Performance Evaluation of Kinetic Hydrate Inhibitors for Well Fluids Experiencing Hydrate Formation
Authors: Park Su-youl , Kim Juneyoung , Choi In-whan , Chang Daejun , Seo Yutaek , Shin Jooyoung , Kang Seong-Pil 
Published: March 2015

Offshore flowlines transporting hydrocarbons have to be operated very carefully to avoid the formation of gas hydrates as they are considered one of the largest concerns for flow assurance engineers. The oil and gas industry is generally relying on chemical injection for hydrate inhibition; however hydrate blockages can occur in many different places of offshore production system due to unexpected circumstances. Once hydrate blockage formed considerable efforts are required to dissociate the hydrate via depressurization. Because residual hydrate structures known as gas hydrate precursors will be present in the aqueous phase after dissociation, the risk of hydrate re-formation becomes extremely high. Although the KHIs are becoming popular in many fields as hydrate inhibitors are considered not effective to inhibit the hydrate formation in the presence of residual hydrate structures, so that the use of KHIs for shut-in and restart operations is not recommended. In this study, new experimental procedures composed of three stages are designed to simulate the dissociation of hydrate blockages and transportation of well fluids experiencing hydrate formation. The obtained experimental results have shown that gas hydrates are rapidly re-formed when the temperature of dissociated water falls into the hydrate formation region. With an injection of KHIs before transporting the well fluids, the subcooling increased significantly indicating the possible use of KHIs for transporting the well fluids after dissociation of hydrate blockage. Moreover, the inhibition performance of KHIs is also investigated with two different gases to study the effect of gas composition. This study is confirmed that KHIs are possible candidate to prevent the hydrate re-formation in well fluids experiencing hydrate formation if the KHI is carefully evaluated.

Introduction

Gas hydrates are nonstoichiometric crystalline compounds that are classified into three structural families of cubic structure I, cubic structure II, and hexagonal structure H. Offshore flowlines transporting hydrocarbons have to be operated very carefully to avoid the formation of gas hydrates as they are considered the largest concern for flow assurance engineers. For many years industrial practice to prevent hydrate-related risks is the injection of thermodynamic hydrate inhibitors (THI) at the wellhead, commonly methanol or monoethylene glycol (MEG), to shift the hydrate equilibrium curve toward higher pressure and lower temperature, so that the operating condition of offshore flowlines are outside of the hydrate formation condition. However as the search for hydrocarbon resources moves into deeper and colder waters further offshore, these conventional techniques are becoming uneconomic due to higher injection rate of inhibitors and accompanying operational issues such as logistics and storage requirement.

Although the MEG injection is considered to be the standard method for the offshore gas production system, Kinetic Hydrate Inhibitor (KHI) is also becoming popular as its dosage rate is expected in the range of 0.5~3.0 wt%, which is much lower than MEG's 30~60 wt%. Kinetic hydrate inhibitors (KHIs) are water-soluble polymers and delay the formation of hydrate crystals. These include homo- and co-polymers of the N-vinyl pyrrolidone (PVP) and N-vinyl caprolactam (PVCap). The KHIs available to date are only effective in subcoolings up to 14 oC and their performance can be affected by the presence of other chemicals such as corrosion inhibitors.

There have been attempts to develop a KHI evaluation method based on a hydrate precursor where the hydrate-forming gas was a binary mixture of methane and propane that forms structure II. In this work, we conduct experiments to investigate hydrate formation in the presence of hydrate precursors and the effects of KHI on the inhibition of hydrate re-formation simulating the cold start-up after remediation of hydratep plug.