Australian operators often compare the Cooper Basin to the Eagle Ford Shale in South Texas when it comes to total organic carbon and kerogen for example. However, there is one key parameter that is rarely, if ever, mentioned—Earth stress.

“There are six to eight different forces that determine if a formation can be hydraulically stimulated. The industry only uses one,” John Davidson, CEO, Predrill Stresses International Ltd., said at the DUG Australia conference Aug. 27. “When you’re trying to produce man-made fractures, you have to overcome the local stresses.”

This type of information is useful to determine prior to drilling. Since hydraulic fracturing is needed to recover unconventional resources, poorly understood Earth stresses in well planning can put a halt to horizontal operations. Finding zones with the lowest stresses will lead to greater success.

Fracturing efficiency can be maximized if porosity increases in the direction of the horizontal wellbore and fractures are directed parallel to the direction of the maximum horizontal Earth Stress (SHmax or SHD), he explained.

Continental Movement

Australia is the fastest-moving continent. For more than 50 million years, it has been heading toward the equator in a north-northeast direction at the rate of more than 60 mm per year. Africa is moving to the northeast at 25 mm per year, while South America is heading north at 10 mm per year.

“There are four, arguably five, more global tectonic forces that control 99% of the world’s hydrocarbon resources,” Davidson said. “Can these forces be understood and harnessed for fracking? You can use 3-D seismic placed as desired to find faults that are the basis of structural geology and stress determination.”

Since Australia is moving toward the equator, it is experiencing an increasing Earth radius. Since the Earth’s crust is basically rigid, this results in a reduction in the crustal curvature by compressing the upper crust and extending the lower crust. This motion lifts the margins like the flanges of a hinge, he explained.

The Cooper and Mesozoic Eromanga basins are being compressed between these flanges. “The strong compression causes an inversion over 300 m of the pre-Tertiary in the northeastern part of the Nappamerri Trough. This results in a reverse-fault stress state, which predicted the lack of success in stimulating parts of the Permian horizon.

“Almost all earthquakes in Australia are strong reverse-fault stress states. This has eliminated the paleo northern Nappimerri Trough in the Cooper Basin for fracturing in horizontal wells. Several wells confirmed this prediction with flows declining rapidly below 1 MMcf/d since there was no vertical fracture propagation for the basin-centered Permian gas,” Davidson said.

Measuring Tectonic Stresses

In 1905, E.M. Anderson described three fault types: 1) normal, which is a subvertical offset resulting from extension or having been pulled; 2) wrench, which is a lateral or strike slip resulting from extension and compression or having been twisted; and 3) reverse, which is a subvertical offset from compression or having been pushed.

Predrill Stresses International (PSI) subdivided these three fault types into seven varieties to extract more stress information from faults. The quality of 3-D seismic allows these varieties to be recognized, he continued.

“PSI has patents covering the derivation of quantified stresses from structures interpreted on seismic lines. Our 4DGeoStress software can map Earth stresses in three dimensions. Quantified stress means a horizontal well can be planned parallel to the smallest component of the Earth’s stress field (ShD). Fractures can propagate at right angles, parallel with SHmax in the direction of extensional failure propagation,” Davidson explained.

This creates detailed pressure-depth graphs for a well planned in any direction or inclination. This means that fractures can be directed and aligned vertically within the potential reservoirs prior to drilling, he emphasized.

The fracture gradient can be extracted from the 3-D seismic and mapped. Reverse fault areas will likely only fracture in the horizontal plane. Normal and wrench fault areas will form vertical fractures.

Cooper Basin vs. Eagle Ford

Although on an in-place barrels of oil equivalent basis, the rock properties and TOC are comparable between the Cooper Basin and the Eagle Ford, the Earth stresses are not. In the Eagle Ford and Bakken plays in the U.S., the wide expanse of low to extensional stress states have led to vertical fracturing from the horizontal wellbore, resulting in increased unconventional production.

The extensive lower stress states in U.S. plays are due to different forces since North America is moving at less than 6 mm per year toward the equator.

“That huge difference in Earth stresses is why the Australian unconventional plays will not be as widespread and successful as the U.S. plays,” Davidson concluded.

Contact the author, Scott Weeden, at sweeden@hartenergy.com.