From Laboratory to Field Applications: A Safer Gas Hydrate Dissolver to Replace Methanol

Abstract

Low-molecular-weight hydrocarbon gases, such as methane and ethane, along with other low-molecular-weight species, such as CO2 and H2S, exist within the gas stream flowing through the pipelines used to transport natural gas and crude oils. Under certain conditions of low temperature and high pressure, and in the presence of free water molecules, gas-hydrate crystals will start to form. These gas hydrate crystals may accumulate and cause partial or complete plugging of the pipeline in the vertical or horizontal sections of the pipe. Methanol was previously found to be the only effective chemical able to clear gas hydrate plugs in many cases. The low flashpoint of methanol makes it unsafe to be pumped or stored in large volumes in hot regions such as in the Middle East. The main objective of the current work is to develop a safer chemical treatment with a higher flashpoint for the dissolution/mitigation of in-place gas hydrate plugs in pipelines. The parameters that make methanol an effective hydrate dissolver were investigated before development of the new hydrate dissolver. Methanol has a very low freezing point (-90°C) and is completely miscible with water. Solvent-based and aqueous-based formulations were chosen while considering parameters such as miscibility with water, freezing point, viscosity, and availability. The performance of these formulations was evaluated using a see-through gas hydrate cell. Pressure and temperature were modified to allow the formation of hydrates at simulated field conditions. Representative gas and water compositions were used in the experiments to form gas hydrate inside the gas hydrate cell. Hydrate formation can be detected either by detecting the change in torque on the stirring shaft or by visual inspection of the cell. The performance of these formulations was evaluated and compared to methanol. Two solvent-based formulations and two aqueous-based treatments met the flashpoint requirement and were effective in dissolving hydrate plugs at similar dosage when compared to methanol. These formulations proved to dissolve gas hydrate plugs in similar or less time than methanol. Two field trials were conducted to test one of the aqueous-based treatments and the outcome showed that the hydrate plug was dissolved in less than 17 minutes from the moment the aqueous solution was injected. The tested formulations have been shown to not only work as a dissolver, but also as an inhibitor to prevent the fresh formation of hydrate downstream. In addition to improving the performance of dissolving gas hydrate plugs in the pipelines, the use of these treatments has enabled safer operations in the field.