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Fig. 11-8. Secor’s composite failure envelope for rocks with 2 MPa tensile strength(above
the axis); conventional, with zero tensile strength, below.

sity of the overburden, h is the ratio of pore-fluid pressure to overburden
pressure, and the factor 8 is the largest difference between ul and u3 in units
of u,, at which u3 can be zero. The requirement for an open fracture is that
the greatest and least principal stresses must be such that the circle on which
they lie must reach the failure envelope while the least principal stress is nega-
tive.
It must be remembered, as Secor was careful to point out, that Mohr dia-
grams are simplifications of a very complex process, and nowhere is the pro-
cess more complex than in the region of very small effective stresses. Secor
concluded that in areas in which the difference between the greatest and the
least principal stresses is small (small circles on the Mohr diagram) and the
ratio of pore-fluid pressure to overburden pressure is large, open fractures
could be created and exist at great depths, and that previous shear fractures
could be opened up.
The existence of open fractures at depth, such as in the Asmari Limestone
reservoirs of Iran (Hull and Warman, 1970) leaves no doubt that the argument
is qualitatively correct. The difficulty with quantitative assessment lies in
determining the value of the tensile strength of rocks in situ.
In the Midland gas field, Louisiana, studied by Fowler et al. (1971), each
fault block in Oligocene sedimentary rocks has its own pressure regime, and
the values of h are known quite accurately from reservoir data. We assume