Systems for collecting and preparation of oil and gas and criteria for their selection

CONTENT

1. INTRODUCTION
1.0 ASSIGNMENT
1.1 Entry data for defining-projecting system for collecting and collective station
1.2 Physico-chemical characteristics of layer fluids..

2.0 SYSTEM FOR COLLECTING PRODUCED FLUIDS
2.1 Introduction
A. Individual / single system.
B. Group system.
C. Central system.
2.2 Basic characteristics of oil field
2.3 The choise of system for collection
2.4 Oil pipeline for the well
2.4.1 Introduction.
2.4.2 Calculation of temperature fall.
A.1 Input data for calculation.
A.2 Procedure of calculation.
2.4.3.1 Equation for pressure fall at isotermal oil flow
a / For laminar flow
b / For turbulent flow
2.4.3.2 Equation for pressure falls at non isothermal flow.
a / laminar flow
b / turbulent flow
2.4.4 Mechanical calculation of oil pipeline for well
2.4.5 Underground corrosion
2.4.6 Isolation of pipeline
2.4.7 Cathodic protection

3.0 COLLECTIVE STATION
3.1 Introduction
3.2 Technical description of collective station
3.2.1 Separators
3.2.1.1 Horizontal cylindrical two-phase separators
3.2.1.2 Vertical cylindrical two-phase separators
3.2.1.3 Spherical separators
3.2.2 Multilevel separation of oil and gas
3.2.3 Selection of separation system at collective station
3.2.3.1Two-phase vertical measuring separator of working pressure 16bar
3.2.3.2 Cumulative two-phase vertical separator of working pressure 16bar
3.2.3.3 Cumulative two-phase vertical separator of working pressure of 3 bar
3.3 Measuring of regulation
3.4 Dehydration of raw oil
3.4.1 Emulsions
3.4.2 Mechanical dehydration of oil
3.4.3 Electrical dehydration of oil
3.4.4 Physical-chemical dehydration of oil
3.4.5 Oil preparation / dehydration / at oil field
3.5 Disposal of layered water
3.5.1 Solving problems of disposal of layered water at considered field
3.6 Reservoirs for collecting the oil
3.7 Despatching the oil
3.8 Supporting system

4.1 Measures of protection of workforce, facilities and equipment
4.2 Measures of protection from explosion, fire and fire extinguishing plan

SUMMARY

LITERATURE

INTRODUCTION

The production of oil is a complex technological procedure consisting of several equally important segments. One of the most important phases is the collection and preparation of oil, which affects the process of production from technical as well as from economical aspects. The choice and method of designing systems for oil collection and preparation is a very complex task that depends on the number of concrete parameters; this carries with it, above all, the difficulty in finding technology that will enable optimal work of wells as well as finding the continually and quality functioning of the entire production process. It is important to mention that each well is a case in itself; there are no broad or typical solutions for oil collection and preparation. Each production project on a field is involves individual solutions dependent upon situation-specific conditions.

The most important factors in finding the best solutions are experience and knowledge of problem-solving methods in similar fields worldwide. As this is the case, this thesis is based on analysis of a selection of different fields from around the worlds that, combined, represent one system of oil collection and preparation and should satisfy even the strictest criteria.

1.0 THE TASK

At oil-gas field (X), 27 wells are drilled and prepared in the development phase for oil and gas production. The average production of peripheral wells involves around 20 m3/24h of oil, 10 m3/24h water, 800 m3 s/24h gas; while central wells involve the production of 35 m3/24h oil 1000 m3 s/24h of gas. Pressure at the well head of the well is 30 [bar], and the temperature is 30° C. The shape of the field and layout of wells are shown in diagram (1).

The following are needed to define the design:

1. optimal scheme for collection of production fluids
2. optimal technological scheme for collective station with basic elements
3. basic parameters of all elements for collective station

When defining the collection scheme, it is necessary to take in account the number of wells, geographic layout of the wells, technical and economical functionality of the chosen scheme of collection etc. In defining the scheme of the collective station, it is necessary to design technology that enables the collection of produced fluids and their separation as well as the preparation of further transport, disposing of and sending the water without endangering the human environment etc. Additionally, the technology of the collective station must ensure the continuous monitoring of elements of station as well as each well in order to observe well behaviour and adjust regimes of production according to conditions in wells.

When assembling a collective station, it is necessary to design equipment that enables functionality of the station. That means designing measuring separators, dehydrators, reservoirs for hydration and storage, systems for collecting and measuring the gas, and onward transport, etc.

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1.1 INPUT DATA FOR DEFINING – DESIGNING SYSTEMS FOR COLLECTION

AND COLLECTIVE STATION

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1.2 PHYSICAL-CHEMICAL CHARACTERISTICS OF Reservoir FLUIDS

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2.0 SYSTEM FOR COLLECTION OF PRODUCED FLUIDS

2.1 INTRODUCTION

Optimal functionality of the production process on one oil field depends on the way fluids are collected and produced. When choosing a system for collection, it is necessary to analyze the entire chain of concrete indicators, and to be mindful and respectful of the basic principles that should be valid for system collection on each oil field. The most important of those principles are:

a) Pressures at the well head should be as low as possible in order to ensure the longer eruptive work of wells, less specific consumption of gas produced by the method of gas lift, higher levels of usage of deep pumps, etc.
b) The losses of carbohydrates in the system must be reduced to minimum.
c) The monitoring and control systems need to be simple in order to ensure the undisturbed process of production and timely intervention in case of problems
d) Single and total measuring of the production, like analysis of content of produced fluids, must be extremely precise.
e) The system must be relatively flexible, designed in such a way so that new wells can be connected without consequences on other production objects.
f) Specific costs per unit of produced fluid needs to be as low as possible.
g) Systems need to be designed to ensure total safety at work, as well as requisite levels of environment protection.

The realization of these principles could prompt the following questions:

I) The reduction of pressure at oil pipeline between the well head and separators must be as low as possible; therefore, it is necessary to avoid sudden changes in direction of the layout of oil pipeline, as well as changes in the diameter along the line. Effective pressure reduction is achieved by correctly dimensioning and routing the oil pipeline, as well as successfully preventing the creation of openings (holes) in the flow because of paraffin segregation, lime scale, sand etc.
II) Production from eruptive wells that have high head pressure needs to be redirected to separator systems of high pressure.
III) At systems with manual or partly automated control, central collection must be used in order to get timely information on the behaviour of the whole system.
IV) Reducing losses in production is achieved by applying a closed system, which maximizes the use of natural energy and creates good working conditions in exchange for mechanical exploitation methods.

The choice of collection system is a very complex task and there are no blanket solutions that can be applied to every situation. Each oil field is unique based on concrete parameters of its chosen optimal collecting system.

There are typically three basic types of collection systems used:

A) INDIVIDUAL (single) SYSTEM

This system is used at wells that produce great quantities of fluid taking into account that within the object there is complete facility for separation, preparation, and measuring of production. These systems are normally very expensive and rarely applied.

B) GROUP SYSTEM

At fields that are spread over a relatively large area and where exploitation requires a large number of wells, collective stations, or the so called “group system” is applied. This system is comprised of a group of wells connected by individual routes with collective stations where, according to the rule, the first level of separation and production is done. More collective stations are connected to the central collective station where a second and third level of separation is done, as well as additional preparation and further oil transport.

The number of group systems and central collecting units is defined based on by minimal investment in their construction, maintenance and functioning. All manually-controlled measuring stations have been replaced by the LAKT system (automatic transport of oil from the field by closed system).

This system is exclusively applied to these objects. Group forms of different types of stations may be used depending on the characteristics of the system, properties of the fluids, climatic conditions etc.

C) CENTRAL SYSTEM

This system of collection with one central unit is generally applied at relatively few wells. There are more options for collecting fluids from oil fields, but complete separation and preparation is done at one central system.

The central system of collection has many advantages that are above all seen as the following:

- Possibility of maintaining minimal pressure at the well head, which increases the eruptive period of the well as well as the production under small pressure at the bottom of well;
- Reduced specific consumption of gas at wells that produce by gaslift method with minimal loss of light fractions and simple and effective system of control of work;
- Possibility of precise measurement of production at each well, convenient for change, modernization, and reparation of all types due to the fact that technical equipment is concentrated in a relatively small area etc. It is important to mention that this system works well autonomously, although it can successfully operate even under manual control.

Based on these facts, it can be concluded that the choice relays on the concrete conditions that prevail at the given oil field which are narrowly connected with technologic requests and economic indicators. Apart from the aforementioned requirements, the system must be flexible to react to the specific conditions arising in the field.

2.2. BASIC CHARACTERISTICS OF THE OIL FIELD

As already mentioned, the choice of collection system depends above all on the concrete conditions that define one particular site. In our case, it is necessary to take into account the following basic characteristics of the oil field:

a) Per area, oil fields are divided into great (30x60 km), middle (10x20 km) and small, up to (10 km2). If the average distance between wells is accepted as 300 m, then the oil field would have a dimension of 2400 x 1200 m so it can be placed into small oil fields.
b) Per shape, oil fields can be categorized into rectangular, circular and oval, taking into account that an oil field is closest in shape to an oval (elliptic).
c) At an oil field, there are 27 production units; according to this parameter, it can be placed into a small field. This conclusion is supported by total production of fluids, in fact, it is expected that the daily production of liquid phase is from 900 m3/24h (810 m3/24h of oil and 90 m3/24h of water), gas phase from 25200 m3 s/24h.
d) According to given conditions, it can be assumed that the constant parameters of pressure and temperature at the well head are:
pu= 30 [bara] and tu= 30°[C]
e) The reservoir in the phase of exploitation and it is assumed that number of wells will not increase.
f) Technological requests enable the individual measuring of each well and the measuring of total production at the oil field.
g) To maintain eruption, it is necessary to maintain minimum pressure at the well head.
h) Based on characteristics of reservoir fluids, it can be expected that the separation of paraffin, scale, etc in the pipelines not problematic.

Therefore, the choice of system for selection is to be done taking into account the above mentioned basic characteristics of the oil field.

2.3. THE CHOICE OF COLLECTION SYSTEM

Based on previously mentioned characteristics of applied systems for collection (individual, group and central), as well as characteristics of oil field, the central system for collection is being chosen. Because of small field dimension, small number of wells, relatively small total production, basic economic considerations, in the end, after using world experiences individual and group systems were not taken into consideration.

Based on characteristics of the oil field, two schemes of collection systems for produced fluids are possible:

A) A central system with combined (collector-beams) method of connecting wells with a final collection unit.
B) A central system with direct beams connecting the wells to the central collection station.

The scheme of collection regarding option “A” is shown in figure (2). The figure display the planned combined connecting of the wells to the central station, and direct connection of the wells that are located at relatively small distances; additional located wells are attached into the collection-measuring system that is laid out across the field. The advantages of this system are seen, above all, in the significant savings in laying out well pipelines, as well as in the fulfilled technological requests (closed system for collection and individual and group measuring of the production). This system is also convenient should further build-up be expected in the existing system. However, in our case the final number of wells is defined.

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This method of collection has certain disadvantages. The most troubling of these is the necessity to introduce a high level of automatization in order to automatically attach wells to the measuring line, which has direct impact on the total economy of connecting wells to the final collecting or measuring collector. This, in turn, disables direct control of work from central unit which could present certain difficulties in the case of an eruption. In the case of disaster or necessary work on the final measuring collector, production would have to be shut down by 50% - which is not recommended at eruptive wells.

Option B (figure 3) with direct connection of wells to the central collective station is the most commonly used method of solving problems of collecting fluids on fields of relatively small dimensions and production units. Considering the aforementioned advantages, this system enables direct control of each well. The level of automatization is not a primary factor for assessing the functionality of the system as problems at well pipelines are solved individually enabling a maximum usage of natural energy for transport of fluids from the well to the collection point etc. This method of collection increases initial investments in wells pipelines and savings result due to the lower level of automatization and continuous maintenance of the units which compensate for a greater initial investment.

The application of this system of collection allows for the fulfilment of all technological requests as well as the aforementioned advantages.

2.4 GATHERING SYSTEM OF OIL WELLS

2.4.1 INTRODUCTION

Steel is normally used for well’s pipelines. These steel pipes are made of varying methods of manufacturing dictated by valid standards; the most commonly applied standards are those according to American Petroleum Institute (API) standards.

API standards for oil transport pipes are the following: std. 5L, related to drawn or cross-welded pipes with or without threads, hardness to elasticity limit of 207 MN/m2; std. 5Lx that is related to drawn steel pipes of hardness to elasticity limit of 290 MN/m2to 448 MN/m2; std. 5LA that is related to aluminium pipes and std. 5Ls that is related to spiral-welded steel pipes. For transport of oil, the most commonly used pipes build to standards 5L, 5Lx.

The following properties of pipelines are applicable for the collection of fluids at examined oil fields:

- standard 5L GRAD B
- nominal diameter: 73,025 [mm]
- wall thickness of the pipe: 5,16 [mm]
- inner diameter: 62,705 [mm]
- hardness to limit of elasticity: 241 MN/m2
- yield strength: 414 MN/m2
- tested pressure: 145 MN/m2
- chemical composition: C–0,21%; Mn–max 1,15%; P–max 0,04%;
- S–0,05%

In table number (1), example lengths of these wells pipelines are provided. For normal function, it is necessary to know the parameters at entrance of the pipelines (pressure, temperature), as well as the parameters at the exit of the pipelines. Therefore, it is necessary to do the calculation of pressure fall and temperature fall for each individual pipeline.

Table (1) lengths Orientation of oil pipeline of wells:

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2.4.2 CALCULATION OF TEMPERATURE DROP

A.1. ENTRY DATA FOR CALCULATION

- Dimensions of pipelines:
- Outer diameter: ds= 0,073025 [m]
- Inner diameter: du= 0,062705 [m]
- Assumed average temperature in pipelines: tsr = 30 ° [C]
- Volumetric flow
- of oil for peripheral wells: Vn= 20 m3/24h
- of oil for central wells: Vn= 35 m3/24h
- water for peripheral wells: Vv= 10 m3
- gas for peripheral wells: Vg= 800 m3 s/24h
- gas for central wells: Vg= 1000 m3 s/24h
- volumetric mass:
- of oil: ρn= 872 kg/m3
- of water: ρv= 1000 kg/m3
- of gas: ρg= 0,754 kg/m3
- dynamic viscosity at average temperature:
- of oil: µn= 0,279 pas
- water: µv= 0,002 pas
- gas: µg= 10,7∙106 pas
- specific heat at average temperature:
- of oil: Cn=1787 J/Kg K
- water: Cv= 4187 J/Kg K
- gas: Cg= 2181 J/Kg K
- coefficient of heat conductivity at average temperature:
- of oil: λn=0,1354 W/m K
- water: λv= 0,568 W/m K
- gas: λg= 0,031 W/m K
- coefficient of heat conductivity for earth: λz=1,0 W/m K
- depth of placing pipelines: H=1,2 [m]
- coefficient of heat conductivity of pipe’s wall: c= 58 W/m K
- earth temperature on depth of pipes placement: Tc= 4 ° [C]
- factor of compressibility of gas: Z ≈ 1

A.2 PROCEDURE OF CALCULATION

In this part, the calculation will be shown here but results per wells will be given in final table.

A.2.1 SEPARATING PROCEDURE OF GAS AT WORKING CONDITIONS

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Where:

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A.2.2 TOTAL VOLUMETRIC FLUID FLOW

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A.2.4 SPEED OF FLOW CURRENT THROUGH PIPELINES

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A.2.5 VOLUMETRIC MASS OF WELL’S FLUID

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A.2.6 DYNAMIC VISCOSITY OF WELL’S FLUID

µ = Abbildung in dieser Leseprobe nicht enthalten

A.2.7 CINEMATIC VISCOSITY OF WELL’S FLUID

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A.2.8 REJNL’S NUMBER

Re=Abbildung in dieser Leseprobe nicht enthalten

A.2.9 SPECIFIC HEAT OF WELL’S FLUID

G=Abbildung in dieser Leseprobe nicht enthalten

A.2.10 COEFFICIENT OF HEAT CONDUCTIVITY OF WELL’S FLUID

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A.2.11 ‘PLANTL’ NUMBER

pr = Abbildung in dieser Leseprobe nicht enthalten

A.2.12 CINEMATIC VISCOSITY AT TEMPERATURE OF PIPE’S WALL

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A.2.13 RELATIONSHIP OF VISCOSITIES AT AXIS AND AT PIPE’S WALL

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A.2.14 ‘NUSELT’ NUMBER

Nu= Abbildung in dieser Leseprobe nicht enthalten

A.2.15 COEFFICIENT OF TRANSFER OF HEAT FROM FLUID ONTO THE PIPE’S WALL

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A.2.16 COEFFICIENT OF TRANSFER OF HEAT FROM WALL INTO THE ENVIRONMENT

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A.2.17 TOTAL COEFFICIENT OF HEAT TRANSFER

K = Abbildung in dieser Leseprobe nicht enthalten

A.2.18 TEMPERATURE OF WELL’S FLUID AT THE END OF PIPELINE

T2 = To + (T1-To) x∙e-al

Where:

a = Abbildung in dieser Leseprobe nicht enthalten

Results of temperature fall calculations are given in table 2.

Table 2

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Table 2 – Calculation of the temperature drop in the gathering system. The well numbers are from 1 to 27

In order to define dimensions of the well’s pipeline, it is necessary to calculate pressure falls when adopting particular sizes of the wells. Different equations are used depending on the conditions of the flow.

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