ReviewReviews of gas hydrate inhibitors in gas-dominant pipelines and application of kinetic hydrate inhibitors in China☆
Introduction
Gas hydrates are solid crystalline ice-like materials in which water molecules bond with each other by hydrogen bonds and form polyhedral water cavities, as well as gas molecules (such as CH4, C2H6, CO2 etc.) that are trapped in these cavities at low temperature and high temperature [1]. Since Hammerschmidt [2] found the hydrate in the pipeline, the threat of hydrate in pipeline became one of the biggest challenges in oil and gas flow assurance. A series of accidents caused by hydrates have drawn worldwide attentions. The oil and gas industry continuously study how to prevent hydrate forming in pipes or they will clog the lines, stop the flow of oil and gas and pose a danger.
Keeping pressure and temperature of operation system out of the hydrate formation region is the base of hydrate inhibition [3]. Heating, using insulated pipelines, depressurization can realize the system conditions transfer from hydrate stable zone to hydrate-free zone [4], [5], [6], [7]. But for the long distance pipes, heating and insulating pipelines become costly. Water removal is an essential method to prevent the hydrate formation. It has been used in many long distance transportation lines. But it is difficult or expensive to remove water totally from the gas and oil, especially in the production well pipes.
A practical method to prevent the hydrate formation is to inject the chemical agent into the pipes to prevent hydrate formation, which is called hydrate inhibitors. Hydrate inhibitors include thermal hydrate inhibitors (THIs), kinetic hydrate inhibitors (KHIs) and anti-agglomerant inhibitors. The last two are named low-dose inhibitors. Anti-agglomerate inhibitors must be used in the water–oil–gas system and water-cut must be in a scope [8], which means anti-agglomerant inhibitors cannot take effect in gas-dominated pipes. Fig. 1 showed a hydrate blockage process in gas-dominated pipelines, which is different with the oil-dominated pipelines [9]. In gas-dominated pipelines, there are three phases, gas, water and hydrate. As the gas and water flow, the gas and water will disperse in each other's phase, forming hydrate particles and hydrate slurry in the pipeline. The formation of hydrate will consume the water in the pipeline. With hydrate increase, the solid–liquid ratio of hydrate slurry increased. When the solid–liquid ratio increases to a certain extent, the pipeline will be blocked due to liquid decline. Additionally, water can form a hydrate film with gas on the pipe wall, which gradually thickens over time and reduces the diameter of the pipe. Thus, pipes will clog. Therefore, preventing hydrate nuclei and growth is more important in gas-dominant pipeline than in oil-dominant pipeline, because once the hydrate starts to grow rapidly, it is hard to keep the pipes from clogging up. THIs or KHIs are necessary in natural gas pipeline, especially in high pressure and low temperature pipelines. This article reviews the hydrate inhibitors used in gas-dominated pipelines, looks back to the efforts of researchers in the past 30 years to improve the KHI performance, and summarizes some application of KHI in Chinese gas fields.
Section snippets
Thermodynamic Hydrate Inhibitors
THIs are a kind of materials that can alter the thermodynamic conditions of hydrate by shifting the hydrate formation region to lower temperature and higher pressure. Typical THIs include methanol (MeOH), monoethylene glycols (MEG), and electrolytes. These compounds can form hydrogen bonds with water molecules to avoid the formation of ordered cages. Just because THIs act with water molecules, the usage of THI commonly is very high, which concentration used is 10 wt%–50 wt% based on water.
Kinetic Hydrate Inhibitors
With the gas and oil well transform from onshore to further offshore, the high pressure and cold deep water create suitable conditions for hydrate formation more easily. The prevention and control of hydrates in pipelines becomes more important. Though the THIs can inhibit hydrate formation completely in the pipelines, the cost of it increases rapidly with the longer pipelines and deeper water. To decrease the hydrate inhibitors usage, Lederhos et al. [50] put forward and extend the metastable
Application of KHIs in China
Research of gas hydrate in China began relatively late, but it has developed rapidly in recent years. As seen from Fig. 5, the number of articles published on KHIs of China is No. 1 among all countries since 2000. In China, institutions that conduct research on hydrate inhibitors are China University Petroleum (Beijing), Guangzhou Inst Energy Convers of Chinese Acad Sci, South China University of Technology, Southwest Petroleum University, Natural Gas Research Institute, China National Offshore
Conclusions
Hydrate inhibition and management is very important to gas industry, relating to the safety and efficiency of production. This issue involves complicated theories, such as thermodynamic, heat and mass transfer, multiphase flow, chemical reaction, phase transfer. Although the studies on hydrate inhibition make a great progress, several challenges and limitations in experiments and application still exist. The main purpose of this paper is to give a review of hydrate inhibition technology in
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Supported by the National Key R&D Program of China (2016YFC0304006 and 2017YFC0307302/03), National Natural Science Foundation of China (51576069, 51876069), the Natural Science Foundation of Guangdong Province (2016A030313488) and Fundamental Research Funds for the Central Universities.