--> Active Geosteering Strategies in Structurally Complex Niobrara Unconventional Reservoirs

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Active Geosteering Strategies in Structurally Complex Niobrara Unconventional Reservoirs

Abstract

The Niobrara is a structurally complex reservoir that requires a multi-disciplined geosteering approach to achieve optimum well placement and maximize return on investment. A successful well plan strikes a balanced approach that not only incorporates the geologic goals of the well, but also takes into consideration factors that affect the operational efficiencies of both the drilling and the stimulation. When Whiting entered the Niobrara play, our geosteering was based primarily on gamma LWD logs. While this approach was somewhat effective, it became evident that the wellbores were not maximizing contact with the most brittle Niobrara chalk layers. As a consequence, Whiting's geosteering philosophy evolved from a passive or “reactive” gamma-based methodology to a proactive “forward-looking” approach to geosteering. Anticipation of, rather than reaction to, structural features would be critical for success. Whiting was able to make this shift in geosteering philosophy, due to the acquisition and interpretation of over 700 square miles of 3D seismic data. A detailed review of early wells using the new 3D seismic volume yielded several compelling observations. Primarily, it exposed an under appreciation of localized structural complexity internal to the Niobrara and an overreliance on regional structure as a guide to geosteering. The structure was much more complex than initially thought, with numerous narrow graben features with highly diverse orientations presenting new challenges for each drilling-spacing unit. A secondary observation was that wells which had the greatest number of frac stages placed in the clean chalk facies had the best individual well performance. Conversely, wells spending a significant amount of vertical section in the marls generally performed more poorly. This trend is likely due to the reduced efficiency of the stimulations in stages that were placed in the more ductile, bentonite-rich facies of the Niobrara. Whiting's current geosteering strategy utilizes a series of TVD and VS waypoints to give a best-fit wellbore path based on 3D seismic interpretation, facies mapping, offset well data and optimum drilling parameters. Narrow grabens are “speared through” without inducing excess wellbore tortuosity. These factors must be considered when employing an active geosteering strategy in a structurally complex unconventional reservoir.