G-function plot of a typical DFIT.

Curving G-function plots and picking closure

G-function plots are routinely used to interpret diagnostic fracture injection test (DFIT) transients. Ideally, a plot of pressure versus G and G*dP/dG versus G should form a straight line. However, the G*dP/dG curve is very often curving. A typical DFIT transient is shown below.

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Derivative of pressure with respect to the G-function

Demystifying the G-function

Diagnostic fracture injection tests (DFITs) are commonly interpreted by plotting pressure versus a function of time called the G-function. The G-function seems rather arcane and is often misunderstood. However, it has

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Diagnostic fracture injection test in a Cartesian plot.

How to plot a DFIT

Aside from the diversity of terminology, the diversity of plotting techniques is the aspect of DFIT analysis that most often causes confusion. Why don’t we just make a Cartesian plot of pressure

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