APPROVAL OF OPTIMAL PIPELINE’S CLEANING METHODS ACCORDING TO MULTIPHASE FLOW PATTERNS

The object of research is gathering and transmission system of gas and gas condensate fields.One of the major problem areas is the lack of an integrated approach and justification for choosing the optimal for pipeline cleaning methods (removing deposit) from the inner cavity of pipelines that form these systems. This leads to inefficient use of pipeline pigging equipment and significant operating costs without visible economic benefits. Based on analysis of gas-condensate life cycles, it is established that different stages of the field development are characterized by a proper gas-liquid flows pattern.During the research, the relief characteristics of the flow and trunk lines, which transmit the gas-liquid flow with different gas contents, are investigated. Based on the analysis of the criteria characterizing the flow pattern, mathematical models of overall pressure drop on the rising and declining sections are presented. The determined pressure differences are formed according to different structures of motion of gas-liquid flows under the influence of hydraulic resistance of each of the studied sections.The estimation of the hydraulic state of the system transporting a multiphase flow, based on a comparison of the actual measured pressure drop and its calculated value, is presented. By experimental researches it is established that the most dangerous factor in the operation of such systems is the possibility of slug moving when changing the baric regime of operation.In order to increase the reliability and efficiency of pipelines operation, an algorithm for determining the structural form of motion and actual hydraulic state of the gathering and transmission system of different types of field is proposed. And a sequence of choosing the optimal method for pipeline cleaning is developed.The proposed algorithm for choosing the best ways to drain liquid from the pipeline cavity will provide an opportunity to discard deliberately inefficient methods, as a result will save time and money for the Company.


Introduction
The gasproducing complex contains in its composition a complex pipeline system, which performs two functionscollection and interindustrial gas transportation. The gas pipeline system receives and transports its own production gas, contains loops industrial pipelines that feed gas to gas preparation unit (GPU), booster compressor stations (BCS) or linear compressor stations (CS). The type of the tech nical characteristics, as well as the performance of the compressor equipment and pipelines depends on the capac ity value of the system as a whole. Thus, the capacity of the gas collection and gas transmission system depends both on the operating modes of the ground infrastructure facilities and on each of the linear sections.
The throughput capacity of a gas pipeline system as a function of mode parameters is the main production indicator characterizing the efficiency of using gas pipe lines for their intended purpose. The value of the capacity of the system is most affected by the technical charac teristics of pipelines and their pressure conditions. If to consider a separate gas pipeline with a certain technical characteristic, then its pressure regime and, accordingly, its throughput capacity are closely related to the operating conditions. In particular, the presence of liquid condensation conditions, plugging, an increase in the hydraulic resis tance of the sections, and, accordingly, the total pressure drop in the gas pipeline as a whole. Accordingly, exces sive pressure drop affects either the level of the initial pressure, reducing gas production at the final stage of development in the gas mode, or the value of the final pressure, increasing the BCS energy consumption for gas transportation. Reducing the pressure drop occurs with a corresponding increase in throughput of gas pipelines, which is achieved by introducing methods of freeing them from liquids, such as cleaning with pistons, switching to selfcleaning mode, tapping drainage devices and drips of various designs.
Depending on the stage of field development, various flow structures form in the cavity of pipelines, respectively, forming various hydraulic supports [1]. So, the methods for clearing the cavity of gas pipelines from these con taminations will be different, and a separate algorithm must be created for their selection.
Accordingly, given the fact that the overwhelming ma jority of the fields in the oil and gas production sector of Ukraine are at the final stage of development, which is characterized by a high gasproducing factor, the question TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 1/2(45), 2019 ISSN 2226-3780 of minimizing pressure losses in the system for collecting and transporting hydrocarbons remains.

The object of research and its technological audit
The object of research is the system for the collection and interindustrial transportation of gas from gas and gas condensate fields.
Today, Ukraine belongs to the countries with a deve loped oil and gas industry and oil and gas transportation industry. The first step in the development of the industry was made a century and a half ago, when the lighting of the streets of one of the cities of Ukraine -Lvivartificial, produced from coal by gas.
Considering the large branching of the system for the extraction and transportation of hydrocarbon products, as well as the remoteness of industrial facilities from each other, the length of the system for extracting and trans porting oil and gas products is about 12 thousand km.
At this stage of operation of the oil and gas sector of Ukraine, most of the fields are operated at the final stage of development, characterized by a significant decrease in reservoir pressure, low well rates, high content of reservoir water and hydrocarbon condensate in reservoir products.
Therefore, taking into account the existing technical condition of the system, as well as the age factor of oil and gas fields under operating conditions, the development and selection of optimal methods for cleaning pipelines for various structures of multiphase flows is a problematic issue.

The aim and objectives of research
The aim of research is determination of a wide range of similarity criteria, which describe the movement of multi phase flow in pipelines of the gascollecting and product collecting systems with a wide range of content of natural and associated gas in the mixture.
To achieve this aim it is necessary to perform the fol lowing objectives: 1. To investigate sections of pipelines on which structural flows follow each other in length and lead to a constant transition of one form to another.
2. To develop an algorithm for determining the structural form of gasliquid flow, assess its type and uniformity, as well as determine the parameters of the hydraulic state.
3. To develop an algorithm for choosing the best ways to drain fluid from the cavity of the pipeline according to their principle of operation.

Research of existing solutions of the problem
Any gas pipeline oil and gas system can be considered as a transported gasliquid flow with different gas content in the flow structure. That is why in world practice these structures are modeled in separate simulation programs, depending on how the flow is considered.
Schlumberger's PipeSim simulator is used for analytical studies, such as well modeling, optimization of mechanized production, modeling of pipelines and process equipment [2].
Weatherford's WellFlo software is a standalone appli cation for designing, modeling, optimizing, and trouble shooting oil and gas wells operating in both the fountain and mechanized mode. This product allows the engineer to build models of wells and pipelines using a convenient user interface. Models constructed in this way accurately reflect the flow of any type of fluid from the reservoir, as well as the flow in tubing (tubing) and land lines [3].
Another of the software tools in which it is possible to create integrated models from the field to the preparation system is Petex of the Petroleum Experts company [4].
It is also like to mention the software simulator of the unsteady multiphase flow OLGA from Schlumberger. The dynamic multiphase simulator OLGA allows to cal culate changes in flow parameters in wells and pipelines as a function of time, that is, to simulate stationary and transient flow regimes [5].
In national practice, for modeling flows, the provisions for calculating hydraulic parameters set forth in [6] are supplemented by methods of the Ukrainian Research In stitute of Natural Gases (UkrNDIGas) [7]. These methods are based on the treatment of theories [8,9] to determine the boundary between liquid accumulations in lower sec tions of the pipeline.
In fact, the use of these methods allows to determine the amount of contamination in the cavity of the pipeline, as, for example, proposed in scientific work [10]. Or to monitor the work of a number of pipelines, form the gas collection system of fields, as provided for by regulatory documents like [11].
To date, much attention is paid to the study of two phase flow. Models of gas and liquid movement in pipes have been developed, new methods for determining the amount of liquid in a gas pipeline cavity and methods for extracting liquid from a gas pipeline have been created, and devices for removing liquid have been modernized. One of such methods is the method of creating a pulsed mode of the working gas flow [12].
Therefore, solving the problem of analyzing the operating modes of pipelines in the oil and gas sector, identifying problem areas from the point of view of the deterioration of hydraulic efficiency, as well as justifying the feasibility of implementing measures to clean gas pipelines are promi sing issues.
When solving the problem of cleaning the pipeline, it is necessary to find out the causes of the ingress of fluid and the amount. This will provide an opportunity to monitor any changes in the process of operation and make a timely decision on the time and method of cleaning. It should also be noted that the amount of pollution in gas pipelines, calculated theoretically, differs from a certain experimental one. Therefore, this problem requires detailed study.
In addition, attention should be paid to the differences in the approach to cleaning the internal cavity of the field pipelines. In accordance with the requirements of regulatory documents, the decision on cleaning this type of pipelines is made solely on the basis of an internal pipe inspection [13], which is virtually impossible to do in Ukrainian specific conditions, which are described in detail in [14].
However, it should be noted that in any conditions, a multiphase medium will form in the pipeline cavity. This medium is considered to be relatively stationary under the operating conditions of mature fields, or it constantly changes its shape when it is localized in low places under the condition that the thermobaric mode of operation is changed [15]. Although, on the other hand, the problem of the behavior of multiphase media under conditions TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 1/2(45), 2019 ISSN 2226-3780 of changing temperature and pressure conditions of the pipeline is mainly highlighted for oil pipelines and col lector threads collecting oil, considering the possibility of formation of both paraffin deposits [16] and tar [17,18].
The processes of fallout and the formation of fluid accumulations in gas collection systems are more specific. Such pollutants are more mobile when the main pol lutant is gas condensate, and more resistant to localization at the final stage of field development, when they will be formed exclusively from water fractions only with traces of condensate. In any cases, experts recommend a com prehensive survey of pipeline sections where fluid accu mulation is possible [19].
In [20] it is presented that to prevent the accumulation of liquid contaminants in the cavity of the pipeline in the ascending sections of pipelines, measures were taken to replace the plumes of large diameters with smaller ones. Accordingly, these measures were carried out with the aim of ensuring the minimum necessary gas velocities to ensure the removal of fluid in the complex gas treatment units (CGTU).
Accordingly, it remains an open question how to choose the best method for removing the formed amount of pollu tion, because according to the stage of field development, the effectiveness of their implementation will be different. First of all, it is necessary to evaluate the structure of the flow, the homogeneity of the liquid formation in the pipeline cavity (define its structure as a mobile dispersed or plug or existing homogeneous mass, which is in a state of relative rest). And at the last stage, choose the most optimal method for draining the liquid from the cavity of the pipeline.

Methods of research
To develop an algorithm for determining the structure of a homogeneous flow and the flow of homogeneous flows, it is necessary to perform a series of successive steps: -to draw up a profile of the pipeline route profile; -to determine the equivalent ascending and descen ding sections of the route and carry out their hydraulic calculation.
The longitudinal planprofile of the pipeline route is designed to determine the effect of the terrain on the formation of zones of increased hydraulic resistance to the movement of a liquidgas stream (localization of water, paraffins, mechanical impurities) and zones of formation of gas caps. These processes increase the rate of pressure change in the pipeline under conditions of a constant flow of fluid in its cavity as a result of mechanical removal from the wellhead or separation equipment [21].
The development of technology now makes it possible to use simple, publicly available software products for the compilation of longitudinal planprofiles of the route, such as Google Earth. These software systems provide only a binding to the terrain of the pipeline route with auto matic profile construction.
The ascending segment of the equivalent route is con sidered to be such a segment between the crossing points of the real profile, along which, regardless of the number and slope of the intermediate sections, the mixture moves only upwards.
The length and the first equivalent ascending section are taken as the total length of the ascending sections of the real route between two crossing points ( Fig. 1). Such an algorithm is completely based on the provisions of [6] and is described in detail in [21].
where l kasc -the length of the kth section, which is included in the ascending section between the crossing points, m. For the angle of inclination of the ascending equivalent area, the average angle of inclination is taken, which is determined by the condition: where H H s e − -marks the starting and ending points of the ascending section, m.
In fact, the use of such an algorithm significantly in creases the time required for the project, since the number of ascending and descending sections, according to the length of the investigated gas pipeline, can reach hundreds or more. This ultimately leads to the need to process large amounts of data for weeks. The use of the above software systems reduces the time required for processing to several hours.
In the future, after determining the geometry of an elongated gas pipeline in space, an assessment of the structural flow of the work should be carried out, which involves the determination of criteria describing the flow structure in dimensionless quantities. TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 1/2(45), 2019 ISSN 2226-3780 6. Research results 6.1. Experimental studies. Conventionally, a multiphase mixture is at least two components: liquid and gas rise from the bottom of the well and move to the horizontal component of the gas production system. Depending on the type of a deposit, liquids may differ in density and visco sity, forming both emulsion formations and separate isolated structures during transportation. In addition, suspensions of the compound may appear in the gasliquid stream as a result of mixing the liquid with solid impurities (clay de posits, sandstones, drilling mud residues, propping agent, etc.).
Conventionally, in the life cycle of a field, there are three main stages, as shown in Fig. 2: 1) early (high pressures, high speeds, leading to dis persion of one type of fluid in another); 2) stage of stabilization (intensification) of production, characterized by the formation of plug structures moving between parts of the system; 3) late, at which the drop in production leads to the separation of the main fluids and the formation of liquid accumulations in the lower generatrix and gas caps in the transfer points of the system. In fact, the main task of the algorithm is to determine what type of structural flow the researcher is dealing with, and also by the quantitative content of a liquid in a gas or gas in a liquid, to determine the uniformity of flow.
The definition of the transfer mode of the liquidgas mixture on the ascending pipeline is performed in the fol lowing sequence [22]: а) V * value is determined: where µ µ µ = 2 1 -reduced viscosity; µ 1 , µ 2 -dynamic viscosity of the liquid and gas phase, respectively, Pa⋅s; where Re 1 -the Reynolds number, which describes the mode of motion of the fluid in the mixture.
The value φ 1 for the descending plug flow is deter mined by the formula: where K -coefficient taking into account the effect of viscosity of the liquid. The actual coefficient of hydraulic resistance in the ring mode is determined by the formula: where λ -coefficient of hydraulic resistance when a ho mogeneous fluid flows.
where y -the combined coefficient of hydraulic resistance.
In case of a plugflow mode, the actual hydraulic re sistance coefficient is determined by the formula: ; β β K is determined by the formulas (11), (12).
In the stratified flow regime, the hydraulic pressure drop for the descending section is determined by the formula [24]: Hydraulic diameter: where Q 2 -the volumetric gas flow rate, m 3 /s; q -half of the central angle to the liquid segment, rad (Fig. 3). The actual fluid content is determined by the formula: φ ρ ρ ρ β a

Re
where Re H µ q -the Reynolds number, which describes the gas flow regime for the hydraulic diameter. Since, by their nature, the flow movement structure can't be expanded in ascending sections, the determina tion of the transfer mode of the liquidgas mixture in the ascending section of the pipeline is based on the deter mination of (5) V * : -at V * ≤ 1 -there is a ring flow of the mixture; -at V * ≤ 1 -there is a plug mode of the mixture flow is realized. Hydraulic calculation of the ascending pipeline section is made according to the formula: The value φ 1 for the ascending ring flow is determined by the formula: The value of K is determined by formulas (11) and (12).
With an ascending ring flow of the mixture, the value λ c is determined by formulas (11)- (13). With an ascending plug flow, the value λ c is determined by formula (14).
6.2. Analysis of operating modes of gas pipelines. The operating modes of the gas production system determine the dynamism of its operating modes, which lead to the movement of liquid formations in the cavity of pipelines, which is especially characteristic of the final stage of field development. For the evaluation, a system of interfield transportation of products between the installations of the fields at the final stage of operation was chosen. In fact, as can be seen from Table 1 based on the processing of the actual operation mode of gas condensate lines with significant gas content in the flow, the structural cur rents follow each other in length and lead to a constant transition of one form to another.
As a result, this leads to an unpredictable rapid release of liquid, because in order to prevent this phenomenon, it is necessary to introduce measures to divert water from the cavity of pipelines. It is possible to estimate the volume of this instantaneous surge due to the movement of liquid plugs under conditions of a decrease in the operating pres sure in the system between industrial pipelines or changes in other operating conditions ( Table 2).  The time required for this plug to be moved by the gas flow from the lowered position of the pipeline to the entrance to the separation equipment is presented in Table 2. For its eva luation, the following calculated data are used: -the maximum speed is formed in the pipeline when the operating pressure drops to 12 at -28 m/s; -the maximum speed is formed in the pipeline when operating at a pressure of 24 at -4 m/s; -the average speed formed in the pipeline when the operating pres sure decreases to 12 at -16 m/s. As can be seen from the Table 2, the expected time of fluid intake will begin 4 minutes after the working pressure is reduced and will end after 30 minutes, during which 15 m 3 of water will be picked up in the sepa rators. If the volume of such a salvo emission is significantly larger than the design of the input separator, this will lead to an emergency stop of the equipment.
Mostly salvo emissions of the li quid are provoked by a sharp change in the pressure mode of operation and can be monitored with a pressure drop in the pipeline, namely by com paring its optimal (nominal) values and actual parameters.
The actual values of pressure and temperature at the initial, final and intermediate points of the pipeline section are simultaneously selected according to the indications of questionnaires or measurement data. Chromatographic analysis allows to remove the gascondensate characteristic in the case of continuous polling with flow meters [26].
For each specific section of the pipeline, in accordance with its orientation in space, according to the above al gorithm, the structure (type) of the liquidgas flow is selected and the corresponding parameters of the hydraulic state are calculated. The actual hydraulic condition of the ascending and descending sections of pipelines is evaluated according to the actual coefficient of hydraulic resistance, which is calculated in accordance with the structure of the liquidgas flow according to an algorithm.
The actual and nominal pressure loss for the present hydraulic condition of the pipeline by adding them to all the studied areas is calculated: Using the values of the initial working pressure and the calculated pressure loss, the calculated final pressure is determined and compared with the measured values ac cording to questionnaires or instrument measurements. If the value of the calculated final pressure is higher than the measured one, the formation of contaminants is recorded at lower points of the pipeline route.
An example of the calculation is given for the existing section of the pipeline pumping gas production between two installations. The results of the comparison of the measured value of the final pressure and its calculated value are summarized in Table 3.
As can be seen from the Tables 1, 3, the presence of various patterns of movement of the gasliquid mixture in the cavity of the pipeline leads to a volatile redistribution of fluid at a certain point in time due to a change in the pressure mode of operation. This necessitates the use of various methods of removal of pollution, and for certain pipelines requires an integrated approach to solving the issue of removal of pollution from the cavity of pipelines.
6.3. The construction of the algorithm for selecting the optimal method of removal of contaminants from the pipeline cavity. From the choice of the method of removal of fluid with the presented algorithm, the hydraulic state of the collection systems and interindustrial transporta tion of products with different gas content with a total length of about 12.0 thousand km were analyzed, which were divided into three groups: 1) for deposits at an early stage of operation -2 % of the total; 2) for deposits with stabilized (or smoothly falling) production -24 % of the total; 3) for deposits at the final stage of operation -74 % of the total.
It is established that the choice of the optimal method of cleaning the internal cavity of the pipeline depends on the following main factors: -structural form of the mixture movement, during pumping of which pollution is formed; -true gas content, which determines the homogeneity of the gas or liquid flow and determines the type of pipeline with which the researcher is dealing; -hydraulic condition, necessitates the removal of fluid. Based on these calculation results, the algorithm for selecting the optimal method for cleaning the pipeline cavity from accumulated pollution results in the following sequence presented in Table 4.
In fact, for the same pipeline, it is possible to apply various methods of drainage, eliminating the obviously in effective, using the data of Table 4 and an algorithm for evaluating the structural forms of motion and the hydraulic state of pipelines pumping a multiphase medium is presented.
The choice of the optimal measure should be based on the assessment of the influence of the three factors listed above, on the basis of its action and based on a mathe matical model of the change in the structural form of the gasliquid mixture or pollution, which is confirmed by experimental studies.
As can be seen from the presented analysis, the most optimal method of cleaning the gas pipelines of the col lection systems and the interindustrial transportation of natural and petroleum gas is the transmission of pistons, since they can be used for any periods of field develop ment and the corresponding structural flow streams. How ever, refining pistons have significant limitations in the use of mature gas fields for gas collection systems, since these systems are equipped with noncorrugated fittings, and the condition of the internal surface of pipelines is eroded and worn. For depleted fields, the optimal point is the selection of liquids, which requires the installation of a large number of traps [14].
The optimal solution is the development and applica tion of new designs of cleaning pistons. One of which is elastic models of pistons, which can undergo both local narrowing and local resistance in the form of noncurved fittings. Table 4 Algorithm for choosing the best ways to drain fluid from the pipeline cavity 1 Definition of flow patterns 1 Stratified with a smooth surface and the formation of waves when changing the technological operation mode 2 Plug typical for ascending areas or salvo emissions of liquid when changing operation modes 3 Ring typical for plumes of flowing wells or gas pipelines with full load 2 Determination of structure homogeneity (phase mode and main phase) by true gas content 1 gas content → 1: gas pipeline with liquid in low places

SWOT analysis of research results
Strengths. Conducting periodic studies of the state of industrial pipelines excludes additional costs for the com pany to use inefficient methods for cleaning the system.
As a result of research to determine the structure of the form of movement and the actual hydraulic state of industrial pipelines pumping a multiphase medium, will allow to determine the sequence of choosing the best ways to drain fluid from the cavity of the pipeline in accor dance with the principle of their action. Such a sequence will make it possible to discard the obviously ineffective methods for one or another type of industrial pipelines from further analysis.
Weaknesses. The research results are somewhat diffe rent from the actual data.
Opportunities. The need to create composite materials that will be able to keep the shape, moving in the form of a tube through the cavity of the pipeline.
Threats. The proposed algorithms require further re search.
It has been determined that the best method of clea ning between industrial gas pipelines and well plumes is the transmission of cleaning pistons, the use of which, however, is limited for gas gathering systems of mature fields. Improvement of the design of such cleaning pistons is possible only due to a significant increase in their elas ticity, which requires the creation of a composite material that would keep the shape moving in the form of a plug through the pipeline cavity with the optimal transit time between the initial and final points.

Conclusions
1. A study of the interfield transportation of products between the installations of fields at the final stage of operation is carried out. The regularity of the flow struc ture transition in the stratified sections of the pipeline is established. It is also noted that when the flow went to the ascending section, the flow structure changed to wave or cork. It is noted that during the transition from the stratified flow into the wave or cork structure of flow, a slight decrease in the hydraulic resistance coefficient is observed in the investigated area. It is established that 85 % of the pipelines of the oil and gas extraction system of mature fields transport raw materials in the form of a stratified structure.
2. An algorithm for determining the structural form of the gasliquid flow is presented. An assessment of its type and homogeneity is carried out, the parameters of the hydraulic state (nominal and actual pressure losses during system operation) are determined. It is determined which processes cause different values of the final pressure in the studied pipeline sections. The proposed algorithm allows to clearly define the structure of the flow at a particular point in the system. Having the data calculated by the presented algorithm, it is possible to localize accumula tions of pollution, as well as to prevent salvo emissions on separation equipment.
3. Based on the algorithm for determining the struc tural forms of the movement and the actual studies of the hydraulic condition of industrial pipelines, oil and gas production systems of various types of fields are formed, and a sequence is developed to select the optimal me thods for removing fluid from the pipeline cavity. Such a sequence makes it possible to discard the obviously ineffective methods for one or another type of industrial pipelines from further analysis.
It is established that the choice of the optimal mea sure should be based on the assessment of the influence of such factors as are presented below: -structural form of the mixture movement, during pumping of which pollution is formed; -true gas content, which determines the homogeneity of the gas or liquid flow and determines the type of pipeline with which the researcher is dealing; -hydraulic condition, which necessitates the removal of fluid.