Oceaneering’s Justin Pizzitola discusses the use of an open water dual coiled riser based system for rigless stimulation.
Back deck of an Oceaneering vessel. Images from Oceaneering International.
For more than two decades, the complexity of the deepwater environment has become more challenging, driving the need for more alternative solutions and advancements in technology. From new wells that come online to aging wells, well intervention is required to stimulate flow and maximize production. An alternative, rigless stimulation method has been developed and used successfully on dozens of wells in the Gulf of Mexico and West Africa. Production enhancement from this alternative method yields results comparable to those achieved using rig-based methods while reducing costs by as much as 50%.
Aging wells need rejuvenation
After a subsea well is online for several years, its production can be impaired by sand migration and/or by deposition of wax, paraffin, asphaltenes and scale. Increased water cut in older wells can contribute to deposits. The first step to restoring production is for oil company production engineers to analyze the produced fluids to determine the cause of the blockage and to select the appropriate treatment chemicals. During this analysis, the engineers also review reservoir characteristics to determine the formation’s injectivity and optimum pumping rates.
High-cost rig-based intervention
Until 2009, the only method of performing subsea stimulations was to deploy a mobile drilling rig and a high pressure pumping vessel. In the current environment of low oil and gas prices, floating rigs may be readily available, but increased drilling activity will likely make scheduling of rigs much more problematic. Even at today’s depressed day rates, contracting a rig can still be an expensive proposition. Once on location, preparations for the stimulation job – including setting a blowout preventer and drilling riser to connect the subsea wellhead to the surface – can take a week or longer. Large crews for the rig, pumping vessel, and asset support teams, add to the cost. Operators can spend as much as US$20 million on a rig-based subsea well stimulation.
The Oceaneering Millennium ROV is connecting the flying leads from the well stimulation tool to the subsea tree.
An alternative method, developed by Oceaneering International, uses a multiple-purpose service vessel (MSV) to safely and efficiently perform well stimulations without a drilling rig or riser. The MSV carries two work-class ROVs, and has a crane for lowering equipment with varying tonnage to the sea floor. Equipment includes one or two open water dual coiled tubing riser units, flying leads, high pressure pumps and associated high pressure piping, and an innovative well stimulation tool. The MSV can carry up to 5000 bbl of treatment chemicals. If a higher chemical volume is required, a second pumping vessel can be employed to avoid additional trips to shore. This method allows for rig-up and testing to be completed in as little as two days so stimulation can begin quickly.
The well stimulation tool is the main interface between the topside and the well tree. To assure safe operation, it provides a double barrier for well control, and includes two “fail-close” valves, that are API 17G compliant, which can be actuated from the surface, or automatically close if unexpected pressures are encountered. The system can be configured to perform 10K psi or 15K psi stimulations as required.
The system has an emergency quick disconnect system (EQD) that enables the vessel to disengage from the well with zero leakage. The EQD can be released using an ROV, via an electric control line, or with a deadman cable that activates the EQD in case of loss of station keeping or a black ship situation.
Advantages of rigless systems
Unlike rig-based interventions that must access the wellhead vertically, there are several options for connecting the well stimulation tool to the subsea wellhead. It can be deployed and landed using a mud mat, or secured to a suction pile installed in advance of the stimulation then attached to the wellhead with a flying lead. On vertical trees, the well stimulation tool uses an H4 connector directly to the tree. On horizontal trees, the H4 connector is connected by a flying lead to a choke interface. Another advantage of Oceaneering’s rigless method over rig-based interventions is its ability to perform the stimulation on horizontal trees without removing the crown plug in the tree. Plug removal is needed for the drill pipe used for rig stimulations to engage vertically with the wellbore. Using this method, a variety of interfaces are available including choke inserts and high flow manifolds, which do not require plug removal.
Rigless intervention capabilities
The well stimulation tool deployed subsea in the Gulf of Mexico.
Currently, the rigless stimulation method can treat wells at high pumping rates up to 15K psi with open water dual coiled tubing riser units. With this process, the subsea tree can be controlled from the vessel through an umbilical distribution system that provides power and communications to the tree locally, without relying on control from the distant host facility, and can close safety valves as required. During operation, real-time topside and subsea data is streamed back to shore, including pump pressure data, ROV video, pressure transducer and flow meter readings.
Project management yields results
Project management is an important aspect of successful subsea well stimulation. Oceaneering’s production enhancement team provides project management, engineering, and offshore service technicians specifically trained to integrate all in-house and third-party services. This provides for the consistent execution of well stimulation activities, strong customer support, and reduces offshore downtime.
Since 2009, more than 30 rigless well stimulations have been completed successfully in waters as deep as 6700ft in single and dual-vessel applications, resulting in average production increases of 50% and savings of as much as 50% compared to rig-based stimulations.
A subsea well in the Gulf of Mexico experienced a significant drop in production due to formation impairment. A rigless stimulation was performed from an MSV, using a wellhead stimulation tool to bullhead 1100 bbl of acid at 11K psi at the tree. Despite bad weather, the stimulation was completed in 18 days (including three days of pumping), for $15 million less than a rig-based operation, and improved the well’s productivity by 75%.
Also in the Gulf of Mexico, production from two wells in 4300ft of water was impaired by fines migration. The operator determined that each well needed a treatment schedule pumped within 15 hours at volumes exceeding a single vessel’s capacity using premixed chemicals. A rigless stimulation was carried out using an MSV and a second, low-pressure pumping vessel. Bulk treatment chemicals were mixed on the fly during stimulation. The operation was completed successfully and produced higher production numbers than originally anticipated.
Offshore Ghana, an operator determined that production from five subsea wells had dropped significantly because of accumulation of fines, calcium carbonate and scale. A dual-vessel, rigless stimulation was performed, combining the capabilities of an MSV and a construction vessel. Four open water dual coiled tubing riser units were used to pump the treatment chemicals at 16 bpm to a specially modified well stimulation tool. Each of the wells in the five-well campaign saw an average production rate increase of more than 70%.