Gas hydrate deposition & remediation during continuous/transient operations

Abstract

Within hydrocarbon production, gas hydrates present a significant issue preventing safe and reliable operation. Plugs from these compounds develop within hours to days from jamming, agglomeration, and deposition processes. In particular, deposition comprises an outstanding and difficult subject to address due to the terminology’s broad coverage of various mechanisms, which include aggregate bedding, particle impingement, liquid splashing, and condensation. This thesis focuses on its potential mitigation with a surface treatment as well as its significance during transient shut-in/restart operations.

Efforts began with a bench-scale evaluation of a smooth, omniphobic surface treatment on its interaction with multiple pipeline liquids and solids. A benchtop interfacial tensiometer, rocking cell, flowloop, and mechanical shear device demonstrated reduced/prevented wetting, deposition, formation, and adhesion of water, crude oil, gas hydrates, asphaltenes, and waxes. This work showed one surface treatment provided passive protection from multiple flow assurance issues.

A scientific understanding of gas hydrate deposition prevention with the surface treatment ensued. Induction time, rocking cell, and flowloop trials highlighted surface roughness/energy controlling effects on gas hydrate nucleation, condensation-driven growth, and aggregate deposition. For optimal prevention, treatments required both smoothened and low energy features, which reduced physical/chemical interactions between fluids and the wall. Next, the surface treatment application scaled to a laboratory flowloop for transient oil-dominated gas hydrate experiments. Baseline tests displayed a prevalence of aggregate bedding. High volume, low conversion deposits, which formed rapidly after cold restart, dominated plugging. Then, application of the omniphobic surface treatment to only 15% of the flowline rectified this problem. The system avoided significant stenosis and thus plugging. The surface treatment successfully scaled to fully flow systems, matching results from benchtop apparatuses. Further transient gas hydrate experimentation transpired on a pilot-scale flowloop with an attached riser. Despite the different setup, analogous plugging mechanisms to the baseline lab-scale flowloop trials occurred. A bedding-based deposition process controlled plugging. Furthermore, possible severe slugging in the riser arose and perpetuated due to the presence of the solid slurry. The riser’s presence emphasized the importance of geometry in laboratory testing setups. Lastly, collected flowloop observations combined with research from key operators and academic groups to generate a conceptual picture for gas hydrate plugging during transient operations. This illustration created a basis for the unresolved modeling efforts of these scenarios.

Overall, the scientific impact of this work emanates from identifying the wall features preventing gas hydrate formation/deposition, connecting solids presence to possible severe slugging, and developing a novel illustration of gas hydrate plugging through deposition during pipeline restart. This information aids researchers in the design of future surface treatment formulations for desired gas hydrate formation and deposition prevention properties. Furthermore, once directly verified, it presents a previously unknown expansion to the envelope of severe slugging occurrence by considering gas hydrates. The thesis work ends by formulating a modeling pathway during highly-complex transient flow conditions for predictive tools. Though each area involved the first recorded instance of such descriptions, the general conclusions applied broadly enough to appropriate scenarios to extend beyond the limited conditions shown in this thesis.