American Oil and Gas Reporter - August 2016 - 64

SpecialReport: Hydraulic Fracturing Technology
FIGURE 2
Well Pad Diagram Showing 6H and 8H Laterals
And Locations of Stage Plugs

0

∆ TVD (feet)

2,000
4,000
6,000

6H
8H

8,000
0
-2,000

∆ Northing (feet)

2,000

-4,000

1,000
0

∆ Easting (feet)

-1,000

FIGURE 3
Production Logging Results for Well 6H Across Producing Clusters
7

% Gas Contribution

Constant
Pump Rate

Variable Pump
Rate

6
5
4
3
2
1
0
27

26

25

24

23

22

21

20

19

18

17

16

15

14 13
Stage #

Figure 2 shows the well pad diagram
highlighting the two laterals (Wells 6H and
8H) that were studied. Another lateral (4H)
extends on the other side of the 8H well,
but is not shown in the diagram. Interwell
separation varies between ~700 and 1,000
feet laterally for wells 6H and 8H. Both
wells were designed to land in the lower
Marcellus Shale. As observed from the
figure, Well 6H had a total of 27 completed
stages and 8H had 28 completed stages.
The frac design involved ~200,000
pounds of proppant (100- and 40/70mesh white sand) pumped with ~200,000
gallons of water for Well 6H. The stages
pumped in Well 8H had about 50 percent
more proppant (~300,000 pounds) and
higher frac fluid volumes. With larger
proppant and fluid pumped in the 8H
64 THE AMERICAN OIL & GAS REPORTER

12

11

10

9

8

7

6

5

4

3

2

1

well, initial productivity in terms of gas
flow rates from the well were expected
to be higher than the 6H well.
Validating The Results
Four methods involving both direct
and indirect diagnostic techniques were
deployed to validate the efficacy of the
variable-rate fracturing technique: production logging, pressure responses to
rate fluctuations, post-completion well
production performance, and post-pumping
shutdown water hammer diagnostics.
Production log results were observed
immediately post-completion for Well
6H where rate fluctuations were introduced
(Figure 3). While there was significant
variability in production behavior over
the entire lateral because of completion

quality as well as exact location of the
lateral compared with reservoir stratigraphy, more often than not, variable-rate
(odd) stages showed higher productivity
than nearby even stages.
Another inherent assumption with
using variable-rate fracturing is that it
should ideally open additional perforations
that may not have been opened fully
during the initial breakdown. This should
allow for a more uniform distribution of
fluid flow into the formation across available perforation clusters. Summing the
skewness measure for flow distribution
across clusters for each of Well 6H's 27
stages shows a very low summed measure
for odd stages (0.5394) compared with
the even stages (4.8528).
Looking at overall productivity from
odd and even stages separately, cumulative
productivity from odd stages is 915.3
Mcf a day compared to 721.1 Mcf/d for
the even stages. There is one additional
odd stage for this well, but the same
trend is evident when evaluating average
productivity per stage (65.38 Mcf/d for
odd stages versus 55.47 Mcf/d for even
stages). This represents 17.87 percent
higher productivity with variable-rate
fracturing.
Since a lower skewness measure was
evident for the variable-rate frac stages,
the study then looked at the behavior of
treatment pressure after introducing pump
rate fluctuations. Figure 4 shows examples
of two odd stages completed in Well 6H,
with drops in treatment pressure ranging
from tens to hundreds of psi, which is indicative of lower entry friction within
the system. This could either mean additional opening of perforations related to
the pressure transient pulse generated by
fluctuating pump rates (i.e., reduced perforation friction), or a consolidation of
nondominant fractures emanating from
the wellbore (reduced tortuosity).
Water Hammer Diagnostics
The wellbore and associated hydraulically created fractures during the treatment
phase of a fracture stage create a complex
hydraulically connected conduit where
unsteady-state transients are common. As
the pumping of proppant and slurry ends
and the pumps shut down, and depending
on the shutdown procedure, the transition
from steady-state constant flow to low or
no flow conditions creates single or multiple water hammers within the wellbore.
Since the decay in energy over the
water hammer cycle is a function of the
American Oil and Gas Reporter - August 2016

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