Sheared Fracture Conductivity

Transcription

1 PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202 Sheared Fracture Conductivity Ravindra Bhide 1, Tyler Gohring 1, John McLennan 1,2 and Joseph Moore 2 1 Department of Chemical Engineering, University of Utah, Salt Lake City, UT Energy & Geoscience Institute, University of Utah, Salt Lake City, UT rjb13@utah.edu Keywords: Shear fractures, self-propping, and in-situ fracture permeability. ABSTRACT Experimental and numerical evaluations have identified features related to conductivity in shear-induced self-propped conduits that could be found in ideal EGS reservoirs. Does reopening or reactivating the natural fractures help to maintain or augment fracture conductivity? Does shear-induced asperity override provides adequate conductivity for geothermal applications? Shearing contributions are difficult to legitimately quantify. Long-term integrity of these fractures needs thorough assessment in order to investigate the potential to alter fluid flow within fracture pathways. The characteristics of hydraulic conductivity change in shearinduced conductive fractures were studied and compared; experimentally and numerically. The intent has been to investigate changes in effective fracture width in-situ which may manifest complex interlocking of influential parameters like lithology, fluid gradients, creep, etc. Baseline laboratory conductivity measurements were completed for an artificially-sheared, self-propped, granite sample subsequently subjected to confining pressure. Internal surfaces of an axially-split core sample were relatively translated to ensure interfering roughness that may cause self-propping. Mechanical stressing and degradation of conductivity associated with changes in fracture width were simulated using a commercially available simulator FLAC 3D (developed by Itasca). The contours of shear-induced fractured surface were re-created from X-ray microtomographic images. Numerical runs have been performed to simulate fluid flow within these fractured surfaces subjected to increasing closure stresses. Fracture conductivity measurements for a shear-induced fracture in granitic sample, at room temperature and a nominal closure stress of 1000 psi were recorded. The temperature was varied from ambient temperature to 200 C. This study introduced a new avenue for in-situ measurements of fracture conductivity in laboratory setup. The insights gained from this study are invaluable in understanding the potential of shear-induced fracture pathways to alter hydraulic conductivity. 1. INTRODUCTION One of the questions of practical importance in EGS (Engineered Geothermal System) development is fracture permeability under various stimulation conditions. Fractures, natural and man-made, are important conduits for fluid flow. An adequate heat exchange surface between the hot rock and cold fluid controls the economic viability of this geothermal operation. Shear stimulation can alter the permeability distribution in the naturally fractured formation. This is achieved through permanent translation of fracture surfaces so that fracture asperities no longer match up during fracture closure. It is generally speculated that shear-induced asperity override provides improved conductivity for geothermal applications. The shear displacement of fracture faces has been comprehensively studied by geotechnical (Perkins and Kern (1961), Brace et. al. (1967), Lawrence (1987), Olsson & Brown (1993), Kotousov et. al. (2011), McClure & Horne (2013)). The residual width formed by fracture asperity mismatch can provide a conduit for high permeability fluid flow (Kranz el.al. (1979), Brown (1987), McClure & Horne (2013)). In this study, the main objective is a better understanding of fracture permeability as function of closing pressures and temperatures that can be found in ideal EGS reservoirs. Fluid flow paths in fractured rock is an area of active research among geothermal as well as petroleum researchers. To date laboratory testing has been done for measurements of fracture permeability. 2. EXPERIMENTAL DETAILS The pressure vessel described in Stoddard (2012) was used in this study. Self-propped granite core sample (2.5 diameter x 5 length) was prepared for this experimental exercise. The sample was first scored along its edges. A wedge was then used to mechanically split it and create a rough fracture between the two mating halves as seen in Figures 1 and 2. Using computerized tomography, surface roughness calculations were performed with MATLAB (Mathworks Inc.). Surface roughness was quantified as the quadratic mean for the vertical deviations of the line profile of fracture surface - as seen in Figure 3. Small amplitude (local) roughness values obtained varied from 1.8 to 1.95 mm along the length of the sample. The sample was modified to account for the interacting roughness that may cause self-propping. The terminology self-propped means that the internal surfaces of sample were relatively translated to ensure interfering roughness. This was done by addition of thin aluminum half disks at opposite ends on opposite halves of a slightly relatively displaced sample as indicated in Figure 4. This would optimistically account for shear movement with limited gouge. 1

2 Figure 1: 3-dimensinal X-ray tomographic image for the wedge-split sample (5 inches in length). Figure 2: This is an individual cross-section from a computerized tomographic three-dimensional scan of the sample. This was taken before any testing. 2

3 Figure 3: Edge view for roughness calculations for self-propped sample. Figure 4: Self-propped granite core sample (5 inches in length). 3

4 In the test arrangement, a Teledyne Isco 500D syringe pump was used to achieve a high precision continuous flow rate. A second Isco syringe pump was utilized to control the confining pressure on the sample. For this self-propped" sample, confined at 1,000 psi, deionized water was flowed through the fracture and the pressure drop through the fracture was measured using a differential pressure transducer with accuracy of psi as seen in the schematic in Figure 5. These measurements were recorded by National Instruments compact data acquisition system with thermocouple and current module. The injection flow rates and pressures at the Isco pump were recorded through the pump controller using analog voltage connections. Flow rates were progressively varied at 5, 10, 15, 20, 25, 30, 40, 50 and 60 ml/minute. Measurements were made at temperatures up to 200 C. These tests were performed at University of Utah. P Data Acquisition Fluid Pump E Sample B P Drain Figure 5: Schematic for experimental arrangement to measure fracture conductivity. Here, ΔP denotes the differential pressure transducer. B is the back pressure valve. P indicates the X-ray transparent pressure vessel accommodating sample with end-cap E (for in-situ fracture width measurement). The position of X-ray enclosure is shown as dotted red line. To investigate in-situ width dependence, conductivity measurements were carried out for self-propped granite sample subjected to confining pressure (nominal normal stress on the self-propped fracture) with real-time CT scanning. These tests involved construction of an X-ray transparent confining pressure vessel with flow-through capabilities provided by TerraTek-Schlumberger. These measurements were conducted at ambient temperature (~22 C) at TerraTek-Schlumberger. The confining stress is varied from 250, 500, 1000 psi in the X-ray transparent pressure vessel. With the exception of X-ray enclosure, the overall experimental setup used at University of Utah and TerraTek-Schlumberger is similar as described in the schematic in Figure RESULTS AND DISCUSSION 3.1 Experimental Results For this self-propped" sample, confined at 1,000 psi (nominal normal stress on the fracture of approximately 1,000 psi), deionized water was flowed through the fracture and the pressure drop through the fracture was measured using a differential pressure transducer. Deionized water was specifically selected to provide one extreme of a flowing fluid because of its potential for increased fluid-rock interactions when compared to water in equilibrium with the rocks. Flow rates were progressively varied at 5, 10, 15, 20, 25, 30, 40, 50 and 60 ml/minute. The permeability (conductivity is permeability times fracture width) of the fracture was calculated using Darcy s law expressed for linear flow, and given in Equation (1). k Q L A p (1) where k, Q, µ, L, A, p are permeability of the fracture (m 2 ), flow rate through the fracture (m 3 /s), viscosity of the fluid (Pa-s), length of the test section (m), cross-sectional area of the fracture (m 2 ) and pressure differential through the test section (Pa), respectively. Fracture conductivity was calculated by multiplying the permeability by the fracture aperture: C kw (2) f 4

5 where C, k, W f are conductivity through the sample (md-ft), permeability of the fracture (md) and width of the fracture (ft), respectively. The head loss through the tubing and end-caps were measured by flowing water at the flow rates indicated above. Differential pressure measurements without a sample (system friction) were used to correct the data obtained in the tests. A back pressure regulator was used to prevent formation of a second phase (steam) downstream of the sample. A confining pressure of 1,000 psi was maintained through testing campaign. Values for the inferred conductivity are shown in Figure 6. Measurements were made at temperatures up to 200 C. Conductivity values were lower at higher temperatures. The one interesting aspect is the ratedependency seen for the self-propped sample. This could be quite an important consideration even if the low-rate darcy conductivity is similar, tortuosity and associated quadratic rate dependency is higher for the self-propped sample. Figure 6: Conductivity summary for a self-propped, rough-surfaced granite core sample with offset asperities, at a closure stress of 1000 psi. The conductivities were calculated using the appropriate viscosities. 3.2 Numerical FLAC 3D (developed by Itasca) is being used to simulate a 3-dimensional fluid flow model for the fracture space under ambient conditions. The governing equations for FLAC 3D are not described here. Interested readers should consult the following reference (FLAC 3D User Manual). In the fluid flow model, the mesh zones represent the actual zones in which the finite difference model is solved. With the recent advances in computer-aided X-ray microtomography, direct measurement of rock frabric and fracture space enclosed within it can be used to quantitatively describe three dimensional fracture surface. The geometries of the finite difference mesh zones correspond to fracture space in the rough contours of granite samples as defined from X-ray microtomographic images as seen in Figure 7. 5

6 Figure 7: Fracture geometry used for numerical calculations for conductivity measurements in self-propped granite core. 4. CONCLUSIONS In this study, fluid flow through the rough contours of fracture in granite was studied at conditions similar to those observed in EGS reservoirs. At an effective closing stress of 1000 psi and temperatures varying from ambient condition to 200 C, it was found that self-propped asperity dominated hydraulic conductivity decreased with increasing temperature. Numerical simulations with geometric description from three dimensional X-ray microtomography images are currently being performed to predict the fluid pressure variation measured in laboratory testing. 5. ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support for this study through U.S. Department of Energy contract number GO REFERENCES Perkins T., and Kern L.: Widths of Hydraulic Fractures, Journal of Petroleum Technology, September 1961, (1961), Brace W., Walsh J., and Frangos W.: Permeability of Granite under High Pressure, Journal of Geophysical Research, 73, (1968), Lawrence W.: Permeability Changes During Shear Deformation of Fractured Rock, Proceedings, 28 th US Symposium on Rock Mechanics, Tucson, AZ (1987). Olsson W., and Brown S.: Hydromechanical Response of a Fracture Undergoing Compression and Shear, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 30, (1993), Kotousov A., Neto L., and Rahman S.: Theoretical Model for Roughness Induced Opening of Cracks Subjected to Compression and Shear Loading, Int. J. Frac., 172, (2011),

7 McClure M., and Horne R.: Is Pure Shear Stimulation Always the Mechanism of Stimulation in EGS?, Proceedings, 38 th Workshop on Geothermal Reservior Engineering, Stanford University, Stanford, CA (2013). Kranz R., Frankel A., Engelder T., and Scholz C.: The Permeability of Whole and Joined Barr Granite, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 16, (1979), Brown S.: Fluid Flow Through Rock Joints: The Effect of Surface Roughness, Journal of Geophysical Research, 92, (1987), Stoddard T., McLennan J., and Moore J.: Residual Conductivity of a Bauxite-Propped Geothermal System- Influence of Self- Propping, Time, and Closure Stress, Proceedings, 46 th US Rock Mechanics/Geomechanics Symposium, Chicago, IL, (2012). 7