Shoe Track

Key Takeaways

• The float collar and guide shoe that define the boundaries of the shoe track serve complementary functions during the primary cementing operation: the guide shoe (also called a float shoe when it incorporates a check valve) at the bottom of the casing string has a rounded or tapered nose that centers the casing in the borehole and guides it past ledges and doglegs during running, and may incorporate a one-way valve that prevents backflow of cement from the annulus into the casing after placement; the float collar, installed one to three joints above the shoe, contains the primary check valve that prevents the cement slurry in the annulus from flowing back into the casing interior after the displacement pumps are shut down, with the float collar valve closing automatically under the backpressure from the column of wet cement in the annulus; the volume of cement slurry that fills the shoe track (the internal volume of the casing between the shoe and the float collar, typically 1 to 3 barrels for standard joint length float collars and larger for two-joint or three-joint spacings) is calculated as part of the cementing job design and must be accounted for when computing the tail slurry volume and the displacement fluid volume to ensure that the wiper plug lands on the float collar at the correct surface pressure that confirms complete displacement.
• The wiper plugs used in primary cementing operations interact with the shoe track and float collar as the primary quality assurance mechanism for displacement completion: the bottom wiper plug (a flexible cup-style plug launched ahead of the cement slurry) wipes the casing wall to separate the cement from the preceding mud and travels down the casing ahead of the leading face of the slurry until it lands on the float collar seat, where it is held in place by the cement slurry pressure behind it and allows the slurry to bypass through the plug's open center (or by rupturing a diaphragm in the plug) and continue out the shoe and into the annulus; the top wiper plug (a solid plug launched behind the cement slurry at the end of the job) is pumped down the casing by the displacement fluid and travels to the float collar where it latches on top of the bottom plug with a characteristic bump in surface standpipe pressure (the plug bump) that signals completion of displacement; after the plugs land, the float collar valve closes, holding the cement pressure in the annulus and preventing backflow, and the shoe track between the float collar and the shoe contains the final volume of tail cement slurry that is now isolated from the wellbore and begins to hydrate.
• Cement compressive strength in the shoe track must reach the minimum value required for shoe integrity before the drill-out is attempted: the standard requirement before drilling out a surface casing shoe track is compressive strength of at least 500 psi (and often 1,000 to 2,000 psi for deep or high-pressure wells) to ensure that the shoe can withstand the weight on bit and rotation forces of the drill-out operation without fracturing; the wait-on-cement (WOC) time needed to achieve adequate strength depends on the cement formulation (accelerated cements with calcium chloride or aluminum sulfate can reach 500 psi in 4 to 8 hours, while standard API class G cement may require 8 to 16 hours at surface temperatures, and longer at low temperatures), the borehole temperature (cement hydration kinetics are strongly temperature-dependent, with each 10 degree Celsius decrease roughly doubling the setting time), and the pressure (higher pressure accelerates cement hydration); the pressure integrity test (PIT) or formation integrity test (FIT or LOT, leak-off test) performed after drilling out the shoe track verifies that the annular cement at the shoe provides an adequate pressure barrier before the next drilling section is opened.
• Shoe track drill-out issues provide early warning of cement job problems: a normal shoe track drill-out encounters firm to hard cement (compressive strength above 500 to 1,000 psi) that requires standard drill bit weight and rotation to mill through, and typically takes 15 to 45 minutes depending on the shoe track length and cement strength; an abnormally soft or mushy drill-out (where the drill bit drops rapidly through the shoe track with little weight on bit and the drill cuttings are soft gray powder rather than hard gray chips) indicates that the shoe cement did not set properly, possibly due to mud contamination (diluted cement with excess water has lower strength and may not set at all), to premature freezing of cement in cold-weather wells before hydration was complete, or to CO2 or H2S attack on the fresh cement (carbonation or sulfate attack reduces compressive strength); the soft shoe drill-out triggers an immediate decision about whether to attempt remedial squeeze cementing of the annular zone near the shoe or to proceed with drilling the next section at elevated risk of cement integrity failure during the casing pressure test.
• Liner shoe tracks in liner cementing operations have the same function as in full casing strings but with additional complexity from the liner hanger and tieback sleeve assembly: a liner (a casing string that hangs from an existing casing string and does not reach surface) requires a liner hanger set at the top of the liner and a float collar and shoe at the bottom, with the shoe track between the shoe and the float collar serving the same cement quality assurance function as in a full string job; however, the liner setting depth (where the liner hanger is set in the existing casing) creates an additional concern that cement must also achieve good bonding at the liner hanger depth (the overlap zone where the liner is inside the larger existing casing), because the annular space at the liner hanger is the zone of critical pressure isolation between the formation below and the existing casing above; the shoe track cement quality is verified during liner drill-out in the same way as for full string casings, but the liner overlap zone cement is evaluated by running a cement bond log through the liner into the overlap zone before drilling out the shoe.