The Gas Research Institute (GRI) initiated a data acquisition and analysis project in 1990 with the goal of evaluating the applicability of horizontal well completions In various gas-productive reservoirs. One of the target gas reservoirs in this study was the Mancos Β interval of the Mancos Shale located in the Southwest Rangely Field in northwest Colorado. The overall objectives of the project were to determine reservoir characteristics from a vertical well, model the reservoir, design an appropriate horizontal well completion and compare the production economics of the horizontal well to the economics of the standard vertical well completion.

Data were initially acquired from a vertical well completion in the Mancos Β to evaluate the extent and strike of natural fractures, in-situ stress magnitude and direction, log-derived reservoir characteristics, pre- and post-frac production rates and hydraulic fracture dimensions. A horizontal well was subsequently designed to intersect the interpreted primary set of natural fractures in the Mancos Β. The horizontal well was air-drilled and intersected approximately 1,500 ft of the Mancos Β "main porosity zone."

Borehole image log data indicated that no significant natural fractures were intersected by the horizontal wellbore. A 4-1/2-in. liner was cemented in the well to facilitate hydraulic fracturing after only minimal gas was produced from the open-hole during a 3-week production test. Reservoir modeling and economic forecasting indicated that 2 hydraulic fracture treatments over the 1,500-ft interval would be the most economical. Two fracture treatments were attempted but both resulted in screenouts during the early sand stages. The early screenouts were believed to be directly related to the higher than normal in-situ stresses that were observed during breakdowns and injection tests. Due to the limited vertical extent of natural fractures and the lack of vertical permeability, horizontal well completions in this area of the Mancos Β reservoir do not appear to be a viable completion alternative to hydraulically fractured vertical wells.

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