In a recent paper (Chandrasekhar, et al., GRC 2024 Proceedings), we presented scaling curves applicable to all enhanced geothermal systems (EGS) and advanced geothermal systems (AGS). We show that a nondimensional parameter, Γ (analogous to the Number of Transfer Units in heat exchanger analysis) scales both thermal performance and decline in EGS and AGS systems. In that work, both thermal performance and decline are evaluated in terms of a nondimensional temperature, θ. We showed that engineered heat extraction systems in both EGS and AGS decline with time (because Γ decreases with time), reaching a pseudo-steady state in one to several years (in terms of θ). The pseudo-steady state θ can sometimes be much lower than θ at initial production, depending upon how the system is operated. This poses problems in operational management of mass rates and exit pressure. The field development plan will have to be engineered such that nameplate power is delivered. However, this cannot be achieved using the θ vs Γ formulation, as it does not give insight into the behavior of enthalpy with time. Given the thermal performance and decline of EGS/AGS systems, it is reasonable to expect that there exists a mass rate (or more generally, a mass rate schedule) that achieves two goals: maximize thermal performance; and minimize decline over a 20 to 30-year life. In this follow-up work, we consider the problem of optimization of mass flow rates, and their operational management to maximize enthalpy generation while minimizing decline. We solve the nondimensionalized governing equations using a semi-analytic approach with appropriate initial condition and boundary conditions. The transient response (temperature gradient in the resource as a function of time) is captured using a time dependent “flux multiplier” which simplifies the solution. From this, a non-dimensional expression for enthalpy (or more specifically, power) as a function of the ratio of Biot and Peclet numbers is derived. The optimization problem is then posed as maximization of power (over a service duration). Several case studies that are representative of different EGS/AGS concepts being currently considered are presented. It is shown that by appropriately managing the mass flow rate, it is possible to optimize enthalpy production over a specified service duration, even though thermal decline is inevitable and will erode thermal performance over time. It is also shown that by being less aggressive with rates and enthalpy production in early life, the decline can be managed. The authors hope that this work, taken together with the previous work, provides EGS/AGS designers and engineers with an easily applied approach to optimize operations. This work does not replace more sophisticated multi-physics simulation models that will need to be customized to the specifics of a given location. However, it provides an approach for a first pass evaluation of different concepts and their operational management. Indeed, it is believed that this is the first paper since Gringarten’s seminal paper (Gringarten et al. (1975)) to systematically examine EGS and AGS systems using scaling analysis, in a manner consistent with current operational feasibility.

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