Shale softening: Observations, phenomenological behavior, and mechanisms

Spurred by the advanced technologies, mainly including horizontal drilling and hydraulic fracturing, shale gas exploration has grown significantly over the past few decades. Upon exposure to the aqueous fracturing fluids in the high temperature and high pressure subsurface, the mechanical properties of shales such as elasticity, hardness, and strength usually deteriorate, a phenomenon termed "shale softening". As a complex, multiphase, and multiscale material, shale is prone to the change in its mechanical properties upon exposure to fracture fluids. It is generally agreed that shale softening has great impact on the design and operation of shale gas exploration and the long-term gas production. This paper provides a critical review of the observed, phenomenological behavior of shale softening, and summarizes the currently recognized potential or hypothesized underlying mechanisms. The former includes: (1) reduction in fracture conductivity and hence the rate of gas production; (2) degradation of mechanical properties and reservoir fracability; (3) creep and long-term damage to the shale formations. The latter consists of clay-fluid interactions, electrical double layer (EDL) repulsion, solid mineral dissolution, short-term unloading, and long-term creep. However, to date, the dominant mechanisms controlling shale softening for a rock with known mineralogical compositions and the chemistry of fracturing fluids still remain unresolved. Our preliminary investigations suggest that the dominant mechanism depend on shale's compositions. Therefore, knowledge of the mineralogy of a shale is proposed as an essential requirement for the development of a framework for probing the mechanisms of shale softening. It is expected that such a newly proposed framework can practically facilitate the design and operation of shale gas exploration and help achieve stable gas production over an extended duration.