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214 Reservoir geomechanics

Stress (MPa)
0 20 40 60 80 100 120
0

S hmin (LOT)
Pore Pressure (RFT)
1000

2000
Depth (mRKB TVD)

3000

4000
S v
HYDROSTATIC
PORE PRESSURE
5000

Figure 7.4. Least principal stress as a function of depth determined from extended leak off tests in
the Visund field (after Wiprut, Zoback et al. 2000). The least principal stress is slightly below the
overburden stress (determined from integration of density logs). The pore pressure is somewhat
above hydrostatic (shown by the dashed line for reference). Reprinted with permission of Elsevier.

to about 1500 m depth (not shown) but at ∼3000 m it is approximately 75% of the
vertical stress. There are three important features to note about the least principal stress
values at depth. First, the measurements are repeatable and indicate a consistent trend
throughout the field. Second, the measurements clearly indicate a compressional stress
state because even at relatively shallow depth (where pore pressure is hydrostatic), the
magnitude of the least principal stress is extremely close to the vertical stress. We show
belowthatthemagnitudeofS Hmax isgreaterthanS v suchthatastrike-slipfaultingregime
exists in this region. However, because S hmin is extremely close to S v if the magnitude
of S v was slightly over-estimated (due to uncertainties in density), or if S 3 is slightly