Differences in fracture characteristics and related production

Reservoirs in the Mesaverde formation in the Piceance Creek basin, northwestern Colorado, consist of naturally fractured, low-permeability sandstones. Permeability along the natural fractures is the primary mechanism of gas production. However, the characteristics of the natural fractures change within the formation at boundaries roughly coincident with changes in depositional environment, as seen in 4,200 ft (1280 m) of core from the U.S. DOE's Multiwell Experiment (MWX) wells. Depositional environment controlled the characteristics of the matrix rock and reservoir heterogeneity as well as the fracture permeability system. Therefore, natural gas production rates from the different zones in the formation can be correlated with the natural fracture characteristics from that zone. The fracture systems consist of unidirectional regional fractures that provide highly anisotropic reservoir permeability trends. Locally, secondary fractures were superimposed on the regional system, and higher production rates resulted. Production rates are low elsewhere, and little well-to-well communication across the fracture trend occurs. Moreover, stimulations of these reservoirs have often been unsuccessful because the hydraulic fractures propagate parallel to the natural fractures.