A review of technologies currently applied in offshore environments to drill and complete subsea wells for hydrocarbon production confirms that many opportunities exist to improve methodologies in ways that can be more economically beneficial and more environmentally sustainable.
Most drilling and completion challenges have been met and overcome on a case-by-case basis although collective knowledge, and industry improvements, have progressed rapidly since the late 2000s.
Subsea completions for gathering hydrocarbons from subsea wells have demonstrated both environmental and economic benefits for offshore oil and gas projects. Barriers and opportunities for expanded use of subsea completions involve both technological and regulatory issues. Advanced technologies are needed to ensure long-lived and serviceable subsea equipment.
Reasonable regulations also are needed to make sure that the best available technologies and practices are considered in rulemaking that affects subsea operations and future of subsea well access.
Well Completions and Subsea Completions
Completion is used in offshore oil and gas activities in two different contexts. A well completion involves a set of actions taken to convert an individual borehole into an operational system for controlled recovery of underground hydrocarbon resources. A subsea completion refers to a system of pipes, connections, and valves that reside on the sea floor and serve to gather hydrocarbons produced from individually completed wells and direct those hydrocarbons to a storage and offloading facility that might be either offshore or onshore.
Subsea completions typically contain an upper completion, a lower completion, and a production tree. Advances in upper and lower completions followed normal developments in materials, pressure, and temperature ratings. However, significant advancements in subsea well access has occurred in the past few years. The later advancement reduced operational costs and led to the capability to develop one-trip, single-system, umbilical-less installation of tubing hangers.
One of the remarkable accomplishments of the upstream offshore petroleum industry has been the development of technology that allows simplified but efficient subsea well access to carry out the operations in a more complex situation.
Despite an increase in complexity, improvements in subsea well access and completion technology have allowed more complex well patterns to be drilled to a greater depth such that additional hydrocarbon resources can be developed at a greater distance from the drilling or production structure, allowing more energy to be produced with less environmental impact.
In the past 15 years, mechanical subsea systems have been developed, which allow deep-water riserless drilling with weighted mud and with fluid returns to the drilling rig. Those systems allow a dual-gradient hydrostatic pressure to be applied, thereby more closely matching the natural deepwater pressure profile. While those systems have been used on a number of offshore wells, there is limited supply of the necessary equipment and other well-control issues must be carefully considered for each particular operation.
The integrity management process (IMP) is the core of the integrity management system (IMS) and should ensure safe, cost-effective, and reliable operation of the subsea production system. It is a continuous and iterative process that should be part of the whole life cycle of the system, starting in the project phase and continuing through the operations phase and abandonment phase.
Environmental Benefits
Subsea completions offer environmental benefits that accrue during the development of the resource (less time over the hole, fewer resources used, less capital equipment requiring resources to develop the field, etc.) as well as during the production and eventual disposal of the production equipment (platforms, manifolds, etc.)
Subsea completions have an economic advantage compared with other field development alternatives such as bottom-founded structures (e.g., platforms). This advantage increases with greater water depth and, in some cases, bottom-founded structures are not possible due to the sheer size potentially required for such a structure. At present, the FPSO cost is approximately one-half of the cost of a bottom-founded structure. Operating costs of an FPSO are approximately $250,000/month compared with satellite subsea trees, which are $25,000/month.
During well construction and installation of the subsea completion, rig costs are paramount. Currently, daily costs run from $200,000 to upwards of $750,000 per day. Operators eager to improve profitability take every opportunity to reduce time over the well and reward contractors who significantly reduce well-construction and completion times. This includes reducing the number of trips to install completions as well as reducing nonproductive time from excessive trips. Specific bottomhole completion methodologies have evolved to minimize the number of trips to complete the well.
Looking Long Term
The true success of a subsea completion lies in its ability to continue to produce over time. Any interruption of the production stream, particularly from deepwater, high-producing wells can quickly affect the economic performance of a project.
The barriers and opportunities for subsea completions fall into five categories:
- Regulatory controls
- Safety management
- Economic advantages
- Technological aspects
- Environmental issues.
The long-term outlook and vision for future of subsea well access and completions is bright. Continuous advances in materials, sensing capability, and control systems will allow more economic recovery of resources. Additionally, field and well architecture developments, including multilateral wells and extended-reach drilling, offer even more potential. Adding to those advances are possibilities for complete field development, production and control including subsea processing, re-injection, and potential waterflooding all controlled without intervention, and matching a predefined model of field drainage.
References
- Paper #2-11 Working Document of the NPC North American Resource Development Study | Subsea Drilling, Well Operations and Completions, prepared by the Offshore Operations Subgroup of the Operations & Environment Task Group
- Optime Subsea Operations and Technical Data.
