Maximizing economics in shale development is dependent on the ability not only to create horizontal wellbores through long segments of tight oil- or gas-bearing formations but also to connect the wellbore to as much of the reservoir as possible through staged perforating and stimulation.

Multizone completion technology has become the accepted standard for long, tight shale wells in the US because of the productivity and cost-saving advantages that come from zone-specific designs. A recent 38-zone hybrid completion in the Bakken shale illustrates the benefits that can accrue from a creative approach and a broad technology portfolio.

In North Dakota, an operating company sought the most efficient way to drain a 3,000-m (10,000-ft), 6-in. lateral at 5,800 m (19,000 ft) measured depth. The operator wanted to treat the maximum number of zones, with an average zone length of 80 m (250 ft). To accomplish the operator’s objectives, the company designed and installed a 38-stage multizone completion using a customized ZoneSelect system.

The system combined the wellbore isolation effectiveness of Fraxsis swellable packers throughout the well with the time-saving efficiency of toe sleeve and single-shot ball-drop fracturing sleeves in the toe section and Frac-Guard composite plugs to increase the number of zones in the heel section. In all, 20 frac sleeves, 18 composite plugs, and 38 swellable packers were used in the well.

Deploying composite plugs with swellable packers in lieu of cement can improve zonal isolation. (Images courtesy of Weatherford International)

Range of options

The fracturing completion system was developed to enable zone-specific completion designs based on an extensive portfolio of zonal isolation and fracturing sleeve technologies and a modular design methodology.

The resulting ability to open, isolate, and fracture specific zones based on length, geologic conditions, configuration, and other characteristics has proven particularly beneficial in North American shale plays. Zone-specific, life-of-well isolation methods help overcome zonal isolation difficulties. Economic viability is enhanced by the ability to perform multiple fracture stimulations in a single trip, with minimal to no through-tubing intervention.

Four zonal isolation methods are offered: swellable packers, mechanical openhole packers, composite plugs, and cement. The annulus swellable packer was selected for zonal isolation on the North Dakota Bakken well. The short, slim packer provides high pressure differential and enhanced performance while improving completion installation.

Proprietary metal backup rings integrated with the rubber element expand with the element and lock against the formation to hold the packer in place and prevent rubber extrusion. As a result, a 1.5-m (5-ft) packer can provide a seal to 7,500 psi. The short length makes the packer easy and less costly to install. Bonded, swellable elastomers activated through natural wellbore fluids provide sealing capability for the life of the well. The packer is available in water-swell, oil-swell, and hybrid elastomer variations; each of these can be customized to meet the pressure and size required by the well.

Pump-down wireline-set composite plugs were used in the heel section of the well. Pumping the plugs into the well accelerates operations and reduces rig time over standard coiled tubing and eliminates the need for expensive wireline tractors. Jet ports wash away debris ahead of the plug, while perforating guns conveyed above the plug make single-trip plugging and perforating possible.

The ZoneSelect SingleShot Fracturing Sleeve was used during a multizone completion in the Bakken shale in North Dakota.

Fracturing sleeves

In the North Dakota well, a hydraulic toe sleeve was used to treat the first zone. The liner was run into the wellbore with the landing collar, allowing circulation during run-in. The first ball was dropped and circulated, falling on the landing collar to close the liner system. Pressure was then applied against the landing collar to shift it closed. Pressuring up further to 2,500 psi opened the hydraulic toe sleeve to actuate it and enable fracturing the first stage.

Single-shot ball-drop fracturing sleeves were used to control access to the frac zones and divert frac treatments in the first 20 zones of the well. The single-shot sleeves use a continuous method of isolating below the sleeve, opening a port, fracturing, then moving to the next zone. Sleeves are opened from the bottom up with successively larger balls. After a ball opens a particular sleeve, it lands above the previous sleeve and seals off the previous zone. After being opened, each sleeve remains open permanently. At the end of the frac job, the balls are produced out of the hole, and the ball seats can be milled out of the tubing if the client prefers.

A volume of 290 bbl of treatment was pumped into the zone. After the first stage was complete, another ball was dropped and landed in the first single-shot sleeve. Pressure of 2,000 psi was applied to shift the first sleeve to isolate and treat the first zone. The ID of the first ball seat was 1.9 in. Balls and seats became progressively larger as the completion system moved uphole.

For this well, the company used sleeves that use aluminum balls rather than the standard composite balls. The aluminum balls enable more sleeves to be installed in the completion by reducing the seat size increments required to shift the sleeves. The perception is that composite balls are easier to mill than aluminum balls. However, in a similar Bakken well in which 28 zones were isolated in a single trip, the aluminum balls were proven to have mill-out times comparable to those with compos- ite balls. A proprietary mill enabled the company to achieve excellent milling times.

Other fracturing sleeve options include multi-array stimulation systems (MASSs), multishift, and monobore frac sleeves. The multi-array sleeves use ball-drop methodology to fracture up to 10 zones, with six sleeves per zone in a single trip (up to 60 individual frac ports per completion). They can be used with packers or cemented in place. After initially being opened with a ball, multishift sleeve balls and ball seats are milled out and can then be closed or opened with a shifting tool, making them well-suited for wells that might require reentering for additional fracturing or zonal isolation.

Monobore sleeves are shifted mechanically with a shifting tool. With monobore sleeves, there are no ball seats to mill out, and zones can be treated in any sequence. The monobore completion is paired with a ZoneSelect “garage” that provides a recess to house the shifting tool during the fracture treatment. The expanded OD provides the necessary flow area around the tool to maximize the flow during the frac.

Choosing the hybrid

While fracturing sleeves are considered the most economic means of completing multizone fractured wells, their application is limited by the progressive ball seats that must be used to add additional zones to a completion.

To ensure the largest possible wellbore to frac through in the lower zones, many operators use frac sleeves at the toe and switch to the plug-and-perf methodology above the toe in a hybrid completion.

By selecting the hybrid completion methodology for its North Dakota Bakken well, the operator was able to maximize contact with the reservoir and increase initial production rates over the average completion while also saving days of rig time.