Microseismic monitoring of the B-sand hydraulic fracture experiment at the DOE/GRI multi-site project Page: 4 of 10
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Microseismic Monitoring of the B-Sand Hydraulic Fracture Experiment at the DOE/GRI Multi-Site Project
detected on as many levels from each well as possible.
Considerable automatic processing was performed to aid in
initial analyses, but all results were quality checked by an
analyst. Although advanced seismic analyses are in progress
(using Nelson and Vidale's algorithms13"4, initial seismic
results using simpler techniques have been found to be quite
accurate because of the large number of receiver stations.
The initial results assume a single p-wave velocity and a
single velocity factor, given by
Vfy p S (1)
which is used to calculate the distance to a microseism based
on the arrival time separation of the p and s waves, given by
d=Vf(ts-tp) , (2)
where t, is the s-wave arrival time and t, is the p-wave arrival
time. The distance and elevation of the microseism are first
calculated using a regression analysis on the ray-path travel
times to all of the receiver stations at which a p-wave arrival
has been determined. Uncertainty analysis of the results
shows that such a calculation is highly accurate for the
elevation (because of the large number of vertically arrayed
receivers), but has considerable error in the distance (because
all of the receivers are vertically arrayed). To improve on the
distance calculation, p-s separations are found wherever
possible and the average distance to the microseism are
deduced. The orientation of the microseism in the horizontal
plane is calculated by averaging all of the levels in which the
orientation of the p-wave particle motion is accurate (usually
8-10 levels for the monitor well and 3-4 levels for the five-
level wireline array). Given the distance, elevation and
orientation, microseismic points can be mapped. As noted
earlier, more advanced analyses are in progress, but initial
comparisons show little difference in the two results.
Figure 4 shows an example microseism detected on the
monitor-well array across 17 levels. In this typical example,
the p-wave arrival is discernible on nearly every level (a few
levels have non-functioning accelerometers) and s-wave
arrivals are evident on many of the upper and lower levels.
Given such detailed data, accurate locations for most
microseisms are routine. One exception to the height
accuracy occurred during the initial injections using KCl
water. During these tests only a single receiver was used in
MWX-3 and elevation locations from a single receiver are
notoriously inaccurate because the velocity structure of the
formation results in bent ray-paths and misleading vertical
locations. To avoid misleading data for fracture height, all
single receiver data were scaled back to fit within the height
determined by the full array of monitor-well receivers for the
same injection. Thus, height on these tests was only
determined by monitor well instrumentation and the single-
level data should only be used for length and azimuth
information. With the minifracs and propped fracture
treatment, the five-level array in MWX-3 was fully capable of
providing accurate height data and no such scaling was
needed for the later tests.
Microseismic locations are not necessarily on the
hydraulic fracture, but can in fact be some distance to the side
or even ahead of the fracture.15 Calculations of the
mechanical response of the formation for B-sand conditions
have been performed to estimate the zone of microseismic
activity anticipated for this interval. These analyses show that
microseismic events at this site can occur as much as 12-15 ft
ahead of the tip of the fracture and 15-20 ft normal to the tip
of the fracture because of the large stress concentrations at the
fracture tip, and several tens of feet normal to the body of the
fracture due to leakoff effects (increase in pore pressure). The
exact distance at which leakoff-induced microseisms might
occur is difficult to deduce because of unknown permeability
of the natural fractures in the formation. Nevertheless, these
analyses provide guidance in interpreting the microseismic
data and show that the microseisms map out an envelope in
which the hydraulic fracture is embedded. By examining this
envelope, the height, length and azimuth of the fracture can be
fairly accurately estimated, but nothing can be inferred about
the width of the fracture or the number of fracture strands or
width of any complex fracture zone that may or may not exist.
Fracture Diagnostic Results
Fracture diagnostic results are presented as plan views and
side views of the microseismic locations. Given the geometry
of the arrays, the most accurate position is the elevation of the
microseism (because of the large number of receivers in
vertical arrays), while the distance from the microseism to the
receiver and the azimuth of the microseismic signal have more
uncertainty. Accuracy of the elevation position is about 5 ft,
but results vary from microseism to microseism depending on
how many receivers have detected the particular event.
Accuracy of the azimuth to the microseism is about 5*, and
since the distance away is approximately 300 ft, the
uncertainty in the azimuthal location is about 25 ft at that 300-
ft distance. Uncertainty in the distance from the receiver to
the microseism varies considerably depending upon the
number of levels on which the s-wave arrival is accurately
detected, but it is on the order of 50 ft.
Given the uncertainties in locations and the interpretation
of microseisms discussed in the previous section, the
microseismic results should be considered an envelope which
surrounds the fracture. Furthermore, since uncertainties on a
few microseisms can become relatively large, the position of
any single outlier microseism should always be questioned.
Step Rate Test (2B). Figure 5 shows the final microseismic
images of the 27 bbl step-rate test (maximum rate of 3 bpm),
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Warpinski, N. R.; Wright, T. B.; Peterson, R. E. & Branagan, P. T. Microseismic monitoring of the B-sand hydraulic fracture experiment at the DOE/GRI multi-site project, article, November 1996; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc678891/m1/4/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.