Abstract
This paper presents the result of a series of three-dimensional numerical studies that investigate the response of enhanced geothermal reservoirs to injection during the stimulation phase. In these studies, the three-dimensional discrete fracture network is explicitly represented in the model. The numerical analyses are hydro-mechanically coupled, and thermal effects during the stimulation phase are disregarded. The sensitivity studies evaluate how the following aspects of the injection method can improve the total shear-stimulated area: (a) staged injection compared to one stage injection; and (b) injection into cased borehole with multiple hydraulic fracture clusters vs. injection through open-hole completion. In addition, the effect of fracture size characteristics of the reservoir on the shear stimulation response is studied. It is observed that the technique used for injection of fluid has a significant effect on the total stimulated area in the reservoirs.
1. INTRODUCTION
This paper presents part of a comprehensive study that investigates viability of Enhanced Geothermal Systems (EGS) using a numerical modeling approach. The purpose of these studies is to evaluate the sensitivity of EGS heat production to various in-situ and operational parameters and to come up with optimum conditions for heat production. Such numerical modelling analyses are valuable in providing insight into the complex mechanisms that are involved in EGS, considering that there is a high level of uncertainty associated with field data.
In a series of previous studies, effects of various in-situ and operational parameters on shear stimulation [1, 2] and heat production were investigated in a two-dimensional framework [3, 4, 5]. These studies first investigated how shear-stimulated area (i.e., the total area of fractures that experience irreversible dilatational opening as a result of pressurization) is sensitive to different parameters, including density of the fracture network, fracture length characterization, orientation of fractures, injection rate and rate history, viscosity of injected fluid, and the positioning of injection and production wells. Subsequently, it was investigated how changes in the aforementioned variables can affect the heat production by evaluating histories of produced fluid temperature and power in typical EGS reservoirs for a five- to ten-year period. The two-dimensional studies allowed for performing numerical investigation with reasonable computational resources and led to valuable insight into the response of the reservoir to injection under different conditions. However, it is believed that the three-dimensional studies are essential to understanding certain aspects of reservoir behavior, including how fluid flows in a three-dimensional discrete fracture network (DFN) and how the length and geometry of the borehole segment through which fluid is injected relative to the reservoir size and DFN orientation can affect fluid flow.
