Seafloor Drilling of the Hydrate Zone for Exploration and Production of Methane

The main cost here is only that of the pipeline used to transport the gas to the pro­duction platform. For subsea systems that do not produce to a fixed platform, a drilling template must be used that connects to a group of wells.

Drilling capabilities developed for conventional deepwater hydrocarbon explo­ration and production are presently sufficient for drilling and completing produc­tion in hydrate and associated gas deposits. Hydrate system deposits are always to be found relatively close to the seafloor. Hydrated sediments are expected in water depths between 500 and 2,000 m. Semisubmersible drilling systems with this depth capacity are currently available; however, the costs are extremely high (Brandt et al. 1998). These systems are capable of deep drilling for oil and gas with multilateral completions into reservoir compartments. Gas production is often

Seafloor Drilling of the Hydrate Zone for Exploration and Production of Methane

avoided because of the cost of transport. To maintain reasonable gas costs, new technologies for recovery and transport are required.

Methane hydrate occurs in marine sediments in water depths greater than about 450 m on continental margins of open oceans. Below this, the temperature-pressure conditions in the sediment are appropriate for hydrate formation (Miles 1995). However, gas hydrates do not exist where the methane flux is too low, regardless of ambient thermodynamic conditions. Gas hydrates exist within seafloor sediments because sufficient gas and water are available to form hydrate in intergranular pore spaces. Oceanic hydrate system deposits occur mainly along continental slopes.

The hydrate economic zone is the combined hydrate, gas, and subjacent sedi­ment zone for which it is important to characterize methane and the geotechnical properties that have a bearing on the gas recovery. It includes the hydrate stability zone and subjacent gas and pore fluid zones that are gas-rich. This is because the gas flux and transport to the hydrate economic zone, as well as methane inter­change between hydrate and the gas phase, affects sediment properties and stabil­ity. In an area where sedimentation has continued over a long period of time, hy­drate at the base of the hydrate stability zone may become unstable and dissociate because of rising geotherms. Where this happens, the hydrate conservation cycle, which is a steady-state, long-term process, conserves and concentrates the meth­ane (Max and Lowrie 1996). Gas produced in or below sediment from dissociated hydrate will rise through buoyancy into the hydrate stability zone and tend to

 

Seafloor Drilling of the Hydrate Zone for Exploration and Production of Methane

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Transport pipeline glyciol injection control lines

 

Seafloor Drilling of the Hydrate Zone for Exploration and Production of Methane

Gathering line

 

Multiple wellheads

 

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Seafloor Drilling of the Hydrate Zone for Exploration and Production of Methane

Geothermal heat flux

 

Fig. 5.5 Riserless drilling and production system for gas hydrates (Max and Chandra 1998)

 

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again form gas hydrate. It is presently estimated that this economic zone is no more than 1.5-2 times the thickness of the hydrate stability zone. Below this zone, sediment compaction and geotechnical properties are more predictable and only gas generation occurs. The hydrate stability zone is relatively shallow, greatly limiting potential overpressures. Methane can normally penetrate the hydrate sta­bility zone along faults (Dillon et al. 1997) and natural blowout of considerable volumes of methane has taken place.

In the case of a hydrate-culmination gas deposit, the drilling strategy can be ei­ther direct or indirect. In the first case, the gas reservoir is drilled directly through the hydrate and in the second case, the gas is drilled and tapped from the side, or possibly from below the gas closure, through the use of horizontal drilling tech­niques. Direct drilling into a normally pressurized gas trap does not usually result in undue safety or blowout problems because of the physical strength of the res­ervoir. Recent advances in drilling technology, which allow for considerable lateral and possible upward return drilling, might be used to avoid problems asso­ciated with direct drilling.

Avoiding technical problems associated with drilling directly down through the hydrate layer may be preferable to attempting to compensate for them. Indirect drilling, for instance, would penetrate the hydrate stability zone to the side of a bathymetric culmination gas trap, minimizing the likelihood of blowout. There is an additional advantage in having a long lateral hole in a gas reservoir: drawdown of gas would take place over a broad area through the reservoir rather than being localized near a vertical hole exposed to a shorter gas section.

A larger drawdown intersection may make it easier to maintain reservoir pres­sure and hydrate cap stability, as well as compensating for low or variable porosity and permeability.

Currently available drill string lengths are up to 8-10 km. In deep water, and to a lesser extent on continental slopes where terrigenous marine sediments are more common, the sediments are generally fine-grained silts and clays. As a result, the theoretical maximum drill string lengths may be realized in practice. Because the maximum water depths at which drilling can take place are no more than 5 km, the excess string length could be applied to curved and horizontal runs to indirectly tap hydrate-trapped gas.

Drilling within the hydrate economic zone, which extends no more than about 1­1.25 km below the seafloor mud line, can be carried out differently from conven­tional drilling. Ordinary drilling must penetrate much deeper below the seafloor and is likely to encounter a much wider variety of drilling conditions, including rocklike materials and substantially higher temperatures. To reduce the cost of completing the wells, a drill-in, telescopic casing system technique would be employed.

A workboat-based, riserless coiled tubing drilling system is proposed for this application. Composite coiled tubing would be used to drill in a horizontal pro­duction casing a few hundred meters beneath the seafloor. Large numbers of these low-cost wells could be tied together to support an offshore production facility and pipeline transport to shore. Figure 5.5 shows a riserless drilling and production system for gas hydrates.

Offshore operators have from time to time reported problems in drilling through gas hydrate zones. Drillers seeking conventional hydrocarbons have whenever possible purposely avoided drilling through natural gas hydrates because the proc­ess introduces two foreign sources of heat, friction and circulated drilling mud, that can cause dissociation of hydrates immediately adjacent to the borehole. When this is not avoidable, the hydrate stability zone is drilled and cased as fast as possible to minimize the risk of wall failure, perhaps leading to loss of the hole. Additionally, the free-gas zone beneath a hydrate cap can be overpressured, such that drilling into it without taking proper precautions can result in a blowout, just as is the case when conventional oil and gas drilling targets are involved. The Minerals Man­agement Service has long maintained maps of the potential offshore natural gas hydrate occurrences to help ensure that this and the next category of risks are avoided or anticipated.

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