They found that the strength of shales increased with the growth of depth and exhibited an obvious declining trend from bottom to top in longitudinal profile, whereas Young’s modulus and Poisson’s ratio were all relatively insensitive to buried depth. Moreover, the dominant failure pattern gradually transforms from multi-fractures to single shear rupture (Fig. 1).

A new brittleness index was proposed by quantifying the complexity of “step-like” post-peak curves in fractal dimension, which could effectively capture the influence of confining pressure on brittleness and distinguish those more brittle layers (Fig. 2).

Considering the great reduction of brittleness and fracture complexity under high confining pressure in laboratory, it could be inferred that the hydraulic fracture network in deep formation might be sparser than that in middle-shallow layer at field scale.

These findings are helpful to understand why past fracking in deep formation usually could not obtain good yield performance. They also provide an important guidance for future horizontal borehole stability control and hydraulic fracturing optimal design in deep shale gas reservoirs.

The results have been published in Journal of Petroleum Science and Engineering.

Paper link: Rock Mechanical Characteristics of Deep Marine Shale in Southern China, a Case Study in Dingshan Block

Fig. 1. Fracture patterns of specimens under different confining pressures along depth.

Fig. 2. Distribution of brittleness under different confining pressures along depth.