A Methodology for the Assessment of Unconventional (Continuous) Resources with an Application to the Greater Natural Buttes Gas Field,Utah |
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Authors: | Ricardo A Olea Troy A Cook James L Coleman |
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Institution: | (1) U.S. Geological Survey, 12201 Sunrise Valley Drive, Mail Stop 956, Reston, VA 20192-0002, USA;(2) Denver Federal Center, Mail Stop 939, Box 25046, Denver, CO 80225-0459, USA |
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Abstract: | The Greater Natural Buttes tight natural gas field is an unconventional (continuous) accumulation in the Uinta Basin, Utah,
that began production in the early 1950s from the Upper Cretaceous Mesaverde Group. Three years later, production was extended
to the Eocene Wasatch Formation. With the exclusion of 1100 non-productive (“dry”) wells, we estimate that the final recovery
from the 2500 producing wells existing in 2007 will be about 1.7 trillion standard cubic feet (TSCF) (48.2 billion cubic meters
(BCM)). The use of estimated ultimate recovery (EUR) per well is common in assessments of unconventional resources, and it
is one of the main sources of information to forecast undiscovered resources. Each calculated recovery value has an associated
drainage area that generally varies from well to well and that can be mathematically subdivided into elemental subareas of
constant size and shape called cells. Recovery per 5-acre cells at Greater Natural Buttes shows spatial correlation; hence,
statistical approaches that ignore this correlation when inferring EUR values for untested cells do not take full advantage
of all the information contained in the data. More critically, resulting models do not match the style of spatial EUR fluctuations
observed in nature. This study takes a new approach by applying spatial statistics to model geographical variation of cell
EUR taking into account spatial correlation and the influence of fractures. We applied sequential indicator simulation to
model non-productive cells, while spatial mapping of cell EUR was obtained by applying sequential Gaussian simulation to provide
multiple versions of reality (realizations) having equal chances of being the correct model. For each realization, summation
of EUR in cells not drained by the existing wells allowed preparation of a stochastic prediction of undiscovered resources,
which range between 2.6 and 3.4 TSCF (73.6 and 96.3 BCM) with a mean of 2.9 TSCF (82.1 BCM) for Greater Natural Buttes. A
second approach illustrates the application of multiple-point simulation to assess a hypothetical frontier area for which
there is no production information but which is regarded as being similar to Greater Natural Buttes. |
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