A laboratory study of hydraulic fracturing at the brittle-ductile transition

Francesco Parisio,Kiyotoshi Sakaguchi,Keita Yoshioka,Eko Pramudyo,Noriaki Watanabe,Ryota Goto,Takahiro Miura,Takuya Ishibashi

doi:10.1038/s41598-021-01388-y

Abstract

Developing high-enthalpy geothermal systems requires a sufficiently permeable formation to extract energy through fluid circulation. Injection experiments above water’s critical point have shown that fluid flow can generate a network of highly conductive tensile cracks. However, what remains unclear is the role played by fluid and solid rheology on the formation of a dense crack network. The decrease of fluid viscosity with temperature and the thermally activated visco-plasticity in rock are expected to change the deformation mechanisms and could prevent the formation of fractures. To isolate the solid rheological effects from the fluid ones and the associated poromechanics, we devise a hydro-fracture experimental program in a non-porous material, polymethyl methacrylate (PMMA). In the brittle regime, we observe rotating cracks and complex fracture patterns if a non-uniform stress distribution is introduced in the samples. We observe an increase of ductility with temperature, hampering the propagation of hydraulic fractures close to the glass transition temperature of PMMA, which acts as a limit for brittle fracture propagation. Above the glass transition temperature, acoustic emission energy drops of several orders of magnitude. Our findings provide a helpful guidance for future studies of hydro-fracturing of supercritical geothermal systems.

Highlights

Developing high-enthalpy geothermal systems requires a sufficiently permeable formation to extract energy through fluid circulation
Solid polymethyl methacrylate (PMMA) has been used as a rock-analogue to experimentally validate the crack tip behaviors predicted by the hydraulic fracturing theory in penny-shape[16] and PKN geometry[17], and molten PMMA has been used as a magma-analogue in dike propagation e xperiments[18]
Experimental apparatuses can reach a temperature close to the ductile transition of certain r ocks[4,22,23] and have been previously employed to study water-based supercritical hydraulic-fracturing[5], testing at lower temperature conditions implies that the propagating fluid is still in its liquid state; in combination with the low permeability of PMMA, it allows to separate the effects of pure solid rheology from the ones of low-viscosity fluid percolation

Summary

Introduction

Developing high-enthalpy geothermal systems requires a sufficiently permeable formation to extract energy through fluid circulation. Hydro-fracturing and dike propagation laboratory experiments are often performed with the aid of rock or fluid proxies at a lower and more controllable temperature and/or pressure[10] To this end, transparent manufactured materials such as polymethyl methacrylate (PMMA) or Polyurethane (PU) are a common choice to study hydraulic fracturing[11,12,13,14]. Experimental apparatuses can reach a temperature close to the ductile transition of certain r ocks[4,22,23] and have been previously employed to study water-based supercritical hydraulic-fracturing[5], testing at lower temperature conditions implies that the propagating fluid (water) is still in its liquid state; in combination with the low permeability of PMMA, it allows to separate the effects of pure solid rheology from the ones of low-viscosity fluid percolation. We assess the implications for hydro-fracturing and stimulation in supercritical and other high temperature geothermal reservoirs and the possible directions of future investigations

Methods

Results

Discussion

Conclusion