PSI can generate accurate quantified minimum Earth horizontal stress (Sh) and frac direction maps from seismic pre-drill. Opposite is the Sh Magnitude (ShM=Sh/SV) map over prospective shale gas acreage in the Cooper Basin, Australia. The map shows Sh/SV contours derived from the stress from seismic map over the same area (Consulting).


Comparing Sh/SV with ‘frac gradient’ (instantaneous shut-in pressure, ISIP/D) shows Sh/SV = ISIP/D = PC/D (closure pressure gradient) for weak, low TO rocks. For harder rocks, ISIP - PC = PNET. High PNET is a reflection of natural fracture complexities greater than tensile strength (TO).


SV can vary from 0.88 to 1psi/ft or 12% in the Permian Cooper Basin, Australia and this degree of variation is no doubt the case in many shallow offshore basins where there are few density logs in the upper section. Thus the distribution of SV over a mapped area, field or regional, is also difficult to measure so PSI can vary SV but uses a single SV over a given area. Variations in SV are greater than the stress from seismic determinations which vary from 0 to 5% when compared with well picked PC measured points on fluid injection pressure decline graphs.


On the map opposite, addition of many more available 2D lines, and 3D data over the Toolochee field, would add important detail to ShM contours and locally to SHD (frac propagation direction) for horizontal shale gas wellbore stability planning and frac design.


In the red areas opposite, Sh/SV>1 and fracs will propagate horizontally. Desirable vertical fracs will occur in the green and amber areas where Sh/SV<1.