Comprehensive Fracture Calibration Test Design
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AbstractThe formation permeability is an essential input for modem hydraulic fracture design. In tight formations estimation of formation permeability and pressure can be impractical or even impossible to determine by conventional pressure buildup transient tests because the formation virtually cannot flow without the fracture stimulation. Traditional fracture calibration tests including minifrac and data-frac provided estimates for the fracture minimum closure stress and the leakoff coefficient.More recently, the continued acquisition of the injection falloff transient pressures after the fracture closure as in the Diagnostic Fracture Injection Test (DFIT) has provided estimates for formation permeability and pressure that could not have been determined from a conventional transient test. After-closure analysis can also yield permeability and pressure in conventional reservoirs and thereby eliminate the need for a pressure buildup test.This paper provides a model for simulating the fracture calibration test to be used for test design and interpretation. The challenge for tight formations is to find an injection volume sufficient to create a suitable fracture while minimizing the closure time in the subsequent pressure falloff. This, in turn, reduces the time required to see after-closure transient features that are used for estimation of formation pressure and permeability. For higher permeability formations, the model enables estimation of the total test time including after-closure radial flow from which estimates of formation pressure and permeability are straightforwardly determined.The design examples included in this paper are based on actual field data and demonstrate that fracture calibration tests provide an efficient and reliable means for determination of not only closure stress and the leakoff coefficient, but also the formation pressure and permeability, in the course of a realistic and viable amount of time.
