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Title: Gas hydrate control by low dosage hydrate inhibitors
Author: Arjmandi, Mosayyeb
ISNI:       0000 0001 3427 338X
Awarding Body: Heriot-Watt University
Current Institution: Heriot-Watt University
Date of Award: 2007
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Gas hydrates are ice-like crystalline compounds, which form through a combination of water and suitably sized' guest' molecules under low temperature and elevated pressure conditioiJ.s. The formation of gas hydrates in subsea pipelines can cause pipeline blockage, resulting in serious economic and safety issues. Gas hydrate formation is generally prevented by employment of so-called 'thermodynamic inhibitors', which include salts and organic compounds such as methanol and ethylene glycol. However, the use of thermodynamic inhibitors can 'become uneconomical when high concentrations are required and/or water cut is high. There are also important associated issues with respect to inhibitor recovery and environmental damage. In the light of this, other methods for hydrate prevention such as making use of natural hydrate inhibitors in oil systems and application of a new family of hydrate inhibitors, - . termed 'Low Dosage Hydrate Inhibitors' (LDHI), are becoming attractive options. In this work both methods have been addressed by investigating the primary mechanism and the parameters involved in hydrate inhibition by the mentioned methods, using novel experimental techniques, and an in-house hydrate model. It is known- that water/oil (W/O) emulsions can reduce gas hydrate blockage risks. Natural surfactants such as asphaltenes and resins in the oil are commonly identified as the agents responsible for stabilising W/O emulsions. In this work, it was shown that oil properties, mixing rate and mixing history, water content, and operational conditions ,- (e.g. pressure) play significant role in reducing hydrate blockage risks in oil/water _systems. The effect of mixing rate on the induction time before hydrate formation was shown to be a function of system mixing history (degree of emulsification of water in oil). Before formation of stable emulsion, the induction time increased with mixing rate. However after formation of stable water/oil emulsion induction time was not a strong function of the mixing rate. Water content found to be the most important factor in controlling the risk. It was shown that for the oils tested, water cuts up to 20% do not pose any risk of blockage in the system tested while at 30% water cut a low dosage hydrate inhibitor will be needed for preventing hydrate blockage. A novel experimental set up (Glass Micromodel set-up) was used to obtain visual information regarding the state of water oil emulsion, size of water droplets in the emulsion, hydrates particle size and morphology and distribution of different phases in the system. The results showed that heavier components in the oil phase are attracted on gas hydrate crystals formed in a water foil emulsion (the oil surrounding the hydrate particles became brighter and more transparent). Furthermore, it was demonstrated that at static condition the agglomeration of hydrate particles appears to be easier than in flowing conditions in the Micromodel set-up. That was in line with the results obtained from the kinetic rig tests (where long shut-in times resulted in stirrer blockage). The principal limitation to curren~ Kinetic Hydrate Inhibitor (KHI) design techniques is a lack of verified molecular mechanisms for LDHI activity. In the framework of a jo~nt project between Heriot-Watt and Warwick Universities, a new approach has been used in the design and testing of new LDHIs. Chemicals designed using molecular dynamic simulation were subsequently synthesised (Warwick University) and tested using novel experimental techniques under simulated offshore pipeline conditions to evaluate their potential for use in offshore operations and factors affecting their performance and to study primary mechanism of hydrate inhibition (Heriot-Watt University). The new KHIs showed mild hydrate inhjbition erfect. In natural gas-water system, their performance was not as good as conventional i<HIs (poly-vinylcaprolactam (PVCap) and poly vinyl pyrrolidone (PVP)), however in methane water system, one of them performed better than PVP. Furthermore, the new KHIs demonstrated good antiagglomeration characteristics (after failure and hydrate formation). Visual observation of hydrate formation and growth in the presence of new KHI showed that it prevents agglomeration of the hydrate particles and cause deformation of the hydrate crystals. In general the performance of the KHIs tested in this study including conventional KHIs were better in structure II hydrate systems compared to structure I hydrates. Identification of the parameters affecting the performance of Kinetic Hydrate Inhibitors (KHI) is crucial for effective design, screening and deployment of them in deepwater applications. in this work, some of the influential parameters on the performance of PVCap were experimentally studied by application of a kinetic rig and a visual rig. The effect of mixing, pressure, polymer molecular weight, the solvent, subzero conditions, and different gas hydrate structures on the performance of a KHI, PVCap were investigated. The negative effect of static conditions on the performance ofPVCap was shown in a visual kinetic rig. By the experiments in a kinetic rig, the negative effect of pressure at constant subcooling on the performance of PVCap was demonstrated. It was shown that the low molecular weight PVCap inhibit hydrate formation better than .high molecular weight PVCap. At constant subcooling, PVCap was shown to inI:tibit structure II hydrate more effectively than structure I hydrate. The negative impact of corrosion inhibitor on the performance of PVCap was shown. The results showed that ethylene glycol (as a carrier fluid) does not have any significant effect on the perfonnance of PVCap. Furthennore, it was shown that PVCap can inhibit hydrate fonnation at subzero conditions in the presence of ethylene glycol. Anti-Agglomerants (AA) are another class of LDHIs developed over the last decade, which prevent hydrates from agglomerating and depositing in pipelines. In this work, after a brief description and literature survey on the development and testing of AA chemicals, a new methodology for testing AAs using the Kinetic Rig and the Glass Micromodel set up was presented. The perfonnance of AAs was evaluated in kinetic rig by torque measurements in different conditions. Two different types of impellers were used for torque measurement and it was shown that the torque llleasurement was improved by using a helical tube instead of paddle-shape impeller.. It was shown that the infonnation obtained from torque measurement technique can be used for screening AAs, however it is not sufficient for selection of an AA for field application. The complementary infonnation such. as hydrate particles size, morphology and their . distribution in different phases can be obtained from Glass Micromodel set-up. Preliminary experiments, using a proven AA chemical in comparison with another similar compound and an un-inhibited system, showed that the techniques developed in this study are suitable and effective for the testing ofAAs. In studying the kinetics of hydrate fonnation and inhibition at different conditions, a thennodynamic hydrate model is essential for predicting hydrate phase boundary from which the driving force for hydrate fonnation can be calculated. In this work an inhouse hydrate model (Heriot-Watt Hydrate Model (HWHYD model) has been used for thennodynamic description of the phases and' prediction of hydrate phase boundary. As part of. the above model, a new approach in modelling phase equilibria and gas . solubility in saline solutions has been proposed. Salts were introduced as components in the EoS by calculating their EoS parameters from corresponding cation and anion parameters. A non-density dependent mixing rule was used for calculating a, b, and c �·,parameters of the EoS. The inclusion of salts in the EoS resulted in the omission of Debye-Huckel electrostatic contribution tenn in the fugacity coefficient calculations. 'Water-salt binary interaction parameters were optimised using freezing point �· depression and boiling point elevation data of aqueous electrolyte solutions. Gas ��. solubility data in aqueous electrolyte solution were used for optimising salt-gas BIPs. �· The predictions of the model have been compared with independent experimental data, .demonstrating the reliability of the approach.-The degree of subcooling is usually used as the driving force for hydrate fonnation; however, it does not encompass the effect of pressure. In this work, by application of the two latest driving force expressions for hydrate fonnation, and an in-house hydrate model the relationships between subcooling and the calculated driving force at different conditions for pure gas-water and natural gas-water systems have been analysed. The relationship between the driving force and the degree of subcooling for methane, ethane and propane demonstrated that subcooling is a good representative of driving force for pure compounds over a wide pressure range.�· For natural gas systems at isothennal conditions, between 5 and 20 MPa, subcooling underestimates the calculated driving force for hydrate fonnation; however, above 20 MPa, subcooling is a�· good�· representative of real driving force. Constant degree of subcooling is an appropriate criterion for up-scaling the tests with pure gas and natural gas. For natural gas-water systems at constant driving Jorce/subcooling conditions, the induction time does not seem to be a function of pressure, while in the presence of PVCap, increasing- the system pressure had a negative effect on the induction time. This was attributed to the effect of KHI and pressure on the kinetic barriers for hydrate fonnation in a system. Therefore, testing KHls at similar field conditions is recommended.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available