Page 198 - Applied Petroleum Geomechanics

In situ stress estimate 193

By analyzing in situ stress data measured in mining industry, Hoek and
Brown (1980) proposed that overburden stress can be estimated from the
overburden gradient of 0.0272 MPa/m. This implies that the rock average
3
bulk density is about 2.77 g/cm , which is too high for most sedimentary
rocks in petroleum basins, where the overburden gradient is about
0.0231e0.0238 MPa/m, as shown in Eqs. (6.12) and (6.14). Therefore, if it
is possible, it is best to compute vertical stress from reliable density log data.

6.2 Minimum horizontal stress from measurements
Ljunggren et al. (2003) reviewed methods for determining in situ stresses,
which can be classified into two main categories. The first category consists
of methods that disturb the in situ rock conditions, i.e., by inducing strains,
deformations, or crack opening. The following methods may be included
in this category:
• hydraulic methods, including hydraulic fracturing and hydraulic tests on
preexisting fractures,
• borehole relief methods, and
• surface relief methods.
The second category consists of methods based on the observation of
rock behaviors without any major influence on the rocks. The following
methods belong to this category:
• statistics of measured data (database),
• core-discing,
• borehole breakouts,
• relief of large rock volumes (back analysis),
• acoustic methods (Kaiser effect),
• strain recovery methods,
• geological observational methods, and
• earthquake focal mechanisms.
In the oil and gas industry, the minimum horizontal stress is commonly
measured from downhole hydraulic fracturing tests, such as LOT, extended
leak-off test (XLOT), diagnostic fracture injection test (DFIT), mini-frac
text, and micro-frac test.

6.2.1 Leak-off tests in normal and strike-slip faulting
stress regimes

The minimum horizontal stress can be determined by direct measurements,
and the commonly used method is hydraulic fracturing (e.g., Haimson and
Cornet,2003),oritsoilfieldequivalent,LOT,XLOT(e.g.,Raaenetal.,2006),