In this paper, novel underground well pattern systems, replac-ng fracturing technology with horizontal wells technology, wereproposed for geothermal energy exploitation. Different from thealready proposed underground closed-cycle heat exchange system[17,18], the proposed underground well pattern systems in thispaper contained multiple horizontal wells with the distance be-tween each wells carefully designed to avoid the thermal break-through, or contained annular wells where the injection andproduction wellhead were close to each other to reduce the pres-sure loss in the transport pipelines on the ground. The proposedwell pattern systems had dramatic advantages in solving scalingproblem due to the flow inside of the wellbores, precisely con-trolling the flow direction and mass flow rate without loss, as wellas decreasing the pressure drop from the injection well to theproduction well which increased the thermal efficiency. Withgreenhouse gas control gaining increasing attention, CO2 can besequestered in deep or shallow aquifers [19e21], be used toenhance CH4 recovery [22] and to extract geothermal energy as theworking fluid in EGS [23]. For EGS with CO2 as the working fluid,CO2 directly entered the turbine for electricity generation from theproductionwellhead, different fromEGS with water as the workingfluid where water firstly entered the evaporator to transfer theextracted heat to the working fluid for ORC. Scaling problem in theturbine in EGS with CO2 as the working fluid severely affected the
long-termoperation of CO2 turbine, and the proposed underground
well pattern systems were beneficial for CO2 system in solving
scaling problem. A numerical model of the presented well pattern
was setup in this paper, taking into account the real thermal
properties of working fluid, the placement and configuration of
injectionwells, horizontal wells and productionwells, and the heat
exchange with the surrounding formation. The temperature and
pressure profiles in the wellbores in different designs at different
production mass flow rates and injection temperatures were pre-
dicted. A comparison of the system with a fractured reservoir and
the well pattern systems with water or CO2 as the working fluid
was conducted based on the geological conditions at the European
EGS site at Grob Schonebekc, Germany.
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