Abstract

Integrated geologic and reservoir analysis of Viola well production in the Marietta basin of Jefferson and Carter counties, Oklahoma indicates natural fractures have a strong control on reservoir performance. A horizontal well was drilled to evaluate fracture development and productive capacity from the lowermost 100-150 ft of the “Chocolate Brown Zone” (CBZ) of the Viola Limestone in the Marietta basin. The Viola CBZ in the study area is a low-porosity reservoir (4-6%) with extremely low matrix permeability (k ≅ 0.0001 mD, measured at overburden stress). Reservoir potential of the CBZ is strongly fracture dependent. The structural model developed for southern Oklahoma predicts the existence of regional extension fractures aligned parallel with the maximum principal horizontal stress. Regional extension fractures originated from far-field regional compression emplaced during late Mississippian to Pennsylvanian tectonism. High secondary silica content in the CBZ ranging from 35-70% was conducive to brittle deformation within the largely compressive structural regime. Development of regional fractures improved permeability within the CBZ while inducing significant permeability anisotropy. Reservoir permeability anisotropy may be economically exploited through production from horizontal wells.

To test the fractured reservoir model, a horizontal well was drilled with 3900 ft (1190 m) of lateral borehole. The well was completed open hole with approximately 1000 ft (305 m) of the lateral within fractured CBZ reservoir. The well initially flowed in excess of 1200 BOPD but declined rapidly and ceased to flow. After installation of artifical lift, production settled at a constant 450 BOPD (water free), until the fracture system was depleted. At this time, matrix permeability within inter-fracture regions was inadequate to supply the fracture network. Thus oil production rapidly declined to an uneconomic rate.

A dual-porosity reservoir simulator using the specific petrophysical and other reservoir properties of the CBZ was used to match, with reasonable accuracy, early production data from the horizontal test well. Draw-down/build-up test data and bottom-hole flowing pressure vs. cumulative oil production were used to calibrate the model to actual well performance. The model was used to predict ultimate recovery and gain a better understanding of reservoir performance. Modeling aided identification of reservoir characteristics necessary for a successful Viola horizontal well play.

The results of the reservoir modeling indicate the reservoir properties fundamental to an economic completion in fractured CBZ reservoir are fracture spacing, fracture aperture, vertical and lateral fracture extent, and sufficient matrix permeability to sustain fluid flow to the fracture network. Simulator results and production forecasts indicate the majority of recoverable reserves will be made very early in the productive life of a fractured Viola CBZ reservoir. This study demonstrates the utility and need for integration of geologic, analytical, petrophysical, and reservoir engineering data for comprehensive evaluation of reservoir behavior and performance forecasts.