INTRODUCTORY PARAGRAPH Collection and preparation of well products for further transportation is the final stage of gas and condensate production. Modern gas enterprises (GE) collect and prepare well products for further transportation at complex gas treatment plants (CGTP) and gas treatment points (GTP), which include: well product collection system, booster compressor stations (BCS), air-cooler exchangers (ACE) and other supplemental processing facilities (heat exchangers, furnaces, flare stacks, etc.), as well as operating procedures for field processing of natural gas and gas condensate for interfield and long-distance transportation. Collection of gas and gas condensate well products is a process of in-field transportation of crude gas from wells or well clusters to facilities for its preparation for long-distance transportation via main gas pipelines (МGP). Preparation of well products from gas and gas condensate fields for further transportation is carried out, depending on its composition, using such main technological processes as: separation (removing of impurities), absorption and adsorption drying and gas purification, stabilization of hydrocarbon condensate, lowtemperature separation and absorption and adsorption drying and gas purification processes, separation of gases, rectification of water-methanol solution, methanol regeneration, etc. Quality parameters of gas industry products are established by international standards (ISO), relevant national standards and technical regulations. GENERAL DESCRIPTION OF FIELD COLLECTION SYSTEMS AND PROCESSING OF WELL PRODUCTS OF GAS AND GAS CONDENSATE FIELDS. BASIC CONCEPTS ON FIELD GAS AND CONDENSATE COLLECTION Purpose of the gas collection system The collection system for gas and gas condensate fields products is designed for pipeline transportation of natural gas and gas condensate to field processing units for their subsequent processing in accordance with the current regulatory documents. Collection of gas and gas condensate well products (i.e., crude gas, unstable gas condensate, reservoir and condensation water, which condense in the well bore, flow lines and collecting pipes, also maybe with oil admixtures, inhibitors of corrosion, salt and hydrate deposition, injected if necessary to the well bore or to the flow line or collecting pipe) is a process for the in-field transportation of crude products. When transporting crude gas from wells or well clusters to facilities for natural gas and gas condensate preparation for long-distance transportation, a productcollecting system or gas-collecting pattern is created. The elements of the gas collection system are common for different fields and usually consist of Christmas trees (bradenheads), gas outlet lines (manifolds, flow lines), cutting off valves, gas collecting pipes, condensate pipelines, and field gas collection points (FGCP). The gas and gas condensate well production collection system in general case is understood as: 1. An extensive network of in-field pipelines connecting wells and clusters (wells, mainly incline or directional (deviating), the well heads of which are grouped at a close distance from each other at a common limited area (base) with field natural gas and gas condensate treatment plants; 2. Systems and devices that ensure the reliable functioning of this pipelines: - systems for distribution and injection of inhibitors of corrosion, salt deposition and hydrate formation; - systems for periodic cleaning of pipeline interior from liquid and solid phases; - well head and track heaters; - gas pre-separation units located at the wells and clusters; - instrumentation systems (control-gauging instruments), including measuring of the temperature, pressure, well rate, temperature along the flow lines, etc. Concepts and definitions of gas collection system In-field gas pipelines are usually divided into flow lines and gas collecting pipes, differing in pipe diameter. It is generally accepted that flow lines − gas pipelines starting from the well head and ending with a tie-in to the gas collecting pipes or at the inlet to the CGTP at the point of pressure control and flow distribution (such a system is called a "comb" or point, switching valve building, etc.). Flow lines usually have a smaller diameter than their collecting pipes. Flow lines from individual wells have internal diameter of 102, 125 or 150 mm, and from well clusters 219, 279, 325 mm, less often 426 and 500 mm. Product flows from gas and gas condensate fields from several flow lines can unite into a gas pipeline, which is called a gas collecting pipe (or gas collection header). Typically, the gas collection headers leading to the facilities of field crude gas treatment at the CGTP have the diameters of 325 mm, 426 mm, and 500 mm. However, it should be noted that this terminology is not always maintained and often the well cluster header is called a flow line, and the well cluster flow lines of a sufficiently large diameter are often called gas collection headers. Characteristics of collection systems of gas and gas condensate fields well products Currently, the following systems for collecting of well products from gas and gas condensate fields have got the greatest practical application: individual, group, centralized, decentralized. The choice of a particular collection system during design or during its modernization is due to a number of objective and subjective factors, the paramount importance of which depends both on technical and economic indicators and on aims facing the industry. The current state of the collection system is directly due to the historical stages of the gas industry development. The classification of the products collection systems for gas and gas condensate fields can be seen in Fig. 1. Systems for collection and in-field transportation of gas and gas condensate well products Individual collection systems (wells, gas collecting pipes, gas pipelines, main gas pipelines) linear radial circular Scope of application At the small and medium fields At the initial stage of field development (gas for own needs) Group collection systems Centralized wells (well clusters), flow lines, GPTP (GE), MGP Decentralized well clusters, flow lines (gas collecting pipes), GCTP, MGP Scope of application At medium and large fields; at several nearby small fields At large fields Figure 1 – The general classification of the products collection systems for gas and gas condensate fields, importance of which depends both on technical and economic indicators and on aims facing the industry. So, during the formation of the gas industry, only small and medium-sized gas fields were put into development and operation. During this period, individual gas collection and preparation schemes were developed and put into practice. (Fig. 2). In such collection schemes for each or several nearby wells were designed and operated their own compact complex of process equipment for gas preparation (separators, condensate collectors, etc.). This complex provided primary separation of well products from mechanical impurities and reservoir water (hydrocarbon condensate released in a small amount was practically not used and was mainly burned in flare stacks). Subsequently, in the 60th of the twentieth century, the gas industry faced a new scientific and practical task of introducing relatively large gas fields into development and operation, such as, for example, Shebelinske in Ukraine and a number of others. Figure 2 – Individual gas collection schemes: а - linear; b – radial (like beams); c – circular (like loops); 1 - well (well cluster); 2 - flow line (in case of well cluster - gas collecting pipe); 3 - gas collecting pipe; 4 - gas pool outline The following deficiencies in the individual gas collection and preparation schemes were identified during operation of the well products collection and preparation systems for gas and gas condensate fields: - the difficulty of reliable operation of remote control systems for the process mode of wells and field equipment for the collection and preparation of well products; - field equipment is distributed over a large area, since it is tied to almost every well, which leads to high metal consumption of pipelines and process equipment in general, significant lengths of field roads, etc.; - a larger number of qualified personnel are needed to operate wells and well products collection and preparation system. Therefore, in the future, technologically more efficient and economically feasible group gas collection and preparation schemes began to be used (Fig. 3). Figure 3 – Group well products collection and preparation schemes for gas and gas condensate fields: а - centralized; b - decentralized; 1 - well (well cluster); 2 - flow line (in case of well cluster - gas collecting pipe); 3 - gas collecting pipe; 4 - gas pool outline In these technological schemes the well products flow via flow line to collection points (CP) where measurements and primary gas separation are performed. Further, the gas is ducted to the system of gas collection headers (sometimes looped to improve reliability), through which it flows to the group or central gas collection point (GCP) - now CGTP. At the GCP, the gas is subjected to final drying and cleaning and flows to the main gas pipeline (MGP). The group centralized collection scheme for gas and gas condensate field is currently the main typical collection scheme for Ukrainian fields. According to the same scheme, a number of relatively small gas and gas condensate fields were mainly engineered. In the 70th and 80th of the twentieth century, the general direction of development of the domestic gas industry was the development of unique gas and gas condensate fields in Ukraine. During this period, a group decentralized collection scheme was developed, which significantly reduced capital and operational costs for the development of large fields. In accordance with this scheme, gas from well clusters flow via flow lines (and/or gas collecting pipes) to high-productiveness GCTP or gas processing plant (GPP), where in accordance with the requirements of regulatory documents, it undergoes full field processing for subsequent transport. At large gas and gas condensate fields may be several GCTP. The choice of a particular collection system is due to the characteristic of the field, a number of technological factors, the socio-production structure of the region, etc., and their current state is directly related to the historical stages of the gas industry development. Current trends in the design of well products collection and preparation systems of gas and gas condensate fields Current trends in the design of natural gas and gas condensate collection and preparation systems are the development of high-power and extra-power GE and GCTP. When enlarging well clusters and increasing the productivity of the GTP, an urgent task is to optimize the quantity and productivity of the GCTP, which would ensure, with minimal capital and operational costs, the production of the required quality products, reliability of the equipment, environmental and industrial safety for the long-term operation of field facilities. Therefore, it is high-potential to design group systems for well products collection, which should optimally combine the advantages of both centralized and decentralized schemes. Such optimal well products collection schemes are presented in Fig. 4. Figure 4 – High-potential flowsheets of high-power and extra-power gas treatment complexes: а - the GCTP has two process modules for field processing of well products - from the near clusters α and the far clusters β; also in this scheme, one module can process the products of wells of a gas productive formation, and the second one - a gas-condensate productive formation or an oil rim; b – the well products of the far clusters are pre-processed at the GCTP -1 and GCTP -2, and the well products from the near clusters are processed at the GCTP at once; 1 - well cluster; 2 - flow line, gas collecting pipe; 3 - gas collecting pipe These flowsheets increase the reliability of the collection system operation and make it possible to significantly reduce the consumption of inhibitors of hydrate formation;. The well products collection scheme from enlarged well clusters and fluid flow through individual flow lines to the GСTP may be promising. This will reduce operating costs associated with the consumption of inhibitors of hydrate formation, corrosion, salt deposition, etc., in gas collection networks. Current trends are associated with the potential of group gas collection systems designing that optimally combine the advantages of both centralized and decentralized schemes (Fig. 4) and the design of high-power GCTPs (and thus, with a decrease of their number at the field). This will be associated with the need to increase the number of wells connected to the GCTP or their flow rates, and significantly increase the distances from the well clusters to the GCTP. The main feature of the well product collection and processing schemes presented in Fig. 4 is the division of well clusters in accordance with the length of their flow lines into three groups: near, middle and far. In this case, field treatment of crude gas coming from each cluster group can be carried out separately on the GCTP process lines. Also, the well products from the far clusters can undergo primary separation at the GPTP, from where it will flow through the header to another GCTP for deeper gas drying and purification and field processing of gas condensate to transport. After the development and choice of a gas collection scheme, choice of the general structure and length of gas collection networks for the designed GCTP, it is necessary to solve the following tasks: to develop methods for laying flow lines and headers and optimize their diameters and thicknesses. The internal diameter of the gas pipeline (flow line, header) is selected in such a way as to ensure the design pipeline-transmission capacity with the minimal hydraulic losses and sufficiently high hydrodynamic efficiency of the flow of well products, as well as to minimize the consumption of hydrate formation inhibitors if possible. It is assumed that gas pressure losses during the flow of well products in the gas pipeline should not exceed 0.05-0.1 MPa per 1 km of the flow line or header, and the gas flow rate should be such that the liquid phase is completely removed from the pipeline. Determination of the optimal diameter and length of flow lines and headers should be achieved by multifactorial and detailed analysis of many variants for forecast thermal, hydrodynamic calculations of the operation modes of pipelines when their pipeline-transmission capacity and composition of the well product of the gas and gas condensate field change. Prediction of the thermobaric mode of fluid flow in flow lines and headers also makes it possible to substantiate the viability of including wellhead gas heaters into the well product collection system at well clusters operating in automatic mode, and during operation at cluster areas of fields of two groups of wells producing gas and gas condensate from different production facilities (pay formations), also substantiate the technology of heating colder gas by heat exchange with warmer gas coming from the underlying pay horizon. MAIN PHYSICAL AND CHEMICAL CHARACTERISTICS OF GAS FIELD PRODUCTS Natural gases Natural gases are characterized by high methane content (67.0-99.0% vol.), and heavy hydrocarbons content (C5+) is low (from complete absence up to 6.0% vol.). Most gases contain 1-5% vol. non-hydrocarbon components: nitrogen, carbon dioxide, hydrogen sulfide, carbonyl sulfide, carbon disulfide, etc. The main physical and chemical characteristics of natural gases of some gas and gas condensate fields are given in Table 1. Таble 1 Content of components, % vol. Field Shebelynske, Ukraine Karachaganakske, Republic of Kazakhstan Gazlynskе, Conden- Content Densate of С3Н8+ sity, efficien- in gas, 3 g/m cy, g/m3 g/m3 СН4 С2Н6 С3Н8 ∑С4Н10 ∑С5Н12+ N2 H2S 92,95 3,85 1,05 0,10 0,21 1,50 - 75,30 5,50 2,60 1,40 6,00 0,70 4,0 2,5 94,60 2,06 0,27 0,32 0,21 2,30 0,07 0,16 0,755 CO2 0,09 0,782 - 10-15 30 - - 15-20 18 Uzbekistan Shatlykske, Turkmenistan 95,05 1,63 0,20 0,07 0,07 1,75 - 1,20 0,759 20-25 7 Quality requirements to final products Sales gas Sales gas quality parameters are based on the following requirements: - gas during transportation should not cause corrosion of pipelines, valves, instruments, etc. (the content of mechanical impurities, hydrogen sulfide, thiol (mercaptan) sulphur and oxygen should be within the permissible limits); - gas quality shall ensure its transportation in a single-phase gaseous state (the parameter "gas dew point by moisture and hydrocarbons" indicates this); - sales gas should not cause problems for the consumer when using it. The presence of moisture, liquid hydrocarbons, aggressive (especially sulphuric) and mechanical impurities in the gas reduces the throughput capacity of gas pipelines, increases the consumption of inhibitors, increases corrosion, increases the required power of compressor units, causes the clogging of monitoring and metering instruments and regulation apparatuses lines. All of this reduces the reliability of process systems, increases the possibility of emergencies at compressor stations and gas pipelines. In addition, dust and mechanical impurities contribute to metal abrasion and sediment accumulation on the surfaces of heat exchangers, thereby impairing their thermal characteristics. Due to the fact that natural gas is transported over long distances from production areas to the consumer through main gas pipelines crossing various climatic zones, the issue of its high-quality treatment and drying to the dew point, which excludes condensation of water from gas, is of particular importance. The dew point in this case is the temperature at which the components of the gas mixture reach the saturation state and condense. Thus, the dew point by moisture is the water condensation temperature and the dew point by hydrocarbon is the hydrocarbon condensation temperature from the gas mixture. If during gas tanportation its temperature decreases, then at constant pressure the equilibrium concentration of components of such gas decreases: the gas becomes oversaturated. In this case, a portion of the moisture or hydrocarbons condenses and precipitates in the pipe. An important parameter of the sales gas quality is the oxygen content in it. The value of this parameter is not more than 1%. With a higher oxygen content, the gas becomes explosive. In addition, oxygen causes the increasing of corrosion in the system. Currently, sales gases supplied to main gas pipelines shall meet the requirements given in Table 2. Table 2 Climate region Parameter temperate frigid s w s w – by moisture 0 -5 -10 -20 – by hydrocarbon 0 0 -5 -10 1,0 1,0 1,0 1,0 0,02 0,02 0,02 0,02 0,036 0,036 0,036 0,036 0,003 0,003 0,003 0,003 Dew point,оС, not higher: Oxygen content, % vol. not more Hydrogen sulfide content, g/mм3, not more Thiol (mercaptan) sulphur content, g/m3, not more Mechanical impurities content, g/m3, not more Note: s – summer period (May 1 to September 30); w - winter period (October 1 to April 30). This classification is done for northern hemisphere, and for the southern hemisphere - on the contrary. Gas condensates Some gas fields with high formation pressure (up to 25-30 MPa) differ in that the gases are saturated with liquid petroleum hydrocarbons from С5Н12 to С20Н42 in amounts of 5-400 g/m3 of gas. During the development of these fields, the pressure decreases, liquid hydrocarbons condense and can be separated from the gas in the form of liquid condensate. After condensate separation, the gas approaches dry gases in composition, and the condensate contains gasoline and kerosene fractions. Gas condensates are a significant resource of hydrocarbon raw materials. Gas condensate (gas condensate) is a mixture of liquid hydrocarbons (С5Н12 and higher) released from natural gases when pressure decreases during their production, preparation and transportation. In appearance, it is a colorless or slightly colored liquid with a density of 600-800 g/cm3. Gas condensates boil off in the temperature range of 25 to 360 °C and above, some ones have a higher initial boiling point (103-210 °C) and some ones have a lower final boiling point (197-234 °C). Condensates from different fields differ greatly each from other in group chemical composition and total sulfur content. The composition of the gas condensate roughly corresponds to the gasoline or kerosene fraction of the oil or mixture thereof. During primary separation in the fields, the mass content of dissolved gas in the gas condensate is up to 10%. This is the so-called unstable condensate. After removal of volatile propane-butane fractions at gas processing plants, stable condensate is obtained, which is a raw material for the production of fuels and other oil products. The technological classification of gas condensates is based on: saturated vapor pressure, sulfur content in gas condensates and their fractions, content of aromatic hydrocarbons in the gasoline fraction with a final boiling point of 200 °C, content of n-alkane hydrocarbons in the diesel fuel fraction (200-320 °C), a fractional composition (final boiling point) and the possibility of obtaining jet engine fuels and diesel ones with and without dewaxing (deparaffination), the combination of values of which offers a glimpse into the physics and chemical nature of a gas condensate with the determination of the effective direction of its processing and qualified use. Depending on the saturated vapors pressure, gas condensates are divided into two kinds: unstable (P1) and stable (P2). Unstable (deethanized) gas condensates are ones with saturated vapor pressure above 93,325 Pa (700 mm. Hg.) containing hydrocarbons С3, С4, С5+ and partially С2. Stable (debutanized) gas condensates are ones with a saturated vapor pressure of not more than 93325 Pa, consisting of hydrocarbons С5+. In order to eliminate the loss of light hydrocarbons, all gas condensates with a saturated vapor pressure above 93325 Pa shall be stabilized. The obtained broad fraction of light hydrocarbons contains propane, butane and partially pentane (i- and n-structures), which are raw materials of the petrochemical industry. Depending on the sulphur content, stable gas condensates are divided into three classes: I - low-sulphur or sulphur-free; II – sulphur; III – high-sulphur. Sulfur content in condensates and their distillation products for I, II and III classes shall meet requirements given in Table 3. Table 3 Class in gas condensate Sulfur mass % in distillation fractions gasoline kerosene diesel fuel (final boiling (jet engine fuel) fraction point is not о (135 - 200 С) (200 - 320 оС) higher 200 °C) І not more than 0,05 ІІ 0,051 – 0,08 ІІІ more than 0,08 not more than 0,03 not more than 0,1 more than 0,1 not more than 0,01 not more than 0,1 more than 0,1 not more than 0,03 not more than 0,5 more than 0,5 If fractions obtained from low-sulphur gas condensate contain sulfur more than the limits specified for class I, these gas condensate shall be classified as class II gas condensate. If the fractions separated from sulphur gas condensate contain sulphur not more than the limits specified for products obtained from low-sulphur gas condensate, it should be classified as class I gas condensates. If one or all distillation fractions obtained from sulphur gas condensate contain sulphur more than the limits specified for this class, it should be classified as class III gas condensates. Class III gas condensates can be classified as class II only when all distillate fuels obtained from them contain sulfur not more than the limits specified for sulphur gas condensate products. Thus, the class of gas condensate is determined not only by the content of sulphur in the gas condensate, but also by the amount of sulfur in its fractions. Depending on the content of aromatic hydrocarbons in the gasoline fraction (up to 200 °C), gas condensates are divided into three sorts: A1, A2, A3. Sort A1 includes gas condensates with an aromatic hydrocarbon content in the gasoline fraction of more than 20 %. Using these gas condensates, it is cost effective preliminary aromatic hydrocarbons extacting using raffinate as a catalytic reforming raw materials to produce aromatic hydrocarbons and high-octane components. Sort A2 includes gas condensates with an aromatic hydrocarbon content in the gasoline fraction of 10–20 %. If the content of naphthenic hydrocarbons is not lower than 38 %, it is reasonable to use the gasoline fraction of these gas condensates as a catalytic reforming raw materials. Gas condensates of sort A3 are characterized by content of aromatic hydrocarbons not more than 10%. This sort of raw materials is used for pyrolysis, but can also be used for catalytic reforming if they have a high content of naphthenic hydrocarbons. Depending on the content of n-alkane hydrocarbons in the fraction of 200-320 °C, which make it possible to obtain fuel for jet engines, winter diesel fuels without dewaxing or using it and liquid paraffins for the microbiological and chemical industries, gas condensates are divided into four types: N1, N2, N3, N4. Another characteristic of the gas condensate type is its pour point. N1 - high-paraffin gas condensates, in the fraction of 200-320 °C of which the content of n-alkane hydrocarbons is more than 25 % by weight. The pour point of these condensates is not lower than minus 15 °C. From these gas condensates, jet engine and winter diesel fuels can be obtained with dewaxing. These gas condensates can be used to produce liquid n-alkanes for the synthesis of protein-vitamin concentrates as raw materials. N2 - paraffin gas condensates, in the fraction of 200-320 °C of which the content of n-alkane hydrocarbons is from 18 to 25 % by weight. The pour point of these condensates is from minus 10 °С to minus 25 °С. From these gas condensates, jet engine and winter diesel fuels can be obtained without dewaxing. Gas condensates of this type are also used for recovering liquid n-alkanes with dewaxing. N3 - paraffin gas condensates, in the fraction of 200-320 °C of which the content of n-alkane hydrocarbons is from 14 to 18 % by weight. The pour point of these condensates is from minus 40 °С to minus 60 °С. From these gas condensates, jet engine and winter diesel fuels can be obtained without dewaxing and are not used for separation of liquid n-alkanes. The diesel fractions of these gas condensates can be used for recovering n-alkanes mixing them with a high-paraffin raw materials. N4 - paraffin-free gas condensates, in the fraction of 200-320 °C of which the content of n-alkane hydrocarbons is less than 14 % by weight. The pour point of these condensates is below minus 60 °C. These include gas condensates of light fractional composition that do not contain diesel fuel fractions or are distilled at a temperature not exceeding 250 °C, and gas condensates of depleted (marginal) fields with a formation pressure less than 98∙105 - 147∙105 Pa (100 – 150 kgf/cm2). From these gas condensates, jet engine and winter diesel fuels can be obtained without dewaxing and they are not used for recovering liquid n-alkanes. Depending on the fractional composition (final boiling point), gas condensates are divided into three groups: F1 – high-boiling gas condensates with final boiling point higher than 320 °С. F2 – gas condensates with intermediate fractional composition which have final boiling point from 250 to 320 °С. F3 - gas condensates with light fractional composition which have final boiling point lower than 250 °С. When indexing each gas condensate, the following shall be indicated: kind – P1, P2; class - I, II, III; sort - А1, А2, А3; type - N1, N2, N3, N4; group - F1, F2, F3. The combination of designations of kind, class, sort, type and group represents the code of technological characteristics of gas condensates.