Critical Chemical-Mechanical Couplings that Define Permeability Modification in Pressure-Sensitive Rock Fractures
Prior studies have examined the important factors controlling the interaction of mechanically- and chemically-mediated processes that modify the transport properties of natural fractures in rock. We have used tightly-constrained laboratory flow-through reactor studies to develop process-based models for observed permeability histories at prescribed normal stresses (0-3 MPa), temperatures (20°C-150°C), flow rates (0-2 cc/min), and chemical potentials (H2O at pH~5-7).
Linkage between changes in porosity and permeability is constrained by multiple independent measurements that define an over-constrained system. Changes in porosity are indexed by measured changes in fluid and dissolved mass efflux, and by periodic imaging by X-ray CT with a voxel resolution to ~O(40 μm). Final porosities are anchored by resin impregnation and thin-sectioning, with some tests including SEM of surfaces and pre-and post-test comparisons of fracture-surface profilometry (resolution ±15μm horizontally, 0.5 μm vertically).
Figure 1. Change in aperture for circulation of water at 2 cc/min in novaculite at incremented temperatures. Experimental data filled circles. Model results as open triangles and circles.
Results on novaculite (99% quartz) have shown that changes in permeability can be both large ~O(×100), and of surprising sense – permeabilities reduce as mineral mass is net removed from the sample by dissolution (Polak et al., 2003). Surprisingly, these changes can occur at modest temperatures (~80°C) and stresses (~3.5 MPa). These results are illustrated in Figure 1 for water circulation at constant flow rate (2cc/min). As temperatures are incremented from 20˚C through 150˚C, the fracture monotonically closes despite net removal of silica at ~1 ppm in the effluent. The fivefold reduction in fracture aperture is measured in the flow response as a permeability reduction of two orders of magnitude, for the 41 day test. Notably, permeability reduces, despite net dissolution throughout the test. This surprising response results from the elevated dissolution of bridging asperities under the constant applied effective stresses. Closure is reactivated as the system temperature is incremented. For each temperature increment, closure ultimately halts as asperity stresses are reduced as the asperity contact area grows, and the excess chemical potential in the contact region diminishes (Yasuhara, et al., 2004). This observed response highlights the complex and poorly constrained behavior where stress and chemical effects are important. Our interest is to further explore these important processes for fractures both far-from-failure, and critically-stressed, and where interactions between mechanical and chemical effects are important.
Figure 2. Change in permeability with time for continuous flow at 2 cc/min through a fracture in Bellefonte limestone. Test at invariant low temperatures (~20°C) and effective stresses (~3.5 MPa). Influent water changed at 935 h from groundwater (pH~8, unshaded panel) to distilled water (pH~6, shaded). At 935 h a brief sharp increase in permeability (seating due to effective stress interruption), is followed by a coherent decrease in permeability, that ultimately arrests as a single flow conduit develops, and permeability climbs. Understanding the interacting phenomena that control this spontaneous response under invariant test conditions is the focus of this proposed study.
Similar effects are apparent in other facies, such as carbonates (Polak et al., 2004), again illustrating the competition between free-face dissolution and stress-mediated dissolution (pressure solution), but now at low relative temperatures. Figure 2 shows changes in permeability of a fracture in limestone that result from the circulation of water at constant rate (2 cc/min), and at constant stress, with a change in pH from ~8 to ~6 at 925 hours into the test. Notably, permeability reduces as calcium is net removed from beneath bridging asperity contacts, and this accelerates as the pH of the influent water drops from ~8 to ~6. Ultimately, the fracture closure is sufficient to localize flow into a single un-occluded flow channel, and free-face etching becomes the dominant process in the system, resulting in a spontaneous switch in permeability, and the development of a throughgoing flow channel (a wormhole). The presence of this feature is confirmed by concurrent X-ray CT (Figure 3), and by post-test sectioning, and is congruent with fluid flux and mass efflux measurements. These constraints allow the principal operative processes – the competition between free-face and stress-mediated dissolution – to be adequately modeled.
Figure 3. Micro-CT-scans of the full core (Figure 2) taken pre-test (center) with marked zone used for zoom-in both before (left) and at the end of the test (right). Graphs show the variation in CT-number across the fracture, and the focusing of dissolution in a single ‘wormhole.’
Spontaneous changes in permeability under invariant effective stresses are also apparent in less-soluble rocks (Yasuhara et al., 2004). Cycling of temperatures and flow rates on an isostatically-stressed natural fracture in novaculite, yields first fracture closure, then fracture widening, with mass net removed from the fracture throughout the test (Figure 4). This illustrates the extreme sensitivity of the long-term permeability of fractures to the ambient fields and paths of stress, temperature, chemical potential, and flow rate.
Figure 4: Evolution of permeability for water circulation in a fracture in novaculite under constant stress, with variable flow rates.
These observed, but anomalous, changes in permeability may be represented by incorporating the role of stress-mediated dissolution at asperity contacts into the overall behavior; closure controls permeability reduction, and incorporates the serial processes of dissolution at contacting asperities, interfacial diffusion, and precipitation at the free face of fractures (Yasuhara et al., 2003). These processes progress over a representative contacting asperity and define compaction at the macroscopic level, together with evolving changes in solute concentration for arbitrarily open or closed systems for prescribed ranges of driving effective stresses, equilibrium fluid and rock temperatures, and fluid flow rates.
Measured fracture surface profiles are applied to define simple relations between fracture wall contact-area ratio and fracture aperture that represents the irreversible alteration of the fracture surface geometry as compaction proceeds. These lumped parameter models are thermodynamically self-consistent, and show agreement with observations in novaculite (Yasuhara et al., 2004), tracking changes in fracture aperture (Figure 1) and concentrations of the interstitial fluid, and confirming pressure solution as the dominant mechanism. Behavior is strongly conditioned by ambient stress and temperature fields, chemical potential of the solute, and its volumetric flux (Yasuhara et al., 2004)(Figure 4). For the dissolution-dominated systems considered, fracture closure rates have been shown to scale roughly linearly with stress increase, and exponentially with temperature increase - taking between days and decades for closure to reach completion.
Articles Submitted and in Preparation
1. Polak, A.B., Landon, M., Grader, A.S., and Elsworth, D. (2004) Detection of bacteria in porous media using x-ray micro computed tomography. Submitted for publication, Env. Sci. and Tech. 30 pp.
2. Liu, J., Sheng, J., Polak, A.B., Elsworth, D., and Grader, A.S. (2004) Fracture sealing and preferential opening under in situ conditions – a prognostic model. Submitted for publication. Water Resour. Res., 40 pp.
3. Yasuhara, H. and Elsworth, D. (2004) Stress- and chemistry-mediated spontaneous switching of permeability in soluble rocks. In preparation.
4. Yasuhara, H. and Elsworth, D. (2004) Mechanisms for porosity destruction and generation in fractured rocks. In preparation.
1. Yasuhara, H., Elsworth, D., and Polak, A. (2003) Compaction and diagenesis of sandstones – the role of pressure solution. GeoProc 2003. Proc. Int. Conf. on Coupled T-H-M-C Processes in Geosystems. Stockholm, Sweden, October, pp. 731 - 736. [pdf]
2. Polak, A., Yasuhara, H., Elsworth, D., Liu, J., Grader, A., Halleck, P. (2003) The evolution of permeability in natural fractures – the competing roles of pressure solution and free-face dissolution. GeoProc 2003. Proc. Int. Conf. on Coupled T-H-M-C Processes in Geosystems. Stockholm, Sweden, October, pp. 719 - 724. [pdf]
3. Liu, J., Mallett, C., Beath, A., Elsworth, D., and Brady, B.H.G. (2003) A fully coupled flow-transport-deformation model for underground coal gasification. GeoProc 2003. Proc. Int. Conf. on Coupled T-H-M-C Processes in Geosystems. Stockholm, Sweden, October, pp. 609 - 614. [pdf]
Keynote, Plenary, and Invited Presentations
1. Elsworth, D. and others (2004) Mechanical and chemical controls on the evolution of fracture transport and chemical properties. Keynote presentation. European Rock Physics Conference. Berlin. September.
2. Elsworth, D. (2003) Hydromechanical and hydrochemical influences on the transport properties of fractured reservoirs. Invited plenary. ARMA-DoE Workshop on Rock Mechanics and Enhanced Geothermal Systems. Cambridge, MA, USA, June 20.
3. Elsworth, D., Grader, A.S., Halleck, P., Liu, J., Polak, A.B., and Yasuhara, H. (2003) Some THMC controls on the evolution of fracture permeability. Keynote Presentation. GeoProc 2003. Proc. Int. Conf. on Coupled T-H-M-C Processes in Geosystems. Stockholm, Sweden, October, p. 58. [pdf]
4. Elsworth, D. (2001) Permeability Changes in Porous-Fractured Media Controlled by Mechanical and Chemo-Mechanical Effects. EOS, Transactions, Amer. Geophys. Union. Fall Meeting. December 10-14. Invited.
Abstracts and Presentations
1. Polak, A., H. Yasuhara, D. Elsworth, Y. Mitani, A. S.Grader, and P. M. Halleck (2004) Quantification of contact area and aperture distribution of a single fracture by combined X-ray CT and laser profilometry. Symposium of Dynamics of fluids in Fractured Rocks. Berkeley, CA, February.
2. Yasuhara et al., (2003), Polak et al., (2003), and Liu et al., (2003). Presentations at GeoProc 2003, detailed above.
3. Elsworth, D. (2003) Coupled THMCB processes in fractured rocks: flow and transport properties. School of Oil and Gas, University of Western Australia, March 18, 2003, and CSIRO Exploration and Mining, Brisbane, March 25.
4. Elsworth, D. (2003) Evolution of permeability in natural fractures: X-ray CT characterization. School of Engineering, Griffith University, Brisbane, March 26.
5. Polak, A. (2003) Quantitative imaging of flow and transport in fractured rocks. Invited Presentations. Israel Geological Survey, Jerusalem; Ben-Gurion University of the Negev; Technion - Israel Institute of Technology, Israel.
6. Polak, A. M. Landon, A. S. Grader, and D. Elsworth (2003) Detection of bacteria in porous media using X-ray computed tomography. Geophysical Research Abstracts, Joint EGS/AGU Meeting, Nice, April.
7. Polak, A., D. Elsworth, J. Liu, and A.S. Grader (2002) Spontaneous switching of permeability changes in a limestone fracture under net dissolution. Eos. Trans. AGU, 83(47), Fall Meeting Suppl., Abstract H62G-10.
8. Elsworth, D., A. Polak, A.S. Grader, H. Yasuhara, P.M. Halleck, and S.L. Brantley (2002) Quantitative Constrained Imaging of dissolution and precipitation in natural fractures. Eos Trans. AGU, 83(47), Fall Meeting, Suppl., Abstract H71B-0801.
9. Elsworth, D. (2002) Some needs and potential benefits related to a national underground science laboratory. NUSL-Geo-hydrology Engineering Team. NSF-ARMA/NeSS Workshop on Potential Benefits of a National Underground Science Laboratory, Washington, D.C., 18 September, and NSF-NeSS Workshop on Neutrinos and Subterranean Science, Washington, D.C., 19-21 September.
10. Elsworth, D., Polak, A., Yasuahra, H., Grader, A.S., Brantley, S.L., and Halleck, P. (2001) X-Ray CT imaging of changes in permeability due to dissolution and precipitation in fractured Berea sandstone. Multiscale Reservoir Symposium, Lawrence Berkeley National Laboratory, December 7-8, 2001.