http://www.petracat.com/energy/Diagnostic-Fracture-Injection-Testing-DFIT-in-Unconventional-reservoirs/page168.html
The DFIT test has become the primary transient test for the ultra-low permeability shales being developed in the USA today. The DFIT goes by many names: mini-frac, mini fall-off, datafrac, et cetera; but all refer to the act of injecting small volumes of a clear fluid (usually 2 or 3% KCL water) at low rates to create a fracture before the wellhead is shut-in and the pressure allowed to fall-off naturally. The fluid contains no proppant so that the fracture can relax and close naturally when pressure is released. Most unconventional zones are overpressured reservoirs that can support a column of fluid in the wellbore. In those cases, upwards of 90%+ of all unconventional reservoirs, the accepted practice is to utilize surface pressure measurements. Multiple direct comparison runs with downhole gauges have confirmed that the DFIT test can be performed using a high quality surface measurement system. This alternative not only produces equally accurate data but is also significantly lower in cost and risk-free.
Conventional pressure transient testing (pressure build up and drawdown/production testing) depends on “transients” or changes in flow rates occurring relatively quickly. Pressure transient testing can be applied from initial completion to the economic producing life of the well. The permeability of the reservoir is the primary determining factor of the duration of the pressure transient test. Because the permeability in shale formations is extremely low, the opportunity to perform pressure transient analysis is severely restricted due to time limitations. For most operators, waiting the requisite time for the transitions in “analyzable” flow regimes to take place is not economical. While a reservoir with a permeability estimated to be 50 md may require less than 24 hours of shut-in time to determine formation properties; a nano permeability reservoir such as the shales might require months or possibly years for this transition to take place. This reality further demonstrates the impracticality of traditional testing methods for these types of ultra-low permeability, unconventional formations.
The oil and gas industry is a dynamic industry and continues to create processes that boost productivity and yield better accuracy while also being more cost and time effective. Knowing this, a pressure fall-off analysis method for the DFIT was developed by Nolte to provide estimates for fluid efficiency, fracture closure pressure or minimum in-situ stress, fluid leak-off coefficient, and fracture gradient, among other important frac calibration parameters. This was followed with a process called after-closure analysis (ACA) to assist in the identification of flow regimes and ultimately to calculate reservoir transmissibility (kh/µ) and initial pressure with results comparable to conventional methods such as pressure buildups and drawdown/production testing. This analysis technique has multiple uses but is routinely employed in ultra-low permeability reservoirs and can be used for reservoir characterization and for successful fracture treatment, evaluation, and design. The ACA technique is comparable to the pressure transient analysis (PTA) technique utilized in conventional higher permeability reservoirs; although, PTA is a technique less preferred.
