Assuming that the overburden stress (Sv), is one of the three principal stresses, the stress tensor (representing the in-situ stress field) in the Earth’s crust can be explained by four independent components:
- Sv magnitude
- maximum horizontal stress (SHmax) magnitude
- minimum horizontal stress (Shmin) magnitude
- and the SHmax orientation
In the previous Geomechanics blog post we explored how SHmax orientation can be estimated using image logs. In this post we explore how the Leak-off Test (LOT) and its extended version can be used to estimate the minimum horizontal stress (Shmin) magnitude.
What is a Leak-off Test anyway?
Leak-off Tests involve increasing the mud pressure in an open and isolated section of wellbore, below the casing shoe, in order to create a small tensile fracture, measuring the strength of the open formation.
What is the Extended Leak-off Test (XLOT)?
The Extended Leak-off Test (XLOT) is more comprehensive and longer leak-off tests in which pumping is not stopped immediately after the leak-off pressure is observed, and are primarily conducted to obtain a fracture closure pressure.
An Extended Leak-off Test (XLOT) will consist of one, two, or more complete cycles of leak-off, formation breakdown, fracture propagation, shut-in and fracture closure. Figure below shows an ideal example of XLOT that shows different stages in a time-pressure test.
How do we estimate Shmin accurately?
In general, the leak-off pressure is less reliable for estimating Shmin, and tends to slightly over-estimate Shmin. However, when Extended Leak-off Test (XLOT) data is available, both Fracture Closure Pressure (FCP) and Instantaneous Shut-in Pressure (ISIP) can be used to provide more reliable information for calculation of Shmin. The FCP information from the subsequent cycles are more reliable than the first cycle because the FCP of the second or third cycles has removed the effect of tensile rock strength and hence provide more accurate estimation of Shmin magnitude.
Mojtaba Rajabi, Scientific Advisor
Below 👇 you are visualizing picking FCP on an XLOT data to estimate Shmin magnitude in the metaKinetic platform.
Want to have access to this simulation and practice estimating Shmin magnitude by picking FCP on Extended Leak-off Test (XLOT) data using a double-tangent method? Contact us!