SPE-113762-MS
SPE 113762
Properties of Polymer Solution and Gel with Different Electrolytes
X.G.Lu, SPE, Key Laboratory of Enhanced Oil Recovery of Ministry of Education, Daqing Petroleum Institute;
W. Wang, SPE, University of Wyoming; and J.Sh. Li, SPE, PetroChina, Co.Ltd.
Copyright 2008, Society of Petroleum Engineers
This paper was prepared for presentation at the 2008 SPE Western Regional and Pacific Section AAPG Joint Meeting held in Bakersfield, California, U.S.A., 31 March–2 April 2008.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract
The polymer clews and flow performance under the effect of electrolytes have been studied. Laboratory experiments were used to determine macroscopic pressure and flow resistance. Optical microscope was used to examine dimensions and shapes of polymer clews.
Experimental results showed that polymer clews and flow resistance force were influenced by the type and concentration of electrolytes. The curling of polymer clews had different optimal concentration scope for different types of electrolytes. Molecular clews became smaller when the concentration of electrolytes was higher under certain concentration scope, and became larger when the concentration of electrolytes exceeded the scope. And the effect of Ca2+ to polymer clews was bigger than Mg2+, both were bigger than that of K+ and Na+. With coagulant it was more likely to form intra-molecular cross-linked reaction when the concentration of electrolytes was higher, which provided a higher flow resistance force and helped to attain a high recovery. When the succeeding water salinity was lower, the polymer Clews would swell, and pressure and resistance force would be further enhanced.
Mechanisms of polymer clews curling and stretching under the influence of electrolyte and coagulant have been analyzed by Stern-Grahame Double-Layer Model and catching and re-catching theory in stratum through coordinating microscopic and macroscopical experiment. The polymer clews would curl and stretch because of the compression of the double layer by ion binding of electrolytes. When succeeding water was diluted, the decrease of electrolyte concentration resulted in the swelling of clews of polymer molecules, re-catching process took place, thus flow resistance in porous media increased.
The objectives of this study was to perform in-depth profile modification by changing molecular clews shape and dimensions, as well as to recycle the produced water in the oilfield for improved oil production.
Introduction
In recent years, polymer powder that has been injected in Daqing oilfield in China has exceed to 9×104t, Increasing oil reach to 1000×104t, polymer flooding technique has been an important technical method to stabilize the oilfield output(Wang et al.2000).With the applicance scope of polymer flooding enlarging, at the one hand, the fresh water that is used to prepare polymer solution becomes less and less, at the other hand, the large quantity of produced water that is produced in the process of producing oil will pollute environment. So it has been necessary choice to adopt produced water to prepare polymer solution in oilfield producing. In additive, main reservoirs have become less and less in the progress of polymer flooding in industry appliance. So it becomes necessary trend to stablize high and stable oilfield output by applying polymer flooding technique to thin and worse reservoirs, which are the second and third type reservoirs, Therefore, changing polymer clew dimension has been significant subject for polymer to adapt the second and the third reservoir development. The routine method is injecting polymer solution with middle and low molecular weight into middle and low permeability stratum, but this method also decreases flow resistance and oil recovery effect. So some people proposed that electrolytes could change dimension and clews of polymer clews(Lu et al. 2006). And polit test that is located in the eastern part of Lamadian oilfield have effectively proved this assuption. When polymer solution that prepared by produced water and diluted by fresh water electrolytes was injected test zone, in which the concentration of electrolytes of fresh water is lower than that of produced water, it is easy to injected stratum for polymer solution, and when water cuts reach to 98%, has a 2.2 MPa higher injection pressure, 8.0% higher decreasing extent of water cut, 0.66×104m3 more accumulative less producing water in average single well, 4.64% higher oil recovery than comparison zone that prepared by produced water and diluted by produced water, and comparison zone has similar reservoir condition and oil saturation before injecting polymer solution with test zones.
This phenomen has attracted people’s attention, some research accomplishments on polymer clew dimension and its affecting factors has been reported(Hou et al.2003; Liao et al.2003; Lu et al. 2006), The types and concentration of electrolytes and cross-linked reactions are main influential factors that affect polymer clew dimension and shape(Hou et al.2003; Liao et al.2003; Lu et al. 2006). If some coagulants are added into polymer solution whose electrolyte concentration is higher, two kinds of cross-linked reaction will probably take place depending on type and concentration of electrolytes, one is intra-molecular cross-linked reaction, whose cross-linked reactions take place in different polymer molecular chains, the other is intra-molecular cross-linked reaction whose cross-linked reaction take place in the same molecular chains( Lu et al. 2006; Lu
2 SPE 113762
et al. 2005). The feature of the former is that apparent viscosity obviously increases along with time. The feature of the latter is that whose feature is that the apparent viscosity is almost the same along with time(Lu et al. 2006; Lu et al.2005), which have the same property with polymer solution under the same condition, so other juding index will be chosed, such as flow performance and optical experiment.
But research on polymer solution flow performamce influence and action mechanism by changing type and concentration of electrolytes and concentration of coagulant has not been reported. Aiming to the problems that supplying of fresh water is insufficient and produced water is need to reused in the oilfield and the practical requirement of the second and third reservoir development, the paper has done some researches on the influence of the types and concentration of electrolytes, cross-agent and tempetature on polymer clew dimension, shape, flow performance and microscopic experiment by using instrumental detection and theory analysis method, and the paper also has done some researches on action mechanism analysis, all of which have practical significance on improving polymer flooding development effect.
Experimental Material Polymer The polymer used in the laboratory tests was polyacrylamide, manufactured by PetroChina Daqing Refining & Petrochemical Company. The relative molecular weight was 2.5×107, active ingredient was 89.67%. C oagulant
The coagulant was aluminum citrate (AlC 6H 5O 7?3H 2O) and the active Al 3+ was 0.6%. Water
The water used in the tests included distilled water (without any electrolytes in water), and electrolyte solutions with different cations, and oilfield produced water(produced from oilfield production) and fresh water(came from the earth surface), which were taken from No.2 Oil Production Company of Daqing Oilfield Co. Ltd. The water quality analysis results were seen in Table 1. Cores
The core samples used were artificial cores which were cemented and pressed by arenaceous quartz and epoxy resin (Lu et al., 2006). The dimensions of the cores were 2.5 cm in diameter and 10 cm in length. The gas permeabilities of the cores were about 1 Darcy.
Experiment apparatuses
The experimental apparatuses used to prepare polymer solution and viscosity-measuring instrument include electronic balance, vernier caliper, dividing ruler, stirrer(Type HJ-6) and RV2 Brook Viscometer etc. Viscosities are measured with RV 2 Brook Viscometer. The polymer molecular clew dimension testing apparatuses(Niu et al. 2001; Liao et al.2001;Lu et al. 2006) are DAWNEOS MODEL 18 ANGLE LASER LIGHT SCATTERING DETECTOR (produced in WYATT TECHNOLOGY CORPORATION, America ，wave length is 690nm) and OPTILAB DSP INTERFEROMETRIC REFRACTOMETER, which is produced in WYATT TECHNOLOGY CORPORATION. Procedure
Fig.1 illustrates the schematics of the apparatus for the flow performance tests. All the procedures were run at 45°C. Viscosity measurements
The polymer solution was stirred for 2 hours before the viscosity was measured. The polymer solution was then stored at 45°C and the viscosity was re-measured periodically. Flow resistance evaluation
a. Each core was weighed, saturated with the water (produced water that is added some electrolytes and the water was also used to make the polymer solution) by vacuuming. Water permeability was measured with 3 different flow rates, 2.0 ml/min, 4.0 ml/min and 6.0 ml/min. Injection pressure 1P δ at flow rate of 0.3 ml/min was calculated from the measurements.
b. Polymer or polymer gel solution was injected at a flow rate of 0.3 ml/min until the injection pressure 2P δ was stabilized (at least 2 PV of polymer was injected).
c. Succeeding water was injected at a flow rate of 0.3 ml/min until the injection pressure 3P δ was stabilized (at least 3 PV was injected).
Resistance coefficient R F and residual resistance coefficient RR F can be calculated with the following equations:
12
P P F R δδ= (1)
1
3P P
F RR
δδ= (2)
SPE 113762 3 Polymer molecular clew dimension testing
The polymer solution was stirred for 2 hours then stored at 45°C for seven days before determing root mean square gyration radius of polymer molecular, which is <Ｒｇ2 >1/2.
Viscosity and flow performance tests
Polymer solutions with different electrolytes were prepared, the composition of flow performance experiment and viscosity data were shown in Table 2. Polymer solutions with different electrolytes and coagulants were prepared, the scheme composition of flow performance experiment and viscosity data were shown in Table 3.
Prilimanary electrolytes analysis
As listed in Table 2, 8 artificial cores with similar gas permeability were used in the tests. The core was first saturated with the produced water solution that were still added some electrolytes, then polymer solution was injected after water permeability measurements. Finally succeeding water was injected. All the injection pressure was recorded.
The curve of injection pressure distribution with PV value depending on Scheme 1, 2, 3 and 4 of Table 2 were plotted in Fig 2. According to Fig 2, we know that electrolyte in polymer solution has great influence on the flow performance. In injection progress, injection pressure in polymer solution that is added electrolytes is lower than that of polymer solution prepared by common produced water. In the progress of succeeding water flooding, injection pressure is decreasing persistently with PV value increasing when using produced water as succeeding water. When using fresh water as succeeding water, injection pressure of common produced water polymer solution is decreasing persistently with PV value increasing, and injection pressure of polymer solution with electrolytes is increasing by a large amount with PV value increasing and show different flow performance. Further analysises of the influences of electrolyte on polymer flow performance were done below.
The type of electrolytes research
The relation curve of injection pressure and PV value of experiment scheme 1, 3 and 5 of Table 2 are shown in Fig 3.
According to Table 2 and Fig 3, viscosity differences of three kinds of polymer solution with the effect of Ca2+, Mg2+ or Na+ are small, however, pressure differences of these three kinds of polymer are greater, in which the pressure of polymer solution with Na+ is higher, and those of polymer solution with Ca2+ and Mg2+ are lower. In the stage of succeeding fresh water flooding, pressures in these three kinds of polymer solutions rise, and the pressure of polymer solution with Na+ is still higher than that of polymer solution with Ca2+ or Mg2+. Experiment results show that the effects of Ca2+ or Mg2+ on molecular clews are stronger, and the curl of molecular clews are more evidence.
The concentration of electrolytes research
The relation curves of injection pressure and PV value of experimental schemes 1, 2, 6 and 7 of Table 2 are shown in Fig 4.
According to Fig 4, Concentration of electrolytes has some influence upon flow performance of polymer solution. In the stage of polymer injecting, injection pressure would decrease when the concentration of Na+ is higher. In the stage of injecting succeeding fresh water, the increasing extent of pressure is becoming larger with the increasing of the Na+ concentration in polymer solution.
The electrolyte concentration of succeeding water flooding research
On the basis of experiment that have done above, we know, when polymer solution prepared by high concentration electrolyte water did succeeding water (that have low concentration electrolyte) flooding, succeed pressure obviously showed increasing trend. In order to do further research on this property, we do some further analysis of flow property on polymer solution with concentration 500 mg/L under different succeeding injection water flooding effect of different electrolyte concentration water solution, which is shown in Fig.5.
Electrolyte concentration scope of succeeding water is high salinity produced water, produced water, fresh water and distilled water. In which the electrolyte concentration of high salinity produced water is bigger than produced water, which are both bigger than fresh water, and all of which are bigger than distilled water. It is obviously showed in Fig 5, when injecting high salinity produced water to do water flooding, pressure show a declining trend, then decrease the concentration of electrolytes in succeeding water, injection pressure gradually increase, which show that when the concentration of electrolytes in succeeding solution is bigger or equal with the concentration of electrolytes in polymer solvent, injective pressure shows declining trend, otherwise, injection pressure first demonstrates increasing then decreasing trend.
Concentration of the coagulant
It is easier to form intra-molecular cross-linked reaction under the condition of high concentration of electrolyte polymer solution after adding coagulant (Lu et al.2006). The paper did some researches on flow performance of polymer solution and intra-molecular cross-linked polymer solution. The influence of coagulant on polymer molecular clews has been researched and analyzed through comparing flow performance with polymer solution. The experiment schemes and experiment results were shown in Table 3.
In intra-molecular cross-linked polymer solutions that are prepared by high concentration electrolyte water with the same electrolytes with correlative polymer solution. There are some influences of content of coagulant on flow performance of Al3+ cross-linked polymer solution. According to the Gel 1, Gel 2 and Polymer 1 comparing research experiment results from Table 3, which is shown in Fig 6, viscosities of Gel 1 and Gel 2 do not have big difference with polymer solution, but resistance factos and residual factors are obviously bigger than that of Polymer 1, which means that Intra-molecular cross-linked reaction
4 SPE 113762 have formed in Gel 1 and Gel 2. And resistance factor and residual resistance factor of Gel 1 are bigger than that of Gel 2, which means that resistant factor and residual resistance factor are increasing with the content of coagulant increasing. So coagulant has great influences on flow performance of polymer solution. And at the certain extent, the influence on polymer clews is greater when the content of coagulant is higher.
Macroscopic experiment tests
Influences of electrolyte and coagulant upon clews of polymer solution are shown in this section. Different electrolyte concentration solvent water is formed by adding different concentration and type of electrolyte in produced water. Then polymer molecular clew dimension of polymer solution with different salinity solvent are determined by the method of LASER LIGHT SCATTERING DETECTOR and OPTILAB DSP INTERFEROMETRIC REFRACTOMETER，whose root mean square radius of gyration results are shown in Table 5.
Table 5 shows that there are some influence of the type and concentration of salinity to the molecular clew dimension of polymer molecules in the polymer solution, which can be shown in root mean square gyration radius <Ｒｇ2 >1/2. The larger the concentration of salinity is, the smaller the<Ｒｇ2 >1/2 is (which are shown in Scheme 4, 5 and 6). In the same concentration of iron, the influence degree of Ca2+ and Mg2+ to the polymer chain dimension are almost the same, while their influence are stronger than that of Na+. In the condition of the same concentration of iron, the polymer molecular clew dimension after the effect of Al3+ is larger than that before the effect of Al3+( which are shown in Scheme 7 and 8).
Preliminary Exploration on Mechanism
According to Stern-Grahame double-layer theory, the type and concentration of electrolytes are the primary reasons to induce polymer gel molecules with different configurations (Lu et al., 2005; Gu et al., 1994; Han, 1993). Cations such as Na+, Ca2+ or Mg2+ in the polymer solution will neutralize negatively charged molecular chains through ion-binding, compressing the diffusion layer thickness and reducing the zeta potential, therefore induce the coiling and contracting of the polymer molecule chains, so root mean square gyration radius <Ｒｇ2 >1/2 of polymer become smaller under the effect of electrolytes. When the concentration of the electrolytes exceeds the upper limit, polymer clews may be re-charged to thicken the hydrated layer, and the dimensions of the polymer clews will increase.
In which the influence of Ca2+ and Mg2+ to molecular clew dimension is greater，which mainly benefit from valence electrons number of Ca2+ and Mg2+ are larger than that of Na+ and K+ with the same concentration condition，and the ability to neutralize negative charge of Stern layer is stronger, leading to the decreasing extent of the thickness of diffusion layer is larger, which further leads to the extent of the molecule chains coil and contract effect is more serious. Ca2+ is more likely to cause ion-binding than Mg2+ because the effectiveness of divalent cations as coagulants usually decreases according to the so-called “lyotropic series”( Israelichivilib, J. 1994).
Within certain concentration range, higher electrolyte concentration enhances the formation of Intra-molecular cross-linked reation gel. And the smaller the contacting probability of different molecule is, the smaller the possibility of inter-molecular cross-linking action taking place is, and it is easier to form intra-molecular cross-linking action under the effect of Al3+, and form the “local” network molecular structure. And the molecular clew dimension of polymer solution under the effect of Al3+ is bigger than that of polymer solution with the same condition besides Al3+.
When conducting experiments of succeeding water flooding, because of the diluted effect of injection water to the concentration of cations in the porous media, the original charge dynamic balance will be broken, some cations are desorption and enter diffusion layer, the number of negative charge in ionogenic group of molecular chain will increase, and original curling molecular chains are trend to stretch because of the action of negative charge excluding from each other, and then leads to molecular hydrodynamics equivalence ball become larger(Han.1993).
In the stage of succeeding water flooding, when remnant polymer are displaced and recovered from the core, injection pressure will decrease. When molecular chain dimension of remnant polymer is increasing, injection pressure will increase. Thereby, injection pressure is affected and restricted by remnant amount of polymer and polymer molecular chain dimension. When carrying out succeeding water flooding using the low concentration of salinity solvent, the quality of polymer that are recovered in the initial stages is larger and the expanding extent of polymer chain dimension are smaller, and the injection pressure will decrease. While the quality of polymer that are recovered in the middle stages is smaller and the expanding extent of polymer chain dimension are larger, and the injection pressure will increase.
Conclusions
1.Electrolytes have influences on polymer molecular chain dimension and shape, the influence extent of Ca2+ and Mg2+ are
larger than that of Na+.
2.Under certain concentration scope of electrolytes, Polymer clew dimension become smaller when the concentration of
become larger. Contrariwise, polymer clew dimension become larger if the concentration of electrolytes in polymer solution become smaller.
3.It is easier to form intra-molecular cross-linked reaction under the condition of high concentration of electrolyte polymer
solution after adding Al3+ coagulant. Compared to polymer solution with the same condition, intra-molecular cross-linked reaction is also easy to enter the cores, and has better flow performance.
SPE 113762 5 4.Action mechanisms of polymer clews’ curling and stretching under the influence of electrolyte and coagulant can be
analyzed by Stern-Grahame Double-Layer Model and catching and re-catching theory in stratum through coordinating microscopic and macroscopical experiment.
5.The displacing pressure of polymer solution and gel in cores are affected by polymer remnant and polymer clew radius of
polymer molecular, which is
References
Wang, J.M.; JI, B.F.; Sui, J.; Guo, W.K.; Ji, B.Y. “Theoretical Research On Polymer Flooding.” Petroleum Geology & Oilfield Development In Daqing, 1999, 18(4):1
Niu, J.G.; Sun, G.; Gao, F. “Experimental Study On Making Polymer With Produced Sewage In Oilfield.”Petroleum Geology & Oilfield Development in Daqing,2001, 20(2):17
Liao, G.Z.; Zhang, C. L.; Zhang, W. X. “Excellent paper assembly of post-doctor admission in Daqing Oilfield.” Beijing: Petroleum Industry Press.2001,12(3):3
Hou, J.R.; Liu, Z.C.; Zhang, S.F.; Yue, X. A.; Yang, J.Z. “Effect of Alkali on Molecular Configuration of Polymer and Its Rheologic Behavior.” Acta phys.-chim.sin., 2003,19(3):257
Liang, G.Z.; Sun, G.; Niu, J.G.; Zhou, J.S.; Liu, F.Q.; Ma, j.M. “Studies on the General Performance of Partial Hydrolyzed Polyacrylamide Used in Oil Flooding.” Acta Scientiarum Naturalium Universitatis Pekinensis, 2003, 39(6):815
Lu, X. G.;Wang, W. “The Molecular Configuration and Its Influential Factors of Al3+ cross-linked Polycrylamide Gel.” Acta phys.-chim.sin., 2006,22(5):631
Lu, X. G.; Hu, Y.; Song, J.S.; Zhao, J.Y. “Molecular Construction Al3+ cross-linked polyacrylamide gel and its identif ication method.” Acta Petroleum Sinica,2005, 26(4):73
Gu, X.R.; Li, Y.L.; Ma, J.M. “Surface Chemistry.” Beijing: Science Press，1994:131-141
Han, X. Q. “The Mechanism of Enhancing Oil Recovery.” Beijing: Petroleum Industry Press, 1993:35
Israelichivilib, J. “Intermolecular & Surface Forces”, Second Editio). Academic Press, lnc,1994
Appendix
Table 3 Experiment data of Al3+ polymer gel solution
Sample No.C Ca2+,
mg/L
Polymer,
mg/L
Polymer_Al3+ ratio
Polymer 1 126 500 0
Gel 1 126 500 20:1
Gel 2 126 500 30:1
Viscosity, mPa?s
Sample
No. 1d 2d 3d 5d 10d 15d 20d 30d Polymer
1 10.5 10.5 9.5 10.0 9.5 8.5 8.0 7.5
Gel
1 9.5 10.5 13.5 25.0 11.5 8.5 6.0 5.6
Gel
2 13.2 9.5 9.0 9.0 8.5 8.2 8.0 13.0
Experiment data of flow performance/tested at the 10th day
6 SPE 113762
Sample No. cores No. Gas permeability,
×10-3μm 2 Resistance Factor Residual Resistance factor
Polymer 1 R1-15 1360 60.8 120.0 Gel 1 R1-10 1395 820.1 920.3 Gel 2
R1-12
1355 433.2 520.0
Table 4 Clew Dimensions of Polymer Molecule
Scheme No.
Concentration, mg/L Type of Solvent
<Ｒｇ2
>1/2
,
nm
1 Produced water 59.5
2 Produced water +216mg/L Mg 2+
56.2
3 Produced water +216mg/L Ca 2+
56.4
4 Produced water +786mg/L Na +
59.3
5 Produced water +1180mg/L Na +
57.4
6 Produced water +1966mg/L Na +
48.3
7 Produced water +72mg/L Ca 2+
58.5
8 150 Produced water +72mg/L Ca 2++7.5mg/L Al 3+
73.3
Fig.1 Experimental Apparatus and Flow Chart
Fig.2 Relation Curve Between Pressure and PV Value (polymer 1000mg/L)
0.00
0.05 0.10 0.15 0.20 0.25
0.30 0.35 0.40 0.00 5.00
10.00 15.00
P
r e s s
u r e , 0.1M P
a
PV Value
472(Na+)590(Na+)72(Ca2+)Pure Polymer solution
Polymer flooding
Fresh water flooding
Produced water flooding Electrolytes(mg/L)
SPE 113762 7
Fig.3 Relation Curve Between Pressure and PV Value (polymer 1000mg/L)
Fig.4 Relation Curve Between Pressure and PV Value (polymer 1000mg/L)
0.00
0.05
0.10 0.15 0.20 0.25
0.30 0.00
5.00
10.00
15.00
P
r e s s
u r e , 0.1M P a PV Value
72(Ca2+)472(Na+)126(Mg2+)
Polymer flooding
Produced water flooding
Fresh water flooding
Electrolytes(mg/L)
0.00
0.05 0.10 0.15 0.20 0.25
0.30
0.00
2.00
4.00 6.00
8.00 10.00
P r
e s s u r e , 0.1M P a PV Value
472590
Polymer flooding Produced water flooding
Fresh water flooding
Na +(mg/L)
8
SPE 113762
Fig.5 Relation Curve Between Pressure and PV (polymer 500mg/l, Ca 2+
108mg/L)
Fig.6 Relation Curve Between Pressure and PV (polymer 500mg/l, Ca 2+
126mg/L)
0.00
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.00
2.00
4.00
6.00 8.00 10.00 12.00 P r e s s u r e , 0.1M P a PV Value
Polymer flooding
High salinity water flooding
Produced water flooding
Fresh water flooding Distilled water flooding