An integrated geological/petrophysical and reservoir engineering study has been performed for a large, mature waterflood project (>250 wells, 80% water cut) at the North Robertson (Clear Fork) Unit, Gaines County, Texas. The primary goal of the study was to develop an integrated reservoir description for “targeted” 10-ac (4-ha) infill drilling and future recovery operations in a low- permeability carbonate reservoir. Integration of geological/petrophysical studies and reservoir performance analyses provided a rapid and effective method for developing a comprehensive reservoir description.

This reservoir description can be used for reservoir flow simulation, per-formance prediction, infill targeting, waterflood management, and optimizing well developments (patterns, completions, and stimulations). The following analyses were performed as part of this study:

• Geological/petrophysical analyses: (core and well log data)

  • Rock typing based on qualitative and quantitative visualization of pore- scale features.

  • Reservoir layering based on rock typing and hydraulic flow units.

  • Development of a core-log model to estimate permeability using porosity and other properties derived from well logs. The core-log model is based on “rock types.”

• Engineering analyses: (production and injection history, well tests)

  • Material balance decline type curve analyses performed to estimate total reservoir volume, formation flow characteristics (flow capacity, skin factor, and fracture half-length), and indications of well/boundary interference.

  • Estimated ultimate recovery analyses yield movable oil (or injectable water) volumes, as well as indications of well and boundary interference.

  • Well tests provide estimates of flow capacity, indications of formation damage or stimulation, and estimates of drainage (or injection) volume pressures.

Maps of historical production characteristics (contacted oil-in-place, estimated ultimate recovery, and reservoir pressure) have been compared to maps generated from the geologic studies (rock type, permeability/thickness, hydrocarbon pore volume) to identify the areas of the unit to be targeted for infill drilling. Our results indicate that a close relationship exists between the rock type distribution and permeability calculated using porosity and other properties derived from well logs.

The reservoir performance data also suggest that this reservoir depletes and recharges almost exclusively according to the rock type distribution. This integration of rock data and the reservoir performance attributes uses existing data and can eliminate the need for evaluation wells, as well as avoiding the loss of production that occurs when wells are shut-in for testing purposes.

In short, a comprehensive analysis, interpretation, and prediction of well and field performance can be completed quickly, at a minimal cost, and this analysis can be used to directly improve our understanding of reservoir structure and performance behavior in complex formations.