Eva South amplitude with locations of seismic lines.

Characterization has shown that Eva South Unit has four compartments divided by the Teepee Creek fault on the west, and by abandoned channel-fill deposits on the south and east. The reservoir is active channel-fill sandstone at the base, overlain by abandoned channel-fill sandstones. The SW compartment is dominantly shaley abandoned channel-fill with tow thin beds of sandstone. Compartmentalization and heterogeneity caused by the abandoned channel-fill deposits and faulting have impeded the sweep efficiency of the waterflood.

Goal

Most of the wells at Eva South Unit have sonic logs. These logs were used to construct models showing that seismic imaging was possible. Surface-generated shear-wave seismic has been used in a Morrow reservoir nearby in Sorrento field in southeast Colorado. Ensign had the high-resolution 3C3D seismic survey ran in May 1999. To date, only the P-wave portion of the survey has been evaluated. The S-wave data had been processed and is being evaluated.

Morrow Structure at Eva South Unit

Figure 5 Net sandstone isopach with locations of cross sections

Within the core area of the Unit the structure was well defined through drilling and log data. In addition to the Teepee Creek fault two fault systems were mapped. The faults west of Teepee Creek do not effect the Unit, but a third fault system on the eastern edge of the field appears to separate one of the injector wells from its producers. Shuting-in the injector for three weeks and running pressure tests indicates that there is effective communication despite the presence of the fault.

The amplitude maps and arbitrary lines show that the Eva South Unit valley system is recognizable on the P-wave data. Discrepancies with the well control include the weak amplitude response of two wells, the best reservoir development and best production in the field and the strong amplitude response west of the Teepee Creek fault where there is currently no reservoir development. Based on the structural enhancement (Figure 5) provided by the seismic survey, Ensign is planning a horizontal well approximately parallel to the east side of the Teepee Creek fault. That well is planned to tap reserves that are trapped south of an existing injection well and improve the overall sweep efficiency of the west side of the field.

Figure 6 Ensign's upper Morrow waterflood units.

Conclusion

The P-wave data has proven to be effective at defining Morrow valleys at an exploration level. S-Wave data is able to further define reservoir heterogeneity and compartmentalization. Ensign believes that an additional 450 MBO (6.2% of

OOIP) can be recovered. These improvements in reservoir characterization increase the possibility of a future CO2 flood that could add an additional 723 MBO (10% of OOIP) of reserves. If the technology is successful at Eva South Unit, Ensign will apply it to three other upper Morrow fields (Figure 6) which have reserves of 1.775 MMBO.

The Class Act Summer 2000 Edition Volume 6/2