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Diagnostic Fracture Injection Test (DFIT) or Mini-Frac

Injection tests for tight or low permeability formations

What is a DFIT or Mini-Frac test?

A DFIT or Mini-frac test is an injection or fall-off test performed using a reservoir compatible fluid. Mini-fracs provide better estimation of reservoir properties in tight or low permeability formations and in formations where fluid inflow is severely restricted by formation damage compared to conventional tests.

The main purpose of a mini-frac is to break down the formation to create a small fracture and observe the pressure response before and after fracture closure. The short fracture allows for better communication between the wellbore and true formation.

Benefits of DFIT Tests:

Why Use DFIT or Mini-Frac Tests?

Mini-frac tests are conducted to obtain:

Fracture design parameters

Reservoir characterization parameters

What is a DFIT-Flowback Test?

The DFIT-Flowback is a newer, faster test than the conventional DFIT test used to estimate in-situ stress, reservoir pressure, and the well productivity index. A traditional DFIT test can take days or weeks to complete whereas a DFIT-Flowback can be completed in hours, making it possible to conduct multiple tests in the same amount of time as one DFIT test.

DFIT-Flowback tests are conducted in two steps. First, an injection is used to create a mini hydraulic fracture. Second, using a choke setting on the wellhead, the flowback of the injected fluid is measured. Rate transient analysis allows operators to use the flowback data to determine flow regimes and closure and reservoir pressure.

Performing a Mini-Frac Test with a DST String

Packers are set above or straddling the test interval to isolate the zone of interest (pay zone). The drill pipe and DST string are filled with injection fluid and a pumper is rigged to inject additional fluid to break down the formation and create a short fracture. The pump is then turned off a few minutes after formation break down to observe the closure of the fracture and the after-closure falloff response (see Figure 1). Northstar uses pressure-temperature downhole gauges along with an electromagnetic tool to monitor the progress of the test in real time in its proprietary app, NorthstarLive. Northstar also offers a downhole shut-in valve to decrease the effect of wellbore storage in under-pressured reservoirs.

Figure 1. Modified Mini-frac test overview – Northstar Real Time Recording Data [1].

Pre-Closure Analysis (PCA) and After-Closure Analysis (ACA)

Pre-Closure Analysis (PCA) is used to determine fracture design parameters (flow is governed by the fracture). PCA is performed with injection and time data collected until fracture closure.  It uses a G-function derivative analysis to determine fracture closure and fluid leak-off (see Figure 2).

Figure 2. Modified Fekete G-function Semilog Plot to Estimate Fracture Design Parameters [2].

After-Closure Analysis (ACA) is used to estimate reservoir characterization parameters. ACA is performed with pressure data collected after fracture closure (flow is governed by the reservoir) and a similar approach to traditional pressure transient analysis (PTA) (see figure 3).

Figure 3. Soliman ACA Derivate Analysis and Conventional Injection/Falloff Derivative Analysis to Estimate Reservoir Pressure and Permeability [3].

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