casing shoe test

A casing shoe test is a pressure test performed on the open-hole formation immediately below a newly drilled-out casing shoe, conducted by closing the blowout preventer, pumping a measured volume of drilling fluid down the drill string, and monitoring the resulting pressure response at surface to determine the formation strength at the casing setting depth, with the result expressed as an equivalent mud weight that defines the upper limit of drilling fluid density that can be used in the next hole section without fracturing the formation at the shoe and losing returns into the annulus. The casing shoe test encompasses two distinct procedures that are often confused but serve different engineering purposes: the leak-off test (LOT), in which pressure is applied incrementally until the formation at the shoe begins to fracture (indicated by a deviation from linear pressure-volume behavior on the pump-in plot), giving the true formation breakdown pressure and the actual fracture gradient; and the formation integrity test (FIT), in which pressure is applied only to a predetermined target pressure (the maximum mud weight anticipated for the next hole section plus a safety margin, expressed as an equivalent circulating density) and the test is terminated at that pressure without intentionally fracturing the formation, confirming only that the shoe can withstand the planned mud weight without confirming the actual fracture gradient. In Western Canada Sedimentary Basin well construction, AER Directive 008 requires a casing shoe test after setting and cementing each intermediate and production casing string before drilling ahead, with the test result documented in the well file and used to confirm that the casing point selection provides adequate margin between the planned maximum mud weight for the next hole section and the formation fracture gradient at the shoe. The test procedure begins after drilling out the float equipment (float shoe, float collar) and drilling 3 to 5 metres of fresh formation below the shoe with the bit and BHA in place, then closing the annular BOP or pipe rams to isolate the annulus, picking up the kelly or activating the top drive, and pumping drilling fluid at a slow constant rate (0.1 to 0.5 bbl/min in WCSB operations) while monitoring surface pressure every 0.1 bbl of volume pumped, plotting pressure versus cumulative pump volume and identifying the linear pressure-volume response that represents elastic compression of the fluid and rock before fracture initiation. The leak-off point on the LOT plot is identified as the pressure at which the pump-in curve departs from the initial linear slope toward a shallower gradient, indicating that the formation has begun to accept fluid into natural microfractures or that a new tensile fracture has initiated; the leak-off pressure converted to equivalent mud weight (LOT EMW = surface pressure divided by 0.00981 divided by test depth plus current mud weight) represents the fracture gradient at the shoe and becomes the maximum allowable equivalent mud weight for the interval below the shoe including the equivalent circulating density during drilling operations. For WCSB Montney horizontal wells where the build section is drilled with oil-based mud at 1.45 to 1.55 sg and the fracture gradient at the intermediate casing shoe must accommodate the equivalent circulating density (typically 1.55 to 1.65 sg at maximum pump rate), a minimum shoe LOT EMW of 1.65 to 1.70 sg is required to maintain a positive mud weight window of at least 0.05 sg between the maximum ECD and the shoe fracture gradient; wells where the LOT result is below this threshold require either a reduction in pump rate (reducing ECD at the cost of reduced cuttings transport efficiency) or an additional casing string to protect the shoe that cannot accommodate the required mud weight. Understanding casing shoe test procedures, the distinction between LOT and FIT, the graphical identification of the leak-off point on the pressure-volume plot, the conversion of LOT pressure to equivalent mud weight fracture gradient, and the implications of shoe test results for mud weight window management gives drilling engineers, wellsite supervisors, and well control specialists the technical foundation to correctly design, execute, and interpret casing shoe tests in every WCSB well construction program.

• LOT versus FIT procedural distinction in WCSB drilling programs: A leak-off test intentionally fractures the formation at the casing shoe to measure the true fracture gradient; the pump-in is continued past the linear elastic response until the pressure plot clearly shows formation breakdown. A formation integrity test is stopped at a target pressure (typically the maximum anticipated ECD for the next hole section plus 0.05 sg safety margin, expressed as surface pressure) without fracturing the formation, confirming the shoe can withstand planned conditions but not measuring the actual fracture gradient. AER Directive 008 accepts FIT results for most WCSB intermediate casing shoes; LOT is required for production casing shoes in HPHT or sour gas wells where knowing the precise fracture gradient is critical to the hydraulic fracturing design.
• Pressure-volume plot interpretation and leak-off point identification: The LOT pump-in plot of surface pressure versus cumulative volume shows three regions: (1) an initial linear region where elastic fluid and rock compression gives a constant pressure-per-volume slope (the test is valid only if this linear region is well-established over at least 3 to 5 pump strokes); (2) a departure from linearity at the leak-off point where formation fracture initiation causes the slope to decrease; (3) a formation breakdown region where pressure drops as the fracture propagates. The LOT EMW is calculated from the pressure at the leak-off point (the last point on the linear trend), not from the maximum pressure, which may be higher if fracture propagation resistance temporarily exceeds initiation pressure.
• Cement integrity implication of low shoe test results: A LOT or FIT result significantly below the predicted fracture gradient (based on offset well data and pore pressure models) may indicate poor cement quality at the shoe rather than a genuinely weak formation, because uncemented annular space above the shoe allows the test pressure to communicate with a weaker formation above the shoe depth. In WCSB Foothills wells where the planned shoe fracture gradient is 1.75 sg but the FIT fails at 1.60 sg, the drilling engineer must evaluate whether the failure reflects a formation weakness, a cement integrity issue, or micro-annulus communication, using a cement bond log re-run or a repeat FIT after a waiting period as diagnostic tools before deciding to squeeze cement or add a casing string.
• Extended leak-off test for WCSB HPHT wells: In WCSB Foothills sour gas wells with complex stress states and naturally fractured formations, the extended leak-off test (XLOT or ELOT) provides additional data beyond the standard LOT: after reaching breakdown, the pump is stopped and pressure decay is monitored (shut-in decline) to measure the fracture closure pressure, which equals the minimum horizontal stress and is the most accurate input for hydraulic fracture design. Reopening pressure (re-pressurization after complete closure) separates tensile strength from formation stress, allowing independent estimation of both the minimum horizontal stress and the formation tensile strength critical for Duvernay and Montney hydraulic fracture azimuth and height prediction.
• Maximum allowable annular surface pressure from shoe test results: The shoe test result directly sets the maximum allowable annular surface pressure (MAASP) for well control operations below that casing shoe: MAASP equals (LOT EMW minus current mud weight) times 0.00981 times shoe depth in metres. For a WCSB intermediate casing shoe at 2,200 m with LOT EMW of 1.68 sg and current mud weight of 1.50 sg, MAASP equals (1.68 minus 1.50) times 0.00981 times 2,200 = 3.88 MPa. If shut-in casing pressure during a kick exceeds MAASP, the annulus must be choked to prevent fracturing the shoe and losing well control to underground blowout.

Shoe Test Failure Leading to Additional Casing String on a WCSB Montney Well

A northeast British Columbia operator set intermediate casing at 2,450 m in a Montney horizontal well program and performed a formation integrity test targeting 1.70 sg EMW (the minimum shoe strength needed to drill the Montney with 1.55 sg mud at 1.63 sg ECD). The FIT was pressured to 3.87 MPa surface pressure (equivalent to 1.66 sg EMW at 2,450 m) and held for 10 minutes with 0.3 MPa bleed-down, indicating the formation could not hold the 1.70 sg target. The drilling engineer ordered a cement bond log re-run which showed a 12-metre micro-annulus section at 2,380 to 2,392 m, potentially allowing test pressure to communicate with weaker Triassic formation above the shoe. A cement squeeze at 2,385 m using 2.0 m3 of neat cement sealed the micro-annulus; the repeat FIT reached 1.71 sg EMW with less than 0.1 MPa bleed-down over 10 minutes. The operator drilled the Montney lateral with 1.54 sg mud at 1.62 sg maximum ECD with a 0.09 sg margin to the shoe, completing all 22 fracturing stages without shoe integrity issues. Cost of squeeze and repeat FIT: $92,000. Cost of an additional casing string (the alternative if the squeeze had not worked): $620,000.

Related Terms

Leak-off test is the specific casing shoe test procedure in which pressure is applied until formation fracture initiation is detected on the pressure-volume plot, measuring the true fracture gradient at the shoe depth and providing the most accurate upper bound of the mud weight window for the next hole section in WCSB well construction. Formation integrity test is the conservative variant of the casing shoe test that pressures the open-hole formation to the planned maximum equivalent mud weight without deliberately fracturing, confirming that the shoe can withstand operating conditions without providing full fracture gradient data; accepted by AER Directive 008 for most intermediate casing shoe tests in Alberta. Fracture gradient is the formation breakdown pressure expressed as an equivalent mud weight that the casing shoe test directly measures via the LOT procedure, defining the upper boundary of the safe drilling fluid density window for the next hole section and the maximum allowable equivalent circulating density during drilling operations. Maximum allowable annular surface pressure is calculated directly from the casing shoe test LOT result and the current mud weight, representing the maximum wellhead casing pressure that can be tolerated during a well control shut-in before the shoe formation fractures and underground blowout occurs; the MAASP is the critical well control parameter that governs when the choke must be used to limit casing pressure during gas kick circulation. Equivalent circulating density is the maximum downhole pressure expressed as a mud weight equivalent during active drilling, which must remain below the fracture gradient measured at the casing shoe test to prevent lost circulation; in WCSB tight-window Montney wells, the margin between ECD and shoe LOT EMW may be as small as 0.03 to 0.08 sg, requiring precise pump rate control and real-time ECD monitoring from MWD sensors.