Inhibition and dissociation of gas hydrates using glycols/alcohols and biodegradable kinetic hydrate inhibitors

• Main Author: Morteza, Aminnaji
• Other Authors: Tohidi, Professor Bahman, Anderson, Mr. Ross
• Format: Thesis
• Language: English
• Published: Heriot-Watt University 2018
• Subjects: permafrost
• Online Access: http://hdl.handle.net/10399/4006

Gas hydrates are a major flow assurance issue that can cause serious operational and safety problems in the oil and gas industry. In the past decade, low dosage hydrate inhibitors (LDHIs) – including kinetic hydrate inhibitors (KHIs) and anti-agglomerants (AAs), have increasingly been used to prevent hydrate formation/blockage. LDHIs offer significant advantages over thermodynamic hydrate inhibitors (THIs); while LDHIs are used typically dosed at <2.5 wt% in produced water, THIs are used at much higher concentrations, e.g. up to 50 wt%. While KHIs are generally known as hydrate nucleation inhibitors, more recent work (Crystal Growth Inhibition (CGI) method) shows they are powerful crystal growth inhibitors for KHIs which can completely inhibit hydrates indefinitely even in the presence of hydrates. Concerns about their biodegradability have hindered their more widespread usage (in Norwegian waters they cannot be used at all), especially when the produced water containing KHIs is potentially released into the sea, sparking a growing interest in the use of green biodegradable chemicals. While only a limited number of potential green KHI candidates have currently been found, the performance of these KHIs is not well known, and they are not yet used for real applications. The main objective of this thesis is to investigate Bio KHIs and their potential combination with THIs for prevention and remediation of gas hydrates. Initial findings of experimental studies using CGI method aimed at investigating three green KHIs which have been reported / are produced (Luvicap Bio, ECO-530, and pectin) show Luvicap Bio has better KHIs properties and was selected for further investigation in this thesis. The performance of Luvicap Bio alone including structure effects and hydrate fraction tolerance was investigated and compared with PVCap. It was concluded that Luvicap Bio is a better inhibitor for s-I methane than for s-II methane, i.e., hydrate growth pattern and structure change studies support the theory of multi-structure hydrate formation in natural gas and s-I and s-II in the methane or ethane system. In addition, inhibition of hydrates using a combination of Luvicap Bio and THIs (MEG, methanol, and ethanol) was investigated to see whether Luvicap Bio could be used to reduce the required THIs dosage for hydrate inhibition. The results showed that Luvicap Bio combines well with MEG and it could reduce the methanol requirement, i.e., methanol is toxic, so green KHI replacing is an application with respect to the environment. Furthermore, work was carried out to investigate hydrate dissociation/remediation using either THIs or KHIs. Hydrate blockage removal in the vertical pipe was investigated using MEG, methanol, and their combination. The results showed that the density of THI mix is a key factor for hydrate plug removal. The initial findings of experimental studies aimed at investigating hydrate dissociation using KHIs demonstrate that, in addition to inhibiting hydrate growth/nucleation, KHI polymers can induce partial or complete hydrate dissociation. It is speculated that it is related to hydrate structure/morphology change. In addition to improving confidence in KHI field use, findings potentially have novel applications with respect to the use of combined THI + KHI (or KHI alone) for hydrate plug remediation and gas production from naturally occurring hydrates in oceanic/permafrost sediments.