Unconventional reservoirs, including shale formations and many tight-gas sands, contain natural fractures, fissures, faults, and microfractures that contribute to the rock flow capacity; thus, core-measured permeability, and especially crushed-core permeability measurements, may not be representative of the true reservoir rock flow capacity. Evaluating the in-situ rock permeability, and simulating production, requires sampling not only the matrix, but also the fractures and fissures that contribute to the unconventional rock system permeability. A properly designed well test in an unconventional reservoir will sample a volume of reservoir rock that is representative of the whole. In other words, a well test should sample a representative elementary volume, which is the smallest volume of rock with properties characteristic of the whole.

Diagnostic fracture-injection/falloff tests (DFIT) have been routinely implemented since the late 1990s to understand leakoff mechanisms, identify fracture closure stress, estimate initial reservoir pressure, and determine permeability-thickness in unconventional reservoirs. In almost every unconventional well completed, a DFIT is the only well test that will be completed during the well lifecycle, but historically, the tests have been designed empirically based on analog formations and without considering the volume of rock investigated.

We demonstrate a new method for calculating the volume of rock investigated by a DFIT, and we show how a DFIT design can allow for sampling a representative elementary volume of the reservoir.

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