SFB 1313 Publication "Modelling and simulation of natural hydraulic fracturing applied to experiments on natural sandstone cores"

A new SFB 1313 publication is published in "Acta Geotechnica". The paper shows in particular how experimental investigations can be combined with numerical simulations of fracturing processes in porous media. The work has been developed outgoing from a research stay of Junxiang Wang (University of Padua), visiting us within the SFB 1313. As a result, this paper was written in collaboration with researchers from the University of Padua (IT) and the University of Stuttgart, involved in project area B.

Modelling and simulation of natural hydraulic fracturing applied to experiments on natural sandstone cores

Authors

• Junxiang Wang (University of Padua)
• Alixa Sonntag (University of Stuttgart, former SFB 1313 researcher, research project B02)
• Dongwon Lee (University of Stuttgart, SFB 1313 research project B05)
• Giovanna Xotta (University of Padua)
• Valentina A. Salomoni (University of Padua)
• Holger Steeb (University of Stuttgart, research projects B05, C05, and Z02)
• Arndt Wagner (University of Stuttgart, research project B02)
• Wolfgang Ehlers (University of Stuttgart, former SFB 1313 researcher, research project B02)

Abstract< 1 , which indicates a strong intensity of physical processes on the boundary, described by the inhomogeneity φ ε (the cases α = 1 and α > 1 were previously studied by the same authors). A complete Puiseux asymptotic expansion is constructed for the solution u ε as ε → 0 , i.e., when the diffusion coefficients are eliminated and the thin network shrinks into a graph. Furthermore, the corresponding uniform pointwise and energy estimates are proved, which provide an approximation of the solution with a given accuracy in terms of the parameter ε .

Under in-situ conditions, natural hydraulic fractures (NHF) can occur in permeable rock structures as a result of a rapid decrease of pore water accompanied by a local pressure regression. Obviously, these phenomena are of great interest for the geo-engineering community, as for instance in the framework of mining technologies. Compared to induced hydraulic fractures, NHF do not evolve under an increasing pore pressure resulting from pressing a fracking fluid in the underground but occur and evolve under local pore-pressure reductions resulting in tensile stresses in the rock material. The present contribution concerns the question under what quantitative circumstances NHF emerge and evolve. By this means, the novelty of this article results from the combination of numerical investigations based on the Theory of Porous Media with a tailored experimental protocol applied to saturated porous sandstone cylinders. The numerical investigations include both pre-existing and evolving fractures described by use of an embedded phase-field fracture model. Based on this procedure, representative mechanical and hydraulic loading scenarios are simulated that are in line with experimental investigations on low-permeable sandstone cylinders accomplished in the Porous Media Lab of the University of Stuttgart. The values of two parameters, the hydraulic conductivity of the sandstone and the critical energy release rate of the fracture model, have turned out essential for the occurrence of tensile fractures in the sandstone cores, where the latter is quantitatively estimated by a comparison of experimental and numerical results. This parameter can be taken as reference for further studies of in-situ NHF phenomena and experimental results.