Measuring Shmin: LOT, XLOT, Minifrac
Learning objectives
- Recognize that Shmin is the one horizontal stress that can be measured directly, by fracturing the rock
- Read a leak-off pressure-time record: leak-off, breakdown, propagation, shut-in, and closure
- Identify fracture closure pressure as the minimum horizontal stress
- Rank the tests by quality: LOT gives a lower bound, XLOT and minifrac give a measurement
The Stress You Can Read Off a Chart
Of the three principal stresses, the minimum horizontal stress is uniquely privileged: it can be measured directly, because a hydraulic fracture opens against it and no other. Pump fluid into a sealed interval until the rock fractures, and the pressures the well records are a direct reading of the stress field. The test comes in grades. A leak-off test (LOT), run routinely below every casing shoe, pumps only until the pressure-time line bends away from its straight fill-up slope, the leak-off point, and stops; that pressure is a lower bound on . An extended leak-off test (XLOT) or a purpose-run minifrac keeps pumping past breakdown, propagates a real fracture, then stops the pumps and watches it close, and the closure gives itself.
Read the record in the figure. Pressure climbs a straight line as the sealed well fills, until the leak-off point where fluid begins entering an opening fracture and the line bends. It rises to a peak, the breakdown pressure, where the fracture initiates and runs, then falls to a propagation pressure as the fracture extends. Stop the pumps and the pressure drops instantly to the instantaneous shut-in pressure (ISIP), then bleeds down more slowly until the fracture closes. That closure pressure is the prize: it is the pressure at which the fracture walls just touch, which is exactly , reading 46.0 MPa on the canon well. The ordering is physics, not convention: closure is below ISIP is below breakdown, always, and a record that violates it is a bad test.
Why This Is the Anchor of the Model
The closure pressure is the single most valuable stress measurement a well can make, because is otherwise the hardest of the horizontal stresses to pin, and because so much depends on it: it is the floor of the mud-weight window's upper bound, the fracture gradient that limits injection, and, through the polygon of the last section, a hard constraint that also narrows the unmeasurable . Picking closure well is a craft, the pressure decline is read in specialized plots, the square-root-of-time and G-function tangents of Part 7's minifrac section, to find the exact departure from linearity. But the principle is simple and worth holding onto: the rock, fractured on purpose, tells you the stress that would close the fracture, and that stress is . With one horizontal stress measured, the next section boxes in the other, which no test can reach directly, using the wellbore observations the polygon is waiting for.
References
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
- Haimson, B., & Fairhurst, C. (1967). Initiation and extension of hydraulic fractures in rocks. SPE Journal, 7(3), 310-318.
- White, A. J., Traugott, M. O., & Swarbrick, R. E. (2002). The use of leak-off tests as means of predicting minimum in-situ stress. Petroleum Geoscience, 8(2), 189-193.