Improving the SWAG-technology with the use of pump-ejector systems

. A set of technical solutions for improving the SWAG-technology using pump-ejector systems is proposed. To effectively influence the reservoir, it is necessary to implement the injection of a water-gas mixture into wells that open reservoir sections with the largest residual reserves (into wells with maximum reserves – a mixture with the highest gas content from the range of rational values). In this case, associated gas, nitrogen, can be used as a source of the gas phase. A description of the pump-ejector system for the use of associated gas from the annulus of production wells is also given. For the efficient operation of the system, it is necessary to select the elements of the system that will be operated with maximum efficiency under field conditions. The results of experimental studies of ejectors are presented: the efficiency of the ejector is directly related to the mineralization of the working fluid, as well as the inlet pressure.


Introduction
Low oil recovery, high water cut of many fields, commissioning of fields of hard-to-recover reserves, as well as damage caused to the environment when associated gas is flared at fields, determine the relevance of research on methods for increasing oil recovery, as well as methods for their implementation.SWAG-technology with the use of pump-ejector systems (PES) is a promising method for increasing oil recovery, which can be implemented in the development of deposits of various geological and geophysical structures both at the facilities being put into development and during the development of residual reserves.

Research methods
An analysis of the literature data showed that there are several technical solutions for obtaining and injecting water-gas mixtures into the reservoir: compressor, boostercompressor, pump-booster, ejector, pump-ejector systems.In [1][2] it is shown that the latter have clear advantages.However, the layouts used earlier do not allow developing sufficient discharge pressures, which led to the separation of the injected mixture and, therefore, significantly reduced the effect of SWAG introduction.Therefore, it is recommended to supply the PES with an additional booster pump.Also, when designing the PES, the discharge pressure of the mixture is calculated, which should be at the outlet of the PES.The booster pump, which pumps water into the ejector nozzle, and the booster pump, pumping the water-gas mixture, must have frequency-controlled drives.
The methods used in the performance of the work included bench studies of the characteristics of liquid-gas ejectors when simulating operating conditions, field studies of the pump-ejector system with subsequent processing of the results and analytical interpretation, calculations of the characteristics of jet devices and pump-ejector systems.In the known methods for the implementation of SWAG there are no recommendations indicating the rational composition of water-gas mixtures.In this regard, a series of experimental studies was also carried out in order to study the effect of various mixture compositions on the efficiency of SWAG.

The discussion of the results
The data of filtration studies of cores of various commercial objects showed that when residual reserves are displaced by a water-gas mixture, the maximum oil recovery factor can be achieved when the gas content of the mixture in reservoir conditions tends to values of 13-20%.For the introduction of SWAG, it is recommended to carry out filtration studies to determine the area of rational gas content for the conditions of a particular field [3].
The study of the influence of mineralization of the working fluid on the properties of water-gas mixtures made it possible to establish that the addition of certain electrolytes helps to suppress the coalescence of gas bubbles [4].It has been found that for each critical concentration (electrolyte concentration at which the effect of coalescence suppression ceases), there are values of the limiting gas flow rate.The presence of a limiting concentration shows that there is a region of rational mineralization, within which, with an increase in mineralization, the effect of coalescence suppression increases.The suppression of coalescence is facilitated by an increase in pressure in the system.The importance of studying the behavior of gas bubbles in a liquid under the influence of a complex of external factors is explained as follows: injection of a water-gas mixture prevents the formation of gas locks in the deposit.
With an increase in the mineralization of the working fluid with the addition of electrolytes (which contribute to the suppression of coalescence), the pressure-energy characteristics of the liquid-gas ejector, the fundamental constituent element of the PES, are improved [5].This phenomenon is explained by the improvement of energy exchange between the working and passive flows in the mixing chamber of the ejector.Also, with a decrease in the gas content β in at the inlet to the booster pump, the pressure P n developed by the pump increases [6].However, in cases where it is impossible to use mineralized water, it is recommended to select the composition and concentration of surfactants, which will increase the pressure developed by the booster pump and effectively affect residual reserves [7].
An urgent problem is the accumulation of APG in the annulus of production wells [8].The PES, which contains lines for supplying gas from the annulus of production wells to the ejector, allows the use of petroleum gas from the annulus for injection into the reservoir.Gas is also separated from well production and fed into the ejector for mixing with water.To control the gas content, the system includes frequency-controlled drives.With an increase in frequency, the supply and pressure developed by the booster pump increase, which leads to a decrease in the pressure at the outlet of the ejector and an increase in its supply.In order to increase the efficiency of the system, the possibility of increasing the efficiency of the ejector with a change in regime parameters was studied.Due to the regulation of the liquid injection pressure, it was possible to achieve an ejector efficiency of 48.2% -the highest value to date (Figure 1).In this case, the increase in pressure expands the working area of the ejector and the system as a whole [9].Because SWAG is a method of developing hard-to-recover reserves, it is necessary to take into account the geological structure of the deposit.Anisotropy in porosity and permeability, the presence of disjunctive disturbances leads to uneven distribution of reserves over the rock [10][11][12].The use of the device, which in its essence represents several PESs, makes it possible to rationally implement the injection of the mixture.The device allows to obtain a water-gas mixture containing low-pressure APG, then the mixture is sent to the separator, part of the water and the separated gas are sent to the booster PES, which creates a water-gas mixture with an increased gas content.The mixture is supposed to be injected into wells that open reservoir zones with the largest residual reserves.At the same time, water is injected into other wells.This solution is especially relevant in cases where obtaining a larger amount of a mixture with the required gas content (for injection into a larger number of wells) is technically impossible or economically unprofitable.
For the most effective SWAG, a method is needed that will allow taking into account the consumption of the injected agent in each well.At the Samodurovskoye field, it was proposed to use Ultraflow multiphase ultrasonic flowmeters, but due to the high cost of the devices, the idea could not be implemented.Therefore, it is recommended to supply pumpejector systems with diaphragms -devices for controlling the flow rate of the injected agent into each of the injection wells.The devices are located according to fig. 2 in such a way that the flow rate of the mixture pumped into each of the selected wells is measured separately for each well.

Conclusions
A new technology has been developed for carrying out SWAG-technology of the reservoir when pumping out annular gas using a pump-ejector system, which has better functionality, a wider range of applications compared to previous solutions, and makes it possible to increase the efficiency of the oil production process, increase oil recovery and ensure higher flow rates wells.
Bench studies confirmed the possibility of implementing the proposed pump-ejector system for SWAG-technology using the annulus gas of production wells in a wide range of working fluid pressure values.During experimental studies, the value of the maximum efficiency of the ejector was 48.2%, which is the highest figure to date.
The proposed set of solutions contributes to the active implementation of this technology in the fields and the expansion of its scope.The use of associated gas makes it possible to drastically reduce the volume of its irrational use, reduce the carbon footprint, increase oil recovery and the profits of developing companies.

Fig. 1 .
Fig. 1.Energy characteristics of the ejector (dependence of the efficiency η on the gas supply at the inlet Q g.in with a relative length of the mixing chamber.component 30.6 diameter, and various operating pressures P op in front of the nozzle.