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EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNO...
EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNO...
ABSTRACT
The current assignment discusses the formation of natural gas hydrates in gas
transmission pipelines. Hydrates ar...
A.Mitsis E.Michailidi F.Zachopoulos - 7 - 2014
TABLE OF CONTENTS
1. CHAPTER 1 INTRODUCTION ..................................
LIST OF FIGURES
Figure 2.1 Three hydrate unit crystals and constituent cavities[14] .....................................1...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
A.Mitsis E.Michailidi F.Zachopoulos ...
Hydrate formation in gas pipelines
Hydrate formation in gas pipelines
Hydrate formation in gas pipelines
Hydrate formation in gas pipelines
Hydrate formation in gas pipelines
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Hydrate formation in gas pipelines

The current assignment discusses the formation of natural gas hydrates in gas transmission pipelines. Hydrates are crystalline compounds, consisting of a gas molecule and water, which form under certain thermodynamic conditions, which include high pressure and low temperature. Natural gas hydrates are responsible for pipeline plugging and corrosion. Thus, handling the issue of the formation is a matter of vital importance for the industry. At the theoretical background of the assignment the topic is presented and analyzed towards the hydrate structure and development, the formation, the consequences and, finally, the solutions as well as the inspection processes. In order to provide the optimal strategy in dealing with hydrate formation, it is of vital importance to have an understanding of the conditions that cause hydrate formation. The most accurate predictions can be conducted with the use of computer software. In the current assignment the chemical simulations software Aspen Hysys is used for studying the formation conditions. Three potential natural gas streams, with different compositions, were modeled and studied towards the conditions of hydrate formation.
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Hydrate formation in gas pipelines

  1. 1. EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY MSc in OIL AND GAS TECHNOLOGY COURSE ASSIGNMENT FOR “OIL & GAS MANAGEMENT” METHANE HYDRATE FORMATION IN NATURAL GAS PIPELINES ELISAVET MICHAILIDI B.Sc. Petroleum Engineering ARISTIDIS MITSIS M.Sc. Civil Engineering FOTIS ZACHOPOULOS B.Sc. Petroleum Engineering SUPERVISOR: ADJ. PROF. NIKOLAOS C. KOKKINOS KAVALA 2014
  2. 2. EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY MSc in OIL AND GAS TECHNOLOGY COURSE ASSIGNMENT FOR “OIL & GAS MANAGEMENT” METHANE HYDRATE FORMATION IN NATURAL GAS PIPELINES ELISAVET MICHAILIDI B.Sc. Petroleum Engineering ARISTIDIS MITSIS M.Sc. Civil Engineering FOTIS ZACHOPOULOS B.Sc. Petroleum Engineering SUPERVISOR: ADJ. PROF. NIKOLAOS C. KOKKINOS KAVALA 2014
  3. 3. ABSTRACT The current assignment discusses the formation of natural gas hydrates in gas transmission pipelines. Hydrates are crystalline compounds, consisting of a gas molecule and water, which form under certain thermodynamic conditions, which include high pressure and low temperature. Natural gas hydrates are responsible for pipeline plugging and corrosion. Thus, handling the issue of the formation is a matter of vital importance for the industry. At the theoretical background of the assignment the topic is presented and analyzed towards the hydrate structure and development, the formation, the consequences and, finally, the solutions as well as the inspection processes. In order to provide the optimal strategy in dealing with hydrate formation, it is of vital importance to have an understanding of the conditions that cause hydrate formation. The most accurate predictions can be conducted with the use of computer software. In the current assignment the chemical simulations software Aspen Hysys is used for studying the formation conditions. Three potential natural gas streams, with different compositions, were modeled and studied towards the conditions of hydrate formation. SUBJECT AREA: Natural Gas Hydrates KEYWORDS: gas hydrates, hydrate prediction, hydrate formation, inhibitors, hydrate structure
  4. 4. A.Mitsis E.Michailidi F.Zachopoulos - 7 - 2014 TABLE OF CONTENTS 1. CHAPTER 1 INTRODUCTION ........................................................................................12 1.1 INTRODUCTION .........................................................................................................12 2. CHAPTER 2 THEORETICAL BACKGROUND............................................................13 2.1 HYDRATES STRUCTURE.........................................................................................13 2.1.1 METHANE HYDRATES ......................................................................................15 2.2 HYDRATE DEVELOPMENT......................................................................................16 2.3 HYDRATE FORMATION............................................................................................16 2.4 HYDRATES EFFECTS IN GAS PIPELINES...........................................................18 2.4.1 HYDRATE FORMATIONS AS INITIATORS OF NATURAL GAS PIPELINE INTERNAL CORROSIONS...............................................................................................19 2.5 SOLUTIONS TO THE HYDRATE FORMATION PROBLEMS.............................22 2.5.1 CHEMICAL INJECTION......................................................................................22 2.5.2 NATURAL GAS DEHYDRATION ......................................................................24 2.6 MAINTENANCE AND DETECTION OPERATIONS ..............................................25 2.6.1 OPTlMAL TRANSMISION OPERATIONS DESIGN.......................................25 2.6.2 REGULAR PIPE INSPECTION AND PIGGING OPERATIONS...................26 3. CHAPTER 3 EXPERIMENTAL PROCEDURE.............................................................27 3.1 PREDICTION OF HYDRATE FORMATION USING COMPUTER SOFTWARE 27 3.2 EXPERIMENTAL PROCEDURE...............................................................................27 4. CHAPTER 4 CONCLUSIONS..........................................................................................33 5. ABBREVIATIONS – INITIALS.....................................................................................34 6. REFERENCES....................................................................................................................36
  5. 5. LIST OF FIGURES Figure 2.1 Three hydrate unit crystals and constituent cavities[14] .....................................14 Figure 2.2 (a) Linking five 512 polyhedra by two 51262 polyhedra to form structure I (b) Two-dimensional view of face-sharing of 512 polyhedra to form 51264 polyhedra voids for structure II[9] .................................................................................................................................15 Figure 2.3 Hydrate development..............................................................................................16 Figure 2.4 Pressure/temperature diagram for methane + water in the hydrate region[18] .......................................................................................................................................................17 Figure 2.5 Gas + single condensate + water systems[18] .....................................................18 Figure 2.6: The "Horseshoe" type erosion-corrosion damage in a copper pipeline[2] .....21 Figure 2.7: Generic Hydrate Formation Curve. The properties of Petroleum Fluids[1] ....23 Figure 2.8: Temperature-Composition diagram for Methane and Water.[18] .....................24 Figure 3.1 Hydrate Formation utility Sweet Gas Design ......................................................28 Figure 3.2 Hydrate Formation Utility Sweet Gas Performance ...........................................29 Figure 3.3 Hydrate Formation Utility Sweet Gas + CO2 Design .........................................29 Figure 3.4 Hydrate Formation Utility Sweet Gas + CO2 Performance...............................30 Figure 3.5 Hydrate Formation Utility Sour Gas + CO2 Design ............................................30 Figure 3.6 Hydrate Formation Utility Sour Gas + CO2 Performance..................................31 Figure 3.7 Sweet Gas Envelope ..............................................................................................31 Figure 3.8 Sweet Gas + CO2 Envelope ..................................................................................32 Figure 3.9 Sour Gas + CO2 Envelope.....................................................................................32
  6. 6. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 11 - 2014 LIST OF TABLES Table 2.1 Properties of gas hydrate structures[13] .................................................................13 Table 3.1 Stream Composition.................................................................................................27 Table 3.2 Initial Conditions........................................................................................................28
  7. 7. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 12 - 2014 1. CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION During the natural gas transportation through pipelines and under certain thermodynamic conditions, considering pressure, temperature and composition, crystalline compounds, consisting of a gas molecule and water, are formed. These ice- like compounds are known as “gas hydrates” or “gas clathrates”. The term “clathrate” has its origins in the Greek language as it derives from the word “khlatron”[3] which means barrier. Gas hydrates are formed from a gas molecule, which is called “guest molecule”, trapped within a hydrogen-bonded structure of water molecules, called “host molecules”[4]. Hydrates were discovered in 1810 by Sir Humphry Davy and experimentally approved by John Faraday in 1820[5]. In 1934 Hammerschmit[6] observed that the formation of gas hydrates was responsible for oil and gas transportation pipelines blockage. The formation of methane hydrates in the pipelines during the natural gas transportation is a major concern for the oil and gas industry as it is responsible for plugging the pipelines and corrosion[2]. Hence, since then, many studies have been conducted concerning a variety of aspects, including the structure, the kinetics[7], the formation conditions, the prediction of formation[8], the effects as well as the inhibition of the hydrates formation which is a highly concerned matter due to its cost for the industry. In order to provide the best possible strategy in dealing with hydrate formation, it is of vital importance to have a comprehensive understanding of the underlying conditions that lead to initial hydrate formation. Computer software are powerful tools for the prediction of the hydrate formation. In the current assignment the simulation software Aspen Hysys is used for the prediction of three different streams of natural gas and the results are presented and analyzed.
  8. 8. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 13 - 2014 2. CHAPTER 2 THEORETICAL BACKGROUND 2.1 HYDRATES STRUCTURE Gas hydrates are crystalline solid compounds consisting of a low molar weight gas molecule trapped within a structure of hydrogen-bonded water molecules. Gas hydrates are formed from a gas molecule, which is called “guest molecule”, trapped within a hydrogen-bonded structure of water molecules, called “host molecules” in a structure which resembles a cage[9]. The general formula is M+NH H2O=M×NH H2O, where NH=the hydration number and M the hydrocarbon molecule[10]. Depending on the type and number of cavities and various sizes of guest molecules - which cause the different arrangements of water molecules, hydrates are categorized into three different structure types[11]. Each structure includes different types of molecules depending to the guest molecule size[8]. Thus, structure I allows the inclusion of methane, ethane and carbon dioxide; Structure II allows inclusion of propane and iso-butane along with methane and ethane. Finally, structure H includes heavier molecules. Natural gas hydrates can consist of any combination of the structures sI and s II depending to its composition[8]. Structure H is yet to be found outside laboratory[12]. The main properties of the different types of structures are presented in Table 2.1. Table 2.1 Properties of gas hydrate structures[13] The unit cell of structure I (sI)[9, 11, 13] is consisted of 46 water molecules. Two types of cavities are formed - two small which have the shape of a pentagonal dodecahedron
  9. 9. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 14 - 2014 (512) and six large which have the shape of a hexagonal truncated trapezohedron (51262). Hydrates of this category are consisted from a single guest molecule of low molecular weight. The average cavity radius is 3.96 Å for the small cavities and 4.33 Å for the large cavities. Hydrates in the category sII[9, 11, 13] are consisted of 136 water molecules which form two types of cavities, 16 small and 8 large which shape a pentagonal dodecahedron (512) and a pentagonal hexadecahedron (51264) respectively. The average cavity radius is 3.91 Å for the small and 4.73 Å for the large cavities. The unit cell of structure H[9, 11, 13] is consisted of 34 water molecules. Three types of cavities are formed- three small with the shape of pentagonal dodecahedron (512), two medium (435663) and one large (51268). The average cavity radius is 3.91 Å for the small cavity, 4.06 Å for the medium cavity and 5.71 Å for the large cavity. The formation of sH types requires the existence of larger organic molecules. Figure 2.1 Three hydrate unit crystals and constituent cavities[14]
  10. 10. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 15 - 2014 Furthermore, in addition to the types I,II and H, two more types were referred from Udachin and Ripmeester[15]. Despite the fact that no hydrocarbon molecules have been found in them, their discovery sets the point of conducting new research studies concerning the hydrates’ structures. 2.1.1 METHANE HYDRATES Methane hydrates are consisted of methane molecules trapped within water molecules. Methane and ethane crystallizes mainly in the sI structure. Type sI structure is depicted in Figure 2.2. Thus, they are consisted of 46 water molecules. Two types of cavities are formed - two small which have the shape of a pentagonal dodecahedron (512) and six large which have the shape of a hexagonal truncated trapezohedron (51262). They are crystallized in the cubic system. The hydration number ranges from 5.77 to 7.41. NH=5.75 corresponds to complete hydration[16]. As it was mentioned above, methane and ethane crystallizes mainly in structure sI. However, Subramanian et al indicated that, under certain conditions, the formation of type sII hydrates is possible[17]. Raman and NMR spectroscopic measurements were used to identify these structures. Figure 2.2 (a) Linking five 512 polyhedra by two 51262 polyhedra to form structure I (b) Two- dimensional view of face-sharing of 512 polyhedra to form 51264 polyhedra voids for structure II[9]
  11. 11. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 16 - 2014 2.2 HYDRATE DEVELOPMENT Hydrates are formed as tiny particles at the interface between water and hydrocarbon phases. These coated water droplets begin to grow, if not removed rapidly, up to agglomerate into larger masses hydrates (Figure 2.3). Figure 2.3 Hydrate development 2.3 HYDRATE FORMATION Under certain conditions, water molecules arrange themselves into structures, which form three-dimensional (3D) polyhedra around the gases. These polyhedra then combine to form specific crystalline lattices which entrap the guest molecule. For systems containing both water and low molecular weight hydrocarbons, hydrates should be taken into consideration in the phase diagram. The formation of clathrate hydrates requires certain thermodynamical conditions which include high pressure and low temperatures. Moreover, the formation is depended on the stream’s composition as certain compounds affect the formation rate. Thus, the analysis of the phase diagrams is required. For a system which includes hydrocarbons in the gaseous phase the pressure- temperature diagram (P-T diagram) is presented and analyzed (Figure 2.4).
  12. 12. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 17 - 2014 Figure 2.4 Pressure/temperature diagram for methane + water in the hydrate region[18] Figure 2.4, schematically represents a methane and H2O system at conditions around hydrate region. The hydrate formation happens to the area at the left of all lines[19, 20]. Due to the fact that methane is the major component of natural gas, this diagram provides adequate information about phase behavior for gas systems without a liquid hydrocarbon phase. Hydrate–methane–water systems involve the following phases[21]:  Hydrate (H)  Water-rich liquid (LW),  Methane-rich vapor (VCH4 ) and  Ice (I). For equilibrium calculations, an equation of state (EoS) can describe all phases. Q1 is the quadruple point which is the starting point for four 3-phase lines:  The LW-H-V line, which has P/T conditions at which water and vapor form hydrates, conditions of most interest in natural gas hydrate systems.  The I-H-V line  The I-LW-H line  The I-LW-V line, which connects Q1 to the pure water triple point (I-LW-V) (273.16 K, 0.62 kPa) and denotes the transition between water and ice without hydrate
  13. 13. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 18 - 2014 formation. Because Q1 is close to 273 K for all natural gas systems, the I-LW-V line extends almost vertically below Q1 to 0.62 kPa. Figure 2.5 describes systems[22] which may include: ethane + water, propane + water, isobutane + water and water + carbon dioxide or hydrogen sulfide[23], which are both common compounds of natural gas. Figure 2.5 Gas + single condensate + water systems[18] Hydrates will form at temperatures and pressures to the left of the region enclosed by the three lines, whereas to the right of this region, hydrates are not possible. Upper quadruple point Q2 often is approximated as the maximum temperature of hydrate formation because line LW-H-LHC is approximately vertical because of the incompressibility of those three phases. 2.4 HYDRATES EFFECTS IN GAS PIPELINES The formation of gas hydrates can be a matter of vital importance for the industry as it is related to blockage of the gas pipelines. Pipeline blockage can be either fluid related or structure related. Problems related to infrastructure of the pipeline, which involves pipe deformation, corrosion deposition and valve abnormal functioning, are interdepended to structure-related blockages. However, these problems can be through careful and
  14. 14. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 19 - 2014 regular inspections along with appropriate maintenance. On the contrary, fluid-related blockages are considered to be more persevering and challenging to control. Fluid related problems are associated with the formation and/or deposition of solids in the gas pipeline. Gas hydrate deposition is considered as the most severe of these depositions. In 1934 Hammerschmit observed that the formation of gas hydrates was responsible for oil and gas transportation pipelines blockage. Light hydrocarbons such as methane, ethane, propane and butane, along with the combinations of longer chain hydrocarbon with methane, act as guest molecules. Furthermore, non-hydrocarbon molecules such as N2, CO2 and H2S, which are more soluble to water are found in hydrates and are known to promote hydrate formation. Gas hydrates are responsible for many problems which include pipeline blockage, jeopardizing the foundations of deep-water platforms and pipelines, plugging blowout preventers during the production and causing tubing and casing collapse[6]. The effects of hydrates in gas pipelines will be examined and analyzed within the current assignment. 2.4.1 HYDRATE FORMATIONS AS INITIATORS OF NATURAL GAS PIPELINE INTERNAL CORROSIONS Hydrate formation is often responsible for various types of internal corrosion in gas pipelines. Internal corrosion can be caused by either physical or chemical processes. It is related to several parameters such as hydrate size, stage and the contact period[2]. Acidic gases, which include H2S, CO2 and Cl-, are components of hydrates which are responsible for increasing the corrosion rate[24, 25]. Methane is also contributing to the metal corrosion phenomena.[26, 27] Moreover, H2O is another agent responsible for corrosion[28]. Interaction between the pipeline and the hydrates initiates internal corrosion. In conjunction with rupturing, corrosion will further lead to gradual degradation of the material and deterioration of its integrity. Eventually, corrosion can lead to leakages and even to full bore rupture (FBR). As a result, the economic repercussions decisively lead to political and environmental ramifications. Corollary to the corrosion, the risk for the pipe’s integrity, can result to a replacement cost which may reach $3 trillion[29]. 2.4.1.1 Corrosion Initiation through Physical Processes As it was mentioned, corrosion can be induced through physical processes. The corrosion types caused by physical processes include cavitations, erosion, pitting,
  15. 15. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 20 - 2014 galvanized and stress cracking corrosions[2].Throughout these stages, interaction between hydrates phase and the pipeline walls initiates a type of corrosion described below. 2.4.1.1.1 Cavitations Corrosion In the first stage of the formation the existence of a semi-solid state, in which the hydrate blocks are having liquid inside the cavities, is observed. During this stage the hydrates are unstable. At areas of low pressure in the conveyed fluid are formed[30]. Cavitations occur when then liquid, moving with high speed, is subjected to a pressure drop. Thus, vapor cavities are formed in the liquid existing in the hydrate. Pressure drop eventuates at point of flow discontinuity, especially joints and bends. The vapor bubbles are imploding, when subjected to higher pressure, and hitting the metal surface, producing an intense shock wave which can remove the protective films. Henceforth, corrosion is accelerated at the damaged surface. 2.4.1.1.2 Erosion corrosion Throughout the time, the hydrates transmute from the semi-solid state into solid blocks. These hydrate chips are moving with high velocity and are hitting the inner surface of the pipeline walls, causing erosion. Erosion is defined as the destruction of a metal by abrasion or attrition caused by the relative flow of fluid against the surface. On condition that there is a constant bombardment of particles on the pipe wall surface, erosion- corrosion occurs, causing the removal of the surface protective film or the metal oxide from the surface, exposing the surface to erosion-corrosion (Figure 2.6) from the fluid properties. Erosion-corrosion can be affected by many factors such as turbulence, cavitation, impingement or galvanic effect and finally result to the pipeline’s rapid failure. Moreover, at later stage, the hydrate chips agglomerate and form bigger blocks which will require more energy to transport across the surface of the pipe-wall[30, 31].
  16. 16. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 21 - 2014 2.4.1.2 Corrosion initiation by chemical/electrochemical processes Natural gas’s composition often includes amounts of CO2 and/or H2S gases in its composition. Moreover, the transported gas contains chloride ions (Cl-), originating from the formation water, and sometimes acetic acid (CH3COOH). During the hydrate formation, the available water reacts with these compounds, producing acids that dissociate to individually yield corrosive electrolysis products (Equation 1): Equation 1 : During the hydrate solidification process, or when the solidified hydrate is melting (during hydrate removal), there will be an interaction between the components of equation (1) and the pipe's inner surface. Since these components are corrosive in nature, corrosion reactions will be promoted over time through electrochemical reactions to yield galvanic and electrolytic corrosions. The corrosion rate will be a function of time, composition of the hydrate, pH and other thermodynamic properties such as temperature, pressure, gas fugacity, etc.[25] Oxidation-reduction (or redox) reaction takes place in electrochemical corrosions with oxidation taking place at anode while reduction takes place at the cathode. However, spontaneous reactions occur in galvanic (voltaic) cells (Figure 5) while non-spontaneous reactions occur in electrolytic cells. The anode of an electrolytic cell is positive (cathode is negative), therefore, the anode attracts anions from the solution (Equation 2,3). However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal. Figure 2.6: The "Horseshoe" type erosion- corrosion damage in a copper pipeline[2]
  17. 17. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 22 - 2014 Equation 2,3 : 2.5 SOLUTIONS TO THE HYDRATE FORMATION PROBLEMS Hydrates formation can cause pipeline plugging due to their inclination of agglomerating and attaching to the pipeline’s walls. Thus, their removal, which is strongly desirable, can be achieved through temperature increase and/or pressure decrease. However, even under these conditions, the hydrate decomposition remains a slow process. Taking this into account, it is unambiguous that the key to confront the problem is preventing the hydrate formation. The hydrate formation is based on three levels of security, listed below: 1. Depressing the hydrate formation temperature. This can be achieved through dehydration. 2. Adjusting the operating conditions in order to prevent the hydrate’s formation. 3. Formation can also prevented by the addition of chemicals that prolonging the hydrate formation time using inhibitors[32]. 2.5.1 CHEMICAL INJECTION During the operation under certain conditions, where the hydrates could be formed, a possible way to prevent the formation is changing the gas composition by injecting chemical. Through chemical injection (a) the formation temperature can be reduced and/or (b) the formation can be delayed. The injected chemical can generally be classified into two types: 1. Thermodynamic inhibitors: The most widely used inhibitors of this type are glycols and methanol. Although both glycol and methanol can be recovered the recovery of ethanol is not preferred from the economical aspect due to the fact that compared to glycols, more losses occur. The most commonly used glycols are the monoethylene glycol (MEG) and the diethylene glycol (DEG). Triethylene glycol (TEG) cannot be used for injection into a gas stream due to its very low vapor pressure. MEG is preferred over DEG for application for applications where the temperature is lower than -10 oC due to its high viscosity at low temperatures[32].
  18. 18. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 23 - 2014 2. Kinetic inhibitors: This type of chemicals is polymers consisted of carbon chains and pendant groups. Partially formed hydrate cages are absorbed from the polymer. The polymer is attached along the hydrate crystal surface and thus the hydrate crystals are growing around the polymer therefore stabilizing the hydrates as small particles in the aqueous phase. Field tests indicated that kinetic inhibitors are effective up to ΔΤ= 20 οF, at concentrations varying from 550 to 3,000 ppm in the aqueous phase[33, 34]. In spite of the fact that it is not the best approach, this is one of the most publicized techniques for handling the hydrate issue as it requires insignificant interruption to the flow and is less meddlesome. The presence of compounds like H2S and SO2 contributes to the hydrate formation due to their high solubility to water. In the same way, other chemicals are added to the natural gas transmission lines in order to cause the inverse effect. These chemicals are altering the hydrate formation line of equilibrium and, as a result, inhibiting the hydrate formation (Figure 2.7) Many studies have been conducted over the years, concerning this issue. The first studies on the hydrate inhibition were conducted by Hemmerschmidt in 1934[6]. In these studies aqueous solutions such as lithium chloride, calcium chloride, zinc chloride, di- ethylene glycol, methanol and glycerin were used. Furthermore, a large number of similar studies were conducted from researchers such as Nakamaya and Hashimoto (1980)[35], Davidson et al. (1981)[36], Makogon(1981)[37], Berecz and Balla- Achs(1983)[38], Svatar and Fadnes (1992)[39], Kelland et al. (1995)[40]. Later studies have been conducted on the mechanisms of gas hydrate inhibition by Sloan (1991)[41] , Lederhos et al.(1996)[42], Sloan (2003)[13]. Despite the fact that these studies have shed Figure 2.7: Generic Hydrate Formation Curve. The properties of Petroleum Fluids[1]
  19. 19. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 24 - 2014 light on the hydrate mechanisms, there is still a gap of knowledge and thus there is no known hydrate inhibitor, until now, that can entirely eradicate the problem of hydrate formation and deposition. 2.5.2 NATURAL GAS DEHYDRATION Natural gas dehydration is a very important process if we consider the fact that gas hydrates cannot be formed without the presence of water. Natural gas dehydration leads to hydrate instability due to the fact that the water molecules (host molecules) are inadequate to form enough lattices to host the gust molecules. This process is the only process heretofore which is completely sufficient in preventing the formation of hydrates in pipelines. A common misapprehension is that free water is the cause of hydrate formation. However, from the thermodynamic perspective, the water dissolved in the hydrocarbons is the cause of hydrate formation which happens at the hydrate-vapor boundaries (Figure 2.8). Figure 2.8 presents the cooling curve (at a constant pressure) of methane - water mixture consisted of 60 mol CH4 and 40 mol H2O. Figure 2.8: Temperature-Composition diagram for Methane and Water.[18] The years following after the discovery of hydrates existence in the pipelines, Hammershmidt[6] and his colleagues published studies concerning gas dehydration with the use of solid desiccants. The most widely used desiccants today are molecular sieves, silica gel and alumina. The most significant of them are the molecular sieves, which are crystalline solid materials with the property of adsorbing particular molecules based upon size and polarity. They are preferred over silica gel due to the fact that it is not damaged by liquid water and it has high water adsorption capacity. Three different
  20. 20. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 25 - 2014 processes for lowering the dew point lowering were proposed by Deaton and Frost in 1946[43]: (a) Hygroscopic solution (b) Chemical adsorption (3) Physical adsorption. However, only hygroscopic solution and physical adsorption methods have been commercially applied. Due to the fact that in the second process the adsorbents cannot be re-generated, it is not feasible and thus, not commercially used. The first process involves lowering the water concentration by contacting the gas with tri-ethylene glycol (TEG) which adsorbs water through hydrogen bonding. Detailed description of this process can be found in Perry (1960)[44], Hall and Polderman (1960)[45], Loomer and Welch (1961)[46] and Bucklin et al. (1985)[47]. Water removal is one of the most effective approaches to prevent hydrate formation. However, it increases the overall cost of transportation of natural gas. Furthermore it requires a technical process which may not always be practically applicable For instance; in deep water production of natural gas, hydrates formation occurs before one ever gets the natural gas to the surface. In such cases the dehydration process cannot be implemented. 2.6 MAINTENANCE AND DETECTION OPERATIONS 2.6.1 OPTlMAL TRANSMISION OPERATIONS DESIGN If we take into consideration the fact that inhibitors cannot completely eradicate hydrate formation and deposition, it is obvious that the knowledge of hydrate forming conditions have to be used in order to diminish the hydrate formation. Hydrates form at or below the hydrate formation temperature for a given pressure and gas composition. Phase diagram analysis is a tool used for the prediction of the lower and upper quadruple points (Q1 and Q2) on the hydrate formation/dissociation line. The first prediction method was proposed by Katz et al. in 1942[48] and is known as the Ki-value Method. In 1945 a second method was introduced by Katz[48], known as the Gas Gravity Method. Both of them allow the calculation of P-T equilibrium curves for systems involving water (liquid phase), hydrate (solid phase) and natural gas (vapor phase). These methods output initial estimates for the calculation and provide qualitative understanding of the equilibrium. The second method provides more precise results (Sloan 1990[49]). Moreover, a third method for the prediction of the equilibrium, which is based on Statistical Mechanic, was introduced. This method is more comprehensive and detailed and finally more accurate.
  21. 21. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 26 - 2014 2.6.2 REGULAR PIPE INSPECTION AND PIGGING OPERATIONS Pigging is a method according to which a device, known as “pig”, is inserted into a pipeline. There is a variety of pig devises in the industry depending on their usage. Pigs are used to remove solid deposits from the pipeline by scraping them off. Additionally, there are types of pigs used to remove condensate and liquids from the natural gas pipelines. These types of pigs are simple devises such as polyurethane foam plug to push along liquids and condensates or include tungsten studs with abrasive wire meshes on the outside to cut rust, scale or paraffin. Furthermore, pigs are used for the pipeline inspection in order to detect corrosion damage, leakages and blockages. Inspection pigs are highly complicated instruments. .
  22. 22. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 27 - 2014 3. CHAPTER 3 EXPERIMENTAL PROCEDURE 3.1 PREDICTION OF HYDRATE FORMATION USING COMPUTER SOFTWARE As it was mentioned to the previous chapter, in order to provide the optimal strategy in dealing with hydrate formation, it is of vital importance to have an understanding of the conditions that cause hydrate formation. The most accurate predictions can be conducted with the use of computer software. The basis of these programs is the Equations of State (EoS). In the current assignment the chemical simulations software Aspen Hysys® is used for studying the formation conditions. 3.2 EXPERIMENTAL PROCEDURE In order to simulate the streams and calculate the conditions, the Peng-Robinson Equation of State (PR EoS) was used as it is the most applicable EoS in oil & gas industry and is ideal for gas systems[50]. As a first step, three streams of natural gas with different composition[51] were created: Table 3.1 Stream Composition Component Sweet Gas Sweet Gas + CO2 Sour Gas + CO2 Methane 0.926700 0.784000 0.842531 Ethane 0.052900 0.060000 0.031494 Propane 0.013800 0.036000 0.006699 i-Butane 0.001800 0.005000 0.002000 n-Butane 0.003400 0.019000 0.001900 n-Pentane 0.001400 - 0.003999 Nitrogen - 0.094000 0.002999 Hydrogen Sulfide - - 0.041792 Water - - - Carbon Dioxide - 0.002000 0.066587
  23. 23. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 28 - 2014 The initial conditions for each of these streams are set as following: Table 3.2 Initial Conditions Sweet Gas Sweet Gas + CO2 Sour Gas + CO2 Temperature (oC) 17 17 17 Pressure (bar) 80 80 80 Molar Flow (kmol) 1000 1000 1000 The Hydrate Formation Utility was used for each stream. It was observed that:  “Sweet Gas” stream: Hydrates will not form under these conditions.  “Sweet Gas + CO2” stream: Hydrates will form under these conditions.  “Sour Gas + CO2” stream: Hydrates will form under these conditions. Furthermore, the tool provides additional information towards the type of hydrates that will form and indicates the Formation Temperature at Stream Pressure and Formation Pressure at Stream Temperature. Thus, if pressure is kept constant the temperature under which the hydrates are formed is indicated. Respectively, if the temperature is kept constant, the tool indicates the pressure over which the formation occurs. For each stream:  “Sweet Gas” stream: o If pressure is kept constant at 80 bar the hydrate formation occurs at temperatures below 16.8943 oC. o If pressure is kept constant at 17 oC the hydrate formation occurs at pressures over 81.3558 bar. Figure 3.1 Hydrate Formation utility Sweet Gas Design
  24. 24. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 29 - 2014 Figure 3.2 Hydrate Formation Utility Sweet Gas Performance  “Sweet Gas + CO2” stream: o If pressure is kept constant at 80 bar the hydrate formation occurs at temperatures below 18.8022 oC. o If pressure is kept constant at 17 oC the hydrate formation occurs at pressures over 59.4216 bar. o Figure 3.3 Hydrate Formation Utility Sweet Gas + CO2 Design
  25. 25. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 30 - 2014 Figure 3.4 Hydrate Formation Utility Sweet Gas + CO2 Performance  “Sour Gas + CO2” stream: o If pressure is kept constant at 80 bar the hydrate formation occurs at temperatures below 19.9749 oC. o If pressure is kept constant at 17 oC the hydrate formation occurs at pressures over 49.2752 bar. Figure 3.5 Hydrate Formation Utility Sour Gas + CO2 Design
  26. 26. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 31 - 2014 Figure 3.6 Hydrate Formation Utility Sour Gas + CO2 Performance The range of conditions in which the hydrates are formed, can be shown in the following diagrams. Figure 3.7 Sweet Gas Envelope
  27. 27. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 32 - 2014 Figure 3.8 Sweet Gas + CO2 Envelope Figure 3.9 Sour Gas + CO2 Envelope
  28. 28. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 33 - 2014 4. CHAPTER 4 CONCLUSIONS Hydrate formation is proven to be a serious problem for the industry as it has negative effects in gas transportation due to plugging and corrosion phenomena. Thus, the industry has to handle great technical difficulties which raise the cost. Hydrate formation is a function of pressure temperature and composition. Low temperatures and high pressures increase the rate of hydrate formation. Moreover, hydrate formation is affected from the composition. Compounds as CO2 and H2S increase the hydrate formation due to the fact that their solubility in water is greater than the hydrocarbon’s solubility. The problem can be handled by adjusting the operating conditions, by dehydration or chemical injection. It is obvious that the prediction of the hydrate formation is mater of vital importance. In the experimental procedure, three natural gas streams with different composition were examined. Pressure was kept constant at 80 bar. For the sweet gas stream the formation occurs under 16.9 oC. For the sweet gas stream which contains CO2 the formation occurs at temperatures under 18.8 oC. Finally for the sour gas stream which also contains CO2, the formation occurs at temperatures below 19.9 oC. It was observed that in the streams which contained H2S and/or CO2 the formation occurred at higher temperatures for a given pressure. As it was mentioned above, the existence of H2S and CO2 is responsible for this.
  29. 29. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 34 - 2014 5. ABBREVIATIONS – INITIALS EoS Equation of State FBR Full Bore Rupture P-T Diagram Pressure-Temperature Diagram Redox Reaction Reduction-Oxidation Reaction MEG Monoethylene Glycol DEG Diethylene Glycol TEG Triethylene Glycol PR EoS Peng-Robinson Equation of State
  30. 30. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 35 - 2014
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  32. 32. HYDRATE FORMATION IN GAS PIPELINES DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY A.Mitsis E.Michailidi F.Zachopoulos - 37 - 2014 19. Sloan, E. D., Khoury, F. M. and Kobayashi, R., Water Content of Methane Gas in Equilibrium with Hydrates, Industrial & Engineering Chemistry Fundamentals, vol.15, no.4, 1976, pp. 318-323. 20. Lasich, M., Mohammadi, A. H., Bolton, K., Vrabec, J. and Ramjugernath, D., Phase equilibria of methane clathrate hydrates from Grand Canonical Monte Carlo simulations, Fluid Phase Equilibria, vol.369, no.0, 2014, pp. 47-54. 21. Yang, S. O., Cho, S. H., Lee, H. and Lee, C. S., Measurement and prediction of phase equilibria for water + methane in hydrate forming conditions, Fluid Phase Equilibria, vol.185, no.1–2, 2001, pp. 53-63. 22. Gernert, J., Jäger, A. and Span, R., Calculation of phase equilibria for multi- component mixtures using highly accurate Helmholtz energy equations of state, Fluid Phase Equilibria, vol.375, no.0, 2014, pp. 209-218. 23. Coquelet, C., Valtz, A., Stringari, P., Popovic, M., Richon, D. and Mougin, P., Phase equilibrium data for the hydrogen sulphide+methane system at temperatures from 186 to 313K and pressures up to about 14MPa, Fluid Phase Equilibria, vol.383, no.0, 2014, pp. 94-99. 24. Norsork Standard, CO2 Corrosion Rate Calculation Model, Norwegian Technological Standards Institute Oscarsgt., Majorstural, Norway, 2005, p.20. 25. Obanijesu, E. O. and Macaulay, S. R. A., West African Gas Pipeline (WAGP) Project: Associated Problems and Possible Remedies, Appropriate Technologies for Environmental Protection in the Developing World,ed., E. Yanful,ed., Springer Netherlands, ch.12, 2009, pp. 101-112. 26. Liang, Y., Shuichi, I., Yomei, Y. and Kunihiko, W., High Temperature Corrosion of Gas Turbine Blade Material in Methane Gas Combustion Environment with the Injection of SO2/NaCl, NACE International, p.1-11. 27. McKee et. al., Carbon Deposition and the Role of Reducing Agents in Hot-Corrosion Processes, Chemistry and Material Science, 2007, pp. 1877-1885. 28. Kritzer, P., Corrosion in high-temperature and supercritical water and aqueous solutions: a review, The Journal of Supercritical Fluids, vol.29, no.1–2, 2004, pp. 1-29. 29. Fingerhut, M., and Westlake, Pipeline fitness for purpose certification, 29/12/2014; http://www.corrosion-club.com/pipelines.htm. 30. Roberge, P. R., "Corrosion Engineering: Principle and Practise, 1st-ed., The McGraw-Hill Companies Inc., New York, 2008. 31. Brown, G. J., Erosion prediction in slurry pipeline tee-junctions, Applied Mathematical Modelling, vol.26, no.2, 2002, pp. 155-170. 32. Gao, S., Investigation of Interactions between Gas Hydrates and Several Other Flow Assurance Elements, Energy & Fuels, vol.22, no.5, 2008, pp. 3150-3153. 33. Fu, S. B., Cenegy, L. M. and Neff, C. S., A Summary of Successful Field Applications of A Kinetic Hydrate Inhibitor, Society of Petroleum Engineers. 34. PetroWiki.org, Preventing formation of hydrate plugs, 28/12; http://petrowiki.org/Preventing_formation_of_hydrate_plugs. 35. Nakayama, H. and Hashimoto, M., Bulletin of the Chemical Society of Japan, 1980, p. 2427.
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