Economic benefit of methane hydrate reformation management in transport pipeline by reducing thermodynamic hydrate inhibitor injection

Abstract

Hydrate reformation is an inherent risk in a methane hydrate production system because dissociated water and methane flow through transport pipelines simultaneously. In this study, comprehensive experiments using high-pressure autoclaves are performed to investigate the effect of reducing the concentration of thermodynamic hydrate inhibitor on hydrate reformation characteristics. In order to simulate the production of methane hydrates, these are formed and dissociated in the preliminary stage; thereafter, the hydrates are reformed from dissociated water. Although dissociated water and fresh water are found to have similar formation rates, they have a major difference in relative torque, especially in the early stage of hydrate formation. The instant rise of the relative torque in the dissociated water is observed to be up to 2.5; thereafter, it remains approximately 3.0 until hydrate conversion reaches 28 vol%. This trend is different from that in fresh water, where a steady increase in relative torque with sudden spikes is observed. These results clearly show the high risk of hydrate blockage in the transport pipelines of a methane hydrate production system; hence, the injection of hydrate inhibitor is necessary. In this study, monoethylene glycol (MEG) is the selected thermodynamic hydrate inhibitor; its concentration is maintained below a specific value to completely avoid hydrate formation. Despite the occurrence of hydrate reformation, a stable relative torque is observed in a 20 wt% MEG concentration, where the conversion of water to hydrate approximately reaches 40 vol%. For a 10 wt% MEG concentration, a sudden increase in relative torque of up to 2.0 is observed in the early stage of hydrate formation; this suggests that the foregoing is less effective in sustaining flowability. Compared to fresh water, the addition of a 20 wt% MEG to dissociated water results in a more stable torque. Once the performance of the MEG concentration reduction is confirmed, the MEG regeneration process is designed accordingly by using both multiphase flow and process simulation models. A life cycle cost (LCC) analysis is conducted by estimating the capital expenditures (CAPEX) and operating expenditures (OPEX) of the MEG regeneration process. Considering both CAPEX and OPEX, the LCC for complete inhibition (35 wt%) is 90.15 MUSD (million United States dollars); however, for the 20 and 10 wt% MEG concentrations, the LCC values are reduced to 86.79 and 84.58 MUSD, respectively. The foregoing suggests the potential economic benefit of methane hydrate reformation management to avoid blockage.