Simulations of pressure fluctuations and acoustic emission in hydraulic fracturing.

We consider a two dimensional lattice model to describe the opening of a crack in hydraulic fracturing. In particular we consider that the material only breaks under tension and the fluid has no pressure drop inside the crack. For the case in which the material is completely homogeneous (no disorder) we present results for pressure and elastic energy as a function of time and compare our findings with some analytic results from continuum fracture mechanics. Then we investigate fracture processes in strongly heterogeneous cohesive environments. We determine the cummulative probability distribution for breaking events of a given energetical magnitude (acoustic emission). Further we estimate the probabilty distribution of emission free time intervals. Finally we determine the fractal dimension(s) of the