Use of a new artificial cohesive material for physical modelling: Application to sandstone intrusions and associated fracture networks

Sandstone intrusions are formed by fluidisation and injection of sand into hydraulic fractures. To experimentally reproduce sandstone intrusion emplacement and to understand mechanisms governing their final morphology, it is necessary to employ a brittle, granular material simulating the intruded medium with water as a pore-fluid. We created a new analogue material made of a mixture of sand and gelatine to simulate overburden behaviour and which is capable of reproducing fracturing in water saturated sediments. The cohesion and frictional coefficient of this material is controlled by gelatine concentration. An increase of gelatine concentration of 1 g/l results in an increase of 490Pa and 0.08 of cohesion and frictional coefficient, respectively. Permeability of sand is sufficiently reduced to prevent fluid-flow prior to hydraulic fracturing (10−14–10−17 m²). Oscillatory tests on sand/gelatine mixture suggest a visco-elastic behaviour with a dominant elastic behaviour. Initial experimental results are presented here and show that the main geometries of sandstone intrusions (sills, dykes, wing-like intrusions and cones) and their network geometry (dyke to sill, dyke to cone or cone to cone) can be reproduced. We show that not all the fractures propagating in the model network are intruded by sand. us in this paper on the emplacement of wing-like intrusions. We make use of a P.I.V (Particle Imaging Velocimetry) technique to analyse plastic deformation, showing that wing-like fractures open in mode I. A cone of vertical displacement was also recorded above the reservoir adding a shear component in the opening mode of wing-like fractures. These results have broad application to the emplacement mechanics of sandstone and igneous intrusions.