Integrated Carbon Sequestration\u2013Geothermal Heat Recovery: Performance Comparison Between Open and Close Systems

Masoud Babaei

doi:10.1007/s11242-018-1042-1

Abstract

In this paper, the lateral boundaries for a theoretical homogeneous, isotropic, horizontal, 3-dimensional sedimentary hot aquifer were prescribed by either unhindered open to fluid and heat flow, or compartmentalised fully close boundary conditions. CO2 injection was administered through three well patterns of varying well spacing and numbers: 3 × 3 (5 injection wells and 4 production wells, 750-m well spacing), 5 × 5 (13 injection wells and 12 production wells, 500-m well spacing), and 7 × 7 (25 injection wells and 24 production wells, 375-m well spacing). To assess the effects of boundary conditions and well spacing on a combined carbon sequestration–geothermal heat recovery process, number of output metrics such as net cumulative CO2 stored, volumetric storage efficiency, geothermal heat recovery and pressure build-up were quantified in 54 simulation runs. Based on the assumption made and conditions that the simulation, it was shown that for a typical and more common fewer well number cases, the open versus close boundaries have critical role in determining the effectiveness of operation. (a) Open boundary conditions may counter-intuitively lead to heat recovery relative deficiencies if the pressure gradient stays largely towards the outside of the system (pressure of system > pressure of surrounding). This, however, will benefit CO2 injectivity and storage efficiency by providing escape paths for fluids (CO2 and brine). (b) Close boundary conditions will universally be beneficial for fluid and heat production purposes; however, the excessive pressure build-up can significantly affect the operation duration length. For economically unjustifiable cases with high number of wells, the effects of boundary conditions were shown to be reduced by effective fluid extraction from the medium, thereby enhancing the geothermal heat recovery and avoiding pressure build-up. The research delineated no significant effect of boundary conditions on salt precipitation.

Highlights

Since 2000, a compelling body of research has been dedicated to numerically evidence the advantages and efficiency of using CO2, alternative to water, as heat transmission fluid for geothermal energy recovery from enhanced geothermal systems (EGS) where the permeability of the underground source is enhanced by hydrofracturing (Brown 2000; Fouillac et al 2004; Pruess and Azaroual 2006; Atrens et al 2008; Xu et al 2015b)
The communication or lack of hydrologic communication of a geothermal system with its surrounding aquifer has profound effects on performance of integrated CO2 sequestration—heat recovery operations, especially if the budget of the operation can be exhausted by large number of operating injection/production wells
If the aquifer is compartmentalised by sealing faults so that boundaries could be assumed as fully close, injecting CO2 can help to produce the pressure required for fluid production by generating a steep pressure gradient between injection and production wells

Summary

Introduction

Since 2000, a compelling body of research has been dedicated to numerically evidence the advantages and efficiency of using CO2, alternative to water, as heat transmission fluid for geothermal energy recovery from enhanced geothermal systems (EGS) where the permeability of the underground source is enhanced by hydrofracturing (Brown 2000; Fouillac et al 2004; Pruess and Azaroual 2006; Atrens et al 2008; Xu et al 2015b). The goal of injecting CO2 is to provide the necessary pressure of fluid production and to displace native formation fluid (e.g. brine or hydrocarbons) In this so-called CO2-plume geothermal (CPG) approach (Randolph and Saar 2011a, b; Adams et al 2014; Ganjdanesh et al 2015) the working fluid is not necessarily CO2 only, and contrary to the first application, a key objective is to provide simultaneous CO2-injection-induced pressure relief by producing hot brine with consumptive beneficial uses (Buscheck et al 2012b). Between the two approaches (EGS where CO2 is used as transmission fluid and CPG where CO2 is used for storage and brine production), a same metric, i.e. amount of produced CO2, is favourable in opposite extremes This metric will be directly dependent on the well spacing, number and pattern administered in the operation

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