Development of a Hydrate Inhibition Monitoring System by Integration of Acoustic Velocity and Electrical Conductivity Measurements
a report by Jinhai Yang,1,2 Antonin Chapoy,1,2 Saeid Mazloum1,2 and Bahman Tohidi1,2 1. Centre for Gas Hydrate Research, Institute of Petroleum Engineering, Heriot-Watt University; 2. Hydrafact Ltd
Hydrate blockage is a major flow assurance issue in the oil and gas industry and inhibitor injection is probably the most popular technique in preventing its occurrence. The amount of inhibitor required is a function of various parameters, including the fluid composition, operating pressure and temperature, types of inhibitor, water production rate, inhibitor partition in hydrocarbon phases, seasonal changes and so on. Currently, inhibitors are injected upstream without any downstream monitoring. Here, we report the development of a novel downstream hydrate inhibition monitoring system. It determines both the inhibitor and salt concentrations, the hydrate phase boundary and therefore, the hydrate safety margin, by measuring the acoustic velocity and electrical conductivity of downstream aqueous samples. It can be used for any inhibition system containing methanol, monoethylene glycol, kinetic hydrate inhibitors, or anti-agglomerants in the presence of salts. Its performance has been extensively evaluated using synthetic samples and real produced water samples by the authors and leading oil companies.
The increasing demand for oil and natural gas has moved the oil industry to deepwater regions worldwide. The high pressure and low temperature conditions in such regions provide favourable conditions for gas hydrate formation. Gas hydrates, apart from waxes, asphaltene and scales, pose serious flow assurance problems to
Bahman Tohidi is a Professor at the Institute of Petroleum Engineering, leading the gas hydrates–pressure, volume and temperature (PVT) research group. He is also Managing Director of Hydrafact Ltd., a Heriot-Watt University spin-out company with its core business in flow assurance and PVT and a consultant to the oil and gas industry. Dr Tohidi is a member of the Society of Petroleum Engineers (SPE) and of the UK Engineering and Physical
Science Research Council Peer Review College for 2006–2009 and 2010–2013. He was SPE Distinguished Lecturer in 2005–2006 with his talk, entitled ‘Gas Hydrates: Friend or Foe?’ His research interests include gas hydrates, flow assurance (wax, salt and asphaltene), PVT, phase behaviour and properties of petroleum reservoir fluids and
CO2-rich systems. He holds a BSc in Chemical Engineering from the Abadan Institute of Technology in Iran and a PhD in Petroleum Engineering from Heriot-Watt University. E:
bahman.tohidi@
pet.hw.ac.uk
Jinhai Yang is a Research Fellow at the Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh and is also a senior consultant for Hydrafact Ltd. He joined the Centre of Gas Hydrate Research, Heriot-Watt University, in 2000. During the past 10 years, he has been involved in several research projects associated with the mechanism of kinetic hydrate inhibitors, hydrate monitoring and warning techniques, geophysical properties of hydrate-bearing sediments,
methane production from gas hydrate reservoirs and CO2 sequestration in marine sediments. He is a principal author or co-author of some 49 refereed journal and
conference publications, and also a holder of four international patents. He has a PhD in Reservoir Engineering from Southwest Petroleum University (SWPU), where he worked as an Associate Professor before moving to Heriot-Watt University.
deepwater drilling, production and processing, impeding hydrocarbon flow by blocking valves, wellheads and flowlines. It is not unusual for hydrate plugs to form in inaccessible sections of pipelines in deepwater, which leads to the partial loss or complete suspension of gas and oil production.1,2
In principle, there are two options for
One option is to remove one of the elements that is essential for hydrate formation. For example, the hydrocarbon fluids in a pipeline can be kept outside the hydrate stability zone (HSZ) by thermal insulation or external heating, or by lowering the operating pressure, or by dehydrating the system. However, these techniques might not be feasible and/or economical for some conditions, especially in offshore and deepwater environments, because of the high cost of insulation, heating and dehydration.4
Application of hydrate inhibitors is the second option to either prevent or reduce the risk of hydrate blockage. Hydrate inhibitors are divided into two categories: thermodynamic hydrate inhibitors (THIs) and low-dosage hydrate inhibitors (LDHIs). THIs, such as methanol (MeOH) and monoethylene glycol (MEG), are chemical additives that shift the hydrate phase boundary to a lower temperature and/or higher pressure by reducing the water activity. It is not unusual for concentrations of up to 60 mass% of MeOH or MEG in the aqueous phase to be needed for the THI to have a sufficient inhibitory effect. For high water-cut systems, the large volume of MeOH and MEG requires huge storage areas on the platform with limited space and/or a separate pipeline, which can result in significant increases in capital and operating expenditure, as well as negative impacts on the environment.5,6
decades, LDHIs have been developed rapidly.7 hydrate inhibitors (KHIs) and anti-agglomerants (AAs).8–12
believed that KHIs delay hydrate nucleation and hinder hydrate growth within a certain degree of subcooling, whereas AAs allow hydrate formation but prevent hydrate crystals from agglomeration (i.e. keep the hydrates transportable).
In current industrial practice, the amount of a hydrate inhibitor is determined based on the predicted or measured hydrate phase boundary for the specific fluid composition (gas or oil and water), water-cut, worst temperature and pressure conditions, and the estimated inhibitor loss to non-aqueous phases. In addition to the above, a safety factor is also considered when calculating the inhibitor dosage and/or pump rate. Consequently, an excessive dose of an inhibitor usually has to be applied to minimise the risk of hydrate blockages. This results in unnecessary additional cost and more severe environmental impacts, although other efforts have been made to optimise hydrate inhibitor injections.13
Recently, Tohidi et al.14 reported a freezing point depression method for
monitoring the degree of hydrate inhibition. It directly determines the hydrate phase boundary by measuring the freezing point depression
36 © TOUCH BRIEFINGS 2011
avoiding hydrate blockage with regard to the nature of gas hydrate formation.3
As a result, during the past two These include kinetic It is generally
Subsea & Pipelines
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