ABSTRACT

A heat extraction process between hot dry rock formation and a cylindrical well with a tubing installed inside is simulation. A wellbore/reservoir coupling formulation is proposed and solutions for temperature, fluid pressure along the annulus and tubing, and the induced pore pressure, temperature and stresses induced inside the HDR formation adjacent to the well bore are calculated. The heat extraction from the geothermal formation by injecting cold water into a tubing and circulating out from the annulus can be evaluated. Surface temperature and cold water injecting rate can be controlled to achieve desired efficiency.

1 INTRODUCTION

Enormous geothermal energy resources exist worldwide (Raymond 2018). It can supply a renewable and clean source of power with the heat removed from a geothermal reservoir being naturally replenished. The high capacity factor of geothermal power makes geothermal energy particularly attractive as a renewable base load energy supply. With the decreasing cost of geothermal installations in deep hot dry rock (HDR), enhanced geothermal systems (EGS) have the potential to replace more costly and environmentally unfriendly technologies (Grasby 2011). EGS in HDR and Enhanced Oil Recovery (EOR) or hydraulic fracturing (HF) of shale gas and shale oil formations using super-critical CO2 are technologies gaining more attention recently for environmental and energy efficiency considerations. In low-permeability HDR or shale formations, HF may further stimulate energy transfer processes by creating a large surface area. Fluid (Δp) and heat flow (ΔT) through intact rock are diffusion processes, and enhanced flux accompanies a larger exchange surface area. Seeing the critical role of hydraulic fracturing (McLennan 1980) in EGS to achieve higher production efficiency, extensive studies are conducted (McTique, 1989; Karshige, 1989, Wang and Papamichos, 1994; 1999, Wang 2017). It has been pointed out that drastic thermal stress changes when massive cold water is injected at a lower temperature into a deep HDR formation contributes to the initiation and propagation of fractures in HDR, which can not be justified by traditional hydraulic fracturing theory and is therefore referred to as thermal fracturing (Clifford et al.1991, Charlez et al. 1996). Calculation of thermal stress changes that induces thermal fractures and their geometry changes including the thermal cracking processes requires a fully coupled thermal-hydraulic-mechanical (THM) model. This model consists of two mass balance equations for the fracture and a matrix systems diffusion process with small portion of free gas flow. A dynamic thermal cracking zone can be created and considered once the effective tensile stresses exceed the tensile strength near a wellbore or hydraulic fractures. In addition, two mass balance equations, in the fracture and matrix systems respectively, and one equilibrium equations are coupled to the two aforementioned energy equations. Parts of these equations are nonlinear and stress dependent. Therefore solving this set of equations is challenging, yet the solution is a key to practical completion design and consequently researchers and companies have allocated effort intensively to find answer to this challenge in different ways. Over the years, there have been quite a few attempts made to simulate thermal fractures (Abousleiman et al. 1996, Tarasovs and Ghassemi 2010, Hofmann et al. 2016). Nevertheless, there are challenges yet to overcome at least from the following perspectives. First, to accurately assess the thermal stress, the thermal convection associated temperature distribution in reservoir must be properly addressed (Wang and Dusseault 2003); secondly, to evaluate the potential zone in the reservoir where thermal fracture could be initiated and propagating, from a boundary condition's perspective, the overburden stress redistribution within the reservoir must also be carefully addressed (Osorio et al. 1999); Thirdly, boundary element method has been proposed and a kernel function corresponding to a temperature change inside the hydraulic fracture is required. Developing such a function and determining the heat transfer process along a hydraulic fracture with thermal leakage term are critical. Fourthly, calculating the radius and evaluating the THM properties of a thermal cracking zone surrounding a wellbore and hydraulic fracture are extremely important for thermal energy extraction and heat exchange efficiency in the EGS. lastly but not the least, to predict the thermal fracture propagation in a fractured deep geothermal reservoir, the disturbance by local natural fractures must be properly addressed (Wang 2017).

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