Hydraulic fracturing is undeniably the crucial technology involved in the development of tight and/or unconventional gas reservoirs. The fracture geometry and pumping execution, as well as the well architecture, can be designed to maximize the well productivity, provided the reservoir permeability is known. Because it is difficult to estimate permeability from conventional pressure transient tests, various authors have shown how fracture-injection/falloff tests designed for final fracture design calibration (i.e. estimation of closure stress, leak-off coefficient and fracture fluid efficiency) can be used to estimate reservoir permeability as well.

This paper presents a new comprehensive model for the analysis of fracture falloff, following a step-rate or constant rate injection test. The derivative of the falloff pressure change with respect to the logarithm of superposition time shows a progression of straight trends that include a newly introduced elastic closure-dominated flow enabling determination of the closure stress (3/2 slope), post-closure formation-linear flow (1/2 slope) and infinite-acting pseudo-radial (0 slope). This model provides a robust complete assessment tool that allows quantification of all fracture parameters (closure stress, fracture extent, leak-off coefficient and fracture fluid efficiency) as well as reservoir permeability, provided that enough time is allowed for the falloff to reach pseudo-radial flow regime. Both oil and gas reservoirs can be effectively evaluated.

While the approach is leveraging well-accepted pressure transient analysis methods, the inclusion of elastic closure-dominated flow is new. This model can be used to design fracture-injection/falloff tests that would allow determination of all the involved parameters, including reservoir permeability. Field data validate the model and demonstrate the value of this approach.

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