A new series of composite plugs known as FracGuard was designed with the intent of facilitating fracturing operations in both single- and multiple-zone completions. Now new plug designs have been created to further accelerate specific operator-identified tasks needed in multiple-zone wells undergoing stimulation completions and remedial cementing operations, three of which are discussed herein.

In general
Generally, the plugs are constructed using proprietary composite material and manufacturing

Figure 1. This photo shows cuttings that look like wood chips. (All figures courtesy of Weatherford International Ltd.)
technology that ensures their operational integrity at higher temperatures and pressures for longer periods of time. Yet they are easier and faster to subsequently drill up. From a material standpoint, the composite construction and absence of metal parts produce super-lightweight cuttings from drill-out with the appearance of wood chips, making the plugs well suited for underbalanced or near-balanced wells or applications using lighter-weight drilling fluids (Figure 1).

The series 300 and 400 plugs can be used in applications having temperatures up to 300ºF and 400ºF (148.7°C and 204°C), respectively, for extended periods of time, permitting the planning of fracture treatment programs with total flexibility and minimal reliability concerns. These composite plugs can be used in downhole situations having differential pressures up to 12,000 psi. Moreover, implementation techniques have been developed that permit pressures to be stacked, with proven applications reaching more than 20,000 psi.

From a design standpoint, the typical composite plug has a beveled bottom with no spinning parts or cast-iron slips, preventing it from spinning during drill-out and potentially damaging the casing. The high-strength composite body and patented element backup system provide its high-pressure capability and performance reliability by keeping the sealing rubber tight against the casing. The new specialized plug designs incorporate the proven design of the company’s existing composite plugs while adding capabilities to meet the needs of operators in their field operations.

In sand
One such plug is the Sand Biter plug, which is designed to accelerate the drill-out of composite plugs in multiple-zone fracturing jobs where large volumes of sand remain in the wellbore as a result of under-displaced sand fracturing. For a typical composite plug drill-out, the plug remnants drop downhole to the top of the next plug. The bevel on the bottom guide locks on to the next plug, which inhibits rotation during drill-out. In certain instances, however, under-displaced sand proppant accumulates below, on top of which the plug remnants eventually rest. Drilling action spins these remnants, impeding an effective drill-out as a result. The new plug has been designed to solve this problem with its specialized biting-tooth profile on the muleshoe bottom, which bites into the sand and grips the remnants, preventing their spinning and permitting a faster and easier drill-out (Figure 2). An adaptation of the already proven composite plug, this biting-tooth profile can be incorporated on all plugs in the FracGuard series.

The specialized plug was run in a 9,918-ft (3,023-m) deep well in Grand Prairie, Alberta, Canada, undergoing a fracture treatment with sand proppant volumes amounting to 10,000 lb. Downhole temperatures measured about 230º F (110º C). Previous fracturing operations in nearby wells undertaken with conventional composite plugs encountered under-displaced sand and the associated problems that inhibited drill-outs and prolonged drill-up times. The novel biting-tooth design solved the problem by preventing the plug from unwanted spin action, thus permitting accelerated mill-up. More than 800 of these special plugs have been run, saving countless rig hours and dollars. As an example, drilling out four stacked plugs used rig time averaging 30 minutes between each of them. The Sand Biter plug has reduced this between-plug drill-out time to an average of 10 minutes, saving 20 minutes per plug.

In cement
Another specialized plug that has been developed is the surface pipe cementing plug, which significantly accelerates remedial cementing operations on shallow surface casing compared to existing conventional methods. Using the plug for remedial cementing eliminates the need for drillpipe, which in turn reduces the number of steps required to achieve a remedial cementing squeeze job, translating into time and cost savings. Its use also does away with the conventional cast-iron cement retainers traditionally used for remedial cementing, serving to decrease the open-hole contamination associated with their drill-out debris and the inevitable operational complications that follow. The bottom of the new cementing plug incorporates a flapper valve instead, which closes and prevents flowback after the cement pumps down the casing and into the perforations, and a wiper plug lands on top of the composite plug (Figure 3). This special plug also eliminates the waiting time for cement to cure when using drill pipe and a retrievable packer. Additionally, the composite construction and superior millability significantly reduce drill-out time; the surface pipe cementing plug can be drilled out on average in about half of the time of a conventional cast-iron cement retainer.

Typically, while drilling a well, the kelly or rotary bushing often can wear a hole in the surface pipe after it and the intermediate and production casing strings are set. This, however, was
Figure 2. Side view schematic of Sand Biter plug.
prevented in a well in the Rocky Mountains, where the surface pipe cementing plug was employed. The plug was set on wireline, so no additional tripping was needed to pump the cement, place the wiper plug, pick up the bottomhole assembly (BHA) and then drill both the wiper and composite plugs before continuing through the cement. The use of the special plug eliminated the need to pick up another BHA and thus the requirement for additional trips. Compared to conventional methods, the main benefit of using this plug in remedial cementing operations is time savings.

In the future
Today there is an entirely new innovation in the composite plug arena. The convertible bridge/frac plug has a unique design that unites together into one tool the respective pressure-control and flowback capabilities of a bridge plug and frac plug. Convertible construction combines the advantages that previously were available only separately with each plug.

When run and set as a bridge plug, the convertible plug allows complete pressure control above the plug during perforating in stack-and-frac operations.

Operating as a bridge plug enables the pressure above it to be controlled to perform the next perforate-and-stimulation procedure, reducing the potential need to control pressure using a costly grease-injection head. Before or during the next fracture treatment, a unique valve assembly enclosed within the plug allows it to be converted to a frac plug without requiring an expensive intervention. Converting on demand to a top check ball frac plug enables the fracturing operations and subsequent flowbacks through the plug to occur without the need for immediate plug drill-out.

This functional combination results in valuable synergies; its operational benefits translate into significant economic gains by eliminating added interventions and drill-outs, thus accelerating the production readiness of a well. No technology comparable to this unique two-in-one capability exists. Its patent is pending; field trials are ongoing, and their results should be forthcoming soon.

Figure 3. Three-part wellbore schematic showing application of surface pipe cementing plug.
While Weatherford has been in the composite plug business for just five years, its completions work using composite technologies has doubled in the past two years. The company foresees the trend of developing oilfield products like composite plugs for special applications designed according to operator-driven needs only to continue to evolve and expand.