Effect Study of Cr on Polymer Molecular Aggregate and Seepage Flow Characteristics

EASTERN ACADEMIC FORUM
Effect Study of Cr3+ on Polymer Molecular Aggregate and Seepage
Flow Characteristics
LU Xiangguo1, WANG Rongjian1, CHEN Xin1, LIU Yigang2
1. Oil & Gas Recovery Ratio Enhancement Key Laboratory of Chinese Education Ministry, Northeast
Petroleum University, Daqing City, Heilongjiang Province, China, 163318
2. Production Department, Tianjin Branch, CNOOC, Tanggu District, Tianjin City, China, 300450
Abstract: Bohai oilfield is characterized by thick layers, high average permeability, serious
heterogeneity, high viscosity crude oil and injection water salinity, which resulted in poor water-flooding
recovery. So there is an urgent need to take new technologies to improve the water-flooding efficiency.
Based on the theories of reservoir engineering, physical chemistry and polymer materials, by means of
chemical analysis, instrumental detection and physical modeling, set evaluating indicators of polymer
molecular aggregate, viscosity, resistant coefficient and residual resistance factor, set the LD10-1 oilfield
reservoir conditions as the study object, conduct performance evaluation and mechanism analysis of the
“salt-resistant” polymer solution and the “high molar mass” Cr3+ polymer gel. The results showed that at
the early time of adding cross-linking agents to the polymer solution, Cr3+ mainly leads to the
cross-linking reaction of different branches (carboxyl) on the same polymer molecular chain. Polymer
molecular chains in the polymer gel are mainly “local reticular” aggregate. At that time, polymer
molecular clew dimension increase is not big, and the viscosity hardly increases. As time went on, Cr3+
begins to actuate the cross-linking reaction among branched chains (carboxyl) on different polymer
molecules, and polymer molecule chains in the polymer gel are mainly “local reticular” aggregate, the
viscosity increases prominently, and its compatibility with the oil reservoir becomes poor. Compared
with the molecular chain in the salt-resistant polymer solution presenting “linear and branched”
aggregate, flexibility of the “local reticular” aggregate in the Cr3+ polymer gel becomes weak, and its
rigidity increases, which showed excellent fluid steering capability. What’s more, when Cr3+ polymer gel
was diluted by injected water, its molecular clew dimension would increase, and its blocking effect to
rock pore was enhanced, which made the residual resistance factor higher than the resistance factor, and
showed peculiar permeability characteristics from common polymer solution.
Keywords: Polymer, Organic chromium, Aggregate, Permeability characteristics, Mechanism analysis
Introduction
Bohai oilfield is characterized by thick layers, high permeability, serious heterogeneity, high viscosity
crude oil and high solvent water salinity. The injected water darts badly along the high permeability
layers, which leads the water cut to increase rapidly and seriously restricted the economic results of
water flooding technology. Compared with onshore oilfield, drilling cost of the offshore field is higher,
whose well spacing density is lower, and whose operation cost is higher. Also, service life of the
producing platform is short, so there is an urgent need to take measures to enhance oil recovery
significantly. Field tests and application of domestic Daqing, Shengli and Henan oilfields show that,
polymer flooding can greatly improve the mobility ratio, enlarge the swept volume [1-6], and greatly
improve the oil recovery ratio. However, due to the high salinity of injected water in offshore oilfield,
current polymer products fail to meet the actual demands of fields in salt resistance and temperature
resistance, thus new flooding agent products and technologies withstanding the high salinity of solvent
water must be sought.
In recent years, much attention has been paid to the polymer gel flooding technology [7-10]. However,
influenced by traditional evaluation indexes, researchers often aim at pursuing the high viscosity of
polymer gel, while ignoring the matching relationship between polymer gel molecules and reservoir
pores, which seriously restricted the actual application effect of polymer gel flooding technology. Based
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on theories of reservoir engineering, physical chemistry and polymer materials science, by means of
chemical analysis, instrument detection and physical simulation, by evaluating indicators of polymer
molecular aggregate, viscosity, resistance coefficient and residual resistance coefficient, relying on the
platform of the LD10-1 oilfield reservoir characteristics and fluid properties, we conducted experimental
study and mechanism analysis on effect of Cr3+ upon polymer molecular aggregate and seepage flow
characteristics. This research achievement plays a significant guiding role in developing new polymer
products and technical decision to screen flooding agents for oilfields.
1 Experiment Condition
1.1 Experiment materials
The polymer used were partially hydrolyzed polyacrylamide powder (HPAM) manufactured by Daqing
Refining & Petrochemical Company, whose relative molecular weights were 1,900×104 (abbreviated as
the “macromolecule” polymer) and 3,500×104 (abbreviated as the “salt- resistant” polymer) respectively.
Their solid contents were both 88.0%, and hydrolysis degrees were both 25.8%. The cross-linking agent
was the organic chromium, whose Cr3+ effective content was 2.7%. The oil used was the mixture of
degassed oil from the LD10-1 oilfield and kerosene, whose viscosity was 16.8MPa·
s at the temperature
of 65℃. The water used was the injected water of the LD10-1 oilfield, whose ion composition analysis
was shown in Table 1.
Table 1 Water quality analysis
Ion Composition (mg/L)
K++Na+
Ca2+
Mg2+
Cl-
SO42-
CO32-
HCO3-
Total Salinity
(mg/L)
2 968.8
826.7
60.8
6 051.6
60.0
0.0
208.7
10 176.6
The core was quartz sand and epoxy resin cemented man made core [11], including three types of
columnar cores, homogeneous square cores and two-dimensional vertical heterogeneous cores.
Columnar cores were used for the liquidity assessment of flooding agents, whose geometric
dimensioning was Ø2.5×10cm, and whose gas test permeability was 2,400×10-3μm2. Square cores were
used for transmission and migration capacity evaluation of flooding agent, whose geometric
dimensioning was 4.5×4.5×30cm, and whose gas test permeability was 1,100×10-3μm2. Two-dimensional
vertical heterogeneous cores were used for the flooding effect evaluation of the flooding agent, which
included 3 permeable layers of high, medium and low, whose permeability were 6,200×10-3μm2,
2,500×10-3μm2 and 1,000×10-3μm2 respectively, and whose thickness was 1cm, 2cm and 1.5cm
respectively. The geometric dimensioning of the cores was 4.5cm in height, 4.5cm in width and 30cm in
length.
1.2 Apparatuses
The viscosity of flooding agent was measured by the DV-ⅡModel Brookfield viscometer with a
sheering rate of 7.35s-1 and a 6r/min rotate speed. The polymer molecular aggregate was scanned by the
S-3400N Hitachi scanning electronic microscope. The molecular clew dimension of polymer (Dh) was
measured by the BI-200SM Model Brookhaven wide-angle dynamic/quiescent light scattering apparatus
(Brookhaven Instruments Cop, USA), whose main components included the BI-9000AT Model laser
correlator and signal processing apparatus, Argon ion laser (200mW, laser wavelength λ=532.0nm),
detecting at a scatter angle of 90°. CONTIN mathematical model was built for data processing. Before
the experiment, samples were leached by the nuclear millipore filter membrane with the pore size of
0.8μm and the KQ3200DE Model numerically controlled supersonic cleaner was adopted to clean the
sample bottles.
The flowing characteristics of flooding agent were tested by the flooding experimental device, which
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includes an advection pump, a pressure sensor, a core holder, a hand pump and a middle container and
so on. (The apparatuses and experimental process are shown in Figure 1.) All equipments, except the
advection pump and the hand pump, were put into the incubator with the temperature of 65℃.
Figure 1 Scheme of the experimental equipment and the flow chart
The polymer solution should be pre-sheered before experiment, so as to get a viscosity retention rate of
60 percent, and then cross-linking agent is added to prepare Cr3+ polymer gel solution.
1.3 Core experimental principles
(1) Flow characteristics
Flow characteristics of the flooding agent are usually described with the resistance factor and the
FR 
P3
P2
, FRR 
P1
P1
residual resistance factor, which are main evaluating indicators that describe the hold-up of flooding
agent in the core (seen in Figure 2(a)), and which are usually expressed with FR and FRR. The
expressions are as follows:
Where, P1 is the core water flooding pressure, P2 is the chemical flooding pressure, and P3 is the
follow-up water flooding pressure. The injection process above must maintain a constant liquid injection
rate, and the injection amount should be between 5PV-6PV.
(2) Transmission capability
Pressure testing points are set in the entrance and middle part of the artificial homogeneous core (seen in
Figure 2(b)), and measure pressures of the entrance and middle measurement points after polymer
solution or polymer gel is injected from one end of the core. Pressure difference in the first and second
half of the core is calculated and the relation curve of pressure difference versus PV is drawn to evaluate
the transmission and migration capability of the flooding agent in porous media.
(3) Flooding effect
First, evacuate saturated water of the artificial heterogeneous cores (see in Figure 2(c)), and then
saturated with oil and conduct the water flooding. Finally, flooding agent is injected into the core and
conduct succeeding water flooding. Produced fluids at each displacement stage are collected, the
injection pressures at different time are recorded, and oil & water volume is measured, so as to calculate
the oil recovery ratio.
Figure 2 Sketch of core and its structure
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2 Results and Discussion
2.1 The impact of Cr3+ on polymer molecular aggregate
Polymer solution and Cr3+ polymer gel system were prepared with injected water (the ratio of polymer
to Cr3+ is 180:1), and the electron microscopy observation results of the polymer molecular aggregates
are shown in Figure 3.
(Polymer solution)
(Cr3+ Polymer gel)
Figure 3 Polymer molecular aggregate
Seen from Figure 3, polymer molecule aggregates showed a “linear dendritic structure” in polymer
solution. When cross-linking agent Cr3+ was added into the polymer solution, part of branches of the
polymer molecule tangled with each other, forming a “local reticular” structure. Thus, Cr3+ can change
the polymer molecular aggregate configuration, and then impact the polymer molecular clew dimension,
permeability characteristics and flooding effect.
2.2 Polymer molecular clew dimension (Dh) and its influencing factors
(1) The dilution effect of injected water
“High molar mass” polymer solution and Cr3+ polymer gel with relatively high concentration were
prepared and diluted gradually with injected water which was used for preparing flooding agent. The
relation experimental results of Dh and polymer concentration before and after dilution are shown in
Table 2.
Table 2 Dh Experimental results (nm)
Concentration of Polymer Cp (mg/L)
Parameters
Flooding agent
200
400
600
“High molar mass” polymer solution
294.8
376.2
533.8
“High molar mass” Cr3+ polymer gel
465.4
324.5
213.0
Seen from Table 2, when the flooding agent was diluted with injected water, Dh of Cr3+ polymer gel
increased gradually while Dh of polymer solution decreased. During succeeding water flooding of core
experiments, due to the dilution effect of injected water on the cation concentration in the pore, the
dynamic balance of original charges of the polymer gel aggregate surface was broken [12-13]. Part of the
cation desorbed and entered into diffusion layer, and the number of negative charges of the ionic groups
of polymer molecule increased. The powerful repulsion of negative charges promoted the original curled
molecular chains to stretch, which made molecular clew dimension increase. With molecular clew
dimension increasing, on the one hand, the plugging effect of polymer molecules trapped by pore was
enhanced. On the other hand, molecules which originally freely migrated along the pore would be
trapped, which caused the channel flow cross-section to decrease further and eventually there were the
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phenomenon that the residual resistance factor was larger than the resistance factor.
(2) The effect of time
The relations of time and Dh of polymer solution and Cr3+ polymer gel are illustrated in Table 3.
Table 3 Dh Experimental results (CP=100mg/L, nm)
Time (day)
Parameters
Initial
Flooding agent
2
5
15
25
stage
“High molar mass” Cr3+ polymer gel
280.6 299.3 366.8 135.9 120.9
“Salt-resistant” polymer solution
230.2 535.2 435.0 194.8 134.0
40
60
113.6
113.2
88.8
93.6
Seen from Table 3, as time went on, molecular clew dimension tended to increase first and then decrease.
Compared with polymer solution, the Dh of Cr3+ polymer gel was larger and more stable.
2.3 Viscosity
Relations of polymer concentration and viscosity of the polymer solution and the Cr3+ polymer gel are
illustrated in Table 4.
Table 4 Viscosity experimental results (MPa·
s)
Concentration of Polymer (mg/L)
Parameters
Flooding agent
1 600
2 000
2 400
2 800
3 200
“High molar mass” polymer solution
10.2
13.8
18.1
43.6
52.6
“High molar mass” Cr3+ polymer gel
9.8
13.2
18.3
42.5
51.8
3+
Note: The ratio of polymer to Cr is 180:1.
Seen from Table 4, viscosity of polymer solution and Cr3+ polymer gel tended to increase with polymer
concentration increasing, but there was not much absolute value difference between them. Thus, there
formed the polymer molecule aggregates which are mainly cross-linked within the molecule in the
Cr3+polymer gel system. As molecular clew dimension hardly changed (see in Table 3), the capability of
polymer molecular chains to envelope water wasn’t affected greatly, so there was minor viscosity
difference between them.
2.4 The resistance factor and residual resistance factor
Testing results of the resistance factor of the “high molar mass” Cr3+ polymer gel and the “salt-resistant”
polymer solution (FR) and the residual resistance factor (FRR) are illustrated in Table 5.
Table 5 Resistance factor and residual resistance factor
Concentration of polymer (mg/L)
Parameters
800
1 200
1 600
Flooding agent
FR
FRR
FR
FRR
FR
FRR
“High molar mass” Cr3+ polymer gel
8.5
9.7
967.0
1 568.0 3 790.3 blocking
“Salt-resistant” polymer solution
8.1
3.2
9.7
4.0
12.9
5.2
3+
Note: the ratio of polymer to Cr is 180:1.
Seen from Table 5, the resistance factor (FR) and residual resistance factor (FRR) of “high molar mass”
Cr3+ polymer gel were far larger than those of “salt-resistant” polymer solution under the same polymer
concentration, and the difference between them enlarged with polymer concentration increasing. The
viscosity of “salt-resistant” polymer solution was a little higher than that of “high molar mass” Cr3+
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polymer gel, however, since the polymer chain in the polymer gel showed “local reticular” aggregates
(seen in Figure 3), compared with the linear branched aggregates in the polymer solution, flexibility of
the former is less than the latter, but the rigidness strengthens, which caused the trapping effect of it to
strengthen in porous media, the hold-up and permeability resistance to increase. Further analysis showed
that, unlike polymer solution, the residual resistance factor of the polymer gel was larger than its
resistance factors, which demonstrated a unique permeability characteristic. Theoretical analysis
indicated that the change of flow resistance was affected by two factors during succeeding water
flooding. One was the amount of polymer detached from pores due to the scouring effect of injected
water and the other was the expansion rate of polymer aggregates still lingered in the pores. If the
reduction rate of flow resistance caused by the former factor was exceeded by the increasing rate of flow
resistance by the latter factor, the injection pressure would enlarge and the residual resistance factor
would increase. The experimental datum provided by Table 2 showed that, the dilution effect of the
injected water would cause the molecular clew of polymer gel to expand (the increase in molecular clew
dimension), the plugging effect to strengthen and flow resistance to increase. Therefore, the residual
resistance factor was larger than the resistance factor.
2.5 Transmission and migration capability
It is shown in Table 6 that pressure difference experimental results in the first and second half of the
core (seen in Figure 2(b)) during the “salt-resistant” polymer solution and the “high molar mass” Cr 3+
polymer gel injecting (0.38PV) and succeeding water flooding processes.
Flooding agent
Table 6 Pressure difference experimental results
Pressure difference (MPa)
Parameters
After succeeding water
Viscosity After flooding agent flooding
flooding
(MPa·
s)
The first
The second
The first
The second
half
half
half
half
“Salt-resistant” polymer solution
7.8
0.018
0.007
0.006
0.003
“High molar mass” Cr3+ polymer gel
4.4
0.350
0.002
0.250
0.004
Seen from Table 6, compared with “salt-resistant” polymer solution, the pressure difference of “high
molar mass” Cr3+ polymer gel in the first half of the core was far larger than that in the second half at the
end of chemical flooding and succeeding water flooding. Thus, the transmission and migration
capability of “high molar mass” Cr3+ polymer gel was worse than that of “salt-resistant” polymer
solution. Therefore, Cr3+ polymer gel was suitable for deep flooding construction of reservoir with high
permeability, high salinity and serious heterogeneity.
2.6 Flooding effect
The flooding recovery ratio results of “high molar mass” Cr3+ polymer gel and “salt-resistant” polymer
solution are illustrated in Table 7.
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Table 7 Recovery experimental results
Scheme
No.
1-0
1-1
1-2
Scheme content
Water flooding until water cut reached
98%
Water flooding until water cut reached
40% then 0.10PV “salt-resistant”
polymer (CP=1 600mg/L) was injected
and succeeding water flooding until
water cut reached 98%
Water flooding until water cut reached
40% then 0.10PV Cr3+ polymer gel
(CP=1 200mg/L, the ratio of polymer to
Cr3+ was 270:1) was injected and
succeeding water flooding until water
cut reached 98%
Viscosity
(MPa·
s)
Oil
saturation
(%)
—
Recovery (%)
Recovery
increment
(%)
Water
flooding
Chemical
flooding
75.9
41.4
—
—
7.6
76.0
28.6
51.7
10.3
4.4
75.5
28.4
56.0
14.6
Seen from Table 7, under the same slug size and injection timing, the recovery increment of “high molar
mass” Cr3+ polymer gel was 14.6% while that of “salt-resistant” polymer was 10.3%, which minus 4.3%.
Thus, the flooding effect of “high molar mass” Cr3+ polymer gel was superior to that of “salt-resistant”
polymer solution.
The relation of injection pressure and recovery versus PV number in the experimental process are shown
in Figure 4.
Figure 4 Relative curves of injection pressure versus PV and recovery versus PV
Seen from Figure 4, the injection pressure of “high molar mass” Cr 3+ polymer gel was far higher than
that of “salt-resistant” polymer solution under the same slug size and injection timing, which indicated
that the swept volume enlarging effect of the “high molar mass” Cr3+ polymer gel was better, and it
achieved larger recovery increment.
The mechanism above can be explained with the model shown in Figure 5. During water flooding, high
permeable layers absorbed large amount of liquid and would absorb more as water flooding proceeded.
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At the initial stage when diverting agent was injected, as the flow resistance of high permeable layers
was smaller, the suction pressure difference was larger, therefore, diverting agent first went into the high
permeable layer and detained in it, leading the pore flow cross-section to reduce, flow resistance to
increase and injection pressure P to enlarge. As injection pressure P increased, the suction pressure
difference (  P3=P-P3,  P2=P-P2) of medium and low permeability layers raised and the amount of
liquid absorbed increased, which indicated that the medium and low permeable layers were effectively
swept. At the same time, the diverting agent would be detained in the medium and low permeable layers,
resulting in the flow resistance to increase in medium and low permeable layers, suction pressure (  P3
and  P2) to reduce, liquid absorption to decrease and entry profile to reverse. Thus, for heterogeneous
reservoirs, when designing the diverting agent strength and slug size, needs of both high permeable
layer and medium-low permeable layers should be taken into account, and the time when entry profile
began to reverse should be delayed. In this way the harm of diverting agent on the medium-low
permeability layers would be reduced and the development effect of medium-low permeability layers
would be improved.
K1, K2, K3—Permeability of high, medium and low permeability layer;
p1, p2, p3—Threshold pressure of high, medium and low permeability layer;
Q1, Q2, Q3—Liquid absorption of high, medium and low permeability layer;
p—Injection pressure.
Figure 5 Sketch of typical model of heterogeneous reservoir
3 Conclusions
3.1 Cross-linking agent Cr3+ can change polymer molecular aggregate configuration, which would affect
the permeability performance and fluid diverting capability of polymer solution.
3.2 When the salinity of solvent water is high and concentration of polymer and cross-linking agent is
low, Cr3+ could push different branches of the same polymer molecule chain occur intramolecular
cross-linking reaction, and form the polymer gel which are mainly “local reticular” aggregates. When
the polymer gel is diluted, the polymer molecular aggregates would expand which leading them to
demonstrate the unique permeability characteristics in porous medium.
3.3 Compared with polymer molecular aggregate of the “salt-resistant” polymer solution, aggregates of
the Cr3+ polymer gel are mainly “local reticular”. Cross-linking has little effect on polymer molecular
clew dimension, thus has little effect to its reservoir compatibility. But as the molecular aggregate is less
flexible and more rigid, it can plug effectively in high permeability layer, which diverts the subsequent
fluids to medium-low permeable layer, and achieves desirable flooding effect.
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Fund:
Funded by the “Efficient Deep Fluid Turning & Flooding and Fine Stratification Injection Technology”
(2011ZX05010-003) and “Marine Heavy Oil Oilfield Thermal Recovery Technology Experiment
Demonstration” (2011ZX05057-005) of the “Twelve-five” oil and gas major national projects.
Author:
Lu Xiangguo (born in 1960), male, graduated from the Development Department of Southwest
Petroleum University in 1989, received a Master’s Degree, and won the D.E. of Waseda University in
2002 at Japan. Now, he is professor and doctoral supervisor in Northeast Petroleum University,
“Longjiang scholar” special employed professor of Heilongjiang Province, mainly engaged in
technology research and teaching work of enhanced oil recovery.
Email: [email protected]
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