Файл: Магистерская диссертация тема работы Оценка эффективности применения полимерного заводнения на месторождении х удк 622. 276. 43 678. 7 Студент.pdf

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Introduction
Polymer flooding is one of the most effective methods of physical and chemical stimulation of the pay zone. The technology of polymer flooding is based on the fact that a high-molecular weight chemical reagent, a polymer, dissolves in water, which, even at low concentrations, is capable of significantly increasing the viscosity of water, reducing the conductivity of the medium, helping align the displacement front, thereby increasing the flooding coverage and extending the water-free period of well operation.
Polymer flooding is widespread because of its advantages. The method is good for oil recovery in the conditions of different stages of the field development with the uneven permeability, different properties, and reservoir structure, is carried out at low reagent consumption, does not require the application of expensive and complex equipment. Disadvantages of the method, such as decreasing stability of polymer solutions at high temperatures (thermal destruction) and mineralization of formation fluids, are usually eliminated by careful selection of polymer composition. However, the limitations of polymer flooding related to reservoir properties (permeability limitations) and limitations related to physical and chemical properties of the oil (oil viscosity) make this method selective when choosing an object for polymer stimulation.
The main property of polymers is the thickening of water, which leads to a decrease in the ratio of oil and water viscosities in the formation and reduces the conditions for water breakthrough due to the difference in viscosity or heterogeneity of the formation. In addition, polymer solutions, having higher viscosity, better displace not only oil but also bound formation water from the porous medium. Therefore, they interact with the skeleton of the porous medium, i.e. rock and a cementing agent. This causes adsorption of polymer molecules, which precipitate from the solution on the surface of the porous medium and block channels or deteriorate water filtration in them. The magnitude of adsorption is greatly influenced by the mineralization of the water and the mineral composition of the rock. To reduce adsorption there is a need to create a fresh water rim. At the

98 same time, the positive role of adsorption in washed formations is obvious, as it leads to permeability reduction and alignment of injectivity profile.
The polymer solutions recommended for application should have favorable rheological and oil displacing characteristics, stability of indicators and other positive properties, the research and regulation of which, as well as the development of the technology of polymer flooding regarding geological and physical conditions of the X field and peculiarities of development of the selected impact areas are devoted in this work.
Polymer flooding can be applied in its pure form as a method of enhanced oil recovery (EOR). In practice, in order to increase the efficiency of the method, polymer flooding is widely applied in combination with flow equalizing compositions: cross-linked polymer systems (CPS), viscoelastic systems (VUS), polymer-dispersed systems (PDS) with the injection of surface-active substances
(surfactants).
1. PECULIARITIES OF POLYMER FLOODING APPLICATION IN
OIL FIELDS
1.1 History of polymer flooding
For the first time application of water-soluble polymers for an increase of oil recovery factor was suggested in the USA in 1959. The Soviet Union lagged behind in this field and already in 1966 at Orlyanskoye field of Kuibyshev region the industrial variant of polymer flooding technology with the application of hydrolyzed polymer acrylamide as densifier was realized.
It should be noted that in those years there was no acrylamide polymer market as such. The oilmen adapted for their purposes the few polymers produced by the chemical industry for other purposes, mainly for use in ore enrichment and industrial and domestic water treatment technologies. In the USA, powdered polyacrylamide of Pusher 500 and 700 grades produced by Dow Chemical was used in polymer flooding technology. In the Soviet Union, the most widespread

99 were polymers produced in the form of 8-12 % solutions by the chemical plant in
Kalush (Ukraine).
High efficiency of polymer flooding technology in the fields of USA and
USSR, as well as the interest of polymer producing firms, have led to the creation of an international market of synthetic water-soluble polymers and to the increase of product assortment tens times. The development of the polymer market in the
Soviet Union is illustrative in this respect. In spite of political barriers, close scientific-political relations were established between Soviet oilmen specializing in polymer flooding technology and foreign chemists-manufacturers of polymers.
Foreign companies promptly supplied samples of polymer prototypes and industrial polymers, which were tested for a number of technological properties in laboratory and field conditions. Such interaction has led to obtaining a series of acrylamide polymers by the end of 70 – the beginning of 80s, which would satisfy the requirements for polymer-thickening agents in the oil industry. A kind of etalon of polyacrylamide in those years was DKS-ORPF-40NT polymer of Dai-
Ichi Koguo Seiyaki Co. Ltd" (Japan). The Ministries of oil and chemical industries had developed a program for the production of domestic polymers with properties similar to DKS-ORPF-40NT for the needs of the oil industry in the amount of 25 thousand tons per year. By early 1990 the program was close to implementation, but with the collapse of the USSR was suspended.
At present, the domestic chemical industry does not produce a single commercial acrylamide polymer suitable for use in polymer flooding technologies.
The commercial market of acrylamide polymers is characterized by a wide range of polymer producers. It should be noted that these firms are represented not only by countries with a developed chemical industry (USA, Japan, UK, France,
Germany), but also by developing countries, primarily China. The range of polymer grades and, accordingly, their physical, chemical, and technological properties, is extremely high. It is the analysis of products of the water-soluble polymers market that should be the first stage in designing the polymer flooding technology, since it allows the preliminary selection of samples promising for

100 industrial application and reduces the volume of experimental research based on accumulated data.
At the same time due to a variety of geological and technical conditions of polymers application as water thickener, there is no universal brand of polymers suitable for industrial implementation in any field. On the basis of market analysis,
10-15 polymer samples are selected, which are potentially suitable for technology implementation on the given site in order to choose the most prospective ones. The selection of these samples is the result of complex analysis of polymer assortment represented in the market by the set of technological, physical and chemical and molecular characteristics.
1.2 Characteristics of acrylamide polymers Molecular characteristics of
acrylamide polymers
As mentioned above, polymer flooding technology was first implemented about 50 years ago. From the very beginning, synthetic acrylamide polymers were chosen as the thickening polymers. Other water-soluble polymers were considered as alternatives: natural polymers (based on cellulose derivatives, biopolymers, polysaccharides) and other synthetic polymers, primarily polyoxyethylene.
Numerous laboratory studies and pilot tests conducted abroad and in Russia, have shown that all of the above classes of polymers, soluble in water, are inferior to synthetic polymers of acrylamide in some or other characteristics (technological, technical, economic). Therefore, the market of polymers suitable for waterflood technology is mainly represented by this class of compounds. Other polymers of biological or synthetic origin are considered only as modifying additives. The structural formula of hydrolyzed polyacrylamide is shown in Figure 1.
Figure 1 - Structural formula of hydrolyzed polyacrylamide

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The value (n+m) in this formula represents the degree of polymerization; the degree of hydrolysis of the characterizes the mole fraction of carboxyl from the total number of functional groups, calculated from formula (1):
(1)
A set of technical requirements for acrylamide polymers is given in Table 1.
Table 1 - Technical requirements for acrylamide polymers for polymer flooding technology
Indicator name
Unit
Norm
Merchandise form
-
Powder
Powder dispersibility:
- fractions with a particle size of less than 0.25 mm
-
- Fraction with a particle size of more than 1.0 mm
% masses no more 10 no more 10
Main substance content
% masses at least 90
Acrylamide content
% masses no more 0,1
Viscosity dl/year
15 - 20
Carboxyl group content
% mole
5 - 30
Dissolution time
- in fresh water
-
- in salt water min no more 60 no more 240
Insoluble residue
% masses no more 0,3
Filterability of PAA solutions in porous at least 5
Resistance factor of mechanically destructed PAA solutions at least 5
Resistance Factor at least 2
Thermal oxidative degradation resistance factor at least 0,8
Shelf life of the polymer months at least 12
Polymer solutions should not (as compared to injected water) cause
Note: filterability not less than 1 (satisfactory) is allowed in special cases.
From the molecular characteristics, Table 1 shows the characteristic viscosity and the content of carboxyl groups (degree of hydrolysis). Usually, the manufacturer characterizes polymers by the value of the degree of hydrolysis and the molecular weight (Figure 1).
The characteristic viscosity (or ultimate viscosity number) is related to the molecular weight by the Mark-Kuhn-Hauvinck equation. The thickening capacity of the polymer, that is, the increase in viscosity of the water in which the polymer

102 is dissolved compared to the viscosity of pure water, depends on the molecular weight of the polymer and its concentration. This relationship is shown in Figure 2.
Molecular weight, mln
Solvent salinity 15 g/l; t=25oC; j=6.1 s -1
Additionally, the viscosity increases as a result of polyelectrolyte swelling due to charged carboxyl groups, increasing with increasing degree of hydrolysis.
Figure 3 shows the change in viscosity with the degree of hydrolysis.

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Figure 3 - Effect of the degree of hydrolysis of PAA (MM=15 mil.) on the viscosity of solutions
Cp=0.2 %; shear rate 6.1 s -1; t=25oC
1 - fresh water;
2 - mineralized water.
The effect of polyelectrolyte swelling is greatest in fresh water, with low salinity. With increasing salinity, i.e. concentration of soluble salts, which are electrolytes, the viscosity of polymer solution due to inhibition of polyelectrolyte swelling decreases.
The range of molecular weights (MM) of industrial polymers is quite wide, from 200 thousand to 30 million. The same applies to the degree of hydrolysis
(agidr), which varies from 0 to 60%.
For the polymer flooding technology, it is advantageous to use polymers with high values of molecular weights and the degree of hydrolysis.
In the USSR the molecular weight of polymers used for water flooding was
10-15 mln. and the degree of hydrolysis averaged 15%. At present, the polymers

104 with the molecular weight of up to 20 million and degree of hydrolysis up to 30% are used abroad.
However, an excessive increase in the molecular weight leads to deterioration of the polymer solubility. The increase of hydrolysis degree above
25-30% leads to polymer salting-out when contacting with hardness salts of formation and pumped water. There is also a risk of polymers leaching with a hydrolysis degree of 20-25%, especially in high-temperature formations. This is due to the fact that at high temperatures (more than 600C) there is spontaneous hydrolysis of amide groups of the polymer, with the formation of carboxylic groups.
Physical and chemical characteristics of polymers
Dissolution time of polymers
More than 90% of acrylamide polymers are available in powder form with a main substance content of about 89-92%. The polymer dissolution process occurs in the injection line, which includes underground pipelines from the polymer dosing unit to the injection wellhead. Pumping of polymer slurry with the following dissolution takes place, in the overwhelming majority of cases, through the tubing. Injection through the annulus can be used to increase the travel time and thereby dissolve the polymer.
As shown in Table 1, the technological requirements for acrylamide polymers stipulate that the dissolution time of the powdered polymer in fresh water should not exceed 60 min., in saline waters - 240 min. Analysis of literature data shows that the dissolution time of the majority of commercial high-molecular polyacrylamides, determined in the laboratory conditions, is sufficiently close to the regulatory time of dissolution of the polymer.
The dissolution time of polymers, as compared to salts and other low molecular weight reagents, is due to the extremely high value of the molecular weight of the polymer and, accordingly, the long length of the macromolecules.
The polymer dissolution process consists of 2 stages: swelling of polymer particles and the dissolution itself - the transition of swollen polymer particles into the

105 solution. The first stage is longer and is determined by diffusion processes. The kinetics of the dissolution process of high-molecular polyacrylamides is approximately the same for most brands and has the form shown in Figure 4.
Time, min.
Figure 4 - Dissolution kinetics of polyacrylamide ROLY-T-101 (MM=10,7 mn, Ag=5,6%) Cp=0,3%, j=6,1 s-1
The dissolution kinetics is reliably enough described by the characteristic change in the dynamic viscosity of the solution during the dissolution of the polymer (Figure 4).
As can be seen from the presented curves, the dissolution kinetics is characterized by a fast stage at the beginning of the process and a slower stage at the end. During the initial stage, approximately 80% of the polymer passes into the solution, then the dissolution process slows down. This is due to the heterogeneity of the powder in terms of particle size (larger polymer particles dissolve in the slow stage) and the macromolecular heterogeneity of the polymer. At the initial stage, smaller molecules move into the solution. In the slow stage, larger molecules and associates (conglomerates of several macromolecules) take considerable time to transfer into the solution.
One should also take into account that the data on kinetics and dissolution time of polymers in particular water obtained in the laboratory conditions in accordance with RD-39-0148311-206-85 differ greatly from the data on dissolution time of polymers in real conditions.

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Numerous field studies during the implementation of polymer flooding technology and technology with the use of cross-linked polymer systems, with sampling along the technological line of polymer compositions movement show that under field conditions the dissolution time is 2-3 times shorter than the laboratory one. This difference is due to the diffusion mechanism of dissolution of high-molecular weight polymers. Individual particles of a polymer powder are close to spherical, with a diameter of the majority of particles in the range of 0.2-
0.4 mm. The presence of larger particles increases the time of polymer dissolution, small particles belong to the dusty fraction, their proportion is limited by technical requirements.
Observations under a microscope show that the particles are porous and penetrated by a network of extremely fine channels (Figure 5).
Figure 5 - Particle of polyacrylamide powder under the microscope
According to data obtained by saturation of particles under vacuum with inert liquid (isopropyl alcohol), the open porosity is approximately 13%. Due to small pore sizes, the total surface area of internal channels is extremely high, exceeding tens or hundreds of times the outer area of a spherical particle.
During the dissolution of the polymer in laboratory conditions water wets the outer surface of the particles. The swelling stage of polymer is quite slow due to the small contact surface of polymer with solvent. Solvent penetration through pore channels is difficult due to the counteraction of capillary forces. In real field conditions, polymer particles in the form of polymer slurry get to the pump intake and high pressure line quickly enough. Pressure rises almost instantly from atmospheric pressure to several tens of kg/cm2. At this pressure drop, capillary

107 forces are suppressed and water fills the polymer particle channels, multiplying the polymer-solvent contact surface and reducing polymer dissolution time.
Viscosity characteristics of polymer solutions
Strictly speaking, the viscosity level of polymer-thickened water is the greatest determinant of the effectiveness of polymer flooding technology. Exact values of viscosity can be obtained only on the basis of laboratory experiments.
As mentioned above, the viscosity of polymer solutions is influenced by the molecular characteristics of the polymer, as well as the salinity of the solvent and temperature. High-molecular-weight acrylamide polymers are characterized by the manifestation of rheological properties - the dependence of dynamic viscosity on the flow regime. In a porous medium, during the flow of polymer solutions, the value of viscosity is also affected by the nature of the structure of the porous medium. Naturally, it is impossible to take into account all these parameters within the framework of the theoretical viscosity model. However, it is possible to calculate the viscosity of polymer solutions with a high degree of approximation, using the accumulated data on the viscosity of polymer solutions under standard conditions.
Standardization provides for the identity of polymer test conditions in terms of solvent salinity, temperature, shear stress range, in which the dynamic viscosity of polymer solutions is measured. For the X field, a model of Alb-Cenomanian water with a total mineralization of 116 g/l, which is close in composition to waters used in waterflooding, can be used as a polymer solvent.
The accumulated database on the technological characteristics of polymer solutions allows calculating viscosity values of polymer solutions of different grades without carrying out experiments and carrying out a preliminary selection of polymers with high thickening capacity.
Mathematical viscosity models such as the Huggins model (formula 2) can be used to calculate the viscosity:

108 where ηotn - is the relative viscosity of the polymer solution; c - concentration of polymer in solution, g/dl;
Kx - Huggins constant;
[η] - characteristic viscosity, dl/g. and V.P. Budtov's model (formula 3): where Htah is the maximum Newtonian viscosity of the solution, mPa*s,
H0 - solvent viscosity, mPa*s, s - polymer concentration in solution, g/dl;
[η] - characteristic viscosity of the polymer in a given solvent, dl/g; γ - parameter of intermolecular hydrodynamic interactions characterizing mutual compression of macromolecular balls in a moderately concentrated solution, which depends on the thermodynamic quality of the solvent.
The models presented reflect the dependence of viscosity on polymer concentration. The equations also include a molecular characteristic in the form of a characteristic viscosity, which is functionally related to the value of the molecular mass (the Mark-Kuhn-Hauvinck equation (formula 4) is most commonly used: where K, a - are empirical constants.
The influence of the degree of hydrolysis on the value of the molecular weight can be calculated using the approach developed at the institute
"Giprovostokneft" under the leadership of L.V. Mineev. Based on direct measurement of the molecular weight by light scattering and parallel determination of the characteristic viscosity, samples with different degrees of hydrolysis was obtained the following ratio (formula 5):
Thus, with the molecular characteristics of a particular polymer grade, it is possible to approximate the viscosity at various PAA concentrations and select the desired concentration level. Calculations using the above formulas give viscosity

109 values relative to a narrow range of shear rates. Acrylamide polymers, especially high-molecular polymers, are characterized by extremely pronounced rheological properties, i.e. dependence of dynamic viscosity on shear rate.
A typical picture of the dependence of viscosity on the flow regime in a wide range of shear rates is shown in Figure 6.
Figure 6 - Dependence of viscosity of PDA-1020 polymer solution in
Cenomanian water on shear rate
Throughout the range, the dependence of viscosity on shear rate can be described by the Sagea equation (formula 6): where μG is solution viscosity at shear rate G, mPa*s,
μtah - the maximum Newtonian viscosity, at G=0, mPa*s, th - relaxation time, s, t - shear liquefaction index characterizing the degree of non-Newtonian behavior.

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The given Sagrea equation is a typical transcendental equation that can be solved only numerically.
The accuracy of the solution depends largely on available experimental data on the values of the highest Newtonian viscosity. This data is not always available due to the lack of viscometers that allow measurements at shear rates of less than 1 s-1 (so called low-shear viscometers).
The values of the highest Newtonian viscosity in real conditions, i.e. in the conditions of a particular reservoir, are practically never realized. The range of real shear rates for filtration in porous media usually exceeds 1 s-1.
Real values of shear rates realized when injecting polymer solutions into a particular well can be calculated from equation (formula 7): where j - is shear rate in the porous medium, s -1 , v is linear filtration rate, m/s, t is formation porosity, k - formation permeability, μm2.
For example, for typical reservoir parameters k<0.2 μm2, t=0.22 and filtration rate in the remote reservoir zone 0.5 m/day, filtration rate will be
(formula 8):
In the vast majority of cases, the range of real shear rates (both averaged and in individual layers composing the product formation) refers to the region of the rheological curve (Figure 6), reflecting the pseudoplastic nature of the flow.
Mathematical description of this area does not require the application of transcendental equations and is quite correctly described using elementary functions (exponential, logarithmic, and, best of all, power functions).
Thus, having the passport data on the polymer molecular characteristics and the data base on the rheological characteristics of polymer solutions under standard conditions one can calculate the range of viscosity properties of polymer solutions

111 of a particular brand as a function of polymer concentration and shear rate in a porous medium.