Minifrac and DFIT
Learning objectives
- Define a minifrac (DFIT) as a small deliberate fracture run to measure Shmin precisely
- Read the pressure falloff after shut-in and pick the closure pressure
- Use the specialized falloff plots, the square-root-of-time and G-function tangents, to find closure
- Recover the canon Shmin of 46 MPa from the closure pick
The Fracture Run to Measure, Not to Produce
The last section showed that any hydraulic fracture, on shut-in, bleeds down to a closure pressure equal to . A minifrac, also called a diagnostic fracture injection test or DFIT, is a small fracture pumped for exactly that reason: not to stimulate the well, but to measure the stress. A modest volume is injected to open a short fracture, the pumps are stopped, and the pressure decline is recorded and analyzed with care. It is the best measurement money can buy, more reliable than a leak-off test because the fracture is fully developed and the decline is watched patiently, and it is standard practice before designing a full stimulation.
The subtlety is picking where the fracture closes, because the pressure decline is a smooth curve with no obvious kink. The trick is to plot the falloff in transformed time so the closure appears as a change in slope. On a plot against the square root of shut-in time, the data fall on a straight line while the fracture is still open, then depart from that line at closure; the departure point is . The more refined G-function method, an industry standard, transforms time to account for the fluid leaking off into the rock, so that fracture closure shows as a clean tangent departure. Drag the closure pick in the figure onto the departure and the tool scores it against the true of 46 MPa: a good pick lands within a fraction of a megapascal, a careless one biases the entire stress model.
Why the Care Is Worth It
The precision matters because is the linchpin. It is the fracture gradient that limits every future injection, the ceiling of the mud-weight window, the anchor that the polygon uses to bound , and the reference against which depletion is tracked. An error in the closure pick propagates into all of them. This is why the DFIT is run and read with a rigor out of proportion to its small size, and why the specialized falloff plots exist at all: they turn a subtle, judgment-laden pick into a defensible, reproducible one. The minifrac is where the theory of this part meets the discipline of measurement, and it is the workhorse measurement that Part 8 will use to anchor the Ogbon-1 stress model. The next section leaves precise measurement behind for scale: growing a fracture big enough to produce through.
References
- Nolte, K. G. (1979). Determination of fracture parameters from fracturing pressure decline. SPE 8341.
- Barree, R. D., Barree, V. L., & Craig, D. P. (2009). Holistic fracture diagnostics: Consistent interpretation of prefrac injection tests using multiple analysis methods. SPE Production & Operations, 24(3), 396-406.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.