Design parameters impacting electricity generation from horizontal multilateral closed-loop geothermal systems in Hot Dry Rock

Abstract

This article presents a parametric study of closed loop geothermal systems (CLGSs) design with multiple lateral horizontal wells, drilled in Hot Dry Rocks (HDRs), by developing a coupled finite element method (FEM) model for the problem of transient heat production in the reservoir and an Organic Rankine Cycle (ORC) model for electricity generation evaluation at the surface. The design parameters are well spacing and geometry, the number of wells, injection flow rate, energy losses along the well, and ORC working and cooling fluid mass flow rates. The study evaluates how these parameters affect the evolution of the spatial distribution of the temperature of the rock mass, the circulation fluid temperature leaving the reservoir, and the rate of electrical power generation with seasonal variations in surface air temperature.Analysis shows that the temperature evolution of the reservoir over a wide range of realistic properties is not influenced significantly by the rock and fluid properties, injection flow rate, nor well length and diameter and is a function of time only, and the initial rock mass and injection fluid temperatures. Therefore, in a CLGS with a hexagonal array of horizontal wells, the distance between the wells, beyond which the performance of a hexagonal array of wells is practically indistinguishable from that of a single isolated well, is 200 m over 30 years; i.e., the produced fluid temperature is practically unaffected by heat extraction from neighboring wells, regardless of reservoir properties. For smaller well spacing (100 m), the system shows practically negligible interaction in the first 15 years and only a small impact (∼1%) during the next 15 years of heat production.The performance of a CLGS mainly depends on the fluid temperature leaving the reservoir (falling rapidly in the first few months) and ambient surface temperature (varying over a year). Using data from the FORGE Utah site, analysis suggests that a CLGS with four 1 km horizontal wells operated at 2 m3/min/well can produce ∼5 MW of electrical power during cold months and none during hot months.