American Oil and Gas Reporter - December 2018 - 52

SpecialReport: Well Stimulation & Completion Technology
FIGURE 2
In Situ Sand within Core (Uphole View)
And Base Sand Before Sample Collection

the lookup table (LUT) so particles were black on a white background;
· Applying the "fill holes" action because proppant tended
to have a darker interior and brighter perimeter (D);
· Applying a watershed algorithm to separate borders of
touching particles (E); and
· Setting "analyze particle" parameters to only count particles
of a certain size (pixel2, depending on the size fraction) and circularity (0.83-0.90, also depending on the sizes being analyzed)
to exclude angular calcite and elongated aluminum, and then
applying a count mask to the image to get a minimum proppant
count (F).
The blue boxes in the A-D panels are examples of proppant
with a darker interior and bright perimeter needing infill after
thresholding. The orange box in D shows proppant touching
aluminum, which then was separated with the watershed
algorithm. The green boxes show automation error, which
counted a piece of aluminum (solid box) and excluded a red
proppant (dashed box).
For total particle counts, the protocol was the same, except
the images were thresholded to include all visible particles
rather than only bright objects. To estimate error, proppant
grains were counted manually and compared with the automated
analysis. The error margin was negligible for larger (250-500
µm and >500 µm) fraction sizes and ~5 percent for the two
smaller size fractions, resulting in a minimum estimate of
proppant abundance by automated counting.
Proppant Packs
While collecting samples from the core, the occurrence of
any in situ sand patches, colored grains and thick proppant
packs was noted. While no evidence of embedment (dimpling)
was evident on fracture faces, panel A in Figure 2 shows one of
the proppant packs found in two locations with complex fracture
geometry containing various arrest features. In this case, the
pack measured 1.0 centimeters thick.
Nearby the proppant packs, a few other fractures contained
undisturbed thinner sand patches on the fracture face (panel B
52 THE AMERICAN OIL & GAS REPORTER

in Figure 2). The lack of drilling mud in these fractures
reinforced the interpretation that the sand was in place in the
subsurface prior to core drilling. The majority of retrieved sand
from the proppant packs/patches found in the Upper Wolfcamp
cores matched the pumped 100-mesh proppant. Panels C and D
show base sand along with potential red and yellow proppant
measured in situ before sample collection.
A large proppant pack also was encountered between cores
five and six in the Middle Wolfcamp, which jammed the core
drilling rig (5 percent sand was observed in mud logger samples
from this area). The proppant pack was interpreted to have been
in a fracture similar to those containing packs in the Upper
Wolfcamp, but it was not preserved because of five feet of
missing core between cores five and six, where it would have
been located.
The 100- and 40/70-mesh proppant pumped during treatment
had a grain size of 149 to 420 µm, yet the majority of recovered
proppant was in the two smallest size fractions, especially the
63-120 µm range. Originally, it was hypothesized that the size
discrepancy between pumped and recovered proppant could be
related to widespread grain crushing of the larger sand within
the subsurface, but inspection of recovered proppant indicates
that this was not necessarily the case.
Figure 3 shows randomly chosen examples of retrieved 63120 µm proppant across all six cores, demonstrating minimal
grain crushing in both Upper (panels A-D) and Middle (E and
F) Wolfcamp fracture face samples. Some grain crushing can
be observed, but the majority of proppant recovered is rounded.
Therefore, most of the recovered proppant is seemingly 100mesh base sand, possibly because the pumped proppant contained
a much larger size range than was indicated by the standard
mesh size.
Colored Sand Diagnosis
As noted, 5 percent of the 40/70-mesh sand pumped during
treatment had included grains coated in colored resins, with different colors pumped in different stages in the adjacent wells.
All recovered colored grains (red, yellow, green and blue) were
noted in an attempt to diagnose the pumping stage linked to
FIGURE 3
Random 63-120 µm Examples from Upper (A-D)
And Middle (E and F) Wolfcamp Samples



American Oil and Gas Reporter - December 2018

Table of Contents for the Digital Edition of American Oil and Gas Reporter - December 2018

Contents
American Oil and Gas Reporter - December 2018 - Intro
American Oil and Gas Reporter - December 2018 - 1
American Oil and Gas Reporter - December 2018 - 2
American Oil and Gas Reporter - December 2018 - 3
American Oil and Gas Reporter - December 2018 - 4
American Oil and Gas Reporter - December 2018 - Contents
American Oil and Gas Reporter - December 2018 - 6
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