Horizontal well development targeting the Niobrara formation in the Denver-Julesburg (DJ) Basin in Colorado and Wyoming is gaining development momentum as the price of gas in the US has fallen and liquid rich plays have come into high demand. The low permeability Cretaceous age Niobrara formation is commonly encountered as three benches of naturally fractured limestones, marls and calcareous shale. The advent of horizontal drilling has allowed exploitation of this previously overlooked target in the DJ Basin.

Thermal maturity varies across the DJ basin, resulting in a variety of reservoir fluids in the Niobrara including dry gas (biogenic and thermogenic), wet gas, condensate, and oil. Two of the main oil fields in the DJ Basin are the Silo field, in Laramie County Wyoming, and the Wattenburg field, which is predominantly in Weld County, Colorado.

Oil has been produced out of the DJ Basin since 1901, however successful horizontal well development is still relatively recent in both fields. With the increased interest in Niobrara development, rate-decline analysis is used extensively by operators to estimate reserves and to evaluate potential locations for future drilling.

Various published models are available for rate-decline analysis. Arps rate-decline relationship has been the most widely used model, however subsequent authors have developed models modifying Arps, or in the case of unconventional and naturally fractured formations, developed completely new models, like Duong, for rate-decline analysis.

This paper will utilize production data for wells in Wattenburg and Silo fields to assess how appropriate different models are for rate-decline analysis in the naturally fractured Niobrara Formation.

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