Variation Rules before and after Water Flooding in Ultra-low
Permeability Reservoir
Siyi Wang
1,2,
*, Lei Song
1,2
, Zhaocai Ren
1,2
, Shuo Song
1,2
, Li Ma
1,2
, and Mingxia Wei
1,2
1
National Engineering Laboratory for Low Permeability Oil and Gas Field Exploration and Development, Xi'an 710018,
China
2
Changqing Oilfield Exploration and Development Research Institute, Xi'an 710018, China
Keywords: Ultra-low permeability reservoir, Water injection development, Reservoir variation, Water flooding
mechanism
Abstract:
After long-term water injection and erosion in water drive development reservoirs, the physical properties
and seepage of the reservoir will change significantly, resulting in complex groundwater flooding laws and
difficult adjustments to oilfield development. Based on the actual core data of inspection wells, this paper
studies the changes in the physical properties and seepage characteristics of ultra-low permeability
reservoirs after long-term water flooding and their effects on oilfield development. Growth, particle
migration, and the illiteization of clastic rocks make the average porosity of the reservoir change little, the
permeability declines greatly, the acid-sensitive minerals in the interstitial decrease, the water-sensitive and
speed-sensitive minerals increase, and the relative permeability curve is medium the bleeding point shifts to
the right, and the wettability changes to be hydrophilic.
1 INTRODUCTION
Water injection development is currently the most
commonly used and most economical and effective
development method for oil fields. However,
long-term water injection development will cause
increased changes in reservoir physical properties,
pore roar structure, seepage characteristics,
wettability, etc., and changes in reservoir parameters
will affect oil and water. The seepage law and
remaining oil distribution affect the oilfield
development effect (Li et al., 1997; Wang et al.,
1999; Zhang et al., 2005; Li & Xu, 2003). Therefore,
studying the changing laws of reservoir physical
properties, seepage flow and other characteristics
before and after water flooding is of great
significance for reservoir development adjustment
and enhancement of oil recovery.
2 VARIATION LAW OF
RESERVOIR
CHARACTERISTICS
After long-term water flooding, reservoir
characteristics will change, and reservoirs with
different physical properties have different changing
laws. In order to study the characteristics of ultra-low
permeability reservoirs after water flooding, the
actual core samples of 8 inspection wells in the
inspection well group in the area A of the ultra-low
permeability typical oil reservoir were selected for
porosity, particle size, full-diameter core analysis,
relative permeability, analysis and testing of mercury
intrusion and wettability. At the same time, in order
to ensure the research results, the core samples
covered 8 inspection wells with different water
flooding directions and different injection-production
well spacings, and for the comparability before and
after water flooding for the cores of the same well
number, try to choose the layers with more
homogeneous lithology.
2.1 Porosity and Permeability Changes
Domestic oilfields have studied the changes in the
physical properties of reservoirs washed by
long-term water injection through indoor
experiments, coring comparisons, and dynamic
monitoring. The results show that the changes in
reservoir physical properties are closely related to
246
Wang, S., Song, L., Ren, Z., Song, S., Ma, L. and Wei, M.
Variation Rules before and after Water Flooding in Ultra-low Permeability Reservoir.
In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 246-250
ISBN: 978-989-758-560-9; ISSN: 1755-1315
Copyright
c
 2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

the physical properties of the reservoir itself. Under
the same sedimentary microfacies conditions, when
the core analysis permeability is above 100mD, the
physical properties of the reservoir become better
after water flooding, that is, the permeability
increase is larger, and the porosity change is smaller;
when the core analysis permeability is
1mD<K≤10mD, the physical properties of the
reservoir become worse after water flooding, that is,
the permeability becomes smaller and the porosity
increases or changes little.
The core analysis permeability of the test area is
1.42×10
-3
μm
2
, which is a typical ultra-low
permeability reservoir. 245 core samples from 8
inspection wells were selected for routine physical
property analysis and compared with the core data
from the initial development of the well group. The
results showed that after long-term water flooding,
the porosity and permeability of the reservoir
decreased to a certain extent. The average drop in
porosity is within 5.0%, and the overall change is not
significant, but the drop in permeability is relatively
large, mainly concentrated in 15-35% (Table 1).
Table 1: Comparison table of porosity and permeability changes before and after water flooding.
Level stage
Φ (%)
K
(10
-3
μm
2
))
Number of
samples
(pieces)
Average (%) decrease (%)
Number of
samples
(pieces)
average(10
-3
μm
2
)
Decrease
(%)
N1
Before 573 12.6
4.8%
573 1.58
33.2
Afte
r
742 12.0 742 1.05
N2
Before 543 12.9
1.0%
543 1.13
17.2
Afte
r
772 12.8 772 0.94
2.2 Pore Structure Changes
Wells W433 and Y180 and wells QJ011-353 and
QJ011-354 respectively represent the pre-wash and
post-wash stages. The test results show that after
water flooding, the sorting coefficient, variation
coefficient, and average value of the pore roar
distribution parameters all show an obvious
downward trend, indicating that during the reservoir
water flooding process, the migration of microscopic
particles blocks some pores and roars, making the
hole roar tend to Uniformity, the microscopic
heterogeneity of pores is weakened; permeability,
pore volume, and median radius of the pore size
characteristic parameters are reduced, and the pore
radius is complicated after water flooding. The
drainage pressure and median pressure of the pore
connection characteristic parameters increase Large,
the mercury inlet saturation and mercury removal
efficiency decrease, indicating that after the reservoir
is flooded, the pore connectivity and seepage ability
becomes worse, and the reservoir capacity becomes
smaller (Table 2).
Table 2: Comparison table of hole roar characteristic parameters before and after water flooding.
stage well
Φ
(%)
K
(10
-3
μm
2
)
Pore
volume
(cm
2
)
Sorting
coefficient
Coefficient
of
Variation
Discharge
pressure
(MPa)
Median
pressure
(MPa)
Median
radius
(um)
Maximum
mercury
saturation
(
%
)
Mercury
removal
efficiency
(
%
)
Before
W433 13.6 1.2 1.5 1.8 0.19 0.57 6.49 0.16 80.3 31.5
Y180 15.1 1.3 1.7 1.6 0.17 0.99 8.51 0.11 87.0 36.3
average 14.3 1.3 1.6 1.7 0.18 0.78 7.50 0.13 83.6 33.9
After
QJ011-353 13.2 0.9 1.2 1.3 0.18 0.81 7.51 0.10 76.6 27.2
QJ011-354 12.4 0.5 1.5 1.5 0.15 1.12 10.21 0.14 80.8 28.8
average 12.8 0.7 1.4 1.4 0.17 0.97 8.86 0.12 78.7 28.0
From the comparison curve of core mercury
injection of exploratory wells (before water flooding)
and inspection wells (after water flooding), the
starting pressure of samples after water flooding
increased significantly, and the flat section of the
curve also changed significantly. Compared with
before water flooding,the mercury ingress curve of
the latter sample became significantly steeper, and
the mercury withdrawal curve also had the same law,
indicating that the pore structure of the reservoir has
increased after water flooding, the overall
permeability has decreased, the seepage capacity has
weakened, and the pore roar structure has become
more complex (Figure 1).
Variation Rules before and after Water Flooding in Ultra-low Permeability Reservoir
247

Figure 1: Comparison of typical mercury injection well
curves before and after water flooding.
2.3 Phase Permeability Changes
Using core samples from 13 exploratory wells in the
test area and 36 inspection wells to carry out water
flooding tests, and normalizing the test results, the
results showed that after water flooding, the
irreducible water saturation increased from 33.1% to
34.5%, with residual oil saturation increased from
26.7% to 30.7%, oil-water two-phase seepage range
narrowed from 40.2% to 34.8%, water saturation at
isotonic point increased from 57.2% to 59.1%, and
oil displacement efficiency decreased from 55.0% to
50.9 % (table 3).
Table 3: Comparison table of phase permeability data before and after water flooding.
stage
Irreducible
water
saturation
(
%
)
Residual oil
saturation (%)
Interval range
(%)
Isotonic
saturation (%)
Isotonic
permeability
(
10
-3
μ
m
2
)
Oil
displacement
efficienc
y
(
%
)
Before 33.1 26.7
40.2
(
33.1-73.3
)
57.2 0.13 55.0
After 34.5 30.7
34.8
(34.5-69.3)
59.1 0.14 50.9
Figure 2: Comparison of phase permeability curves before
and after water flooding.
Judging from the normalized phase permeability
curves of the core water flooding test before and
after water flooding, the isotonic point of the sample
after water flooding moved to the right, indicating
that the wettability of the rock after long-term water
flooding changed to more hydrophilic (Figure 2);
And there is a certain difference in the relative
permeability curves of cores with different washing
degrees, that is, the higher the washing degree, the
higher the isotonic point of the two-phase seepage,
the faster the oil and water seepage changes, the
faster the water content rises, and the two-phase
seepage of the medium-water washing is isotonic
The spot is closer to the right than the weakly
washed one and is more hydrophilic (Figure 3).
Figure 3 Comparison of phase permeability curves of
different water-washed intervals
3 CHANGE MECHANISM OF
RESERVOIR
CHARACTERISTICS
The main reason for the change of reservoir
characteristic parameters after water flooding is
caused by the interaction of injected water with
reservoirs and fluids during the water injection
development process (He & Xu, 2010). The changes
are mainly manifested in the migration of formation
particles and changes in clay minerals. It is the
hydration and expansion of clay minerals, and the
continuous erosion will cause the stratum particles to
fall off, migrate and block pores or roars. This is
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 102030405060708090100
Kro、Krw
Sw(%)
After water flooding-Kro
After water flooding-Krw
Before water flooding-Kro
Before water flooding-Krw
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 102030405060708090100
Kro、Krw
Sw(%)
Unwashed-Kro
Unwashed-Krw
Weak washing-Kro
Weak washing-Krw
Medium washed-Kro
Medium washed-Krw
WRE 2021 - The International Conference on Water Resource and Environment
248

particularly prominent for ultra-low permeability
reservoirs with high clay mineral content; secondly
in the long-term water injection process, some
authigenic microcrystalline minerals will accumulate
in the pores or roars, blocking the roars, resulting in
enhanced reservoir heterogeneity; at the same time,
due to the chemical reaction between the injected
water and underground minerals, some clastic rocks
Yili petrification increased the content of
water-sensitive minerals in the reservoir, resulting in
weakening of the reservoir's seepage capacity
(Figure 4, Figure 5).
Figure 4: The remaining dissolved pores are filled with
authigenic quartz.
Figure 5: Yili petrochemical alteration of clastics.
At the same time, studies have shown that the
degree of cementation between different clay
minerals and rock particles is very different. Chlorite
is generally attached in a film-like form or surrounds
the particles. Its crystallite structure is relatively
strong and is not easily damaged during water
injection. After flooding, the water cut declines
slightly (2-5%). The content of clay minerals varies
greatly. The illiteization of clastic rocks causes the
content of illite to increase by 6-8%. The Yi/Meng
interlayer has strong water swelling properties,
resulting in a substantial increase in content. The
relative content increases from 5% increased to
14-16%; these clay minerals have different reaction
mechanisms during water flooding, so the damage to
the formation is also different. Among them, the
illite and the mixed layer of I/M are water-sensitive
minerals, and the damage to the bottom layer is
mainly due to water Swelling, blocking the roar;
kaolinite is a speed-sensitive mineral, which is
mainly manifested as migration and blockage during
the water flooding process, and is affected by the
reservoir water flooding speed and intensity;
Chlorite is an acid-sensitive mineral, except for
transportation In addition to migrating blockages,
other deposits can also form under the action of acid
to damage the formation (Peng et al., 2006; Gao,
2003).
The wettability is an important characteristic of
the interaction between rock and fluid, and it is also
the basis of whether the injected fluid can drive oil.
In the water injection development process, the
wettability of the rock can have an important impact
on the water flooding process, but long-term water
flooding can also change the wettability of the rock.
The wettability change is mainly caused by the fluid
in the porous medium. It is determined by factors
such as the flow of water, the injection of water, the
production of oil and water, and the changes in the
physical and chemical properties of oil-water-rock
(Ji et al., 2009). During the oilfield water injection
process, with the extension of the development time,
a relatively strong water-rock reaction will occur in
the reservoir. Particles and interstitials will generally
dissolve, break and migrate under the action of the
injected water, resulting in the reservoir variety. On
the one hand, because the oil film on the particle
surface is scoured and migrated, more hydrophilic
rocks are leaked; on the other hand, the temperature,
pressure, formation water, crude oil composition and
oil saturation of the formation have all changed,
which together affect wettability. The test data also
shows that when the water saturation of the oil layer
is greater than 40%, the wettability of most of the
rocks changes to be weak hydrophilic. When the
water saturation of the oil layer is greater than 60%,
it becomes hydrophilic.
4 CONCLUSION
1. During the water injection development process,
the reservoir parameters will change significantly.
These changes will affect the later development and
Variation Rules before and after Water Flooding in Ultra-low Permeability Reservoir
249

adjustment of the reservoir. For the ultra-low
permeability reservoir, due to its poor reservoir
physical properties, the pore roar structure of the
reservoir has deteriorated, the seepage capacity has
been weakened, the water-sensitive and
fast-sensitive minerals have increased, and the
acid-sensitive minerals have decreased. Therefore,
during water injection development, the formulation
of water injection technology policies should be
based on the formation. The critical flow rate is
determined, and the fluid that does not exceed the
critical flow rate and is highly compatible with the
formation for different reservoirs which is injected to
help improve the oil displacement efficiency of the
reservoir.
2. After water flooding in ultra-low permeability
reservoirs, the physical properties of the reservoir
become worse, the porosity changes little, and the
permeability decreases by about -30%; After water
flooding, the displacement pressure and median
pressure increase, and the median radius decreases.
The maximum mercury inlet saturation and mercury
removal efficiency decrease, the microscopic pore
structure deteriorates, the water drive efficiency
decreases, the oil-water two-phase flow interval
narrows, and the isotonic point shifts to the right.
3. Due to the long-term scouring effect of the
injected water, the surface of the hydrophilic
feldspar and quartz is dissolved and exposed, and the
wettability of the rock changes in the direction of
"lipophilic, weak lipophilic, neutral, weak
hydrophilic, hydrophilic", and the higher the degree
of water washing, the more the isotonic point of the
two-phase flow moves to the right, and the more
hydrophilic the rock is.
ACKNOWLEDGMENTS
The project is supported by Major Science and
Technology projects of CNPC. Technologies of 50
million tons sustainable and high-efficient
production stabilization in Changqing oilfield (No.
2016E-0508).
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