Hydrates Prevention and Removal in Ultra-Deepwater Drilling Systems
Authors: Botrel Thierry 
Published: October 2008

With more and more deep water wells drilled and the water depth increasing, Hydrate blockages impairing well control equipment is a possibility that can no longer be ruled out. Chemical inhibition of drilling mud is only partially effective therefore, a sound strategy to prevent and remedy hydrate incidents during drilling operations must be implemented. Various scenarios of hydrate blockage with their possible consequences are examined. Procedures, methods and equipment modifications are proposed in order to avoid or to mitigate the problem. The possibility of hydrate formation involving Nitrogen gas and the water content(s) of Hydraulic control fluid inside subsea accumulators is also discussed. Depressurization, thermal, chemical and mechanical methods to remove a hydrate plug are studied with their pros and cons. The use of existing equipment and requirements for specific tool and well control equipment design is highlighted.

Introduction

With an increasing number of wells drilled at water depths approaching 3000 meters or in frontier environments where sea bottom temperature is below 0°C, chemical inhibition of drilling mud can no longer be considered as a safe and practical solution to be fully protected against hydrates formation. An important section of the well bore and the drilling riser is exposed to hydrate clogging especially during long cooling off periods. Even if the number of events reported is still low, the economical and environmental risk of a hydrate incident escalation is serious enough to justify the development of a strategy for hydrate prevention and removal.

Hydrates

Hydrates are ice like compounds of gas and water. Natural gas hydrates are formed when pressure increases and temperature decreases. Depending on pressure and gas composition, natural gas hydrates may remain stable well above water freezing temperature. Mechanical and physical properties of methane hydrates are very similar to the ice. Hydrates can be formed with any gas and even condensate providing the right combination of pressure and temperature and existence of a water phase either liquid, vapor or solid. Some non-hydrocarbon gases, even at low concentrations help hydrates to form at lower pressure and higher temperature, this is especially the case with hydrogen sulfide and carbon dioxide. When dissociating hydrates generate high volumes of gas, around 170standard M3 per M3 for a methane hydrate. Once formed, hydrates are difficult to remove, it takes around twice more heat to dissociate hydrates than to melt ice. If hydrates melt in an enclosed space pressure can built several hundred bars, well above burst and collapse pressure of standard drilling tubular equipment.

Hydrates related hazards

The following hydrates related incidents have been reported or suspected to be possible during deep-water drilling and completion operations (We will not address drilling through hydrates neither formation testing).Hydrates form by gas channeling at mud line clogging wellhead hydraulic connector and prevent disconnectionDuring well control situation, hydrate plugging kill and choke line and impairing proper actuation of Blow Out Preventers (BOP) rams[1]Hydrates formed by a gas leaking in a casing tubing annulus leading to a stuck tubing string.