Gas hydrates are of great importance for a variety of reasons (Figure-1). In offshore hydrocarbon drilling and production operations, gas hydrates cause major, and potentially hazardous flow assurance problems.
Naturally occuring methane clathrates are of great significance in their potential for as strategic energy reserve, the possibilities for CO2 disposal by sequestration, increasing awareness of the relationship between hydrates and subsea slope stability, the potential dangers posed to deepwater drilling installations, pipelines and subsea cables, and long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release, and global climate change.
Figure-1 Major issues of gas hydrates.
Source : http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm
Hydrates in Offshore Hydrocarbon Production Operations
Drilling
In drilling, record water depths are continuously being set by oil companies in the search of hydrocarbon reserves in deep waters. Due to environmental concerns and restrictions, water based drilling fluids are often more desirable than oil based fluids, especially in offshore exploration. However, a well-recognised hazard in deep water offshore drilling, using water based fluids, is the formation of gas hydrates in the event of a gas kick.
In deep-water drilling, the hydrostatic pressure of the column of drilling fluid and the relatively low seabed temperature, could provide suitable thermodynamic conditions for the formation of hydrates in the event of a gas kick. This can cause serious well safety and control problems during the containment of the kick. Hydrate formation incidents during deep-water drilling are rarely reported in the literature, partly because they are not recognised, Two cases have been reported in the literature where the losses in rig time were 70 and 50 days.
The formation of gas hydrates in water based drilling fluids could cause problems in at least two ways:
Gas hydrates could form in the drill string, blow-out preventer (BOP) stack, choke and kill line. This could result in potentially hazardous conditions, i.e., flow blockage, hindrance to drill string movement, loss of circulation, and even abandonment of the well.
As gas hydrates consist of more than 85 % water, their formation could remove significant amounts of water from the drilling fluids, changing the properties of the fluid. This could result in salt precipitation, an increase in fluid weight, or the formation of a solid plug.
The hydrate formation condition of a kick depends on the composition of the kick gas as well as the pressure and temperature of the system. As a rule of thumb, the inhibition effect of a saturated saline solution would not be adequate for avoiding hydrate formation in water depth greater than 1000 m. Therefore, a combination of salts and chemical inhibitors, which could provide the required inhibition, could be used to avoid hydrate formation.
Production
The ongoing development of offshore marginal oil and gas fields increases the risks of facing operational difficulties caused by the presence of gas hydrates. A typical area of concern is multiphase transfer lines from well-head to the production platform where low seabed temperatures and high operation pressures increase the risk of blockage due to gas hydrate formation (Figure-2). Other facilities, such as wells and process equipment, can also be prone to hydrate formation.
Different methods are currently in use for reducing hydrate problems in hydrocarbon transferlines and process facilities. The most practical methods are:
- At fixed pressure, operating at temperatures above the hydrate formation temperature. This can be achieved by insulation or heating of the equipment.
- At fixed temperature, operating at pressures below hydrate formation pressure.
- Dehydration, i.e., reducing water concentration to an extent of avoiding hydrate formation.
- Inhibition of the hydrate formation conditions by using chemicals such as methanol and salts.
- Changing the feed composition by reducing the hydrate forming compounds or adding non hydrate forming compounds.
- Preventing, or delaying hydrate formation by adding kinetic inhibitors.
- Preventing hydrate clustering by using hydrate growth modifiers or coating of working surfaces with hydrophobic substances.
- Preventing, or delaying hydrate formation by adding kinetic inhibitors.
Figure-2 A large gas hydrate plug formed in a subsea hydrocarbon pipeline. Picture from Petrobras (Brazil)
Source : http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm
Hydrates and Seafloor Stability
A significant part of the gas hydrate geohazard problem is related to how they alter the physical properties of a sediment. If no hydrate is present, fluids and gas are generally free to migrate within the pore space of sediments. However, the growth of hydrates converts what was a previously a liquid phase into a solid, reducing permeability, and restricting the normal processes of sediment consolidation, fluid expulsion and cementation. These processes can be largely stalled until the BHSZ is reached, where hydrate dissociation will occur. Dissociation of hydrates at the BHSZ can arise through an increase in temperature due to increasing burial depth (assuming continued sedimentation) or an increase in sea bottom-water temperatures, and/or a decrease in pressure (e.g., lowering of sea level). Upon dissociation, what was once solid hydrate will become liquid water and gas. This could lead to increased pore-fluid pressures in under-consolidated sediments, with a reduced cohesive strength compared to overlying hydrate-bearing sediments, forming a zone of weakness. This zone of weakness could act as a site of failure in the event of increased gravitational loading or seismic activity (Figure-3).
The link between seafloor failure and gas hydrate destabilization is a well established phenomenon, particularly in relation to previous glacial-interglacial eustatic sea-level changes. Slope failure can be considered to pose a significant hazard to underwater installations, pipelines and cables, and, in extreme cases, to coastal populations through the generation of tsunamis.
Figure-3 Potential scenario whereby dissociation of gas hydrates may give rise to subsea slope failure and massive methane gas release
Source : http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm
Source :
http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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