Hydraulic fracture and flow modeling in fluvial tight gas sand reservoirs are common methods for predicting completion and reservoir performance. However, geocellular modeling of the fluvial architecture used in these models frequently relies on assumptions of channel aspect ratios using well logs as the basis for " channel" thickness or using geostatistical methods with closely spaced well logs. In contrast, this study uses 3D geologic observations from a lower Mesaverde outcrop belt north of Rangely, CO to construct two detailed geocellular models based on the documented stratigraphic facies differences across a 400 feet thickness of fluvial deposits (Figure 1). The two geocellular models (upper and lower) contain different sandbody and overbank architecture.
The geocellular modeling strategy is dictated by the stratigraphic variability in facies heterogeneity. The lowest sequence is characterized by pancake-like high aspect ratio channels and crevasses splays with limited facies heterogeneity and is modeled deterministically. The associated overbank mudstones and thin sandstones show significant facies variability; retained using stochastic techniques. In contrast the upper stratigraphic sequence had ribbon-like lower aspect ratio channels with significant facies variability encased in overbank mudstones with limited facies variations. To retain the character of this sequence the channels were modeled stochastically and the overbank was modeled deterministically. Rock properties were populated by facies in both models based on subsurface data from an analog producing field (Figure 1).
The upper and lower models were the framework for hydraulic fracture modeling. Because of differences in the geocellular models, two different completion strategies are studied: one by treating both models as a single stage treatment, and a second approach by treating each model in two different stages. These strategies test how variations in fluvial architecture in a vertical sequence affect fracture propagation and sandstone body connectivity, and imply changes in the completion technique (treatment volumes and perforation strategies).
Modeled hydraulic fractures in both geocellular models are incorporated in a flow simulator to study and optimize common well-spacing scenarios. Simulation results indicate that these subtle differences affect drainage results from both an addition and acceleration standpoint, and incorporation can benefit overall flow and drainage understanding.