The American Journal of Engineering and Technology
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TYPE
Original Research
PAGE NO.
01-07
10.37547/tajet/Volume07Issue06-01
OPEN ACCESS
SUBMITED
26 March 2025
ACCEPTED
22 May 2025
PUBLISHED
01 June 2025
VOLUME
Vol.07 Issue 06 2025
CITATION
Green, & Willhite, G. P. (2025). Modeling Ion Exchange and Wettability
Alteration during Low Saline Water Flooding in Sandstone Reservoirs. The
American Journal of Engineering and Technology, 7(06), 01
–
07. Retrieved
from https://theamericanjournals.com/index.php/tajet/article/view/6214
COPYRIGHT
© 2025 Original content from this work may be used under the terms
of the creative commons attributes 4.0 License.
Modeling Ion Exchange
and Wettability Alteration
during Low Saline Water
Flooding in Sandstone
Reservoirs
Green D. W.
Department of Petroleum Engineering, Colorado School of Mines, Golden,
Colorado, USA
Willhite, G. P.
Department of Petroleum Engineering, Colorado School of Mines, Golden,
Colorado, USA
Abstract:
The behavior of ion interactions during low
saline water flooding in sandstone reservoirs plays a
crucial role in enhanced oil recovery (EOR) processes.
This study investigates the impact of ion interactions on
the displacement efficiency and fluid dynamics during
low saline water flooding (LSWF) using a numerical
modeling approach. A comprehensive model was
developed to simulate ion exchange, electrostatic
forces, and permeability alterations in sandstone
formations. Results indicate that LSWF significantly
alters the pore structure and wettability, leading to
improved oil recovery. This work provides insights into
the potential for low saline water flooding as a viable
EOR method, emphasizing the role of ion interactions in
optimizing recovery processes.
Keywords:
Low saline water flooding, ion interactions,
ion exchange, sandstone reservoirs, enhanced oil
recovery, numerical simulation, wettability alteration,
electrostatic forces, relative permeability, capillary
pressure, reservoir modeling, ionic composition, oil
displacement, fluid dynamics, reservoir engineering.
Introduction:
Enhanced oil recovery (EOR) techniques
are widely employed to maximize hydrocarbon
production from mature and underperforming
reservoirs. Low saline water flooding (LSWF), a subset of
water-based EOR methods, has gained attention for its
potential to improve recovery in sandstone reservoirs.
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The process involves injecting water with lower salinity
than the native formation water, which can induce
changes in reservoir rock properties and fluid behavior.
One of the key mechanisms behind LSWF is the
alteration of ion interactions at the rock-fluid interface,
affecting wettability, capillary forces, and relative
permeability.
Sandstone reservoirs are typically composed of a
mixture of quartz and other minerals that are sensitive
to the ionic composition of the injected water. Ion
exchange reactions between the brine and rock surface
can lead to changes in pore structure, thereby
influencing the effectiveness of water flooding. While
experimental studies have provided valuable insights,
there is a need for a more detailed understanding of the
ion
interactions
through
advanced
numerical
approaches. This study aims to assess the role of ion
interactions during LSWF in sandstone reservoirs using a
numerical modeling framework.
Enhanced oil recovery (EOR) techniques are critical in
improving oil recovery from mature reservoirs that have
already undergone primary and secondary recovery
methods. Among various EOR techniques, water
flooding is one of the most widely employed, with its
efficiency being highly dependent on factors such as the
water chemistry, injection rates, and reservoir
properties. While conventional water flooding uses
formation water or brines with high salinity, a relatively
newer approach known as
low saline water flooding
(LSWF)
has gained attention in recent years for its
potential to increase recovery efficiency in sandstone
reservoirs.
In traditional water flooding, the injected water
generally has a salinity comparable to the formation
water, often with ionic concentrations in the range of
30,000
–
50,000 ppm (parts per million). However, LSWF
uses water with significantly lower salinity, typically
around 5,000
–
20,000 ppm, to alter the rock-fluid
interactions. These low saline waters are expected to
induce favorable changes in the reservoir rock
properties, such as wettability alteration and improved
oil displacement efficiency, by altering the ionic
interactions between the brine and the mineral surface
of the sandstone.
The primary mechanism driving the effectiveness of
LSWF is the
alteration of the wettability of the rock
surface
. In many sandstone reservoirs, the rock surface
tends to be oil-wet or mixed-wet, which means that
water has a poor ability to displace the oil from the pore
spaces. By injecting low saline water, ions in the brine
exchange with ions on the rock surface, particularly
calcium (Ca²⁺) and sodium (Na⁺), causing a shift in
wettability towards a more
water-wet
condition. This
wettability alteration is believed to facilitate more
efficient oil displacement during the water injection
process.
The interaction between the injected brine and the rock
surface is heavily influenced by the
electrostatic forces
at the interface. When low saline water is injected, the
reduction in ionic strength weakens the electrostatic
double layer that forms at the rock-fluid interface, which
in turn modifies the capillary pressure and relative
permeability. The ions in the brine interact with the
mineral surfaces of the rock, causing
ion exchange
reactions that alter the surface charge density and
structure of the porous media. For example, divalent
ions such as Ca²⁺ tend to replace monovalent ions like
Na⁺, which can lead to changes in surface chemistry,
swelling of clays, and changes in the pore network.
The challenge, however, is to predict and quantify these
ion interactions and their impact on the overall
displacement process. While laboratory experiments
have provided valuable insights into the ion exchange
processes, there is still a lack of a comprehensive
understanding of how these interactions play out in real
reservoir conditions. Moreover, sandstone reservoirs
are not homogeneous, and their mineralogical
composition, pore structure, and initial brine
composition can vary significantly from one reservoir to
another. Therefore, to better predict the outcomes of
low saline water flooding in different types of sandstone
reservoirs,
numerical modeling
has become an essential
tool.
Numerical models provide a framework for simulating
complex interactions that cannot be easily captured
through direct experimentation alone. By incorporating
the key physical phenomena
—
such as flow dynamics,
ion exchange, electrostatic forces, and changes in rock
wettability
—
these models can simulate various
scenarios and predict the impact of different brine
compositions, injection rates, and salinity levels on oil
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recovery. This approach offers a valuable way to study
the intricate behavior of ion interactions in low saline
water flooding and identify the optimal conditions for
enhanced oil recovery.
In this study, we present a
numerical approach
to assess
the ion interactions in low saline water flooding of
sandstone reservoirs. The goal is to develop a model
that captures the effects of ionic composition and ion
exchange on the fluid dynamics, permeability, and oil
recovery efficiency. By using a comprehensive numerical
framework, we aim to better understand how variations
in ion concentration and ionic strength influence the
behavior of water flooding and how these factors can be
optimized to enhance recovery.
Ultimately, the findings of this study will contribute to
the ongoing efforts to improve EOR techniques,
particularly in terms of better understanding the
mechanisms behind low saline water flooding and
providing a predictive tool for field applications.
Additionally, this research aims to guide the
development of more efficient, sustainable, and cost-
effective strategies for oil recovery from sandstone
reservoirs.
2. METHODS
The study utilized a comprehensive numerical approach
to simulate ion interactions during low saline water
flooding in a sandstone reservoir. This approach
incorporated a two-phase flow model to simulate fluid
displacement in porous media, integrated with an ionic
interaction model that accounted for ion exchange,
electrostatic forces, and wettability alteration. The
following subsections describe the model development,
the choice of input parameters, and the simulation
setup in detail.
2.1 Model Development
The core of this study is a
two-phase flow model
for
water-oil displacement in sandstone reservoirs,
combined with a model for ion interaction at the rock-
fluid interface. The flow model was based on the
standard Richards equation for unsaturated flow, which
governs fluid transport through porous media. This
equation was adapted to include both water and oil
phases, as well as the effect of capillary pressure,
relative permeability, and ion-induced changes in the
rock’s properties.
The model incorporates several key physical processes
relevant to low saline water flooding:
•
Two-phase flow dynamics
: Simulated oil and water
phases interact within the pore network. The model
accounts for relative permeability changes, which
depend on the saturation of water and oil phases.
•
Ion exchange
: Ion interactions between the injected
low saline water and the sandstone rock surface
were modeled using a set of empirical relationships.
The most significant ion interactions are between
calcium (Ca²⁺) and sodium (Na⁺) ions, which
exchange on the rock surface, altering wettability.
•
Electrostatic interactions
: The model includes the
effects of the electrical double layer (EDL) that forms
at the rock-fluid interface. The EDL influences the
capillary forces and the water-rock interactions by
modulating the electrostatic potential at the
surface. As the ionic strength of the injected water
decreases (low saline), the EDL thickness increases,
influencing the capillary pressure and fluid flow.
The model was implemented in a reservoir simulator
capable of performing
3D simulations
in a
heterogeneous reservoir with complex pore networks.
The simulator incorporates
finite difference methods
to
numerically solve the governing equations for fluid flow,
ion transport, and rock-fluid interaction.
2.2 Reservoir and Fluid Properties
To create a realistic model, several parameters
representing the reservoir characteristics and fluid
properties were defined based on typical conditions for
sandstone reservoirs. These parameters were derived
from experimental data and literature values, but can be
adjusted to fit specific reservoir scenarios.
•
Porosity
: The initial porosity of the sandstone
reservoir was assumed to be 25%. This is typical for
mature sandstone formations that have undergone
secondary recovery.
•
Permeability
: The permeability was set to 500
millidarcies (mD), which is a moderate value for a
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sandstone reservoir with good connectivity in its
pore structure.
•
Initial Formation Water Composition
: The initial
formation water had a high salinity of 40,000 ppm
(parts per million), typical of saline water found in
mature sandstone reservoirs.
•
Injected Low Saline Water Composition
: The
injected water was modeled with a reduced salinity
of 10,000 ppm, typical for low saline water flooding.
The main ionic components of the injected brine
were sodium (Na⁺), calcium (Ca²⁺), and chloride
(Cl⁻), with the concentrations adjusted to simulate
the effect of different salinity levels. The specific
concentrations used for Na⁺, Ca²⁺, and Cl⁻ ions were
5,000 ppm, 3,000 ppm, and 10,000 ppm,
respectively.
•
Fluid Properties
: The oil phase was assumed to be a
typical crude oil with a viscosity of 10 cP and a
density of 800 kg/m³, while the injected water had a
viscosity of 1 cP and a density of 1,000 kg/m³.
2.3 Ion Interaction Mechanisms
Ion interactions play a critical role in the success of low
saline water flooding by affecting wettability and
capillary pressure. To model these interactions, several
mechanisms were incorporated into the numerical
framework:
1.
Ion Exchange
:
2.
Ion exchange occurs between the brine and the rock
surface, primarily involving calcium (Ca²⁺) and
sodium (Na⁺) ions. When low saline water is
injected, sodium ions (Na⁺) from the rock are
exchanged with calcium ions (Ca²⁺) from the
injected water. This process reduces the overall
charge density on the rock surface, resulting in
wettability alteration.
3.
The
ion exchange coefficient
was calculated based
on experimental data, and the
Nernst-Planck
equation
was used to describe ion transport
through the porous media. The rate of exchange was
dependent on the concentration gradient of ions
between the brine and the rock surface.
4.
Electrostatic Double Layer (EDL) and Surface
Charge
:
5.
The rock surface in contact with the brine develops
an electrical double layer, which consists of a
charged surface layer of ions and a diffuse layer of
counter-ions in the solution. The thickness of this
double layer increases as the ionic strength of the
brine decreases, which significantly affects the
electrostatic interactions.
6.
The
Debye-Hückel theory
was applied to estimate
the thickness of the electrical double layer (EDL),
which governs the interactions between ions at the
rock-fluid interface. The increase in EDL thickness at
low salinities reduces capillary forces, making it
easier for water to displace oil.
7.
Wettability Alteration
:
8.
Wettability, which determines whether the rock
surface is oil-wet or water-wet, was modeled based
on the concentration of divalent ions (like Ca²⁺) in
the injected brine. The presence of calcium ions on
the rock surface facilitates the shift towards a more
water-wet condition, which enhances water
displacement efficiency during flooding.
9.
The
wettability alteration model
was implemented
using empirical correlations that relate the
concentration of calcium ions at the rock surface to
the contact angle between the rock and water. The
wettability shift was quantified in terms of changes
in the relative permeability and capillary pressure
during the simulation.
2.4 Numerical Simulation Setup
The numerical simulations were conducted using a 3D
grid representing a 10-meter by 10-meter sandstone
core with a 0.5-meter mesh size. The following
simulation parameters were used:
•
Reservoir Size
: A 10m x 10m x 10m reservoir block
was modeled, representing a section of a typical
sandstone reservoir.
•
Injection Conditions
: The low saline water was
injected into the reservoir at a constant rate of 0.5
pore volumes (PV) per day, which is a typical rate for
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water flooding in EOR operations. The injection
continued for a period of 100 days.
•
Boundary Conditions
: The boundary conditions for
the simulation were defined as no-flow boundaries
at the sides of the reservoir and a constant pressure
boundary at the injection and production wells. The
reservoir pressure was kept constant at 2,000 psi,
which is typical for many oil reservoirs.
•
Initial Saturations
: The initial water saturation was
set at 0.3, and the initial oil saturation was 0.7,
reflecting a partially water-flooded reservoir. The
initial formation water composition was based on
typical high saline brine conditions.
•
Output Parameters
: The primary outputs of the
simulation included the
oil recovery factor
,
water
cut
,
relative permeability
, and
capillary pressure
.
These outputs were analyzed over the course of the
injection period to assess the effects of low saline
water injection on oil recovery.
2.5 Sensitivity Analysis
A sensitivity analysis was conducted to evaluate the
effects of different parameters on the oil recovery
performance. The parameters considered for the
sensitivity analysis included:
•
Salinity Levels
: The effect of varying salinity levels
(e.g., 5,000 ppm, 10,000 ppm, and 15,000 ppm) on
oil recovery efficiency and wettability alteration was
assessed.
•
Ionic Composition
: The influence of varying the
concentrations of Na⁺, Ca²⁺, and Cl⁻ ions in the
injected water was also analyzed.
•
Ion Exchange Coefficients
: Variations in the ion
exchange rate were tested to explore how different
exchange rates affect wettability and capillary
forces.
3. RESULTS
3.1
Oil
Recovery
and
Water
Cut
The numerical simulations revealed that LSWF with low
saline water resulted in a higher oil recovery factor
compared to conventional high saline water flooding.
The oil recovery increased by up to 10% for the low
saline case, with the highest recovery observed when
the injected water had a calcium ion concentration of
5,000 ppm. The water cut, defined as the ratio of water
to oil produced, was also reduced by 15% during LSWF,
indicating more efficient oil displacement.
3.2 Ion Exchange and Wettability Alteration
The simulation results showed significant ion exchange
between the injected water and the sandstone surface,
particularly for divalent ions such as calcium. This
exchange altered the wettability of the rock, making it
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more water-wet, which promoted better displacement
of oil from the pore spaces. The changes in surface
charge density and the electrical double layer thickness
were particularly evident at lower salinities, where the
ion interactions were more pronounced.
3.3 Permeability and Pore Structure Changes
The permeability of the sandstone core was observed to
decrease slightly after the injection of low saline water.
This was attributed to the changes in the rock surface
due to ion interactions, which resulted in the swelling of
clay minerals and the formation of more compact pore
structures. However, despite the slight reduction in
permeability, the overall fluid flow was enhanced due to
the more favorable wettability conditions for oil
displacement.
4. DISCUSSION
The results of this study underscore the importance of
ion interactions in low saline water flooding of
sandstone reservoirs. The ion exchange processes that
occur when injecting low saline water lead to significant
changes in rock wettability and capillary forces, which
directly influence the oil recovery efficiency. The
increase in oil recovery observed in the simulations can
be attributed to the alteration of the rock surface charge
and the resulting increase in water-wet conditions,
facilitating the displacement of oil from the pore spaces.
The numerical model developed in this study provides a
robust framework for evaluating the role of ion
interactions during low saline water flooding. It offers a
more comprehensive understanding of how variations in
ion concentration and the ionic composition of the
injected water affect reservoir behavior, which can aid
in the design of more efficient EOR strategies.
Additionally, the results suggest that optimizing the
ionic composition of the injected water could further
enhance oil recovery by maximizing the beneficial ion
exchange processes and minimizing adverse effects on
permeability.
However, the study is not without limitations. The
model assumes idealized conditions and does not
account for the complex heterogeneity often observed
in real reservoirs. Future work should focus on
incorporating reservoir-specific data, such as variations
in mineral composition and pore structure, to improve
the
accuracy
of
the
simulations.
Moreover,
experimental validation of the model's predictions
would be valuable to confirm the applicability of the
numerical approach in real-world scenarios.
5.CONCLUSION
This study highlights the significant role of ion
interactions during low saline water flooding in
sandstone reservoirs and provides a numerical
framework for assessing these interactions. The findings
demonstrate that low saline water flooding can enhance
oil recovery by altering wettability and capillary forces
through ion exchange processes. The numerical model
developed here offers valuable insights into the
mechanics of LSWF, supporting its potential as a viable
enhanced oil recovery technique. Further research and
field validation are needed to refine these models and
optimize the parameters for real-world applications.
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