RU2733969C1 - Method of refined estimation of quality parameters of natural gas during its transportation by gas transport system - Google Patents

Abstract

FIELD: gas industry.

SUBSTANCE: invention can be used in gas industry and relates to pipeline transport of natural gas by large-scale gas supply systems. Essence of the invention consists in the fact that one or several gas quality indicators are measured, the measured values are analyzed and the quality of transported gas is assessed. According to the invention, measurements are taken at all gas measuring stations and gauging stations of the gas transportation system and the whole set of measurements is processed. Gas quality assessment is carried out using the maximum likelihood method, and based on the assessment, actions are taken to arrange gas supply to different consumers with given gas quality indices, as well as the location of gas chemical complexes. Proposed method allows to make decisions on flow maneuvering during on-line control of system. When designing programs for long-term development, the method enables to compare versions of gas transportation by GTS, providing delivery of high-calorie gas to points of expected location of gas-chemical complexes.

EFFECT: higher quality of supplied gas taking into account requirements of each consumer.

1 cl, 3 dwg, 2 tbl

Description

The invention relates to the field of pipeline gas transportation and is aimed at improving the information support of a large gas transmission system (GTS). In principle can be applied to other distributed network piping systems.

The proposed method refers to the cumulative measurements of RMG 29-201 [State system for ensuring the uniformity of measurements. Metrology. Basic terms and definitions, term 4.21] natural gas quality indicators: component concentrations GOST 31369-2008 (ISO 6976: 1995) [Natural gas. Calculation of calorific value, density, relative density and Wobbe number based on component composition], gross calorific value, net calorific value, density, relative density and Wobbe number [GOST 31369-2008 (ISO 6976: 1995)], water dew point temperature GOST R 53763-2009 [Natural combustible gases. Determination of dew point temperature for water] and hydrocarbons.

Known [RU 2580909 C2] a system for compounding high-sulfur oil in several directions of pumping a mixed flow. The invention relates to the compounding of oil in two or more directions of transportation, containing a meter installed in each of the oil pipelines measuring parameters of the flow of oil (flow rate, oil density), as well as means for regulating oil flow The system includes devices for calculating the coefficients of the ratio of oil consumption, oil quality (sulfur content), a plan controller, the outputs of which are connected to control valves. However, the known invention is focused on the compounding of liquids (oils), the mixing processes of which are not non-equilibrium. The invention does not take into account the probabilistic nature of the measurement results.

Known [RU 2248031 C2] control system for the compounding of oils by several quality parameters. The system refers to the means of automation of oil transportation processes through various pipelines with different parameters of oil quality. The system contains computing and regulating devices allowing to control compounding by sulfur content, by oil density, by the content of chloride salts, water, as well as by product consumption. The known system aims to regulate liquid pipelines and does not involve systematic processing of cumulative interrelated quality measurements.

Known inventions [RU 2158437 C1, RU 2270472 C2] related to sequential pumping and compounding of oils. The invention [RU 2158437 C1] relates to automation equipment and is intended for use in pipeline transport when pumping oil from several pipelines into a common pipeline through which the oil mixture is transported to the consumer. EFFECT: maintaining the specified quality indicators of the compounded oil, ensuring pressure control in the pipeline with high-sulfur oil entering the common pipeline, ensuring the accounting of the amount of oil sent to the consumer through the common pipeline, and the amount of high-sulfur oil. The invention [RU 2270472 C2] relates to a means of automating the processes of transporting oil through various pipelines with different oil quality and allows you to control the quality indicators of the mixture when combining flows. These inventions have the same essential differences from the proposed one, as well as those mentioned above [RU 2580909 C2, RU 2248031 C2].

The known method [RU 2615092 C9] for processing natural gas with low heating value. The method involves dehydration and purification of natural gas from impurities, cryogenic splitting in order to extract helium, nitrogen and a wide fraction of hydrocarbons. The method consists in the fact that methane homologues are isolated from the feedstock and sent to the production of marketable products. The remainder is sent for compounding with methane. However, the known method does not imply systemic procedures for processing aggregate interconnected measurements and taking into account the probabilistic nature of the measurement results.

The invention [US 20010007915 A1] consists in the fact that when ethane or propane is added to natural gas, the molecular weight of the mixture and the coefficient of compressibility change, and as a result, the energy required for pumping the mixture through the pipeline is reduced. The efficiency of the method is estimated within the limits of change in temperature and pressure parameters at which the mixture remains in a gaseous state. The known invention should be taken into account when preparing information for calculating the quality indicators of the pumped product for the GTS (calculating the flow rate of the fluid for all gas pipelines of the system).

Method and measuring device [RU 2184367 C2] for determining the composition of a multiphase liquid. The invention relates to a method and a measuring device for determining the composition of a multiphase liquid by passing a photon beam through it. In particular, the invention relates to a method and a meter for determining the composition of a multiphase fluid mixture produced by one or more crude oil production wells, where the crude oil is usually accompanied by some natural gas and / or water. The known invention can be used in addition to the proposed invention to address issues of modernization of the system of metrological support of hydraulic structures in case the proposed invention reveals the feasibility of modernization.

Known telemetry systems related to various fields of technology. For example, the invention [RU 2378509 C1] relates to field geophysics and is associated with a data transmission system from the bottom of oil and gas wells. The main goals of telemetry systems are to improve the accuracy of the information received, to increase the speed of information transfer in real time, etc. However, no telemetry systems have been identified that would be based on the proposed method or would allow obtaining the same results as the proposed method.

The essence of the proposed method is as follows.

GTS transport a multicomponent fluid - natural gas - coming from gas and gas condensate fields, gas processing plants, underground storage facilities, oil fields (associated gas), etc. The components of the fluid include methane, other hydrocarbons - methane homologues - ethane, propane, butane, etc. others, as well as water vapor, carbon dioxide, helium, etc. Gases from different sources differ in their component composition. They are mixed during transportation.

In the operational management of the UGSS of the Russian Federation, it is required to know the quality indicators: the component composition of gas, its density, relative density, Wobbe number, heating value, dew point temperatures for water and hydrocarbons - at each object of the system, primarily at large consumers and at export points.

The basis of the proposed invention is a method (information technology) for the refined determination of quality indicators at each GTS facility (gas pipeline) by processing the entire set of measurements at GTS metering points.

The composition of the gas is characterized by the concentrations of the components r _ij , (i, j) ∈ E. Here and below, G (V, E) is a graph describing the structure of the pipeline system, V is the set of its vertices (nodes) x _k , E is the set of arcs (i , k) the graph [(i, k) is an arc connecting the nodes (x _i , x _k ); x _i , x _k ∈ V]. In accordance with GOST 31369-2008 [Natural gas. Calculation of calorific value, density, relative density and Wobbe number based on composition (ISO 6976: 1995)] concentration of natural gas components, can be set in molar, mass or volume fractions. GOST 31369-2008 contains rules for converting from one unit to another. In this document, it is assumed that the values r _ij , (i, j) ∈ E are mass concentrations.

In accordance with GOST 8.009-84 [State system for ensuring the uniformity of measurements (GSI). Standardized metrological characteristics of measuring instruments] each measurement is considered a random variable, which is the sum of the true (deterministic, but unknown) value of the quality indicator and a random error, for example,

- множество пунктов замера, замеренные концентрации и ошибка замераWhere

- many measuring points, measured concentrations and measurement error

среднеквадратическое отклонение которой σ_ij определяется классом точности измерительного прибора ГОСТ 8.009-84 и ГОСТ 8.401-80 [Государственная система обеспечения единства измерений (ГСИ). Классы точности средств измерений. Общие требования].respectively. The results of aggregate measurements are a set of random variables, which are used to construct point estimates of the quality indicator (s). Evaluation is performed using the maximum likelihood method. In accordance with the theory of errors, the measurement error is considered to be a normally distributed quantity

the standard deviation of which σ _ij is determined by the accuracy class of the measuring device GOST 8.009-84 and GOST 8.401-80 [State system for ensuring the uniformity of measurements (GSI). Accuracy classes of measuring instruments. General requirements].

The maximum likelihood method leads to the problem of minimizing the likelihood function, which is the sum of squares,

with restrictions in the form of equalities and inequalities. Constraints in the form of equalities follow from the conditions of material balance. For the concentrations of each of the components of the two-component fluid, they have the form

Constraints in the form of inequalities for the concentrations of each of the components of a two-component fluid have the form

In ratios (2 - 4) V _joint is the set of butt nodes of the graph G (V, E), Г (x _k ) is the set of vertices into which the arcs outgoing from x _k enter, Г ^-1 (x _k ) is the set of vertices , from which the arcs go into x _k , q _ik - the flow rate of the fluid along the arc (i, k).

Constraints in the form of inequalities follow from the fact that when gas flows in a pipeline, the mixing process of components at each node is nonequilibrium. The non-equilibrium of the processes of mixing of natural gases during flow through large pipeline systems is expressed in the fact that at the joints of pipelines the concentrations of components (as well as other quality indicators) along the outgoing pipelines can be and, as experience shows, are different. Inequalities (4) limiting the concentrations at the mixing nodes are explained by the following physical effect: for several pipelines entering the butt and several outgoing pipelines from the butt joint, the maximum concentration along the outgoing lines cannot exceed the maximum concentration along the incoming lines, and the minimum concentration along the outgoing lines may be less than the minimum concentration on the incoming lines.

The degree of non-equilibrium - the degree of closeness of the component composition of the fluid in the outlet lines to complete mixing - depends on many factors, first of all, the flow rates and the local structure of the pipeline system in the mixing unit. FIG. 1, for example, a schematic flow diagram of pipelines in a butt joint of a GTS is shown.

To determine the desired concentrations, a nonlinear programming problem is obtained (2-4). The computational complexity of the problem depends on the structure of the GTS and the initial data. In some cases, the problem could be solved using standard software and computing systems, in other cases its solution requires the development of special algorithmic and software.

To assess such indicators of the quality of natural gas as density, calorific value, dew point temperature for water and hydrocarbons, the same mathematical model (2-4) is used with the replacement of designations: instead of the concentrations of the components, an appropriate quality indicator should be put.

The main advantage of the proposed method is that the simultaneous values of all aggregate measurements of the indicator (indicators) of the quality of natural gas (density, concentrations, calorific value, dew point temperature for water and hydrocarbons) are taken into account in interrelation. The assessment is made by a scientifically sound method based on aggregate measurements of interrelated quantities.

The proposed method has the following advantages:

• Suitable for any composition of the metrological support of the GTS;

• Allows to evaluate the effectiveness of modernization of the system of metrological assurance [the validity of assessments increases with the improvement of metrological assurance: an increase in the number of measuring points and an increase in the accuracy (reliability) of measuring instruments (GOST 8.009-84)];

• Allows you to identify those fragments of the GTS, where taking into account the interrelation of measurements does not lead to a more accurate assessment (due to the insufficient number of devices on the fragment);

• Equally suitable for tasks related to the calculation of any indicator of the quality of natural gas (density, concentration, calorific value, dew point temperatures for water and hydrocarbons);

• Allows generalization: the transition from one-time aggregate measurements to the history of the indicator change;

• Can be (with appropriate modification) used to solve the problems of compounding oils.

Example

Construction of a design scheme and calculation of the distribution of ethane concentrations over the gas pipeline system using the proposed method.

Let us estimate the concentration of the two-component fluid using the GTS shown in Fig. 2. For simplicity of presentation, let's call the components of the fluid methane and ethane. The example also shows how to move from the technological scheme to the design scheme of the proposed method. The example is illustrative.

концентраций компонент флюида, %, приведены в Таблице 1.Costs q _i and measurements

concentrations of fluid components,%, are given in Table 1.

Construction of the design scheme. FIG. 2 and table 1 show the actual (operational) data on costs (all costs in the example in mln.m ³ / day). Balance ratios are not accurate. The imbalance in the system as a whole is equal to 0.4 (the total daily gas supply is 57.1, and the total outflow is 56.7). An imbalance of inflows and outflows is also observed in the compressor shops (CC) of the compressor station (CS). The inflow to the inter-shop bulkhead from CC 1 (Fig. 2) is 26.5, and the inflow from the bulkhead to CC 2 is 26.1. The imbalance of 0.4 is the same as for the system as a whole; it can be explained by technological losses and consumption for own needs.

The technological features of the system make it possible to make some simplifications to the scheme. The loading of the gas pipeline approaching CC 1 from the southeast is very low.

The gas pipeline operates mainly in reverse mode with a flow rate of 0.1. The loading of this gas pipeline is incomparably less than the loading of other gas pipelines of the fragment, therefore, without prejudice to the accuracy of the calculation, the gas pipeline can be removed from the technological scheme.

The flow rate through the inter-shop bulkhead KTs 1 - KTs 2 is significant, but its length is much less than the lengths of the other pipelines in the scheme. In addition, there is no concentration measurement point on it, therefore, in the developed design scheme, it is possible to combine the shops of KC 1 and KC 2. The flow rate for gas pipeline 5 (KC 1 - GIS 2 - GIS 1) is significantly less (by about 25 times) the costs in other areas pumping. If you do not take into account the flow through this gas pipeline in the technological scheme, then the calculation results will change insignificantly. Taking these simplifications, we obtain a schematic diagram of the system shown in Fig. 3.

FIG. 2 shows 7 points for measuring the composition of the gas. On each of directions 4 and 5 there are 2 points. In principle, their readings should duplicate each other. The concentration values assigned in Table 1 to directions 4 and 5 are the average for each of these directions.

Table 1 presents the initial data on the concentrations (in%) of all three components of the fluid: methane + ethane + propane and the following high molecular weight homologues of methane. Moreover, the third "component" is comparable in mass concentrations with the second, for example, for direction 1, the component composition is 92.71 + 3.68 + 3.61. The transition from a three-component formulation to a two-component formulation allows a quantitative demonstration of the effectiveness of the invention. This transition can be done in two ways:

• option 1: methane + ethane (component 1) and propane and the following high molecular weight homologues of methane (component 2);

• Option 2: methane (component 1) and high molecular weight homologues of methane (component 2).

These options are presented in Table 1 in columns 7 and 8, respectively. The optimality criterion in the considered example has the form

замеренное значение концентрации 2-й компоненты и дисперсия ошибки на i-м направлении перекачки. Дисперсии ошибок на всех замерных пунктах считаются одинаковыми

Here

measured value of the concentration of the 2nd component and the variance of the error in the i-th pumping direction. The variances of errors at all measuring points are considered the same.

To determine the unknown values r ₁ ,…, r _4, we have a quadratic programming problem. Using the equality constraint, we reduce the calculation to the unconstrained minimization problem.

Calculation of the distribution of ethane concentrations over the gas pipeline system using the proposed method

The numerical solution of the problem is presented in Table 2.

по предлагаемому способу существенно отличается от замеренных значений

The table shows that the concentration estimate

according to the proposed method differs significantly from the measured values

Thus, the proposed method makes it possible to estimate each concentration value not by a single measurement, but to use all cumulative measurements, and obtain refined estimates of concentrations. Discrepancies between measured and calculated values are significant and reach 16%. The scatter in the results is explained by systematic measurement errors. The largest discrepancies are obtained for the values of r ₂ and r ₄ , which indicates that the highest probability of systematic errors on GIS 5, GIS 3 and GIS 4. Detection of the very fact of the presence of systematic errors is one of the advantages of the proposed method.

Claims (1)

A method for the refined assessment of the quality indicators of natural gas during its transportation through the gas transmission system, which consists in measuring one or more gas quality indicators, analyzing the measured indicators and assessing the quality of the transported gas, characterized in that measurements are made at all gas metering stations and metering points of the gas transmission system, the entire set of measurements is processed, while the gas quality is assessed using the maximum likelihood method, and on the basis of the assessment, measures are justified for organizing gas supply to various consumers with specified gas quality indicators, and the location of gas chemical complexes is also selected.

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