American Oil and Gas Reporter - April 2019 - 58

SpecialReport: Drilling Technology
FIGURE 5

11,000

12,000

13,000

14,000

15,000

16,000

17,000

Toe Stages

Heel Stages

11,000

12,000

13,000

14,000

15,000

16,000

17,000

Toe Stages

Heel Stages

11,000

12,000

13,000

14,000

15,000

16,000

17,000

Toe Stages

Heel Stages

2
1

Well C

Well B

Well A

Well-to-Well Comparison of Mechanical Quality
And Prestage ISIP Gradients

Track 1: Measured Depth (ft)
Track 2: UCS
Brittleness

High Mechanical Quality

Figure 4 shows the mineralogic model
and the calculated UCS and brittleness
for each of the three wells. To better
understand their relationship to each
another, the petromechanical workflow
displays UCS and brittleness together
in Track 2. The authors consider intervals
of relatively high brittleness and low
strength as high mechanical quality.
Therefore, these intervals (shown in
green) have a positive crossover. Conversely, values of relatively low brittleness and high strength (gray) have a
negative crossover (lower mechanical
quality). Intervals of high mechanical
quality are generally easier to fracture
and prop, which would result in a more
conductive fracture network than intervals of lower mechanical quality.
The analysis shows that approximately
75 percent of Well A's lateral has high
mechanical quality compared with 25
percent for the laterals in both wells B
and C. These observations are consistent
with the production responses. Of the
wells with consistent production mechanisms, Well A is the best producer in the
development area. After 100 days of normalized production, Well A produced
about 30 percent more oil than wells B
and C. It was hypothesized that Well A's
hydraulic fracturing treatment created a
better fracture network than those of the
other two wells because of A's superior
mechanical rock quality.
58 THE AMERICAN OIL & GAS REPORTER

Low Mechanical Quality

Pre-Stage ISIP Gradient

Initial Shut-In Pressures
The step-down rate tests performed
before pumping proppant in each stage
collected initial shut-in pressures (ISIPs).
The three wells were completed in a
zipper fashion, so the prestage ISIP measurement is not a perfect comparison with
near-wellbore mechanical properties because it can contain effects of interstage/interwell stress shadowing. Despite this, a
general trend was observed between mechanical quality and prestage ISIP.
Figure 5 shows the mechanical quality
curves for each well with prestage ISIPs.
The ISIPs were converted to a gradient
to normalize by depth. In general, intervals
with higher mechanical quality (green
shading) had lower ISIPs, while intervals
with lower mechanical quality (gray
shading) had higher ISIPs. Assuming
high prestage ISIPs indicate reservoir
rock that is more difficult to fracture,
this correlation validates the hypothesis
that Well A's higher-quality rock resulted
in better productivity.
The first few stages of the well near
the toe are expected to have lower ISIPs
because of limited stress shadowing
effects. In toe stages, the ISIPs increase
gradually as each stage introduces more
pressure and greater stress to the formation.
The variation in ISIPs for toe stages
likely is driven by completion order rather
than mechanical properties, but after these
first few stages, ISIP variance probably

is driven by mechanical quality.
Note that in all three wells, the heel
stages had the greatest percentage of high
mechanical quality and ISIPs were lowest
in the heel (left side of the image). From
the toe (right side of the image), Well A
had lower mechanical quality in the first
25 percent of the well and Well B had
consistently lower mechanical quality
through the first 40 percent. In both cases,
ISIPs are higher in these intervals, suggesting greater fracturing difficulty. Well
C had sporadic intervals of high and low
mechanical quality and sporadic ISIP
fluctuations.
In the Permian pilot well, drill bit
geomechanics data compared favorably
with and complemented mechanical
properties derived from more traditional
wireline log and core reservoir characterization data. Combining these measurements can inform the best decisions
for landing zones and horizontal well
development.
The case study of the three neighboring horizontal wells with notably different production highlights the importance of geomechanics. Blindly extrapolating geomechanical properties from
a vertical pilot well to all nearby horizontal wells can skew assumptions and
interpretations. One cannot assume these
properties are constant across each geologic formation.
In field development, it is common
to find a wide range of production responses from wells that are geographically close and share the same landing
zone. Significant lateral heterogeneity
exists, occasionally on the scale of a
few hundred feet, as illustrated in the
three parallel Permian wells with similar
lateral lengths and completion designs
in the same formation. Drill bit geomechanics offers a more economic and operationally simple way to measure mechanical properties along horizontal laterals. By measuring lateral heterogeneity
and relating it to the petrophysical understanding of a pilot well, operators
can improve completion quality and ultimately increase production.
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American Oil and Gas Reporter - April 2019

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