Development of a hydrate early warning system based on tracer technique application

Abstract

Flow assurance in the oil and gas industry is a very important factor that has proven to have a huge effect on the industry economically. It is well known that any unparalleled method to ensure efficient uninterrupted flow of fluids from the reservoirs to processing facilities is still non-existent. The issues associated with flow assurance in terms of fluid delivery include gas hydrates, wax, asphaltenes etc. Over the years, methods of reducing the risks posed by hydrate formation and deposits in flow lines have proved to be very capital extensive. Conventional ways to prevent and reduce hydrate risks during gas and oil transportation in transfer lines and process facilities is an important requirement in the oil and gas industry. The more effective practise is to inject hydrate inhibitors at the upstream end of pipelines based on certain factors including calculated/measured hydrate phase boundary, water cut, worst pressure and temperature conditions etc. Of recent, new methods have been developed that can monitor downstream inhibitor concentrations and shield the system against hydrate formation due to unforeseen circumstances. This thesis presents the research work done on a new technique that has been developed to detect early signs of hydrate formation which is based on tracer application. The tracer technique presents a unique concept based on the assumption that a non-hydrocarbon compound which can readily form hydrate at low concentrations and be detected at these concentrations, can be used to monitor compositional change in the gas phase of a flow line. Sulphur hexafluoride (SF6) meets all the requirements to facilitate this technique. Significant contributions by the JIP research program both in past years and current or future research projects have made it possible to better develop the application of the technique. However, a proper understanding of experimental methods and actual sample and overall compositions is very important for an accurate interpretation of the results. The centre for Gas Hydrate Research in the Institute of Petroleum Engineering, Heriot Watt University is a facility built to study the production and properties of gas hydrates. Three significant experiments were carried out for the development of this technique; the first experiment determined the hydrate phase boundary of SF6 in a high-pressure rocking rig. The SF6 hydrate dissociation points were measured at three different temperatures in the presence of V-LW, V-LW-H, V-LW-LHC and V-LW-LHC-H. The second experiment investigated the partitioning behaviour and sensitivity performance of SF6 in CH4-SF6/NG-SF6 gas mixture and deionised water using a high pressure kinetic autoclave to be able to simulate realistic pipeline conditions. Systems with chemical additives including PV Cap, ethylene glycol, salt, methanol and light oil (condensate) were also investigated. The effect of hydrate structure and low water to gas ratio were also investigated. Results showed that SF6 has the potential as a tracer for detecting early signs of hydrate formation. The final experiment investigated injecting the SF6 along with a chemical additive (a commercial corrosion inhibitor) acting as a carrier fluid to transport the SF6 tracer into a pipeline as an alternative injection method. The feasibility of this method depended mainly on the solubility of the SF6 tracer gas in the corrosion inhibitor. Results showed that it is possible to inject SF6 together with a corrosion inhibitor which would ultimately make the process more cost effective as no equipment will be required for the injection process.