Thermal drawdown and late‐stage seismic‐slip fault reactivation in enhanced geothermal reservoirs

Quan Gan, Derek Elsworth

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

Late-stage seismic slip in geothermal reservoirs has been shown as a potential mechanism for inducing seismic events of magnitudes to ~2.6 as late as two decades into production. We investigate the propagation of fluid pressures and thermal stresses in a prototypical geothermal reservoir containing a centrally located critically stressed fault from a doublet injector and withdrawal well to define the likelihood, timing, and magnitude of events triggered by both fluid pressures and thermal stresses. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low- or high-relative flow rates. At low relative dimensionless flow rates the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative non-dimensional flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate QD that scales with fracture spacing s (m), injection rate q (kg/s), and the distance between the injector and the target point L∗ (QD ∝ qs2/L∗). This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. These results illustrate that the dimensionless temperature gradient adjacent to the fault dTD/dxD is exclusively controlled by the factor QD. More significantly, this temperature gradient correlates directly with both the likelihood and severity of triggered events, enabling the direct scaling of likely magnitudes and timing to be determined a priori and directly related to the characteristics of the reservoir. This dimensionless scaling facilitates design for an optimum QD value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.
Original languageEnglish
Pages (from-to)8936-8949
Number of pages14
JournalJournal of Geophysical Research: Solid Earth
Volume119
Issue number12
Early online date11 Dec 2014
DOIs
Publication statusPublished - Dec 2014

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drawdown
reactivation
Flow rate
flow velocity
Thermal stress
Thermal gradients
Fluids
pressure pulses
fluid pressure
time measurement
thermal stresses
injectors
temperature gradient
seismicity
temperature gradients
Waste heat utilization
injection
Hot Temperature
scaling
induced seismicity

Cite this

Thermal drawdown and late‐stage seismic‐slip fault reactivation in enhanced geothermal reservoirs. / Gan, Quan; Elsworth, Derek.

In: Journal of Geophysical Research: Solid Earth, Vol. 119, No. 12, 12.2014, p. 8936-8949.

Research output: Contribution to journalArticle

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title = "Thermal drawdown and late‐stage seismic‐slip fault reactivation in enhanced geothermal reservoirs",
abstract = "Late-stage seismic slip in geothermal reservoirs has been shown as a potential mechanism for inducing seismic events of magnitudes to ~2.6 as late as two decades into production. We investigate the propagation of fluid pressures and thermal stresses in a prototypical geothermal reservoir containing a centrally located critically stressed fault from a doublet injector and withdrawal well to define the likelihood, timing, and magnitude of events triggered by both fluid pressures and thermal stresses. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low- or high-relative flow rates. At low relative dimensionless flow rates the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative non-dimensional flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate QD that scales with fracture spacing s (m), injection rate q (kg/s), and the distance between the injector and the target point L∗ (QD ∝ qs2/L∗). This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. These results illustrate that the dimensionless temperature gradient adjacent to the fault dTD/dxD is exclusively controlled by the factor QD. More significantly, this temperature gradient correlates directly with both the likelihood and severity of triggered events, enabling the direct scaling of likely magnitudes and timing to be determined a priori and directly related to the characteristics of the reservoir. This dimensionless scaling facilitates design for an optimum QD value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.",
author = "Quan Gan and Derek Elsworth",
note = "Date of Acceptance: 04/11/2014 Data supporting Figures 3–6 and 8–11 are available as in Supporting Information Datasheet for Figures 3–11.xlsx S1. This work is the result of partial support from the US Department of Energy under project DOE-DE-343 EE0002761. This support is gratefully acknowledged. The authors would like to thank the anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper.",
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N2 - Late-stage seismic slip in geothermal reservoirs has been shown as a potential mechanism for inducing seismic events of magnitudes to ~2.6 as late as two decades into production. We investigate the propagation of fluid pressures and thermal stresses in a prototypical geothermal reservoir containing a centrally located critically stressed fault from a doublet injector and withdrawal well to define the likelihood, timing, and magnitude of events triggered by both fluid pressures and thermal stresses. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low- or high-relative flow rates. At low relative dimensionless flow rates the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative non-dimensional flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate QD that scales with fracture spacing s (m), injection rate q (kg/s), and the distance between the injector and the target point L∗ (QD ∝ qs2/L∗). This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. These results illustrate that the dimensionless temperature gradient adjacent to the fault dTD/dxD is exclusively controlled by the factor QD. More significantly, this temperature gradient correlates directly with both the likelihood and severity of triggered events, enabling the direct scaling of likely magnitudes and timing to be determined a priori and directly related to the characteristics of the reservoir. This dimensionless scaling facilitates design for an optimum QD value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.

AB - Late-stage seismic slip in geothermal reservoirs has been shown as a potential mechanism for inducing seismic events of magnitudes to ~2.6 as late as two decades into production. We investigate the propagation of fluid pressures and thermal stresses in a prototypical geothermal reservoir containing a centrally located critically stressed fault from a doublet injector and withdrawal well to define the likelihood, timing, and magnitude of events triggered by both fluid pressures and thermal stresses. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low- or high-relative flow rates. At low relative dimensionless flow rates the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative non-dimensional flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate QD that scales with fracture spacing s (m), injection rate q (kg/s), and the distance between the injector and the target point L∗ (QD ∝ qs2/L∗). This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. These results illustrate that the dimensionless temperature gradient adjacent to the fault dTD/dxD is exclusively controlled by the factor QD. More significantly, this temperature gradient correlates directly with both the likelihood and severity of triggered events, enabling the direct scaling of likely magnitudes and timing to be determined a priori and directly related to the characteristics of the reservoir. This dimensionless scaling facilitates design for an optimum QD value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.

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