OS6C-4：FLOW LOOP INVESTIGATION OF HYDRATE FORMATION AND FLOW BEHAVIOUR IN UNDER-INHIBITED CONDITIONS

Mauricio Di LORENZO, Karen KOZIELSKI
CSIRO Earth Science and Resource Engineering, 26 Dick Perry Avenue, Kensington WA 6151
Zachary M. AMAN, Eric F. MAY, Michael L. JOHNS
Centre for Energy, School of Mechanical and Chemical Engineering, University of Western Australia
   
    The cost of avoiding hydrate plugs by injection of hydrate inhibitors such as monoethylene glycol (MEG) or methanol significantly increases the operational and capital expenses of deepwater oil and gas developments. There are substantial economic incentives to reduce the volumes of such inhibitors, by managing hydrates instead of completely suppressing them. In addition, transient operations (e.g. restart) may result in uneven MEG distribution through the flowline, with some locations operating in an under-inhibited region. Hydrate growth rates and plug formation mechanisms in under-inhibited conditions represent a critical path to successful operation in deepwater fields.
   
    In this study, the effect of under-inhibition on hydrate formation and flow behaviour in gas-dominant pipelines was investigated using a one-inch flow loop that simulates temperature and pressure conditions found in deepwater environments. Aqueous solutions of MEG in a range of concentrations (0-40% w/w) and domestic gas were used in these tests. They were conducted at the same initial pipeline pressure (10 MPa) and a range of pipeline temperatures to investigate the effect of the subcooling.  Constant flow rates for the gaseous and liquid phases were maintained throughout the experiments to mimic steady-state transport in a gas pipeline with a limited amount of liquids (5% volume fraction). The hydrate formation rate was estimated from the overall gas consumption during each test and the time-dependent pressure drop across the loop was examined as the main indicator of hydrate deposition in the flowline.
   
    The analysis of the data indicates that hydrate plugging risks are effectively managed in the pipeline when the system is fully inhibited, in line with accepted thermodynamic model predictions. When MEG is under-dosed, the pressure drop behaviour over time  is consistent with a proposed conceptual description for hydrate plugging in gas-condensate pipelines based on the mechanisms of stenosis build-up (formation of a hydrate coat at the pipe wall) and sloughing (shear breaking of the hydrate deposits). In experiments where a steady, monotonic increase of the pressure drop was observed, a constant value for the average deposition rate over the entire flowline was estimated by applying the Beggs-Brill correlation for the steady state pressure drop in a horizontal pipe with a variable hydraulic diameter.