Tight gas reservoir simulation: Modeling discrete irregular strata-bound fracture network flow, including dynamic recharge from the matrix Page: 4 of 70
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intersection/termination frequencies of clusters could be observed. Furthermore, suitable simple
schemes to model clusters of fractures in strata-bound rocks were not found in the literature.
Therefore, we focused on making two-dimensional models of strata-bound fractures that were
located either randomly or in swarms.
To estimate fracture lengths, the initial plan was to start modeling with short fractures and
gradually increase the fracture length with each new series of fracture networks that would be
generated, until flow simulation results began to match the well production or well test results.
Fracture center-point density would be decreased while fracture length would be increased so that
fracture density would remain constant. Cluster lengths might be estimated in a similar manner
with longer-term production data. Intersection/termination frequencies would be ignored because
these attributes could not be determined with data from wells. This initial plan was not imple-
mented because the flow simulator was not yet operational.
Later, a more successful reservoir modeling effort began with an attempt to simulate a well test at
the U.S. Department of Energy's Multi-Well Experiment (MWX) site near Rifle, Colorado. Infor-
mation on fracture lengths and intersection/termination frequencies was available for some sets
from Lorenz et al. (1989, 1991) and from their analog outcrop fracture map (Figure 3A of Lorenz
and Finley, 1991). It became apparent that this analog outcrop could provide essential bits of
information for the generation of synthetic fracture networks that match the real networks in
nearby deeply buried reservoirs.
It also became apparent that, even where analog outcrops are unavailable and where reservoir
fracture lengths cannot be readily estimated from traditional correlations with bed thickness,
geological interpretation of fracture sets and well test data could suggest certain characteristic
intersection/termination frequencies that would require determinable fracture lengths.
Intersection/termination frequencies not only are useful for estimating fracture length distributions
in reservoirs; they also are essential to properly model flow and network appearance. Our early
flow simulation work demonstrated that it was insufficient to merely know the fracture length
distributions in reservoirs and to plot fractures with those lengths in a Poisson or mixed Poisson
process. It is imperative also to control intersection/termination frequencies, because a network's
connectivity affects flow rates through time. Therefore, modeling connectivity became the third
objective of the fracture network modeling effort.
Many tight gas reservoirs exist in relatively thin strata, where many of the largest and most con-
ductive fractures extend from top to bottom in individual beds, but do not extend into adjacent
beds because of contrast in rheological properties. Therefore, two-dimensional representations of
strata-bound fracture networks may adequately represent many reservoirs. Where many fractures
extend into adjacent beds and where a large flux of gas occurs across bed boundaries, three-
dimensional models are needed.
Approach: Statistical Models
The currently popular techniques of geostatistics (e.g., Kriging and sequential indicator simula-
tion) depend on the application of a certain support area, usually a square or rectangle. Unless an
extremely fine-grid representation is used, these techniques fail to accommodate the irregular
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McKoy, M.L., Sams, W.N. Tight gas reservoir simulation: Modeling discrete irregular strata-bound fracture network flow, including dynamic recharge from the matrix, article, October 1, 1997; United States. (https://digital.library.unt.edu/ark:/67531/metadc690581/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.