Rupture Disk Valve Improves Plug-and-Perf Applications
Plug-and-perf completions have proved to be the most flexible multistage well completion schemes. The main reason is that each stage can be perforated and treated optimally because options can be exercised right up to the moment the perforating gun is fired. This allows the engineer to apply knowledge from each previous stage to the stage under treatment. For example, using real-time microseismic fracture mapping techniques provides a view of where previous fractures have propagated, which enables adjustments to be made to perforating depth and pumping schedules for subsequent stages.
Some people have argued that the plug-and-perf technique is too time consuming and that it is too slow at the start because of the need to run the first stage perforating guns in with coiled tubing, stick pipe, or a downhole tractor, thus adding an unnecessary expense.
A new technique introduced by Schlumberger adds significant operating efficiency and lowers the cost of cemented, multistage fracturing by eliminating the initial perforating run. The technique uses the KickStart pressure-activated rupture disk valve to enable the first stage treatment to be combined with casing operations. In addition to reducing downtime, the technique has been shown to lower fracture initiation pressure, thereby eliminating the risk of closed perforation tunnels caused by shale swelling, and to increase the likelihood of fracturing the stage to completion.
The extensive study of geomechanics has shown that rock formations fracture at their weakest point and that the weakest point aligns with the plane of maximum horizontal stress whenever the overburden (vertical) stress exceeds lateral stresses, which occurs most of the time. The pressure-activated rupture disk valve has a unique helical port design that exposes the formation outside the casing to fracture pressure, thereby ensuring that the fracture will align with the maximum horizontal stress plane and propagate at the weakest point in the formation. A major benefit of this alignment is the reduction of tortuosity at the fracture initiation point, which significantly reduces the probability of a premature screenout of the initial fracture treatment at the toe of the completion.
Valve Installed in Casing String
The pressure-activated rupture disk valve is installed in the casing string, spaced one or two casing joints above the float shoe to align with the toe-stage fracture point in the formation when the casing is landed. Cementing operations proceed normally. After the cement is set and tested, all that is required to begin the first stage fracturing treatment is to open the valve. This is accomplished by increasing the bottomhole pressure to rupture one of the two rupture disks. The rupture disks are typically made up in the field district, but they can be selected on site and are available in step increments from 250 psi to 300 psi.
It is unnecessary to have both disks rupture. Rupture pressure is typically set at approximately 500 psi above casing test pressure. When a disk ruptures, pressure is applied to an internal piston to open an internal sleeve, exposing the fracture ports to the internal flow regime. The ports, which are slots arrayed helically around the valve, expose nearly the entire circumference of the valve to the cement sheath (Fig. 1). Total slot area is equivalent to 12 ft of perforations at 6 shots per ft. At this point, the first stage is ready for treatment with no intervention by a wireline tractor or coiled tubing to perforate.
The operator can initiate the first stage treatment by breaking out the cement sheath with internal pressure and fracturing the formation through the valve. There are two options for completing a well using the rupture disk valve. The first is to fracture through the cement and run the pump down plug-and-perf operation afterward. The second option is to use the valve as a pumping conduit and begin the pump down plug-and-perf operation right away. Pump down plug-and-perf is more economical than coiled tubing or tractor-run plug-and-perf for the first stage. No fracture balls are required to activate or isolate the rupture disk valve.
The pressure-activated rupture disk valve is rated to 20,000 psi and 325°F (163°C). Treatments can be pumped through the valve at rates of up to 120 bbl/min with 10 lbm/gal of proppant added. Valves are available in 4½- and 5½-in. base pipe sizes. By eliminating pretreatment intervention, considerable time and money are saved. Subsequent stages are perforated and treated normally, retaining the flexibility to change treatment depths, pumping schedules, and proppant mix as needed, based on observations from previous stage treatments.
The valve can be torqued to substantially more than 10,000 ft-lbf. A standard open hole size for 4½- or 5½-in. casing will suffice when using the pressure-activated rupture disk valve. The valve can be cemented through, and a cement wiper plug can pass through the valve and be bumped on bottom. No special cement chemistry is required. Fractures can be initiated through any type of standard oilfield cement when the valve is used.
Streamlined Completions in Eagle Ford Shale
The Eagle Ford shale in south Texas is notable among shale plays for its production of hydrocarbons at a high liquid-to-gas ratio. The formation, which has been developed mostly with horizontal wells, contains a high percentage of carbonate shale. The most common completion designs in the Eagle Ford use cemented laterals with plug-and-perf fracturing techniques. These techniques require mechanical conveyance of perforating guns for the initial fracturing stage at the toe of the well by means such as coiled tubing, stick pipe, or tractor.
Cabot Oil & Gas has been very active in the Eagle Ford and has adopted innovative strategies such as a 55-acre well spacing instead of the customary 80-acre spacing. The company has pioneered the use of the pressure-activated rupture disk valve in the play to improve the logistics and economics of well completions. Fig. 2 shows the fracturing treatment plot from the first stage of one of the wells on which Cabot used the pressure-activated rupture disk valve. More than 250,000 lbm of proppant were pumped through the stage at a rate of 68 bbl/min. The company has used the valve so far to complete 14 Eagle Ford wells. This eliminated coiled tubing intervention to perforate the first stages of the wells and allowed Cabot to streamline the completion process, reducing cost by more than USD 100,000 per well.
Elsewhere, the pressure-activated rupture disk valve has been used globally by various operators in more than 300 wells. North American locations have included unconventional formations such as the Marcellus, Fayetteville, Barnett, Montney, Three Forks, Horn River, Avalon, Woodford/Cana, and Granite Wash. Internationally, the valve has been used in the Baltic Basin of Poland and the Bohai Basin of China.