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The application of Nano-silica gel in sealing well micro-annuli and cement channeling

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Abstract

The possibility for hydrocarbon fluids to migrate through debonded micro-annuli wells is a major concern in the petroleum industry. With effective permeability of 0.1-1.0 mD, the existence of channels in a cement annulus with apertures of 10-300 micrometer constitutes a major threat. Squeeze cement is typically difficult to repair channels-leakage with small apertures; hence, a low-viscosity sealer that can be inserted into these channels while producing a long-term resilient seal is sought. A novel application using nano-silica sealants could be the key to seal these channels. In the construction and sealing of hydrocarbon wells, cementing is a critical phase. Cement is prone to cracking during the life cycle of a well because of the changes in downhole conditions. The usage of micro-sized cross-linked nano-silica gel as a sealant material to minimize damaged cement sheaths is investigated in this study. Fluid leakage through channels in the cement was investigated using an experimental system. With a diameter of 0.05 inches, the impact of the cement channel size was explored. The sealing efficiency increased from 86 percent to 95 percent when the nano-silica concentration of the sealing gel increased from 13 percent to 25 percent. This demonstrates that the concentration of nano-silica in the sealing gel affects the gel's ability to seal against fluid flow. This research proposes a new way for improving cement zonal isolation and thereby lowering the impact of cement failure in the oil and gas industry.
1
Abstract
The possibility for hydrocarbon fluids to migrate through
debonded micro-annuli wells is a major concern in the
petroleum industry. With effective permeability of 0.11.0 mD,
the existence of channels in a cement annulus with apertures of
10300 µm constitutes a major threat. Squeeze cement is
typically difficult to repair channels-leakage with small
apertures; hence, a low-viscosity sealer that can be inserted into
these channels while producing a long-term resilient seal is
sought. A novel application using nano-silica sealants could be
the key to seal these channels. In the construction and sealing
of hydrocarbon wells, cementing is a critical phase. Cement is
prone to cracking during the life cycle of a well because of the
changes in downhole conditions. The usage of micro-sized
cross-linked nano-silica gel as a sealant material to minimize
damaged cement sheaths is investigated in this study. Fluid
leakage through channels in the cement was investigated using
an experimental system. With a diameter of 0.05 inches, the
impact of the cement channel size was explored. The sealing
efficiency increased from 86 percent to 95 percent when the
nano-silica concentration of the sealing gel increased from 13
percent to 25 percent. This demonstrates that the concentration
of nano-silica in the sealing gel affects the gel's ability to seal
against fluid flow. This research proposes a new way for
improving cement zonal isolation and thereby lowering the
impact of cement failure in the oil and gas industry.
Introduction
Cementing engineering is one of the most important issues in
the drilling operations, which operates in tough and challenging
settings with temperature and pressure restrictions
(Wojtanowicz et al., 2016). When a well fails, various factors
come into play, including leakage, cementing, and so on. In this
scenario, a variety of solutions can successfully improve
industrial processes, with a focus on cement characteristics,
crack sealing, shielding permeability reduction, gas migration
management, and so on. During hydrocarbon production,
external influences such as temperature and pressure cause the
well cement to fracture, allowing fluid to leak at the wellbore
contact (Dusseault et al, 2014). As a result, the leak has a
substantial influence on the ecology, putting oil workers' and
other aquatic wildlife's safety in jeopardy (Davies et al, 2014).
Cement is prone to cracking during the life of the well because
of changes in downhole environment. During drilling or
production operations, the cement integrity must never be
jeopardized. If the cement is not properly finished and
abandoned, leaks can occur at any time during the well's life
(Watson and Bachu, 2009). Fluids (water or hydrocarbons) can
move through channels within the cement or between the
cement and its environment. These channels form when the
wellbore integrity is compromised, allowing formation fluids to
travel between formations and/or from the formation to the
surface. Personnel and the environment are put at risk when
water, gases, and hydrocarbon substances leak through cement
routes (Davies et al., 2014).
Mechanical failures as a result of pressure and temperature
cycles, chemical degradation owing to corrosive formation
fluids (Zhang and Bachu, 2010), or inappropriate slurry design
can all provide leakage paths in the cement annulus. The use of
low-density slurries, which may allow formation fluids to create
channels within the cement sheath, high-fluid-loss slurries,
which may affect the mechanical properties of the set cement,
and rigid cement, which can get fracked as a result of casing
expansions and contractions, are all examples of poor slurry
design. Furthermore, failures might arise as a result of incorrect
mud clearance. The mud on the formation's and casing's surface
can hinder the cement from adhering with its surroundings,
The application of Nano-silica gel in sealing well micro-annuli and cement
channeling
Olatunji Olayiwola1, Vu Nguyen1, Randy Andres1, Ning Liu1.
1 University of Louisiana at Lafayette, USA
2
resulting in micro-annuli. These and other defects can occur,
allowing formation fluids to pass through the cement barrier.
Cement squeeze remedial operations are commonly used to fix
cement leaks. However, due to low cement injectivity, pressure
restriction, pin-hole leakage, micro-channels and fractures
inside the leaked formation, and micro-cracks within primary
cement, cement squeeze cannot be employed successfully to
reduce and prevent leakage in some instances (Jones et al.
2014). Although micro fine cement is designed to promote
penetration, the underlying problem of cement and steel casing
adhesion versus cohesive bond failure in the downhole
environment is not addressed by most, if not all, current repair
materials. Adhesion failure is characterized by brittleness and
unpredictability. A typical adhesion failure is cement-steel
failure.
However, delivering the gel fluid to the fracture zone before it
gels is a major challenge in remedial operations. The rheology
of the gel controls the occurrence. (Zareie, C., 2019). To
address this problem, the gel mixture's composition is designed
to have a long gelation time and low viscosity, allowing it to be
pumped and crosslinked during oil well workovers (Zhang, X.
et. al, 2015). Because of their low thermal conductivity, low
refractive index, high optical transparency, and large surface
area, Nano-silica particles have gotten a lot of attention in well
remedial operations. Cross-linking of polymers contributes to
the formation of the silica network. The gel state, a colloidal
intermediate phase between the solution and the solid state, has
a three-dimensional structure and has promising applications in
a variety of fields.
State of the Art
Cement squeeze operations are used to repair wells with poor
primary cementing jobs or suspected leaks, in which additional
cement is injected through perforations produced in the casing
near the suspected source of leakage to ensure appropriate zonal
isolation. Squeeze cementing is a corrective procedure that
includes applying differential pressure across the cement slurry
in order to dehydrate the cement (Goodwin, 1984). In theory,
the slurry is supposed to reach and fill the problem area,
immobilizing the area until compressive strength can be
developed.
However, because to a miscalculation of the leakage problem,
cement slurry is frequently inadequately positioned or planned.
Even if an annular gap and/or cracks with apertures on the order
of 0.010.3 microns are present, effective permeability in the
region of 0.11 mD can be significantly increased (Jung et al.,
2014). Fractures or leakage paths with small apertures, in
particular, are difficult to repair using ordinary Portland cement
because the cement slurry is screened out of the dispersion fluid
and cannot penetrate the fracture. As a result, squeeze
cementing is frequently unsuccessful, resulting in rig time
waste and excessive expenses.
The particle size is the key factor limiting the sealing
effectiveness of traditional Portland cement. Class H oil well
cement comprises large particles in the range of 100-150
micrometres, making it difficult to penetrate small channels,
micro-annuli, or thin mud channels, and often resulting in
unsatisfactory results. When the slurry is squeezed to penetrate
fractures thinner than 400 micrometres, bridging and cement
dehydration will occur. Ultra-fine cement technologies have
been developed to reduce the particle size of cement slurries,
such as Halliburton's Micro Matrix cement and Schlumberger's
Squeeze-CRETE. These new slurries have a better ability to
penetrate through tiny spaces.
Many studies have been carried out to investigate the rheology
and variables that influence gel resistance to water flow.
Grattoni et al. (2001) carried out a series of experiments to
investigate the relationship between polymer gel parameters
(such as gel strength and polymer concentrations) and flow
behaviour. Permeability is a function of both water flow
velocity and polymer concentration, they discovered. Yang et
al. (2002) established a mathematical model for water flow via
polymer gel-impregnated tubes. Zhang and Bai (2010)
demonstrated that millimetres-sized particles produced a
permeable gel pack rather than full blocking in opening
fractures.
Gel rheology and extrusion properties of gels in fractures are
linked, according to Seright (2003). Ganguly et al. (2012)
conducted a series of studies to see how fluid leakage affected
gel strength when placed in fractures. The transfer of partially
produced gels in fractures was studied by Sydansk et al. (2005).
Wang and Seright (2006) investigated whether employing
rheology measurements to determine gel characteristics in
fractures is a cost-effective alternative to extrusion
experiments. Wilton and Asghari (2007) investigated how
fractures could improve bulk gel placement and performance.
The effect of shear on flow characteristics during the
installation of sealants in fractures was examined by McCool et
al. (2009).
However, none of these previous works investigate the potential
application of nano-silica gel to seal cracked and fractured
cements. This study will use extensive experimentation to better
understand and evaluate the mechanics of nano-silica gel
growth and placement in cement fractures and channels. The
study will begin by assessing the impact of various parameters
on sealant characteristics and rheology. The gel injection will
3
next be evaluated using a variety of fracture and channel sizes
in a series of studies. After that, several experiments will be
carried out to analyse water and oil leakages using nano-silica
gel.
Experimental Investigation
The capacity of Nano-silica gel to inhibit fluid leakage through
channels in the cement was investigated using an experimental
setup. The use of crosslinked micro-gel in cement zonal
isolation has received very little experimental attention. This
research will also give petroleum engineers a basic grasp of how
to use Nano-silica sealer for wellbore integrity applications.
The impact of Nano-silica concentration on the gelation
properties of the sealant, the sealant's capacity to block routes,
and the sealant's ability to halt leakages will be investigated in
this study. Rheological testing, gelation time estimations,
blocking efficiency, and mechanical qualities are also part of
the laboratory experiments.
Materials
Nano-Silica Gel - For all studies, a commercial super-
absorbent silica gel (supplied by Nouryon Pulp and
Performance Chemicals Inc.) was chosen as the sealant
material. The material was used exactly as it was supplied from
the company, with no chemical changes. The gel was a dry
white granular powder before it swelled. Chloride of sodium
(NaCl). A brine solution was made using commercially
available NaCl with a purity of 99.99 percent.
Class H Cement - All of the cement specimens used in this
investigation were made with Class H cement and distilled
water. Using a gas pycnometer, the specific gravity of the
cement was determined to be 3.18. X-ray fluorescence
spectroscopy was used to determine the chemical composition
of Class-H cement (XRF)
Material Preparation
Preparation of Cement Paste - A two-speed bottom-drive
blender was used to mix the cement slurry at room temperature.
After pouring a precise amount of water into the blender, dry
cement was added at a uniform rate and mixed on low speed for
around 15 seconds. The blender was then covered and the
mixing continued at high speed for an additional 35 seconds
(API RP 10B-2). The water/cement ratio (WCR) of the cement
slurry was 0.38, as per API specification 10A for API cement
Class-H. (API 2010).
Preparation of Fractured Cement Cores - To create the
cracked cement cores, a two-inch diameter hose was filled with
the cement slurry and left to harden for over 72 hours, closed
on one end and open on the other. Following the hardening of
the cement core, it was removed from the hose. The dried core
was then gently pushed against the blade of the running tile-saw
until it was chopped into two half pieces. The two parts of the
core are then separated to the required fracture width using
proppant or coins, as shown in Fig. 1.
Fig 1. The Fractured Cement Core
Gel Preparation - A 50 percent original concentration of
Nano silica solution was utilized. Brine solution was added as
a gelation accelerator to aid in the development of gel in the
combination. Using Nano silica concentrations ranging from
15% to 40%, salt concentration, and water, a straightforward
experimental approach was employed to make silica gels at
ambient temperature, as shown in Fig. 2. The Nano-silica
particles were first produced at the necessary concentration.
The brines with a concentration of 3% were then introduced to
the Nano-silica particle and allowed to gel. Then, using an
Anton Par Rheometer, the rheological and elastic properties of
the resultant gel particles were measured. The Injectivity test
employed NS Gel samples of 15% concentration for the gel
sealing effectiveness evaluations.
Fig 2. 15% Nano-silica gel (Before and After Gelation)
Methodology
Rheological Measurements
Anton Paar Modular Compact Rheometer MCR 302
Instruments with a parallel plate system (using a 100 microns
gap) was used to measure the gel strength, the yield point and
elastic properties. The measurements were conducted at 25 ⁰C
to measure the gel properties of these Nano-silica gel samples.
4
Experimental Procedures
Fig.3 shows the gel propagation through the cement setting. two
syringe pumps, two accumulators with pistons, and a 2-inch
Core holder make up the device. The fracture width was
measured by inserting a 0.05-inch proppant between the two
parts of the fragmented core. After that, the two pieces of the
core are taped together to produce a whole core. The proppant-
filled core is then inserted into the core holder's rubber sleeve
and confining pressure is applied to prevent fluid leakage
around the fractured core and to mimic the confining pressure
conditions. Nano-silica Gel and brine were fed into the core-
holder via the accumulator. To monitor the gel performance
along the fracture, pressure transducers were installed at the
cell's intake, middle, and outflow.
Fig 3. Experimental set up for the Injectivity Test
To simulate the circumstance when the cement was already
filled with water before the sealant injection, brine was pumped
into the core-holder using an accumulator. After the brine was
in place, a steady flow rate of 1 cc/min of nano-silica particle
gel was injected into the core-holder. The gel injection was
continued until the gel was formed at the cell's exit and the
injection pressure across the fracture was steady. The injection
pressure was measured with pressure sensors, and the angle of
gel propagation across the fracture was monitored with a high-
resolution camera. To test the gel's water sealing efficiency,
brine was introduced into the gel particle packed crack after it
was in place. The brine was injected at a continuous flow rate
of 1 cc/min through an accumulator. The brine injection was
continued until the pressure of the brine injection was
stabilized. Transducers were used to record pressure data for all
of the trials.
Results and Analysis
Nano-silica Concentration effect on Gel properties
After sample preparation and testing, the observed gel
generated at various nano-silica concentrations is shown in Fig.
4. The appearance of the gels demonstrates that the gel structure
becomes stronger as the concentration of nano-silica particles
increases. Furthermore, the gelation time reduces as the
concentration increases. Gelation time is the length of time it
takes for a silica solution in NP liquid form to turn into a gel.
The shear stress is measured at a low shear rate after the NP
Solution has sat quiescently. An NP Gel's yield point is defined
as the stress at which it deforms significantly for a small
increase in stretching force.
Fig 4. Nano-Silica gel at different concentration
Tab 1. Nano-Silica gel at different concentration
NP Conc.
Gel. Time.
Gel. Str.
Yield Point
5
(%)
(mins)
(lb/100ft2)
(lb/100ft2)
15
185
504
27.9
18
90
679
189
20
76
920
257
22
60
1546
452
25
20
3006
536.9
30
6
6160
1372
35
0
8484
8398.9
40
0
8492.6
8358.8
According to the experimental results in Tab. 1, the gel strength
and yield point values increase as the nano-silica concentration
increases. This is owing to the Nano-silica particle's aggressive
filler network, whose rheological property increases as its
concentration in the solution increases (Havet G et al, 2003).
This increase in rheological feature raises the corresponding gel
strength and yield point values by raising the ionic strength of
the mixture solution (Gallagher & Mitchell, 2002).
Furthermore, the greater the crosslinking density with the
crosslinking agent, and therefore the stronger the gel strength,
the higher the nano-silica concentration.
In addition, looking at the texture of the generated gel in Fig. 4
demonstrates a link between gel texture and nano-silica
concentration. Increasing the nano-silica concentration
accelerates the rheological property, which has a major impact
on the solution's thickening and elastic properties. At a nano-
silica concentration of 15%, the observed gel has a light texture
in liquid form, as shown in Fig. 3, which also correlates to gel
strength and yield point values of 504 100lb/ft2 and 27.9
100lb/ft2, respectively as seen in Fig. 5, with a prolonged
gelation time of 185 minutes. The texture of the related gel
thickens as the concentration of nano-silica particles in the
solution increases due to the higher molecular weight caused by
the inclusion of the nano-silica particle concentration in the
solution. The presence of hydroxyl groups in nano-silica
particles structure leads to an easy filler network in the polymer
chains and caused the gel strength and yield point to increase
(Zareie, C. et al. 2019).
Fig 5. Gel. Strength & Yield Point at different Nano-silica
concentrations
Gel - Cement Sealing Efficiency
The Gel-Cement Plugging Efficiency setup was used to
examine the gel's ability to plug cement fractures and limit
water generation in this region (Fig. 3).
Establishing a Stable Pressure Prior to Gelation
Table 2 depicts the pressure of gel injection at various
flowrates. In this study, a cement fracture of 0.05 inch was used.
As the flowrate increases, the pressure of the gel injection
increases. The highest-pressure value of 265 psi was achieved
with the highest flowrate of 4 cc/min. The injection stable
pressure declined to 173, 113, and 75 psi, respectively, when
the flowrate was reduced to 3, 2, and 1 cc/min. These findings
show that as the flowrate increases, the pressure drop increases,
following the Darcy law of flow.
To estimate the fracture conductivity during the injection test,
Eqn.1 is used.
  
 (1)
0
2000
4000
6000
8000
10000
10 15 20 25 30 35 40
0
2000
4000
6000
8000
10000
10 20 30 40
Yield Point ( lb/100ft2)
NP (%)
6
Where q (flowrate) in cc/min, µ (viscosity) in cp,
󰇛 󰇜 in inch, D (core diameter) in inch and
(fracture conductivity) in mD-ft.
The hydraulic aperture of the fracture, B is calculated using
Eqn. 2
 󰇛 
 )1/3 (2)
Where B is measured in inch, Q in cc/min, is in cp, L in inch,
w in inch and dP in Psi.
Tab 2. Experimental result at different flowrates
Flowrates
Proppant
Dime
(cc/min)
P (psi)
CF (mD-ft)
B (inch)
P (psi)
CF (mD-ft)
B (inch)
1
75
0.54
0.073
64
0.628
0.077
2
113
0.71
0.081
94
0.855
0.086
3
173
0.69
0.080
140
0.861
0.086
4
265
0.61
0.076
195
0.824
0.085
Average
0.64
0.08
0.79
0.08
From Tab. 2, the average values for fracture conductivity and
hydraulic aperture are calculated as 0.64 mD-ft and 0.08inches
respectively using proppant and dime of equal thickness of 0.05
inches. The purpose of using dime and proppant is to double-
check the effect of proppant embedment which reduces the
fracture width and conductivity.
Establishing a Stable Pressure After Gelation
(Permeability Test)
This process involves measuring the resistance to waterflow
after Gel placement. To evaluate the Nano-Silica gel blocking
behavior against water, brine was introduced at a steady
flowrate of 1cc/min after the gel was placed into the cement
fracture. The flow resistance at various silica concentrations is
shown in Figure 6. It was discovered that the gel with the
greatest concentration (25%) had the highest pressure to reach
a water breakthrough at 1400 psi, which dropped to around 520
psi when the Nano-Silica concentration was reduced to 13
percent. The pressure dropped once the water broke through
until it reached a stable level. In comparison to gels with lower
concentrations, gels with higher concentrations showed higher
steady pressure. The gel's ability to plug cement fractures and
manage water generation may also be controlled, with the nano-
silica concentration boosting this ability.
7
13% Nano-silica 15% Nano-silica
18% Nano-silica 21% Nano-silica
25% Nano-silica
Fig 6. Flow resistances at different Nano-silica concentrations
0
100
200
300
400
500
600
020 40 60
Pressure (psi)
Time (mins)
0
100
200
300
400
500
600
700
020 40 60
Pressure (psi)
Time (mins)
0
200
400
600
800
1000
020 40 60 80
Pressure (psi)
Time (mins)
0
200
400
600
800
1000
1200
020 40 60 80
Pressure (psi)
Time (mins)
0
500
1000
1500
020 40 60 80
Pressure (psi)
Time (mins)
8
4.2.3. Water Residual Resistance Factor
(FRRW)
The ratio of water phase permeability before and
after particle gel treatment is known as the Water
Residual Resistance Factor. The gel's plugging
efficiency improves as the FRRW value rises, hence
a high FRRW value is advantageous. In this study
article, FRRW was determined using Eqn. 3.
FRRW= 
 (3)
The percentage of permeability reduction can be
computed from Equation 4 to reflect the Nano-silica
gel sealing efficiency (E) (Imqam et al. 2015)
Efficiency=[1-(
)]*100% (4)
Tab 3. Sealing Efficiency at different flowrates
NS Conc.
%
Pressure before Gel
(psi)
Pressure After Gel
(psi)
FRRW
Efficiency
(%)
0
75
75
1
0%
13
75
520
6.933333
86%
15
75
635
8.466667
88%
18
75
805
10.73333
91%
21
75
1044
13.92
93%
25
75
1400
18.66667
95%
Tab. 3 demonstrates that as nano-silica
concentration rises, FRRW rises as well. This
suggests that raising the nano-silica concentration
makes water shut-off applications more successful.
FRRW rose with increasing nano-silica
concentration; for example, FRRW of 13 percent
nano-silica gel was 6.93, while FRRW of 25 percent
nano-silica gel was 18.667.
Fig 7. Sealing Efficiency at different Nano-silica
concentrations
When a nano-silica gel was used as a sealant agent,
the results showed that the sealing efficiency rose as
the concentration of the gel increased. For all nano-
silica concentrations, the sealing efficiency varies
from 86 percent to 95 percent. A summary of nano-
silica concentrations, FRRW, and sealing efficiency
may be found in Table 3.
Conclusions
Several findings were produced in this study by
investigating the impact of infusing nano-silica gel
into cement fractures. Gel rheological
measurements, gel strength, gel transportation
testing, and gel sealing efficiency trials were used to
arrive at these conclusions. The most important
conclusions are listed below.
1. Smaller fracture widths and crack
characteristics suggest that nano-silica gel
particles have appropriate injectivity. Their
water leakage plugging performance, on
the other hand, is up to 1400 psi. The gel
strength and concentration might be
changed to control the sealing pressure.
2. Gel with a 25 percent nano-silica
concentration had the highest sealing
efficiency of 95 percent, while gel with a
0%
25%
50%
75%
100%
0 5 10 15 20 25
Sealing Eff (%)
Nano-silica Conc. (%)
9
13 percent nano-silica concentration had
the lowest sealing efficiency of 86 percent.
3. Gel sealant material selection is influenced
not only by plugging efficiency but also by
gel injectivity. When compared to a low-
strength gel, the high-strength gel had
superior blocking efficiency but poorer
injectivity, and vice versa.
4. The sealing capacity of the selected nano-
silica gel increases as the concentration
increases. This is attributed to the fact that
the concentration of nano-silica is directly
proportional to its gel strength values,
which also govern the sealing capacity.
Acknowledgement
The author would like to express his gratitude to the
University of Louisiana at Lafayette (ULL) for
providing him with a scholarship. Thank you also to
the Department of Energy in the United States for
supporting the research.
Nomenclature
% - Percent
API America Petroleum Institute
B Hydraulic Aperture
cc/min Cubic centimeter per minute
CF Fracture Conductivity
D Darcy
FRRW Water residual resistance factor
ft Feet
in Inch
Inc. International
MCR Modular Compact Rheometer
mD Millidarcy
mD-ft Millidarcy feet
NaCl Sodium Chloride
NP Nano-particle
NS Nano-silica
PSI Pounds per square inch
oC Degree Centigrade
WCR Water Cement ratio
XRF X-ray Fluorescence
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