Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)
Abstract
:1. Introduction
2. Materials and Methods
2.1. Rock Samples
2.2. Experimental Procedures
- (1)
- The purpose of stage one was to investigate the potential transient fracture permeability degradation after pressure build-up, which exerts a force on the fracture surfaces. Such initial fracture permeability decline was widely observed at the first dozens to hundreds of hours of continuous fluid flow through fractured granitic rocks [4,21], shale [22], novaculite [15,17], limestone [16], and dolomitic anhydrite [18]. However, this time-dependent fracture permeability decay did not occur in some fractured sandstones and mudstones [23,24] with the intermittent flow (flow-stop-flow with a certain time interval). To monitor the influence of pressure on fracture permeability evolution in slates, we continuously measured permeability for several to dozens of hours, followed by stopping the flow for dozens of hours and measured fracture permeability again. This initial stage was performed at room temperature immediately after pressurization.
- (2)
- The second stage was to reveal thermal effects on fracture permeability evolution and to eliminate any irreversible fracture permeability changes upon thermal expansion of the rock matrix. The temperature was increased and decreased stepwise between room temperature and 70 °C for sample SM2 and between 25 °C and 90 °C for sample SM1 and SM3 (Table 2). Fracture permeability was measured after stabilization of temperature in each step.
- (3)
- In the last stage, the temperature was kept at the highest value (70 or 90 °C), and permeability was measured regularly after a time interval of 6 days. In between the time intervals, the valve of the upstream pump was closed, and the downstream pump maintained constant pressure so that the pore fluid could be considered as a semi-closed system. Before each permeability measurement, the effluent was sampled through the relief valve at a constant flow rate of Q = 0.1 mL/min. Each time, seven to nine subsamples with a volume of V = 1.0 mL at constant pore fluid pressure of Pp = 1 MPa were collected (downstream side). Each sample was acidified by addition of 0.01 ml super-pure HNO3 to minimize any potential precipitation or alteration of the fluid. The purpose of the chosen sampling strategy with collecting small-volume subsamples (V = 1.0 mL) was to better specify the fluid composition within the fracture. Otherwise, a large volume of effluent would have mixed the fluid within the fracture with the fluid in the capillaries connected to the sample.
2.3. Analytical Methods
2.3.1. Effluent Element Concentrations
2.3.2. Fracture Surface Topographies
2.3.3. SEM-EDX
3. Results
3.1. Variations of Fracture Permeability
3.1.1. Initial Fracture Permeability Decline with Continuous Flow (Stage 1)
3.1.2. Temperature Effects (Stage 2)
3.1.3. Time Dependence of Permeability with Intermittent Flow (Stage 3)
3.2. Fluid Chemistry Evolution
3.3. Fracture Surface Topography
3.4. Microstructures
4. Discussion
4.1. Mechanism of Initial Permeability Decline under Constant Conditions
4.2. Thermal Effects on Permeability
4.3. Potential Fluid–Rock Interactions on Time-Dependent Permeability Changes
4.4. Implications for EGS
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A
References
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Mineral Content (wt %) | Quartz | Muscovite | Chlorite | Plagioclase | Chalcocite | Ankerite | Pyrite |
---|---|---|---|---|---|---|---|
SM1 | 36 | 33 | 12 | 8 | 5 | 5 | 1 |
SM2 | 36 | 35 | 11 | 8 | 5 | 4 | 1 |
Sample | Temperature (°C) | Flow Rate a (mL/min) | Flow Type | Duration b (Days) | Permeant Fluid |
---|---|---|---|---|---|
SM1 | Room temperature | 0.3~0.5 | Continuous | ~3 | DI |
25 → 50 → 70 → 90 → 70 → 50 → 32 → 90 | 0.3~0.5 | Intermittent | 3 | ||
90 | 0.05~0.3 | Intermittent | 34 | ||
SM2 | Room temperature | 0.3~0.5 | Continuous | <1 | DI |
25 → 50 → 70 → 50 → 32 → 70 | 0.3~0.5 | Intermittent | 3 | ||
70 | 0.1~0.3 | Intermittent | 34 | ||
SM3 | Room temperature | 0.02~0.3 | Continuous | ~3 | 0.5 M NaCl |
25 → 50 → 70 → 90 → 70 → 50 → 32 → 90 | 0.02~0.1 | Intermittent | 3 | ||
90 | 0.1 | Intermittent | 34 |
Sample | Max T (°C) | kf (Initial) (10−12 m2) | kf (Final) (10−12 m2) | Duration (Days) | Fluid |
---|---|---|---|---|---|
SM1 | 90 | 4.76 | 1.02 | 40 | DI |
SM2 | 70 | 4.98 | 2.66 | 38 | DI |
SM3 | 90 | 3.49 | 2.01 | 40 | 0.5 M NaCl |
Parameters | SM1_A | SM1_B | SM2_A | SM2_B | SM3_A | SM3_B | Stage |
---|---|---|---|---|---|---|---|
Rp (µm) | 41.18 | 51.08 | 78.13 | 80.97 | 24.00 | 119.26 | Before |
65.45 | 70.98 | 78.57 | 116.80 | 52.96 | 106.55 | After | |
Rm (µm) | 2.35 | 1.82 | 10.99 | 1.46 | 1.56 | 10.31 | Before |
3.52 | 1.78 | 10.69 | 3.47 | 1.98 | 9.12 | After | |
Rrms (µm) | 3.00 | 2.41 | 14.31 | 2.16 | 2.05 | 17.94 | Before |
4.33 | 2.44 | 13.69 | 4.93 | 2.60 | 16.27 | After |
Sample | k0 | α/b0 | n | Adjusted R2 |
---|---|---|---|---|
SM1 | 1.95 × 10−15 | 7.3 × 10−2 | 0.43 | 0.99 |
SM2 | 2.0 × 10−15 | 4.5 × 10−2 | 0.61 | 0.99 |
SM3 | 1.13 × 10−14 | 0.6 | 0.072 | 0.98 |
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Cheng, C.; Herrmann, J.; Wagner, B.; Leiss, B.; Stammeier, J.A.; Rybacki, E.; Milsch, H. Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS). Geosciences 2021, 11, 443. https://doi.org/10.3390/geosciences11110443
Cheng C, Herrmann J, Wagner B, Leiss B, Stammeier JA, Rybacki E, Milsch H. Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS). Geosciences. 2021; 11(11):443. https://doi.org/10.3390/geosciences11110443
Chicago/Turabian StyleCheng, Chaojie, Johannes Herrmann, Bianca Wagner, Bernd Leiss, Jessica A. Stammeier, Erik Rybacki, and Harald Milsch. 2021. "Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)" Geosciences 11, no. 11: 443. https://doi.org/10.3390/geosciences11110443