Abstract

The trapping of hydrocarbons in the Dollarhide Fusselman and Montoya reservoirs is both structural and stratigraphic. The structural trapping mechanism is a north to south trending anticline bound on its eastern flank by a major reverse fault with 2,000 feet of throw. The field is cut by two transverse faults which run Northeast to Southwest across the field. The stratigraphic components are related to the environments of deposition.

Four cores were taken in the field during primary development. There were 200 feet of Fusselman and 40 feet of Montoya core. These were described as “highly fractured lime”. From this it was assumed that the entire reservoir was highly fractured and the main production mechanism was bottom water drive. So as wells began to produce water they were plugged back to keep them producing water free.

Reservoir performance is not typical of a fractured reservoir with a bottom water drive as first thought. First, oil recovery from the field has been less than 25 percent. Second, offset wells produce with variable oil cuts at similar depths in the reservoir. Third, twin wells drilled in the same pad had DST’s and shows in porous intervals which did not exist in the Silurian producer only 200 feet away. These factors led us to believe that the reservoir is much more heterogeneous than originally thought.

In fall of 2000, Pure Resources cored and ran image logs in the DHU 25-2 S to obtain a better understanding of the actual interval geometries within the Fusselman and Montoya reservoirs. There was 333 feet of core taken and 281 feet recovered. Of the core recovered, 193 feet was Fusselman and 88 feet was Montoya. The entire Fusselman and Montoya formation were logged with image logs. They revealed tectonic fractures exist within sections of the reservoir but they were not dominant throughout. The stratigraphic component was much more complex than originally thought. There were definite vertical barriers within the Fusselman and Montoya reservoirs. These barriers were the result of complex stratigraphic changes during deposition. These barriers formed effective internal traps for hydrocarbons.

A workover program and infill drilling program was implemented to recover these bypassed reserves. Sixteen wells were reentered over the past 3 years and relogged with behind pipe hydrocarbon logs. These identified the existence of perched oil zones throughout these reservoirs in several wells. Selective perforations and an effective acid treatment resulted in an average of 57 BOPD per workover. Four infill wells on 20 acres spacing have been drilled to date. Only the Montoya has been completed in each which have an average IP of 238 BOPD and 264 BWPD. The estimated average recovery per well for both the Fusselman and Montoya is 235 MBOE per well. Critical data such as open hole electric logs and mudlogs, and combining a good cement job with an effective completion have proved invaluable in making good wells.

Several future recompletions and infill wells are planned at this time. To date this program has added 2.9 MMBOE of reserves to the field. This project is an example of applying a new model to an old field to recover additional reserves.