The experience of implementation of polymer flooding technology at Zaburunye oil filed as a method for developing mature fields

Толық мәтін

Introduction

Nowadays, many oil and gas fields of Kazakhstan are at the late stage of development and related to the category of “brownfields”. In Kazakhstan, about 70% of oil remains in the interior of the earth, according to the estimate of the RK Ministry of Energy. In the world practice of developing brownfields, there is an increased focus on the tertiary methods for enhancing oil recoveries, such as chemical, thermal, gas, and microbiological ones. The widespread use of new oil recovery methods would make it possible to increase the extracted oil reserves at least by 15-20% [1].

Searching for new technologies and development methods, which are cost-effective for the development of the brownfields under the conditions of advanced water flooding, is a first-priority task for the oil industry specialists. The gas, thermal and chemical methods for enhancing oil recovery have gained the most widespread acceptance.

 

Results and discussion

To evaluate the current state of development, screening of the top-27 fields of NC KazMunayGas JSC has been carried out for assessing the depletion of the reserves by the current and accumulated technological indicators. Taking into account an advance of the water content by 10%, as compared with the oil recovery from the initial recoverable reserves (hereinafter referred to as the IRR), the following fields can be distinguished: Karazhanbas, Kalamkas, Asar, Uzen, Karamandybas, Zuburunye, B. Zholamanov, Kenbai (the V. Moldabek site), UAZ, Akingen, Nuraly, Alibekmola (Fig. 1). Low reserve recovery with high water cut is characteristic for the given fields.

 

Figure 1. Depletion of the reserves of the 27 fields of the subsidiaries and affiliates of NC KazMunayGas JSC

 

Successful industrial introduction of the methods for enhancing oil recovery (hereinafter referred to as the MEOR) in the fields of the subsidiaries and affiliates (hereinafter referred to as the S&A) of NC KazMunayGas JSC will make it possible to ensure enhancing the oil recovery factor (hereinafter referred to as the ORF) up to 5-10% or more (Fig. 2). With an increase in the ORF only by 5% in the fields with the advanced rate of water flooding, belonging to Ozenmunaigaz JSC, Mangistaumunaigas JSC, Karazhanbasmunai JSC, Embamunaigas JSC, additional recoverable reserves can make up more than 164 million tons of oil, which is equivalent to the discovery of a new large-scale field.

 

Figure 2. Assessment of oil production potential when applying the MEOR with an increase in the ORF by 5%

 

It is common knowledge that water flooding remains the main method for affecting oil formations, that is why the improvement of its efficiency is one of the main tasks of modern oil engineering.

Among the tertiary methods for enhancing oil recovery, chemical methods for influencing oil formations are most widespread. These methods are applicable at low and medium temperatures up to 90°C, with a wide range of viscosities from 10 to 900 cP. Polymer water flooding (hereinafter referred to as the PWF) is one of the promising solutions to optimize the existing water cut.

Upon injection into formation, the polymer solution is usually moving through the highly permeable layers of the collector due to the lowest resistance in them, arising upon filtration, and creates a combination of two effects – an increase in the viscosity of the displacing agent and reduction of the conductivity of the porous medium due to decreasing the dynamic heterogeneity of the fluid flows, resulting in an increase in the coverage of the formations by flooding. The main specificity of the filtration of the polymer solution consists not only in increasing water viscosity but also in reducing its mobility, in increasing a resistance factor in the porous medium at low filtration rates of the solution, which is due to adsorption of the polymer in the porous medium [2].

The PWF technology is widespread abroad, in China, India, Canada, Oman, France, and other countries. The positive results of the field tests have been obtained, in some cases, PWF is used on an industrial scale [3]. In the world practice, PWF has been used in the Canadian fields, such as Pelican lake, Mooney, Seal, with the oil viscosity of more than 1000 cP (3000-80000 cP), with the integrated approaches for development (dense drilling of horizontal wells in combination with polymer water flooding, PWF modification, etc.).

Taking into account the generally accepted criteria of the MEOR (Fig. 3) for the conditions of the Zuburunye field of Embamunaigas JSC, chemical methods for enhancing oil recovery are most suitable, in particular, the polymer water flooding technology.

 

Figure 3. Express screening of the Zaburunye field, belonging to Embamunaigas JSC

 

General information on the field and polymer water flooding project

The Zaburunye gas and oil field was discovered in 1981. By the size of the recoverable reserves the field is related to the medium fields and has a complex geological structure. The main productive horizon is the II-ne horizon. The II Neocomian horizon lies in the depth range of 889-960 m. The temperature and pressure conditions are as follows: temperature and formation pressure on average, for the II Neocomian horizon, are 38.9°C and 10.0 MPa, respectively; oil viscosity under the reservoir conditions, on average over the horizon is 15.3 MPa*s (Table 1).

 

Table 1. Geological and physical characteristics of the II Neocomian target horizon of the Zaburunye field

Parameters	Zaburunye, II Neocomian horizon
Average depth of occurrence, m	942
Collector type	Terrigenous
Log-derived porosity, unit fraction	0.303
Thickness weighted average oil saturation, unit fraction	0.638
Core permeability, mkm2	0.526
Original reservoir temperature, °С	39
Original reservoir pressure, MPa	9.18
Saturation pressure, MPa	5.4
Oil viscosity in-situ, mPa*s	15.3
Oil density in-situ, g/cm3	0.811
Oil density at surface, g/cm3	0.895
Reservoir water viscosity, mPa*s	1.6
Formation water density, g/cm3	1.092
Total salt content of the formation water, g/l	141.5

 

Pilot tests (hereinafter referred to as the PT) for the injection of the polymer solution with the purpose to enhance oil recovery on the II-ne horizon have been carrying out at the Zaburunye field since November 2014. The current number of the production and injection wells in the PWF section is 62 and 6, respectively. The pilot tests of the PWF technology have started from the injection of a polymer solution into disposal wells No. 11 and 55. Based on the PT results at wells No. 11 and 55, the works for expanding the PWF technology have begun at the Zaburunye field since September 2017, by additionally transferring the four wells, No. 14a, 34, 42, 48, for injecting the polymer solution (Fig. 4).

 

Figure 4. The section of PWF tests at the Zaburunye field

 

The produced (waste) water is used as a source of water for the preparation of the polymer solution. A storage tank for the precipitation of water, used for the preparation and injection of the polymer solution into the well, is available at the PT section. Based on the results of the chemical composition studies, the injected waters are weakly acid, middle brines. They are represented by the average total mineralization of 134657.5 mg/l and density of 1.0914 g/cm³. The water type according to Sulin is calcium-chloride. Calcium and magnesium ions have the content of 2405 mg/l and 1216 mg/l, respectively, therefore the waters are very rigid. The content of dissolved oxygen varies from 7.30 to 7.80 mg/l. The content of trivalent iron is within the range of 0.98-1.54 mg/l, and that of bivalent is within the range of 24.92-32.76 mg/l, hydrogen sulfide (H₂S) is not detected. The content of the suspended particles is at the level of 18-27 mg/l, and that of the petroleum products is 43.7-75.4 mg/l.

It is noteworthy that treatment of the produced waters is recommended with the purpose to optimize the technology by reducing the degree of degradation, increasing viscosity, and reducing the consumption of dry polymer powder for thickening the injected fluid. In addition to high mineralization, increased content of bivalent metals Ca₂ + and Mg₂ +, oxygen iron FE₂ +, dissolved oxygen, is marked. The presence of these components in water creates a predisposition for oxidative chemical destruction, and, as a result, the loss of viscous properties of the polymer.

The polymer of the grade FLOPAAM 5205 VHM AL-888, used for injection, is related to the type of terpolymers of acrylamide, acrylic acid, and acrylamide-tertbutyl-sulfonate (hereinafter referred to as the ATBS) and is a triple copolymer (acrylamide/ATBS/acrylic acid). The addition of such a monomer as the ATBS to the polymer expands the range of polymer use under the conditions of high mineralization and rigidity of the formation waters.

The FLOPAAM 5205 VHM Al-888 polymer is a solid granulated substance, of a slightly yellow color, with a molecular weight of 13.53 million Da. The content of the main substance is 90.82%, of the insoluble sediment - 0.00%. The characteristic viscosity of the polymer is 20.43 dl/g, the degree of hydrolysis is 19.54%, and the dissolution time of the polymers in low-mineralized water is 120 minutes. The physicochemical parameters of the FLOPAAM 5205 VHM AL-888 polymer correspond to the stated technical characteristics and generally accepted requirements for polyacrylamide for the oil industry.

The rheological studies of the polymer solution have been carried out, the dependences of the viscosity of the polymer aqueous solutions on the concentration and the shear rate have been obtained at a temperature of at 25°C and 39°C (the reservoir temperature). An assessment of the leak-off coefficient of the polymer solution of the working concentration (1950 ppm) has been carried out. The polymer solutions for PWF should have good filtration characteristics - this is the basis for ensuring their good injection and ability to move through the reservoir. Unfavorable filtration characteristics can result in serious pore plugging and pollution of the formation. Based on the results of testing the filtration characteristics of the solution of the FLOPAAM 5205 VHM AL-888 polymer with the concentration of 1950 ppm through the membrane filters, the filtration coefficient <1.5 is considered complied with the specifications.

The studies of the thermal stability of the polymer solution for 30 days at the reservoir temperature of 39°C has been carried out by the method of parallel flasks (in the sealed containers), with the time intervals of 0, 5, 15, 30 days, by measuring the initial and intermediate viscosity of the polymer solution. For these studies, freshly prepared polymeric solutions with polymer concentrations of 500 ppm and 1950 ppm have been used. According to the results of the studies of the thermal stability, after 30 days a decrease in the viscosity of the polymer solution with the concentration of 1950 ppm and 500 ppm is observed by 31% and 23%, respectively.

Also, to evaluate the oil-sweeping properties of the polymer solution, the studies of filtration have been carried out under the real thermobaric conditions on the available core of the Botakhan field, which is most suitable by the porosity and permeability properties, represented by fine sandstone, sections of medium-fine-grained sections, with the aleurolite streaks. The average values of the porosity factor are ​​within 0.25-0.3 unit fraction, the permeability coefficient is in the range of 176-344 mD.

According to the obtained results of filtration experiments, an increase in the displacement coefficient by 5.6% is observed upon the injection of the polymer solution with the concentration of 500 ppm, and by 14.1% with the concentration of 1950 ppm, which indicates the efficiency of oil displacement with the polymer solution under the experimental conditions as compared with water flooding.

Taking into account the obtained results of laboratory studies, the original file has been updated, with the indication of the polymer properties for the hydrodynamic model. Certain parameters of polymer water flooding have been obtained during adaptation. Thus, the following parameters have been selected: adsorption - 0.0001 kg/kg, inaccessible pore volume - 0.2 unit fraction, residual resistance factor - 10.

The results of the PWF technology implementation

Based on the results of the calculation of the efficiency of the PWF technology as of 01.01.2021 (Fig. 5), the overall effect since the beginning of the Project implementation is 184.635 tons, 14.7% of the pore volume is pumped over, 3,640 tons of polymer are spent, a decrease in the water cut in relation to the base level is 2,0%, the additional oil production per 1 ton of polymer is 50.7 tons.

 

Figure 5. Dynamics of additional PWF oil production at the Zaburunye field

 

An analysis of the section for applying the PWF technology by various diagnostic charts and integral displacement characteristics shows the improvement of oil reserve recovery and oil displacement processes after starting the injection of a more viscous displacing agent. The stabilization of the dynamics of water flooding of the products and oil production as a whole in block III of the II-ne horizon of the Zaburunye field is the confirmation of the change in the filtration processes and increase of the reservoir coverage by the displacing agent. The results of the analysis of the PWF efficiency are presented in Fig. 6-7. The current increase in the ORF upon polymer water flooding as compared with the basic option is 2%.

 

Figure 6. Dependence of the cumulative oil production on the cumulative liquid production

 

Figure 7. Dependence of water cut on the IRR depletion

 

An analysis of the polymer yield in the producing wells

Various types of commercial and laboratory studies are used as the tools for assessing the efficiency of the PWF technology monitoring, one of which is the so-called express test of the polymer yield, with the use of kaolin clay, to qualitatively determine the presence of the polymer in the producing wells at the experimental PWF section.

The results of the laboratory tests for the quantitative analysis of the polymer present in the produced waters of the Zaburunye field show that the polymer is present in all wells, but to a greater extent in the samples from wells No. 100, 101, 126 (Fig. 8), located in the area of injection wells No. 11 and No. 55, and in a lesser extent - in the samples from wells No. 8A, 67, 70, 100, 101, 126 and 151.

 

Figure 8. Results of the analysis of the producing wells for the polymer yield at the PWF section

 

To further improve the process of displacement, involvement of the additional oil reserves, and cost optimization, it is necessary to consider the expediency of shutting-in of non-commercial producing wells with the high polymer concentration in their products.

To increase the PWF reservoir coverage under the condition of the polymer breakthrough in producing wells No. 100, 101, and 126, it is recommended to carry out treatment with the use of flow deviation technologies for blocking the high-permeable sublayers, possible channels and performing conformance control of the solution to be injected into disposal wells No. 11 and 55. Before the performance of the given works, it is necessary to carry out the tracer studies to clarify the volume of channels, the rate of fluid travel, and other filtration features of the injected fluid travel in the direction of the producing wells.

Construction of the sector hydrodynamic model

A hydrodynamic model (hereinafter referred to as the HDM) (Fig. 9) with the adaptation of the model indicators to the historical data for the period from 01.05.1989 to 01.01.2020 has been constructed at the Zaburunye field to calculate the forecast indicators. The HDM is a three-dimensional model of black oil (BlackoilModel) with a fund of 73 wells and a number of active cells, equal to 62,822 units.

 

Figure 9. General view of the 3D HDM of the reservoir

 

The average permeability for the entire model is 703 mD, the average porosity is 0.29 unit fraction. Histograms of the permeability distribution (PERMX) and porosity (Poro) are presented in Fig. 10.

 

Figure 10. Distribution of permeability (a) and porosity (b)

 

The HDM has been initialized using the J-function. Irreducible water saturation is taken as the envelope according to the porosity logging data in the pure oil zone. The residual oil saturation is taken based on the displacement coefficients for the analogous fields. An example of the relative phase permeability curves, adopted in the model, is presented in Fig. 11.

 

Figure 11. Phase permeabilities in the oil-water system

 

The three-phase black oil model has been chosen as the PVT model. The results of the study of the reservoir oil, which have been used in the calculations for the correlations of the PVT models, are presented in Fig. 12, a) and b).

 

Figure 12. Dependence of the gas content and formation volume factor on pressure (a) and dependence of the oil viscosity on pressure (b)

 

HDM adaptation

The constructed HDM reproduces the dynamics of the main development indicators at a high level (Fig. 13). The errors in the convergence of the model and historical cumulative oil and liquid indicators are 0.5% and 0.05%, respectively.

 

Figure 13. Results of the adaptation of the main development indicators

 

Due to insufficient information on the reservoir pressure, the quality of the well-to-well adaptation of the bottom hole pressures has been analyzed. A cross-plot of the comparison of the historical and model BHP values with an acceptable error of 20% is shown in Fig. 14.

 

Figure 14. A cross-plot of the actual (history) and model (calculation) BHP indicators

 

The function of the polymer in HDM

As part of the construction of a hydrodynamic model with the injection of a polymer solution, the laboratory studies have been carried out to determine the dependence of viscosity of the polymer solution on concentration, the dependence of viscosity on shear stress, and on the change of the displacement coefficient (the residual oil saturation). The above parameters have been successfully implemented in the hydrodynamic model. The adjustment of the properties of the polymer solution in the HDM, which have not been obtained from the results of the laboratory studies, has been carried out by adapting the dynamics of the polymer concentration yield at a number of producing wells at the PWF section (Fig. 15). Upon the adaptation of the polymer yield history, the quality of the adaptation of the actual water cut to the model water cut has significantly improved during the injection of the polymer solution both for the wells and for the PWF section.

 

Figure 15. Comparison of the polymer yield concentration (actual and model) for the producing wells

 

To test the function of the polymer solution, an option of the model has been reproduced in the HDM, which is identical to the option of the polymer solution injection, except for the injection agent itself. Thus, water has been used instead of the polymer for injecting into 6 disposal wells. The resulting difference in the oil recovery is an indicator of the PWF efficiency in the model (Fig. 16). It should be noted that when water is injected instead of the polymer solution, the adaptation of the water cut and, accordingly, the oil output rates have deteriorated noticeably.

 

Figure 16. Evaluation of the efficiency of the PWF technology in the HDM

 

Description and results of the calculations on the sector model

The constructed sector hydrodynamic model satisfactorily reproduces the dynamics of the main development indicators and on its basis makes it possible to evaluate and analyze the efficiency of the polymer flooding technology with its further optimization.

To determine the strategy for the further implementation of the polymer flooding technology, the calculation has been made on the prepared HDM for several options in the T-navigator software. The 8 development options have been calculated, differing from each other by:

• the agent injected into the disposal wells (water and polymer);
• the concentration of the injected polymer solution;
• the wells, transferred for the water injection.

A description of the calculations of the main forecast development options is presented in Table. 2.

 

Table 2. Description of the forecast options

Option No.	Description (all forecast options start from 2021)
Option 0	Basic option - water injection
Option 1: (Continuation of PWF – 1950 ррм)	Continued injection of the polymer solution in 6 wells without changing the polymer concentration
Option 2: (Forecast – water)	Transition to the water injection at 6 wells. Shutting-in of the polymer solution injection
Option 3: (Water injection into wells No.11 and 55)	Transition to the water injection in 2 wells (No. 11 and 55). Continued injection of the polymer solution in 4 wells without changing the polymer concentration
Option 4: (shutting-in of in wells No. 11 and 55)	Shutting-in of 2 wells (No. 11 and 55). Continued injection of the polymer solution in 4 wells without changing the polymer concentration
Option 5: (Continuation of PWF – 1500 ррм)	Continued injection of the polymer solution in 6 wells with changing the polymer concentration by 1500 ppm
Option 6: (Continuation of PWF – 1000 ррм)	Continued injection of the polymer solution in 6 wells with changing the polymer concentration by 1000 ppm
Option 7: (Water injection into well No.55)	Transition to the water injection in 1 well (No. 55). Continued injection of the polymer solution in 5 wells without changing the polymer concentration
Option 8: (Shutting-in of injection in well No.55)	Shutting-in of an injection in 1 well (No. 55). Continued injection of the polymer solution in 5 wells without changing the polymer concentration

 

To assess the PWF technological efficiency, a base option has been calculated with fixing the last liquid flow rate at the wells since November 2014, and with the water injection (option 0). The obtained indicators of the additional oil production are divided into 2 periods (5 and 14 years) and calculated in relation to the base option 0 (Table 3, Fig. 17). 

 

Table 3. Results of the calculations of the forecast option

Calculation period	2021–2025
Options	0	1	2	3	4	5	6	7	8
Cumulative oil production, thousand tons	254.3	410.9	366.3	395.8	386.1	405.8	399.2	395.4	398.0
Additional production, thousand tons		157	112	142	132	152	145	141	144
Calculation period	2021–2034
Options	0	1	2	3	4	5	6	7	8
Cumulative oil production, thousand tons	585.1	1024.9	776.4	946.5	870.2	991.6	962.2	978.0	957.8
Additional production, thousand tons		440	191	361	285	407	377	393	373

 

Figure 17. Comparison of the technical indicators of the 1 and 2 options of development (red - option 1, blue - option 2).

 

As it is shown by the calculations, in all options of the transition for water injection an insignificant difference in oil production is marked at the initial periods, which may be a consequence of the residual effect (post-effect) and preservation of the screen in the reservoir from the injection of the polymer solution since 2014 at the section of wells No. 11 and 55. When comparing the forecast options of the injection of polymer and water (options 1 and 2) for 5 and 15 years, a noticeable difference in oil production is marked between the amounts of 45 thousand tons and 249 thousand tons, respectively.

At present, the most technologically effective option is to continue injecting the polymer solution in the 6 disposal wells with the current concentration of 1950 ppm (option 1). All additional options of the continuation of applying the PWF technology with various modifications have higher oil production indicators as compared with the full transition to water injection, differing between them by the volumes of polymer injection.

Conclusions and recommendations

1. The PWF Project at the Zaburunye field is of high technological efficiency.
2. The HDM has been constructed for the PWF section of the Zaburunye field, the polymer properties have been successfully introduced, and the model has been adapted to the historical development indicators.
3. The forecast options for the PWF technology implementation have been calculated based on the HDM, according to the results of which the option of the continued injection of the polymer solution into all 6 disposal wells is technologically the most effective.
4. With the purpose to prevent the further breakthrough of the polymer into producing wells No. 100, 101, and 126, it is recommended to consider a possibility of carrying out treatments to block the flushed zones or channels (based on the results of the tracer studies).
5. With the purpose to increase the efficiency of the PWF technology, it is advisable to shut off the unprofitable wells with a high polymer content in their products.
6. With the purpose to control the implementation and improve the efficiency of the PWF technology, it is necessary to carry out the full scope of the research program for the PWF section of the Zaburunye field.
7. With the purpose to optimize the PWF technology, it is recommended to consider the issue of the treatment of the produced waters at the Zaburunye field.
8. The conducted laboratory studies, as well as the further study for the evaluation of the polymer efficiency in the laboratory and the field data will be taken into account when updating the model, thus contributing to the improvement of the quality of the forecast calculations.
9. In the current economic conditions, taking into account constantly varying oil prices, the task is to find the best options for implementing the injection technology (the duration and volumes of the injected polymer, injection design, polymer solution concentration), the correlation between the economic indicators of the injection project and an increase in the oil recovery factor. The fulfillment of such a task requires constant multivariant calculations on the HDM, taking into account the comparison with the actual work results, laboratory and field studies, and forecasts of the macroeconomic indicators.

Авторлар туралы

Marlen Mussayev

KMG Engineering LLP

Хат алмасуға жауапты Автор.
Email: m.mussayev@niikmg.kz

Master of Science, senior engineer of the enhanced oil recovery methods unit of the Integrated Modeling Department

Қазақстан, Nur-Sultan

Darya Musharova

KMG Engineering LLP

Email: d.musharova@niikmg.kz

Master of Science, research worker of the enhanced oil recovery methods unit of the Integrated Modeling Department

Қазақстан, Nur-Sultan

Birzhan Zhappasbayev

KMG Engineering LLP

Email: b.zhappasbayev@niikmg.kz

Doctor Ph.D., research worker of the enhanced oil recovery methods unit of the Integrated Modeling Department

Қазақстан, Nur-Sultan

Yermek Orynbassar

KMG Engineering LLP

Email: y.orynbassar@niikmg.kz

Manager of the enhanced oil recovery methods unit of the Integrated Modeling Department

Қазақстан, Nur-Sultan

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