Almost all geology undergraduates are taught about Mohr diagrams because they are a simple and convenient way of understanding the relationships between stresses and fluid pressure (effective stresses), the mechanical properties of the rock and the resultant fracturing. Sadly, most geologists seem to forget those lessons and do not appreciate how Mohr diagrams can be used to predict under what conditions rocks will fracture and what types of fracture will be produced by certain effective stresses. We have gone back to basic principles and used Mohr diagrams to make predictions about the conditions in the Variscan rocks at reservoir depths beneath Göttingen. In particular, we have made predictions about the fluid pressures required for stimulation, which is envisaged to be necessary to obtain viable fluid flow rates (https://www.mdpi.com/2076-3263/11/8/349).
This simple approach is useful because it helps geoscientists consider, model and predict the ranges of mechanical properties of rock, stresses, fluid pressures and the resultant fractures that are likely to occur in the sub-surface. As such, it provides a critical and necessary link between a range of approaches, including field analogue studies, rock deformation experiments and discrete fracture network modelling.
Mohr diagram showing the effects of different fluid pressures on the state of effective stress, using our model for the Devonian and Carboniferous greywackes with a geostatic stress ratio of 0.41, at a depth of 4.5 km and with no applied tectonic stresses. The fluid pressures are zero, hydrostatic (44.15 MPa) and overpressured (138 MPa). While the stress system is stable (no fracturing) at lower fluid pressures, the model predicts that a fluid pressure of 138 MPa is needed to generate new extension fractures.