As more operators drill and complete wells with long horizontal segments requiring multistage hydraulic-fracture stimulation treatments, traditional plug-and-perf technology is being left behind by newer options that offer economic, safety, and logistical benefits.

Consistent manufacturing and operations procedures enhance the success of frac sleeve completion systems, such as the DirectStim system shown here at a well in West Virginia.



One option that has been field-proven in thousands of wells around the world is frac sleeve completion technology. Although this technology is not new, recent improvements in design, planning, manufacturing quality control, and operations process management have greatly improved system reliability and completion success by optimizing five key functions of these systems.

Measuring success

Frac sleeve systems were developed to replace the slow and expensive process of running individual bridge plugs to isolate segments of a well with multiple stimulation targets. The newest systems comprise as many as 20 packer/sleeve sets run in a single trip. Sleeves are opened during the stimulation treatment with a ball dropped at the end of each frac stage to open the sleeve for the next zone and seal off lower zones.

To be successful, a frac sleeve completion system must:
• Provide an economic benefit — or at least no penalty — compared with traditional technology;
• Run into the well smoothly and easily;
• Isolate each zone by providing a hydraulic seal against the formation;
• Open each sleeve sequentially during the stimulation treatment while sealing off lower zones; and
• Allow the well to flow after all treatments are completed.

Economic benefits of successful implementations include faster time to sales; elimination of liner, cementing, and perforating costs; and reduced pumping costs and standby charges when fracs are pumped continuously. In many cases, production can also be increased because the economics of pack-and-frac operations previously limited the number of zones that could be stimulated; isolating and stimulating additional pay provides more access to reserves.

It’s important to note, however, that these benefits accrue only when the other four requirements are met. This means meticulous quality assurance and systematic job planning and execution are necessary to reach the operator’s economic goals.

Smooth trip

The first step in planning a multizone frac sleeve completion is to understand the well conditions. Temperatures, pressures, and geology can affect tool running and stimulation processes.

Generally, the best candidates are openhole wells in low-permeability, consolidated formations, such as tight sands and shales. Ideally, wells should be drilled with stabilizers to produce a circular hole for best packer sealing. Swellable packers can be used in wells with irregular holes under certain temperature, fluid, and chemical conditions. However, they are slow to set, requiring days or weeks of waiting time, especially when well conditions and fluids are not ideal.

Reamer and caliper runs are strongly recommended to aid in ensuring the tool can reach the bottom of the well and provide a good seal by avoiding washed-out areas.

BJ Services recommends using a specific reamer before standard DirectStim system installations because the distance between the reamer’s blades matches the distance between the two packer elements in the standard Bearfoot DS packer, the longest tool in the frac sleeve system. If needed, the running assembly can also be rotated to work through tight sections.

Durable packer designs are important for ensuring consistent operation and reliable sealing against the formation so that stimulation fluids and proppants can be pumped as designed. The Bearfoot DS packer, for example, uses metal backup rings to support the two sealing elements and prevent extrusion. Because of the backup rings, the packer has been tested in 61?2-in. casing to hold 10,000 psi differential pressure.

To further improve reliability, the two sealing elements are about 3 ft (1 m) apart, which provides maximum system stability and redundancy in case the tool lies across a washout area. Finally, to ensure that a set packer remains set, a ratcheting mechanism locks the packer in the set position.

Opening frac ports

An ideal continuous stimulation operation in a well with a frac sleeve completion system. It’s obvious from spikes and hills in treating pressure (blue line) that each ball seats and each fracture is initiated.



The next critical function of a frac sleeve completion system is to enable pinpoint stimulation. This requires a mechanism for opening one frac sleeve that will accept the stimulation treatment while isolating lower zones. The most economical method of opening sleeves is dropping balls.

Historically, these completion systems have used sequential sets of balls in 1?4-in. increments for as many as 12 zones, with newer 20-zone systems relying on ball increments of just 1?8 in. Poor quality assurance during tool manufacture and conditions created while pumping the stimulation job can prevent proper operation of these systems.

A systematic approach to quality assurance can eliminate manufacturing errors, even on the newest tight-tolerance systems. For example, each BJ tool arrives on location accompanied by written verification of step-by-step quality assurance measures taken at the factory. Final verification measures are performed on location as part of the written operation procedures and the results logged in a quality assurance binder provided to customers for their records.

To validate frac sleeve performance under simulated downhole conditions, BJ personnel have performed a variety of stress tests.

In one test, 3.5 million lb of 20/12 proppant were pumped at 52 bbl/min. for eight hours through a 31?4-in. DirectStim frac sleeve to simulate the erosion it would experience during a typical nine-zone stimulation. The test allowed BJ to optimize the frac sleeve design for both erosion resistance and drill-out times.

In another test, an air cannon was used to fire frac balls into a frac sleeve at the equivalent of 68 bbl/min. to simulate the effect of a ball drop during the high-rate portion of a slickwater frac. This test was designed to determine the impact resistance of various frac sleeve shifting balls. In fact, tests determined that BJ balls withstand four times as much pressure as those used by other frac sleeve system suppliers.

Anecdotal evidence from operators suggests that competitive balls may fragment on impact with frac sleeves and other downhole objects and then are unable to shift their respective sleeves, especially in systems with small size increments between balls. If the surface pressure chart does not show a large spike when the sleeve shift is expected, then the lower zone may not be isolated and the frac may re-treat the same zone rather than the intended one.

Additional ball testing also provided data related to the fourth requirement of a frac sleeve system, the ability to re-open the well after stimulation. The largest balls may become stuck in their seats upon application of enough pressure to shift the sleeve. Extensive lab testing has determined the maximum reservoir pressures required to unseat the balls. This knowledge may not prevent a milling run in an underpressured reservoir, but it eliminates a potentially expensive delay in achieving production and enables pre-planning for coiled tubing or other milling operations.

In fact, some operators mill out the seats in a frac sleeve system even if the balls flow back normally. The DirectStim system is designed for this contingency, providing full-bore inside diameter and easy drill-out because the seats lock upon shifting and will not spin during a drill-out operation. One operator reported using coiled tubing to drill out the seats in a six-zone DirectStim system in 2 hours, compared with two days required to drill out another supplier’s six-zone system.

In summary, methodical testing and an understanding of real-world tool requirements has led to development of new technologies to improve reliability and economics of frac sleeve systems.