On Numerical Modelling of the Hydraulic-Fractured Closed Loop Systems: Single Producer

Abstract

There are large regions of the subsurface where the temperature is sufficiently hot to generate geothermal electricity within reasonable drilling depths, but the formation does not have enough permeability to create heated water productivity. The primary bottleneck is the difficulty of achieving adequate flow rate in such geothermal reservoirs. These formations are referred as hard dry rock (HDR). In these formations two common applications are conducted: Enhanced (or Engineered) geothermal systems (EGS) and Advanced geothermal systems (AGS). AGS is a closed-loop system that are built on wells drilled that connect with each other allowing a heat exchange-type to set up beneath the surface. There has recently been a lot of attention to the concept of ‘closed-loop geothermal’. The concept is still evolving and few startups such as E2E Energy Solutions, Eavor Technologies, and Green Fire Energy suggested different types of the closed-loop to optimize the technology. Since the technology only relies on conduction as the only source of heat transfer to improve the economics of the closed-loop E2E Energy Solutions suggested to use the hydraulic fracture to increase the surface area between the well and the geothermal reservoir. Such process is called Enhanced Geothermal Reservoir Recovery System (EGRRS). In the E2E's EGRRS process, a fluid would be produced from an existing hot subterranean aquifer reservoir, close to a favorable geothermal zone, instead of creating the whole loop from the surface. The fluid withdrawn from a hot subterranean reservoir would be contacted with a hydraulically fractured zone in the geothermal zone, which would result to additional energy transfer and therefore a higher enthalpy once the fluid reaches the surface. The plan is to reach temperatures above 200°C that greatly increasing the electrical generation potential. Although to implement the EGRRS process we are using the common oil and gas practice but to optimize and design the process using current modelling techniques is not achievable. Current simulation schemes cannot model such complex system, and to resolve this, a new framework must be designed to resolve the challenge. In this paper we present a new method that calculate rate to each branch by a new iterative approach that resolve the problem on how fractions of different pipe should be solved. Since the closed-loop wells are connected to the subterranean aquifer reservoirs and operator required to keep the WHP at constant pressure, there is another layer of iteration that required to solve for fraction on each pipe and BHP at the aquifer. Finally, to model the heating in the fractured zone, a new resistance is added to the model to mimic the heating exchange between the fracture and the fluid? and also the fracture and the radiator formation.