American Oil and Gas Reporter - June 2014 - 78

SpecialReport: Offshore & Subsea Technology
sists of the high-strength solid carrier with
a phosphonate-based scale inhibitor that
slowly and steadily comes back as water
production commences. The phosphorous
content that is released from the composite
into water can be monitored by laboratory
or field analysis techniques. The inhibitor
offers excellent inhibition against bariumand strontium-sulfate scale formation and
good inhibition against calcium-carbonate
and calcium-sulfate formation.
The inhibitor is added with conventional
intermediate or high-strength proppant at
any ratio, typically (for practical economic
considerations) at 10-20 weight percent
of the proppant concentration in deepwater
treatments. This results in inhibitor being
placed along the entire volume of the
proppant pack, and because the inhibitor
is retained in the proppant pack without
plugging the pore throats, it can be used
in a range of reservoir conditions. The release profile is designed to provide immediate availability as well as sustained,
long-term availability of the inhibitor.
In contrast to a regular solid-scale inhibitor composite that quickly releases
the treatment chemistry to the produced
fluid, the high-strength solid inhibitor
composite provides a slower and lower
dosage inhibitor release to keep scale
deposition under control for a much
longer time, as shown in Figure 3. The
minimum inhibitor concentration (MIC)
for this graph is 0.1 ppm.
In laboratory and field tests, the nanotechnology-based material has demonstrated superior crush resistance with no
conductivity loss when added at up to 20
percent by weight to high-strength (bauxite) proppant, even at the high closure
stresses encountered in deepwater wells
(Table 1). Continuous monitoring of the
scale inhibitor residual in the produced
water shows a concentration above the
MIC (typically 1-3 ppm for calcium carbonate and 5-10 ppm for barium sulfate)

for more than 1 million barrels of cumulative water in a well with high water
production. The technology can be applied
to any type of formation and any completion technique using proppant.
Deepwater Application
A Gulf of Mexico operator planned
to complete a subsea oil well in deep
water using a frac pack as part of a seawater flood program, and needed a longterm treatment option to mitigate any potential flow assurance issues. An integrated
program that offered stimulation and remediation services in a seamless manner
to optimize field operations was deployed
to analyze flow assurance risks and offer
suitable solutions.
The process began with an analysis of
the well's specific reservoir mineralogy,
lithology, production fluids and stimulation
source water. This analysis incorporated
field history and offset well experiences
to predict post-stimulation problems
related to the deposition of paraffins, asphaltenes and scale. The analysis confirmed
the operator's expectations that barium
sulfate scale deposition was likely.
If seawater, which is high in sulfate,
mixes with connate water with high barium
content, barium sulfate scale will form.
This scale is not soluble in acid, and once
formed, is difficult to remove. Remediation
measures include using chelating chemistry,
sufficient downhole temperature, and long
periods of soaking. Even under optimum
conditions, barium scale removal often
calls for mechanical remediation means
such as coiled tubing.
A plan was developed to proactively
treat the well against barium scale. The
operator decided to use the nanotechnology-based solids technology, mixing inhibitor with proppant on-the-fly during
the frac-pack operation.
A comprehensive laboratory screening
process was followed to develop the appropriate substrate and inhibitor package

TABLE 1

for the application. This included testing
the new scale inhibitor composite for release,
using a length of stainless steel pipe with
tubing attached to each end so that fluids
could be injected at one end and exit at the
other. The column was filled with 20/40mesh proppant containing 10 wt. percent
of the composite of 30-mesh substrate. The
proppant and composite were mixed thoroughly to disperse composite throughout
the entire proppant column.
The pore volume of the test column
was 12 milliliters. A synthetic brine with
100 percent carbon dioxide was pumped
through a high-pressure liquid chromatography pump into the column at the rate
of 2.0 ml/minute. The effluent from the
column was collected periodically and
analyzed with a ultraviolet photometer
for the presence of scale inhibitor.
For a liquid inhibitor, the inhibitor return concentration in the first few pore
volumes was above 200 ppm, but fell
rapidly to around 10 ppm and then dropped
to less than the detection limit of 5 ppm
after 200 pore volumes of return fluid. In
comparison, the solid inhibitor composite
lasted more than 2,000 pore volumes.
Dynamic tube-blocking tests were carried
out also to determine the minimum concentration of scale inhibitor required with
50 percent seawater breakthrough, which
was considered the worst-case scenario.
Under the conditions of the test, a blank
scaling time of 28 minutes was observed.
Previous lab tests showed that the highstrength solid inhibitor would provide complete inhibition at a concentration of 15
ppm. A slow buildup of pressure was observed as the solid inhibitor concentration
dropped to 7.5 ppm, indicating that the
MIC for this well would be between 7.5
and 15 ppm under the worst-case scenario.
Fluid compatibility tests, in which
cross-linked fracturing fluids were tested
rheologically at 200 degrees Fahrenheit
in both the presence and absence of 10
ppg solid scale inhibitor, indicated that
the inhibitor had no compatibility issue
with the fracturing fluid formulations.

Crush Resistance and Fines Generation at Different Closure Pressures
Proppant Sample

Crushed Fines (%)
5,000 psi
Closure
Pressure

6,000 psi
Closure
Pressure

8,000 psi
Closure
Pressure

10,000 psi
Closure
Pressure

30-mesh commercial
intermediate-strength
proppant

4.9

5.3

10.1

14.7

30-mesh nanotechnologybased substrate

0.7

1.5

4.4

11.6

78 THE AMERICAN OIL & GAS REPORTER

Crush Resistance
Proppant crush resistance tests were
performed to evaluate the suitability of a
particular proppant at a given closure stress.
Forty grams of the 30-mesh sample were
placed in a crush cell. The piston was
added and rotated 180 degrees. The sample
then was placed on a press, and the specified
stress was applied over one minute, held
for two minutes, and then released. The

American Oil and Gas Reporter - June 2014

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