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In situ stress estimate  203


                                        Stress/pressure (psi)
                                0    2000   4000  6000  8000  10000
                             7200
                                    Pore pressure  Sh in sandstones/siltstones
                             7300   Sh measured
                                    Sh calculated
                             7400   Hydrostatic Pp
                                    Overburden
                             7500   Sh measured, shale
                           Depth (ft)  7600               Sh in shales
                                    Pp interpolated
                             7700
                             7800
                             7900
                             8000
                             8100
              Figure 6.9 Pore pressure, measured and calculated minimum horizontal stresses (S h ),
              and overburden stress versus depth in Well MWX-2 based on the data of Warpinski
              et al. (1985).

              match the measured results in sandstones and siltstones, where measured
              pore pressure data are available. The calculated results do not match the
              measured results in shales, and this may be caused by the unreliable pore
              pressure estimate in the shales. There were no measured pore pressures in
              shales, and the pore pressures provided in shales were interpolated from the
              pressures measured in adjacent sandstones (Warpinski et al., 1985). The
              mismatch in the minimum horizontal stress calculation may also be caused
              by the tectonic stress.

              6.3.2 Minimum horizontal stress with tectonic impact
              The uniaxial strain model is oversimplified for estimating the minimum
              horizontal stress in a tectonic region. Therefore, modification is needed.
              If the tectonic stress is considered (as proposed by Daines, 1982), the
              minimum horizontal stress can be written as:
                                     n
                              s h ¼     ðs V   ap p Þþ ap p þ s min      (6.27)
                                                           tect
                                   1   n
                     min
              where s tect is the tectonic stress in the minimum horizontal stress direction.
              It is also named as the excess minimum horizontal stress, and
                                            E
                                   s min  ¼    ðε h þ nε H Þ             (6.28)
                                    tect
                                         1   n 2
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