The stress path represents the change in horizontal stress as the pore pressure changes. In a producing reservoir the horizontal stress is reduced as the pore pressure is reduced, leading to an increase in effective stress. The relationship between the initial and present state of these two factors is known as the stress path (Q) and can in its simplest form be described as:
Q = (Sinitial – Spresent) / (PPinitial – PPdepleted)
A simply way to understand this mechanism is by comparing it with the arching principle. During the construction of a bridge, the material weight is supported by scaffolding. After removing the scaffolding, the structural support is taken away and stresses are diverted along the boundaries (i.e. along the bridge itself). The same holds for a depleting reservoir: as pore fluid is removed less support is given to the surrounding structure, hence a reduction in stress occurs internally in the reservoir. This internal stress relief is counteracted by a stress increases above and below the reservoir.
Some use the same expression to describe the change in fracturing pressure as a function, but studies have shown that this is a more complicated process. The effect of physical properties is in this case much more pronounced.