Rock mechanical characteristics of deep marine shale in southern China, a case study in Dingshan block

The success of commercial exploitation of shale gas has already been achieved in middle-shallow layers of China. But more than 65% of shale gas resources is buried in deep formation (vertical depth >= 3500 m), which has not been effectively developed yet. Deep buried depth is the source of a series of challenges. To clarify the influence of buried depth on mechanical characteristics of shale, tri-axial compression tests considering high confining pressures (80 MPa) are conducted with core samples collected from the gas bearing section of a deep marine shale gas well. The results show that the peak deviatoric stress significantly increases with the growth of confining pressure and exhibits 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. Unlike conventional smooth tri-axial test curves, many of the post-peak curves are in "step-like" shape, and their corresponding failure patterns are always multi-fractures, all of which are originated from the activation of bedding planes or natural fractures during yielding process and are indications of high brittleness. However, the proportion of specimens with these features decreases from 66.7% to 22.2%, with the confining pressure increased from 30 MPa to 80 MPa. As most of existed indices could not properly describe the brittle characteristic of this type of deep marine shale, a new brittleness index (BI) is proposed by quantifying the complexity of "step-like" post-peak curves in fractal dimension. It is demonstrated that BI could effectively capture the influence of confining pressure on brittleness and distinguish those more brittle layers. Owing to the great reduction of brittleness and fracture complexity under high confining pressure, the fracking network in deep formation might be sparser than that in middle-shallow layer at field scale, leading to the poor performance of past fracking deep wells.