Since 2008 more than 150,000 horizontal wells have been drilled and hydraulically fractured in North America. A large portion of these wells were either understimulated using early fracturing techniques or are underperforming because of declining production. In most unconventional shale and tight oil reservoirs significant hydrocarbons remain in place and could be recovered through infill drilling or by refracturing existing wells. Although drilling additional wells is a good option in plays with shallow reservoirs and low well construction costs, restimulating existing wells in deeper, higher cost basins is a potentially better alternative.

During the last several years expandable casing patches have become more commonplace in the repair of multiple well integrity issues in both vertical and horizontal wellbores. Most recently, longer expandable liners have been utilized to totally recomplete both new and old completions. These expandable liners seal off existing perforations, damaged casing sections or fracture sleeves in existing horizontal wells. New small-diameter composite plugs suited for multistage plug-and-perf (PNP) operations inside the expanded liner allow these wells to be properly stimulated.

One Oklahoma operator used expandable liners up to 1,390 m (4,562 ft) and new small-diameter composite fracture plugs to stimulate six wells with multiple integrity issues in its horizontal sections. Successful results on these wells demonstrate that the expandable liner-fracture plug combination has the potential to efficiently refracture understimulated or underperforming horizontal wells to improve production rates and increase ultimate recovery.

System description

The Mohawk Fracpatch expandable liner system incorporates high-performing materials and a fracture sealing system that handles high treating pressures as well as the induced loads from large temperature cool-downs. The system is designed for efficient liner running and expansion using a workover rig and uses a combination of hydraulic force and the lifting power of the rig to expand the liner from bottom to top.

During mechanical expansion no hydraulic pressure is applied to the connections and inside diameter (ID) of the expandable liner, reducing the stress loads during expansion.

No balls or darts are dropped down the ID of the work string to initiate expansion. The remainder of the liner is expanded by tripping pipe to the surface using the power of the drawworks at about 304.8 m/hr (1,000 ft/hr). When higher forces are required due to debris or tight spots, hydraulic expansion can be resumed, which is valuable in horizonal wells.

The expandable liner is made from a high-performance steel that has a high yield strength and collapse rating after expansion. The bottom and top joints of the expandable casing include metal-to-metal seal sections that, when expanded, hold fast to the ID of the well’s casing.

Well cleanout

Wellbore cleanout is an important preliminary step to running the expandable liner. The operator should make adequate cleanout trips prior to installing the liner. A Mohawk-specific drift run with a specialized cleanout assembly also is required to determine the cleanliness of the lateral.

During the well cleanout operation a sensor is placed on the load line and a pressure transducer is attached to the standpipe to monitor and record weight and pressure, taking 20 readings per second. This allows drag factors to be calibrated to confirm the long liner can be run to total depth. When this bottomhole assembly is retrieved, if there is predetermined volume of debris, the assembly is run again until the hole is well cleaned.

Liner expansion

Liner deployment begins with making up the running tool assembly including—from the bottom up—the fl ow control device, which activates and deactivates the tool; the drive unit that provides force; filters to keep tool internals clean; the expansion cone; anchor sub; debris cup; and stabbing sub. This assembly is hung off in the rotary table and made up to the first joint of expandable liner. The running tool assembly and liner are run into the well using standard liner- running procedures. Threaded connections do not require pipe dope.

When the liner reaches bottom, fl ow is directed through the work string to the fl ow control valve, building up pressure to engage the internal anchor. Pressure is further increased to 3,000 psi, activating the pistons to expand the pipe. Pressure is bled off at surface to reset the tool. Hydraulic expansion is repeated, securing the lower liner seals to the casing ID.

Once the lower seal is achieved and the liner is anchored into the host casing, expansion resumes by mechanical over-pull. The liner is expanded by tripping pipe to the top of the expandable liner, and then the top seal is expanded by the hydraulic method. The top metal-to-metal seal allows liner movement induced by temperature cycles and fracking pressure.

The Mohawk liner expansion system running tool leverages the hydraulic force and lifting power from the rig to expand the liner. (Source: Mohawk Energy)


Small-diameter composite fracture plugs

Advanced composite fracture plugs from Downhole Technology are suited for PNP operations inside expanded liners. Specially designed 3.06- outer-diameter (OD) refracture plugs were designed to complete stages within the expanded liner. The plug’s tapered nose facilitates maneuvers in tortuous wells and guides the plug into the smaller diameter of the expanded liner without hanging up.

The Boss Hog fracture plug is half the length and weight of typical fracture plugs of similar diameter with the same 10,000-psi pressure rating. It is constructed from filament-wound composite material rather than from composite sheets or extruded components and has a modular design with far fewer parts than previous plugs. During drillout the filament-wound composite is reduced to small hair-like cuttings that are easily returned to the surface.

To prepare for this application, the fracture plug supplier performed tests in its engineering laboratory to prove that the 3.06-OD plugs were compatible with the expanded 3.4-in. liner ID.

Oklahoma recompletions

In Kingfisher County, Okla., an operator drilled and completed six horizontal wells with 4.5-in. 11.6-lb casing. Caliper logs and pressure tests confirmed they all had multiple integrity issues that would prevent their completion and stimulation. To resolve these problems, the operator elected to run expandable liners over as much of the horizontal sections as possible and to complete the wells using the PNP method.

Wellbore preparation and cleanout took an average of 64 hours per well, and liner makeup and expansion took an average of 29 hours per well.

All six wells were recompleted with expandable liners with lengths ranging from 853 m to 1,389 m (2,800 ft to 4,560 ft). A total of 6,814 m (22,356 ft) of liner was expanded, and 185 stages were pumped across all six wells. Slickwater fractures at 100 bbl/min were pumped, placing up to 3.3 million pounds of proppant per well. Fracture plugs were all drilled out successfully and all wells brought on production.

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