Process Zone Stress (PZS) is a directly measured pressure taken from the extension pressure and the closure pressure in a fracture injection test. Because the PZS includes effects of fluid lag, intact rock strength (tensile strength), and other non-linear stress dissipations around the tip of the fracture, it is not related to only one property. The combined effect of all these mechanisms that restrict frac growth can be directly measured, quantified, and input directly to GOHFER as the PZS.
PZS can also be estimated from log data. In most cases we use the function:
PZS_VSHALE = a1 * VSHALE + a2
a1 = 500
a2 = 300
to estimate PZS where a2 is the measured difference between ISIP and closure pressure, or the net frac extension pressure. The value of a1 needs to be locally calibrated based on tracers and other diagnostics of height growth. The inclusion of Vshale provides a link to effective porosity because it is the crossplot porosity multiplied by (1-Vshale).
In some special cases, like CBM, the relationship between GR (Vshale) and PZS from sands to coals does not give the correct result. In these cases (and possibly in carbonate sections) it is recommended to use a linear transform on RHOB instead of Vshale to calculate PZS.
PZS_RHOB = (YME * RHOB) / (2 * PR + 2) * r1 + r2
r1 = 120
r2 = 100
The goal is to find constants that give PZS=ISIP-Pclosure as the value in the tested interval and provides approximately 300-1000 psi contrast between the “weakest” and “strongest” zones.
