Tortuosity is defined as “something winding or twisted” or “full of twists and turns”.
Well tortuosity is typical of today’s unconventional wells (see Figure 1) and is now acknowledged to result in casing deformation such as collapsed inner diameters (“ovalized casing”). There is also growing awareness among shale operators that excessive tortuosity can predispose the casing to rupture which results in fracturing fluids going into the wrong stages. Rupture occurs in sections which are deformed and therefore more vulnerable to cyclic fracturing pressures, pressure-induced bending, and earth loads.
Casing failure can also result from a frac plug that was damaged during conveyance. Forcing a nonmetallic frac plug through ovalized casing, or subjecting it to frequent stick-slip, will damage it (see The Pump-In Process). Forcing a long perforating gun-running tool-frac plug assembly through a tight bend in casing will also damage the frac plug (see The Plug and Perf Process). Damaged frac plugs seal poorly with the casing ID, which leads to leakage, jetting of fracturing fluids through the leaking packing element, erosion of the casing wall, and eventual rupture.
As was stated in Frac Plug Reliability, a frac plug cannot have tolerance for an infinite range of wellbore conditions. So, what can be done to mitigate against frac plug damage from tortuosity?
Frac plug geometry is based on permanent packer geometry, meaning that the plug OD-to-casing ID clearance is small. Higher clearance metallic packer products are commercially available but are unsuitable for unconventional well completions. A high-expansion composite version would be less sensitive to wellbore tortuosity. This is a commercial opportunity.
Modelling with caliper log and high-resolution gyro data can now generate 3D casing paths showing locations of restrictions along the path. These “hot spots” determine the maximum diameter for a given length of completion assembly. With this information, an OEM can adjust assembly lengths and diameters to safely traverse hot spots. This method is also being used for long completion tools such as electric submersible pumps (ESPs) which are ~65 ft. long.
TRUTH: Well tortuosity not only damages frac plugs, but also predisposes casing to fail.
This is the 5th in a series of five articles on why Frac Plugs fail in Plug and Perf applications.