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Cellular geomodel for a
giant gas field, Hugoton, Midcontinent, U.S.A.
Martin K. Dubois, Alan P. Byrnes, Saibal
Bhattacharya, Geoffrey C. Bohling and John H. Doveton,
Kansas Geological Survey
The Hugoton geomodel provides a
comprehensive lithologic and petrophysical view of a mature
giant Permian gas system, the 70-year-old Hugoton Field,
which is the largest gas field in North America. Fine-scale
cellular models are particularly important for modeling
thin-layered, differentially depleted reservoir systems
(Hugoton) and methods used in building the model demonstrate
the construction of a cellular petrophysical model for a
giant field. The study also illustrates the benefits of
pooling proprietary geologic and engineering data in
settings with multiple operators. Both the knowledge gained
and the techniques and workflow employed have implications
for understanding and modeling similar reservoir systems
worldwide. As giant fields mature, high-resolution modeling
at the full-field scale in data-rich environments will
become increasingly important and the Hugoton model is a
large-scale example for developing such models.
Building an accurate static model for the
entire Hugoton field (Hugoton and Panoma in Kansas and
Guymon-Hugoton in Oklahoma) was the primary objective of a
2-1/2 year collaborative project sponsored by ten industry
partners and the State of Kansas through the Kansas
Geological Survey. The goal was to develop a model with
sufficient detail to represent vertical and lateral
heterogeneity at the well, multi-well, and field scale,
which could be used as a tool for reservoir management
including accurate prediction of original and remaining
gas-in-place. This required that the model be finely layered
(169 layers, 3-foot (1 m) average thickness) and have
relatively small XY cell dimensions (660x660 ft, 200x200m;
64 cells per mi2). These criteria resulted in development of
a 108-million cell model for the 10,000-mi2 (26,000 km2)
area modeled. Although lithofacies geobodies tend to be
laterally extensive, covering multi-section to township
scales, small XY cell dimensions were required to allow the
extraction of portions of the model for local reservoir
simulation. Water saturations needed for original
gas-in-place (OGIP) determination were estimated using
capillary pressure methods and not measurements from
wireline logs because accurate determination of water
saturations using conventional wireline logs is complicated
by deep mud filtrate invasion for typical drilling programs.
Material balance methods for estimating OGIP are equally
problematic because the reservoir is layered and
differentially depleted and wellhead shut-in pressures (WHSIP)
are strongly influenced by high-permeability interval
properties and |
do not accurately
represent all interval pressures; and pressure data
for individual layers are sparse. The Hugoton geomodel may
be the largest model of its kind (lithofacies-controlled,
property-based water saturations). Core-based
calibration of neural-net prediction of lithofacies using
wireline-log signatures, coupled with
geologically-constraining variables, provided lithofacies at
wells. Stochastic methods were employed to estimate
lithofacies between wells. Differences in petrophysical
properties among lithofacies and within a lithofacies among
different porosities illustrate the importance of integrated
lithologic-petrophysical modeling and of the need for
closely defining these properties and their relationships.
An accurate lithofacies model, coupled with lithofacies-dependent
petrophysical properties, allowed the construction of a 3-D
geomodel that has been effective at the well, section (1mi2,
2.6 km2) and multi-section and field scales. Multi-well,
multi-section simulations validate the property model and
illustrate differential depletion relationship to layer
property variability.
The model provides
a detailed three-dimensional view of the 160-m reservoir
comprised of thirteen shoaling-upward cycles vertically
stacked in a low-relief shelf setting and is an analog for
similar thin layered, stacked-cycle reservoir systems,
including those in the Paradox and Permian basins and the
Khuff Formation in Gwahar and North fields in the Arabian
Gulf. Unprecedented 3-D views of shifting lithofacies
patterns on a large stable shelf document sedimentary
response to climate change during the transition from
icehouse to greenhouse conditions in the Lower Permian. The
model is a tool for predicting properties, water saturations
and OGIP, is a quantitative basis for evaluating remaining
gas--in-place, particularly |
in
low-permeability intervals and has helped direct field
management and field rules changes that could enhance
ultimate recovery. The model and study also provides a
"fast-forward" view of similar giant reservoir systems
worldwide. Static model construction was completed in 2006.
The consortium is now focused on the characterization of
remaining gas-in-place and developing alternative drilling
and completion practices for more efficient extraction of
remaining reserves.
Key findings:
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The Kansas-Oklahoma portion of the field
has yielded 35-tcf gas (963-billion m3) over a 70-yr
period from over 12,000 wells and an estimated 65% of
the original gas-in-place in the central portion of the
field.
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Most remaining gas is in lower
permeability pay zones of the differentially depleted,
layered reservoir system.
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The Chase (Hugoton) and Council Grove (Panoma)
behave as a common reservoir.
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Lithofacies bodies are laterally
extensive and reservoir storage and flow units exhibit
extensive lateral continuity.
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The Hugoton reservoir free-water level (FWL)
is sloped, ranging, west to east, from subsea depth of
+1000 ft (+300 m) to +50 ft (+15 m).
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Base on reservoir simulations,
production is sustainable through 2050, provided the
integrity of 40-70-yr old wells can be maintained.
For more information and references, see the full report
online at the Kansas Geological Survey website
www.kgs.ku.edu/PRS/pub
lication/2007/OFR07_06/index.
html). 
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Fence diagram from the 108-million cell,
80 x 120 mi, model for the Chase Group (Hugoton field, solid
outline) and the directly underlying Council Grove Group (Panoma
field, dashed outline), flattened at the top of each
interval. Production is from thirteen upward-shallowing
marine carbonate intervals (cooler colors) separated by
continental siliciclastics (warmer colors). |
Midcontinent
Region