G-function analysis is a type of fracture fall-off data analysis technique that is used to identify pressure leakoff mechanisms and evaluate if fracture closure has occured. G-function is basically a dimensionless time function which was designed in such a way that when pressure data is plotted against this time function, linear flow in the wings of the fracture is plotted in straight line. Further derivative (dp/dG) and G.dp/dG curves are also used to magnify deviation from straight line and pick frac closure event.
This means that when injection is stopped in frac pumping, the fluid first flows from the frac (fluid which is held with-in the open frac) that this flow is linear (in the two wings of the fracture). This would create a straight line on t G.dp/dG curve most often starting from the origin. If we track this this line with a user drawn line from origin, we can estimate the exact point where the original curve deviates from this user line. This point can be identified as the frac closure event, and pressure associated with this point is the formation closure pressure which is analogues to minimum horizontal stress of the formation. Following curve presents a G-function plot annotated:
The above plot is from a real mini-frac test conducted in sandstone reservoir. We can easily pick the first closure event the white curve deviates from red tangent line, however an interesting aspect to see is how the white curve behaves after the first closure event. Note the trend at G-function time 6 (x-axis), are we observing a second closure event? Are there multiple closures in this formation? Interesting phenomenon.
Thats the end of this MiLog. I will be writing a second part to G-function MiLog where i'll discuss how G-function plot can be used to evaluate the type of leakoff in the formation and incorporate mitigation in main frac design to counter this fluid leakoff.
