Tight reservoirs with low and ultralow permeability must be successfully stimulated to produce at economic oil or gas rates. For this reason, costs of drilling and completing wells are very high in tight reservoirs. In order to reduce these costs, operators have often tried to replicate the same or similar hydraulic fracturing designs that have been successfully used in previous wells in the same geological area. This strategy sometimes results in unexpected surprises and operational challenges leading to unsuccessful stimulations and poor production performance. The major reason behind these challenges is that tight reservoirs exhibit a localized behavior with changes in reservoir quality such as mineralogy, hydrocarbon content, and thickness across the same reservoir.
In order to study the localized behavior of tight reservoirs; three wells that penetrated the Eaglebine formation in Texas were evaluated. The Eaglebine formation contains both the Eagle Ford and the Woodbine reservoirs. The combined Eagle Ford and Woodbine (Eaglebine) reservoir can sometimes exceed 1,000 feet in thickness. These reservoirs are present at depths between 6,500 and 15,000 feet in East Texas. In some areas, the Eaglebine contains a large percentage of silica-rich sands interbedded in organic rich shale and carbonate layers.
This paper investigates the reasons as to why same hydraulic fracturing techniques should not be applied necessarily for every well in the same geological area. Furthermore, it demonstrates how we can exploit the localized reservoir behavior to plan for future wells despite limited data availability. Data from mud logs, well logs, and cores, including mineralogy and geomechanical data are integrated to build the localized reservoir characterization model that can be used to plan how each individual well should be hydraulically fractured. The model provides information such as location of organic-rich zones, brittle zones, and ductile zones in a geological area. Lastly, it recommends the type of fracture fluid that can yield a successful stimulation operation in ductile or brittle zones.