Shales can serve as pressure barriers in basins, as top seals, and as reservoirs in shale gas plays. This paper emphasises the role of geomechanics in governing shale fracturing. In many basins, the fluid pressure of the aqueous system becomes significantly elevated, leading to the formation of a hydrofracture, and fluid bleed-off. Natural hydrofracture is an unlikely process in the circumstances that exist in most basins. The ideas that underpin hydrofracture thinking are briefly summarised as: a given state of stress such that two in-plane (normally a 2D analysis) principal stresses are almost equal in magnitude; an existing flaw in the material contains a highly pressurised fluid, and a stress concentration develops at the sharp tip of the flaw (which is normally assumed to be slit-like); the stress concentration locally causes a tensile stress to develop in a small region (on the order of mm) in front of the crack tip, causing the material to fail, and hence lengthening the crack; in the elastic equations, the stress concentration depends on the crack length, so the process can continue by feedback. In a P-Q diagram, the hydrofracture conditions plot in a tiny region near the origin. Those states can be reached in Nature, but only by peculiar paths. It seems likely that the conditions of fluid-related yielding (in low effective stresses) are not those of hydrofracture, but instead are associated with dilational, shear-related deformations. This type of deformation increases the pore volume of the material, and, locally, the fluid pressures will be decreased (at least temporarily) as a result. Fluids will flow into the dilated region, and may leave evidence in the form of veins or sand-filled intrusion swarms. Such physical features are widely observed, but usually attributed to hydrofracture. My analysis suggests that they may be better interpreted as dilational yielding of basin geomaterials. Shale gas plays require the manufacture of the reservoir by inducing hydraulic fractures within the shale. Experience suggests that the outcome can be a classical bi-wing, single hydraulic fracture or the creation of a fracture network. Geomechanical simulations, involving approaches that are based on discontinuum methods, help to understand these processes.
|Publication status||Published - Oct 2011|
|Event||AAPG International Conference and Exhibition 11 - Milan, Italy|
Duration: 23 Oct 2011 → 26 Oct 2011
|Conference||AAPG International Conference and Exhibition 11|
|Period||23/10/11 → 26/10/11|