The Physical and Mechanical Properties of Carbonate Reservoirs: Implications for Sustainability and Injection Induced Seismicity of Deep Geothermal

The Dinantian carbonate sequence (DCS) is a proven geothermal reservoir in Northwest Europe. In the UK, evidence points towards it being an important thermal spring source and a major aquifer in many basins. In many locations in NW Europe the DCS is buried to such depths that makes it a prime target for deep (> 500 m), low to intermediate enthalpy (up to 200 °C) geothermal system development. Major limiting factors to deep geothermal development are related to issues of sustainability and safety. Deep geothermal resources often have low permeability and require reservoir stimulation to achieve sustainable heat production. However, fluid injections related to reservoir stimulations at geothermal sites have been shown to cause felt earthquakes, raising questions about the safety of such energy developments. In present thesis, laboratory measurements and triaxial loading tests were conducted on 20 mm diameter cores of low porosity limestones and porous dolostones taken from the DCS. The aim of this thesis was to provide a framework for geothermal exploration and reservoir development for the dolomitized sections within the DCS. The characterization of reservoir petrophysical properties such as porosity and permeability have shown that reservoir stimulation would be required for geothermal resource development within porous dolostones. Triaxial loading tests demonstrated the effect of reservoir stimulation on permeability and induced seismicity within the carbonate reservoirs. Triaxial loading tests on composite samples were designed to show the effect of triggered slip on the seismic response of the reservoir. Both induced and triggered seismicity have been shown to be affected by certain geological parameters (e.g., porosity, effective pressure, pore pressure etc) previously overlooked during the design of seismic risk mitigation plans. The petrophysical and mechanical characterization of a new, alternative play (porous dolostones) within the DCS provides important input for potential future geothermal exploration and reservoir design.