Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

Chaojie Cheng,Jessica A Stammeier,Erik Rybacki,Bernd Leiss,Johannes Herrmann,Harald Milsch,Bianca Wagner

doi:10.3390/geosciences11110443

Abstract

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee well (Harz Mountains, Germany) were performed. The purpose was to investigate fracture permeability evolution at temperatures up to 90 °C using both deionized water (DI) and a 0.5 M NaCl solution as the pore fluid. Flow with DI resulted in a fracture permeability decline that is more pronounced at 90 °C, but permeability slightly increased with the NaCl fluid. Microstructural observations and analyses of the effluent composition suggest that fracture permeability evolution is governed by an interplay of free-face dissolution and pressure solution. It is concluded that newly introduced fractures may be subject to a certain permeability reduction due to pressure solution that is unlikely to be mitigated. However, long-term fracture permeability may be sustainable or even increase by free-face dissolution when the injection fluid possesses a certain (NaCl) salinity.

Highlights

Geothermal energy is ubiquitous at a certain depth of the Earth’s crust and may provide great potential to meet the energy demand [1]
Fracture permeability (Equation (3)) evolution was explored upon continuous fluid flow after pressurization at ambient temperature, heating–cooling cycles, and intermittent flow over a long-time duration (Section 3.1)
To evaluate sustainability of fractures within slates, three long-term flow-through experiments with Wissenbach slate samples containing a macroscopic saw-cut fracture were conducted under constant pressures (i.e., Pc = 10 MPa and Pp = 1 MPa) and varying temperatures

Summary

Introduction

Geothermal energy is ubiquitous at a certain depth of the Earth’s crust and may provide great potential to meet the energy demand [1]. To utilize such hot formations for district heating or electricity generation, a sufficient amount of fluid needs to be injected and extracted into/from the reservoirs. Fracture permeability changes associated with, e.g., effective stress, fracture surface roughness, fracture offset/shear displacement, and the mechanical properties of the rock matrix have been widely investigated in experiments e.g., [3,4,5,6,7,8,9,10,11]. Such short-term experiments mainly focused on the stress-dependent permeability variation to shed light on the critical factors

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Conclusion