Intermediate Stress - The Neglected Middle Child of Geomechanics?

(My big brother, a middle child, who no doubt broke his leg because everyone, including our big sister, was too busy tending to little tiny me instead of looking after him. Image used without any permission whatsoever.)

Last time I wrote about a common question we get from clients. This time I'm going to expand on a question I ask myself at least twice a week:

Why do so many people think they are in a normal faulting environment??

There are several answers, all of which make my life difficult. I've written before about one of the biggies, which is that people are applying "old" stress calculation workflows from conventional days when many plays were indeed in a normal faulting environment. Those plays were in passive basins with minimal tectonic stresses. Vertical stress (overburden) could be calculated from logs and used, with Poisson's ratio, to calculate the horizontal stresses, which were equal to each other and lower than the vertical stress. This workflow does not work in most, if not all, unconventional plays.

A new, and probably pretty important, reason just occurred to me the other day - if their hydraulic fractures are vertical, people are inferring (correctly) that the minimum stress is horizontal, but they are then incorrectly extending that to say that the vertical stress is the maximum stress, as would be the case in a normal faulting environment. They forget, or don't know, that you will also get vertical hydraulic fractures in a strike-slip faulting environment where the vertical stress is intermediate - that is, lower than the maximum horizontal stress and higher than the minimum horizontal stress.

Where in the world do we see strike-slip faulting environments? All over the place. Just have a wee gander at the World Stress Map. All that green? That's strike-slip.

Who cares? The geologists should care, because it has implications for the types of faults and fractures that may be present and might influence where the most prospective parts of the reservoirs are. The drillers should care, because it will change their wellbore stability situation. The engineers should care, because it affects their hydraulic fracture models and every prediction and calculation that follows. To name just a few.

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