Abstract
The G-function analysis was first introduced by Nolte (1979). Castillo (1987) developed the G-function plot based on Nolte’s pressure decline analysis. Fracture closure pressure in a mini-frac test can be found by using the G-function plot, which is also used to assess the fluid efficiency, fracture geometry and leak-off coefficient. This method has become a standard for fracture calibration in oil and gas industry and is used in applications from low efficiency (high permeability) to high efficiency (low permeability) tight gas. For this analysis, pressure derivative and G-function derivative (G function multiplied by the pressure derivative, GdP/dG) are plotted versus the G-function time. Deviation from straight line on the G-function derivative plot is an indication of the fracture closure pressure. Barree et al. (2009) also suggested to use the Log-Log plot of pressure change to confirm the post-shut-in flow regime and fracture closure time and pressure. This paper presents the application of the G-function analysis and the Log-Log plot of pressure change to the determination of fracture closure pressure and flow regime after shut-in. Practical examples of mini-frac testing will be provided and presented in order to better understand the potential fracture characteristics.