Pushing the envelope in deepwater cementing
As operators continue exploring and developing deepwater resources, cementing challenges have increased exponentially due to the very narrow pore pressures and fracture gradients typically seen in deepwater wells. Not only do operators have to ensure they’re achieving the necessary zonal isolation and sufficient cement coverage, they’re also now dealing with increasingly thicker casing and the resulting tighter restrictions. Thicker casing has come about due to the industry’s desire to improve safety, said Iain Levie, Vice- President of Global Technical Services for Antelope Oil Tool. “Casing designs require more robust burst and collapse thresholds, so operators are moving toward thicker-walled casing.”
At the same time, operators are looking to maximize the final production string, said Tim Dunn, Well Construction Product Line Specialist at Weatherford. As a result, casing diameters are increasing, as well. “Casing sizes coming out now are pushing the envelope for the outer diameter (OD) to maximize the final production string,” he said. To get bigger and thicker casing strings deep into the well will require new technologies for cementing and running casing, he added.
In response to this challenge, service companies are developing close-tolerance slip-on centralizers and centralizer subs that can fully compress and allow casing to pass through tight restrictions – often with annular clearances less than half an inch – while still achieving good standoff when it enters the open-hole section below.
In addition, thicker casing is driving the development of sensors for cement evaluation that can accurately evaluate the cement bond. R&D efforts are also focusing on maintaining a low equivalent circulating density (ECD) to avoid exceeding the pore pressure and fracture gradient window. Innovative solutions, from reverse cementing techniques for deepwater to additives for lightweight cement, are also under development and will be commercialized later this year.
“Technology has progressed so much that we can drill in the most extreme environments, and the industry is constantly innovating and developing technologies,” said Crystal Wreden, Senior Technology Advisor at Weatherford. “So as drilling progresses, so will cementing equipment.”
Bringing reverse cementing to deepwater
The narrow pore pressure and fracture gradient windows common in deepwater wells often necessitate a lower bottomhole ECD while cementing to avoid taking losses. “Being able to reduce ECD during placement reduces your risk of exceeding the fracture gradient and breaking down the formation and losing cement,” Ms Wreden said.
Because conventional cementing techniques can require pressure that exceeds the fracture gradient in lost-circulation and weak zones, Weatherford has embraced reverse cementing in deepwater. Although the technique has been used on land and in shallow water, the CrossStream Subsurface Reverse Cementing system will be the first application of this technique for liners in deepwater.
The system, which remains under development, pumps fluid down the work string and diverts fluids into the lower annulus through a crossover tool. Returns are circulated up the inside diameter (ID) of the liner, and the crossover tool diverts them back into the upper annulus, resulting in a lower ECD bottomhole.
In conventional cementing, after cement is pumped down through the ID of the liner and then flows into the annulus, pressure is applied to lift the cement up into place within the annulus. “In the event that you have a weak formation and narrow pore/frac gradient, you run the risk that the pressure needed to place the cement conventionally will exceed your fracture gradient and you’ll break down the formation, then lose your cement,” Ms Wreden said.
The crossover tool, which is the heart of Weatherford’s reverse cementing system, can be set to a conventional flowpath or a reverse flowpath using radio frequency identification (RFID) tags. “One of the benefits of RFID is that it’s interventionless,” she said. RFID tags are programmed with a specific command for a specific tool and pumped downhole. When the tag reaches the tool its programmed for – in this case, the crossover tool – it will transfer the programmed command to the tools. “If you have the tool in a conventional position to run your casing in hole, you then pump down a tag to tell it to switch to the reverse position so that you can inject fluids down the annulus.”
Weatherford is currently running field trials on the CrossStream in the US, with additional field tests possible in Asia Pacific, West Africa and Brazil. The system is expected to become commercially available within 12-18 months.
In addition to developing technology to lower ECD when cementing, the company is focusing on how to achieve good centralization while running casing. As operators opt for increasingly thicker-walled casing to increase the casing’s collapse rating, it compounds the challenges around centralization, Mr Dunn said. “Well design requirements are driving the OD of the casing further out and leaving very tight clearances,” he said. Centralizers need to be able to flatten to pass through these narrow restrictions, sometimes as narrow as 0.125-in., and then open up in the open-hole section with sufficient restoring force to give the required standoff, he explained.
In late 2015, Weatherford launched the VariForm line of bowspring centralizers to address the challenges of passing centralizers through increasingly narrow restrictions. These centralizers can be slipped onto the pipe and have the capability to nearly flatten during the run-in-hole through the previous casing before expanding in the open hole. The contoured bow profile reduces drag when running through casing.
The VariForm line also includes centralizer subs that are made up and run as an integral part of the casing string. They feature a profiled recessed area where the centralizer sits. As it runs through the restriction, it can be fully compressed into the recess. This means the OD of the centralizer does not exceed the OD of the casing. “If there’s anything your casing can pass through, the centralizer can pass through it, as well,” Ms Wreden said.
In 2015, the centralizer sub was run in a 3,000 ft of water in the Mississippi Canyon in the Gulf of Mexico. The well required an 11 7/8-in. casing to pass through a 12 ?-in. previous casing before opening in the 14 ?-in. open hole with a 60° deviation. The bowsprings on the centralizer sub collapsed into the recess, so the VariForm centralizer was able to pass through the ID of the previous casing. The bowsprings then expanded back out in the open hole and centralized the casing with a 70% standoff at a depth of 12,147 ft.
Proper centralization can reduce drag as one section of casing runs through the previous section. “When operators have problems running casing to total depth, it’s typically noted by an increase in drag because the pipe isn’t centralized; it’s being pushed down the hole against the pipe,” Mr Dunn said. “The drag forces create a lot of compression loading in casing and connections, so an operator’s ideal scenario is to minimize the amount of drag force generated when they’re running casing, especially in highly deviated wells.”
The VariForm centralizers have been shown to help minimize drag by providing good standoff. In February, the centralizers were deployed offshore Trinidad, where a casing string with a 9 5/8-in. OD was run through a 12 ?-in. deviated wellbore. The centralizers provided 80% standoff and eliminated wiper trips, saving the operator 76 hours of rig time and an estimated $2.21 million compared with similar operations that required wiper trips.
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