Dry tree versus wet tree trends examined
Mark J. Kaiser, Center for Energy Studies, LSU
Inthe finalpart of this five-part series on Gulf of Mexico well trends, subsea wells, also known as wet wells, are examined.
The first subsea wells in the Gulf of Mexico were drilled in the late 1950s. Operators experimented with subsea production systems in shallow water in the mid-1970s to prove-up the technology in anticipation of moving into deeper water. The first deepwater subsea well was drilled in 1988 and remains one of the most important tools in the deepwater development toolkit.
Through 2017, there have been 1,443 subsea wells drilled in the Gulf of Mexico – 112 in shallow water and 1,331 in deepwater. Circa 2017, eight subsea wells were producing in shallow water and 375 were producing in deepwater.
Dry tree vs. wet tree
In subsea completions the tree resides on the seafloor requiring mobilization of an intervention vessel or MODU whenever well work is required. In direct vertical access wells, a subclass of wet well, the tree resides on the seafloor but is accessible from a rig on the platform. The Auger and Ursa TLPs and the Perdido spar employ direct vertical access wells.
Dry tree wells by comparison maintain the tree on the platform and connects to the seafloor wellhead via conductors or tensioned risers. The host facility has the ability to drill, intervene, and monitor each well. On floater structures a heave optimized platform is required, and the inclusion of a rig increases the payload, design, and operational complexity.
Subsea wells are identified separate from dry tree wells because they differ in several fundamental ways in terms of their capital expenditures, operational requirements and limitations, recovery rates, and decommissioning liability.
Subsea wells are more expensive to equip, workover, operate, and decommission than dry tree and direct vertical access wells, by perhaps an order-of-magnitude or more. They are expected to be abandoned at a higher production rate relative to dry tree and direct vertical access wells, for all things equal, because of the back pressure that arises delivering the fluid to the host and less frequent interventions to maintain the well’s productivity.
Subsea wells also have difficulty flowing with high water cuts because of hydrate formation; and if gas lift or subsea compression is used to flow to a lower abandonment pressure, then flow assurance issues such as asphaltene deposition may result.
The cost to drill and complete a subsea well, for all things equal, may be comparable to a dry tree well depending on how the wells are drilled and completed (e.g., with or without a platform rig, number of stages, number of casing strings, well complexity, market rates, etc.).
Subsea wells are not needed or desirable in shallow water because well protectors or fixed platforms can be employed for isolated small reservoirs and there is no advantage using wet wells.
Most shallow-water wet wells are old and about two-thirds of shallow-water wells have been permanently abandoned. Circa 2017, the shallow-water active wet well count was 39 with eight wells producing. Since 2008, only three shallow-water subsea wells have been drilled.
In deepwater, subsea wells are an important component in field development and began to be drilled in large numbers in the mid-1990s when deepwater development concepts began to mature.
Since 2008, about 400 subsea wells have been drilled in deepwater. Circa 2017, there were 1,153 active deepwater wet wells with 375 producing.
Trends in subsea wells spud, abandoned, and producing are shown in the accompanying figures. •