reservoir geomecanics

382 Reservoir geomechanics
Figure 12.2. Variation of stress change with pressure as a function of Biot coefficient, α, and
Poisson’s ratio, ν (after Chan and Zoback 2002). The normal faulting line (A = 0.67) is explained
in the text. The gray area represents the possible combination of α and ν such that stress path leads
to production-induced normal faulting. Observed stress paths in different reservoirs are shown on
the right-hand side of the diagram. For the fields listed in italics,itisnot clear whether the reported
stress path indicates a change of stress with depletion or variation of stress with pore pressure in
different part of the fields. C○ 2002 Society Petroleum Engineers
in Figure 12.2 corresponds to values of the stress path above which depletion would
ultimately lead to movement on pre-existing normal faults as explained below. This is
derived below.
Depletion data from several wells studied in a Gulf of Mexico oil field (Field X) are
shown in Figure 12.3 (from Chan and Zoback 2002). All of the wells in this field (indicated
by the different symbols) deplete along the same path, indicating that where the
reservoir is penetrated by wells, it is interconnected and not sub-compartmentalized.
The least principal stress, measured during fracpac completions, indicates a stress path
of ∼0.55 (Figure 12.2), easily explained by reasonable combinations of α and ν. Data
from this field will be considered at greater length below. Note that considerable depletion
occurred, from an initial pore pressure of ∼80 MPa prior to significant production
in 1985 to ∼25 MPa in 2001, after ∼15 years of production.