Cracks in Saturated Porous Media: Desiccation Cracks, Hydraulic Fracturing, and Microseismicity

Abstract

Desiccation of fluid-infiltrated porous media is a highly non-homogeneous process that in most cases is bound to generate cracks. Under sufficiently slow crack propagation in an elastic fluid-saturated porous medium, the tip of a steadily running crack is drained. On the other hand, the tip of steadily running cracks that propagate fast enough in regard to the local diffusivity and length scale is not drained. In fact, the pressure there is negative and large. During soil and gel desiccation, the suction increases in time close to the evaporation surface. The developments of cracks due to desiccation, on one side, and hydraulic fracturing, on another side, are both considered in fluid-saturated porous solids. While elements of fracture mechanics of brittle elastic solids with and without cohesive zone are exposed, focus is on the enhancement of geothermal reservoirs via hydraulic fracturing, rather than on petroleum engineering. The microseismic effects resulting from fluid injection into, or extraction from, rock formations during waste water disposal, geothermal operations, and carbon dioxide sequestration are reviewed. Simplified models defining the motions of the free surface resulting from these operations are developed using the Mindlin solution for a half-space and illustrated.