Using Tracers to Evaluate
Propped Fracture Width, S.A. Holditch, David Holcomb, Zillur Rahim, SPE
26922, November 1993.
ABSTRACT
Using Tracers to Evaluate
Propped Fracture Width
Many production engineers are
beginning to use three-dimensional (3-D) fracture propagation models to
design and analyze hydraulic fracture treatments. To use a 3-D model, one
must define the layers that comprise the reservoir and develop detailed
datasets that accurately describe the layers. The data that are critical for
designing and analyzing hydraulic fracture treatments are in-situ stress,
formation permeability, formation porosity, reservoir pressure, and Young's
modulus. Many times, these parameters can be determined from logs and/or
correlated to lithology.
Once the datasets are
obtained, one can use a three-dimensional fracture propagation model to
estimate values of created or propped fracture length, width, and height. To
understand and improve the fracture design process, the engineer must
confirm the estimates of fracture dimensions that are predicted by a
fracture propagation model. To verify the model, one must analyze field data
to be sure the field data are consistent with the model results. For
example, the net pressure predicted by the 3-D fracture propagation model
should closely match the net pressures observed in the field. When net
pressure is adequately matched, we usually find that the overall created
fracture dimensions predicted by a 3-D fracture propagation model are
reasonable. To determine estimates of propped fracture length, one must also
analyze post-fracture production and pressure transient data. Because of
fracture fluid cleanup problems, we often find that values of propped
fracture length generated by analyzing field production data are much
shorter than the created fracture length predicted by the fracture
propagation model. Detailed engineering studies are often required to
reconcile the differences.
To directly measure values of
fracture width, one must perform a fracture treatment in openhole, then use
a downhole imaging tool to "see" the fracture. Such an approach is
not usually pratical. In this paper, we will describe a method to
qualitatively estimate the propped width profile at the borehole that uses
radioactive tracers. Confirming the propped width profile generated by a
model with field data can be very beneficial and informative.
We have found that the use of
zero wash radioactive tracers can help us learn both (1) where the fracture
fluid is going and (2) where the proppant resides in the fracture near the
wellbore. Assuming the level of radioactivity is proportional to volume,
then the level of radioactivity will also be proportional to the propped
fracture width. As such, one can obtain qualitative estimates of propped
fracture width at the wellbore using a radioactive tracer where the strength
of the radioactive signal is proportional to fracture volume near the
wellbore.
The objectives of this paper
are to discuss what factors control the fracture width profile and how to
obtain data to compute fracture width. We also explain how one can use
radioactive tracers to develop data that can be analyzed to determine
qualitative estimates of propped fracture width. Finally, we provide several
examples to illustrate how one can estimate values can be used to calibrate
a 3-Dimensional fracture propagation model.
The information described in
this paper can be used by a production engineer to obtain a better
understanding of a specific hydraulic fracture treatment. As our
understanding of hydraulic fracturing improves, we should be able to design
the optimal fracture treatment with more certainty. When we design and pump
the optimal fracture treatment, we maximize the economic return on
developing oil and gas properties.