An Experimental Study on Conductivity of Unpropped Fractures in Preserved Shales
- Weiwei Wu (The University of Texas at Austin (now with Apache Corporation)) | Junhao Zhou (Qmax Solutions) | Pratik Kakkar (Siemens Corporation) | Rodney Russell (The University of Texas at Austin) | Mukul Mani Sharma (The University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- May 2019
- Document Type
- Journal Paper
- 280 - 296
- 2019.Society of Petroleum Engineers
- mechanical properties, unpropped fractures, fracture conductivity, shale, fluid-shale interaction
- 11 in the last 30 days
- 393 since 2007
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A great deal of evidence shows that hydraulic fracturing creates a large surface area of induced unpropped (IU) fractures that are too small to accommodate commonly used proppants and that, subsequently, close during production (Sharma and Manchanda 2015). Because of their enormous surface area, IU fractures can play an important role in hydrocarbon production if they can remain open during production. Therefore, the conductivity of these IU fractures under different stress conditions and when exposed to different fracturing fluids is of great importance.
In this study, core-scale IU fractures were created with preserved shale samples from the Eagle Ford and Utica formations. Samples with different mineralogies were selected to represent a broad cross section of representative samples. Great care was taken to ensure that the shale samples were preserved because large changes in shale mechanical properties caused by sample desiccation have been observed. The fracture conductivities of unpropped fractures created in each of the shale samples were measured as a function of closure stress by using nitrogen or brine. The unpropped fractures were exposed to several water-based fracturing fluids including neutral brine, alkaline brine (pH 11, 12), and acidic brine (pH<1), with or without clay stabilizers. The effects of fluid type, pH, clay stabilizers, shale mineralogy, and cyclic stress on IU-fracture conductivities were investigated. Batch tests also were performed to study the change of mechanical properties and fines production caused by fluid-shale interaction.
Our results show that unpropped fractures yielded conductivities that were 2 to 4 orders of magnitude lower than those of propped fractures, and were more susceptible to closure stress. Exposure to water-based fracturing fluids decreased the unpropped-fracture conductivity by one order of magnitude. The primary mechanism for the decrease was shale softening caused by exchange of water and ions between the native fluid of shale and the exposed fracturing fluid. Shale softening was observed in exposure to all brines tested, regardless of their pH. In addition to shale softening, fines generation also contributed to the reduction of unpropped-fracture conductivity when shales were exposed to alkaline or acidic brine. Amine-based clay stabilizers were able to control the unpropped-fracture conductivity impairment by reducing the amount of clay-based fines. However, they were not as effective at stabilizing nonclay fines. Shale mineralogy affected the unpropped conductivities in two ways: It controlled the mechanical properties of the native preserved shale, and also affected the fluid-shale interactions. A clear correlation was observed between mineralogy and stress dependence. Clay-rich samples showed the most stress sensitivity in the presence of water or brine at neutral pH, whereas the calcite-rich samples showed less stress sensitivity. High clay content also resulted in lower restored conductivity after cyclic stress. Mechanical properties of shale such as hardness and Young’s modulus, before and after fluid exposure, strongly correlated with the mineralogy of shales. Unpropped conductivity was more sensitive to cyclic stress than propped conductivities, and it dropped by 80% after one cycle of closure stress between 300 and 4,000 psi of closure stress. Clearly, it is shown that water-based fracturing fluids can affect conductivities of IU fractures in shales significantly, and these impacts need to be considered in the selection of fracturing fluids.
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Akrad, O. M., Miskimins, J. L., and Prasad, M. 2011. The Effects of Fracturing Fluids on Shale Rock Mechanical Properties and Proppant Embedment. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 30 October–2 November. SPE-146658-MS. https://doi.org/10.2118/146658-MS.
Alotaibi, M., Nasralla, R. A., and Nasr-El-Din, H. A. 2010. Wettability Challenges in Carbonate Reservoirs. Presented at the SPE Improved Oil Recovery Conference, Tulsa, 24–28 April. SPE-129972-MS. https://doi.org/10.2118/129972-MS.
Alramahi, B. and Sundberg, M. I. 2012. Proppant Embedment and Conductivity of Hydraulic Fractures in Shales. Presented at the 46th US Rock Mechanics/Geomechanics Symposium, Chicago, 24–27 June. ARMA 12-291.
API RP 19D/ISO 13503-5. 2008. RP for Measurement of Long-Term Conductivity of Proppants. Washington, DC: American Petroleum Institute.
ASTM E10-15a. 2015. Standard Test Method for Brinell Hardness of Metallic Materials. West Conshohocken, Pennsylvania: ASTM International. https://doi.org/10.1520/E0010-15A.
Auradou, H., Drazer, G., Hulin, J. P. et al. 2005. Permeability Anisotropy Induced by the Shear Displacement of Rough Fracture Walls. Water Resources Research 41 (9). https://doi.org/10.1029/2005wr003938.
Beg, M. S., Kunak, A. O., Gong, M. et al. 1998. A Systematic Experimental Study of Acid Fracture Conductivity. SPE Prod & Fac 13 (4): 267–271. SPE-52402-PA. https://doi.org/10.2118/52402-PA.
Chen, L., Zhang, G., Wang, L. et al. 2014. Zeta Potential of Limestone in a Large Range of Salinity. Colloids and Surfaces A: Physicochemical and Engineering Aspects 450: 1–8. https://doi.org/10.1016/j.colsurfa.2014.03.006.
Chenevert, M. E. and Amanullah, M. 2001. Shale Preservation and Testing Techniques for Borehole-Stability Studies. SPE Drill & Compl 16 (3): 146–149. SPE-73191-PA. https://doi.org/10.2118/73191-PA.
Deng, J., Mou, J., Hill, A. D. et al. 2012. A New Correlation of Acid-Fracture Conductivity Subject to Closure Stress. SPE Prod & Oper 27 (2): 158–169. SPE-140402-PA. https://doi.org/10.2118/140402-PA.
Derjaguin, B. V. and Landau, L. 1993. Theory of the Stability of Strongly Charged Lyophobic Sols and of the Adhesion of Strongly Charged Particles in Solutions of Electrolytes. Progress in Surface Science 43 (1): 30–59. https://doi.org/10.1016/0079-6816(93)90013-L.
Fan, L., Thompson, J. W., and Robinson, J. R. 2010. Understanding Gas Production Mechanism and Effectiveness of Well Stimulation in the Haynesville Shale Through Reservoir Simulation. Presented at the Canadian Unconventional Resources and International Petroleum Conference, Calgary, 19–21 October. SPE-136696-MS. https://doi.org/10.2118/136696-MS.
Fjar, E., Holt, R. M., Raaen, A. M. et al. 2008. Petroleum Related Rock Mechanics, second edition, Vol. 53. Elsevier.
Fredd, C. N., McConnell, S. B., Boney, C. L. et al. 2001. Experimental Study of Fracture Conductivity for Water-Fracturing and Conventional Fracturing Applications. SPE J. 6 (3): 288–298. SPE-74138-PA. https://doi.org/10.2118/74138-PA.
Gangi, A. F. 1978. Variation of Whole and Fractured Porous Rock Permeability With Confining Pressure. In International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 15 (5): 249–257. https://doi.org/10.1016/0148-9062(79)90824-6.
Gong, M., Lacote, S., and Hill, A. D. 1999. New Model of Acid-Fracture Conductivity Based on Deformation of Surface Asperities. SPE J. 4 (3): 206–214. SPE-57017-PA. https://doi.org/10.2118/57017-PA.
Grieser, B., Wheaton, R., Magness, B. et al. 2007. Surface Reactive Fluids Effect on Shale. Presented at the SPE Production and Operations Symposium, Oklahoma City, Oklahoma, USA, 31 March–1 April. SPE-106815-MS. https://doi.org/10.2118/106815-MS.
Gross, M., Fischer, M., Engelder, T. et al. 1995. Factors Controlling Joint Spacing in Interbedded Sedimentary Rocks; Integrating Numerical Models With Field Observations From the Monterey Formation, USA. In Fractography, ed. M. S. Ameen, 215–233. Geological Society Special Publication, London: Geological Society. https://doi.org/10.1144/GSL.SP.1995.092.01.12.
Gu, M., Kulkarni, P., Rafiee, M. et al. 2016. Optimum Fracture Conductivity for Naturally Fractured Shale and Tight Reservoirs. SPE Prod & Oper 31 (4): 289–299. SPE-171648-PA. https://doi.org/10.2118/171648-PA.
Gulbis, J. and Hodge, R. M. 2000. Fracturing Fluid Chemistry and Proppants. In Reservoir Stimulation, third edition, ed. M. J. Economides and K. G. Nolte, Chap. 7. New York, New York: Wiley.
Gupta, J. K., Zielonka, M. G., Albert, R. A. et al. 2012. Integrated Methodology for Optimizing Development of Unconventional Gas Resources. Presented at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 6–8 February. SPE-152224-MS. https://doi.org/10.2118/152224-MS.
Jansen, T., Zhu, D., and Hill, A. D. 2015. The Effect of Rock Mechanical Properties on Fracture Conductivity for Shale Formations. Presented at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 3–5 February. SPE-173347-MS. https://doi.org/10.2118/ 173347-MS.
Jung, C. M., Zhou, J., Chenevert, M. E. et al. 2013. The Impact of Shale Preservation on the Petrophysical Properties of Organic-Rich Shales. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 30 September–2 October. SPE-166419-MS. https://doi.org/ 10.2118/166419-MS.
Kamali, A. and Pournik, M. 2016. Fracture Closure and Conductivity Decline Modeling—Application in Unpropped and Acid-Etched Fractures. Journal of Unconventional Oil and Gas Resources 14: 44–55. https://doi.org/10.1016/j.juogr.2016.02.001.
Kamenov, A., Zhu, D., Hill, A. D. et al. 2013. Laboratory Measurement of Hydraulic Fracture Conductivities in the Barnett Shale. Presented at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 4–6 February. SPE-163839-MS. https://doi.org/10.2118/163839-MS.
Kumar, V., Sondergeld, C., and Rai, C. 2012. Nano to Macro Mechanical Characterization of Shale. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8–10 October. SPE-159804-MS. https://doi.org/10.2118/159804-MS.
Lee, H. S. and Cho, T. F. 2002. Hydraulic Characteristics of Rough Fractures in Linear Flow Under Normal and Shear Load. Rock Mechanics and Rock Engineering 35 (4): 299–318. https://doi.org/10.1007/s00603-002-0028-y.
Leone, J. A. and Scott, E. M. 1987. Characterization and Control of Formation Damage During Waterflooding of a High-Clay-Content Reservoir. SPE Res Eng 3 (4): 1279–1286. SPE-16234-PA. https://doi.org/10.2118/16234-PA.
Manchanda, R. and Sharma, M. M. 2013. Time-Delayed Fracturing: A New Strategy in Multi-Stage, Multi-Well Pad Fracturing. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 30 September–2 October. SPE-166489-MS. https://doi.org/10.2118/166489-MS.
Manchanda, R., Sharma, M. M., and Holzhauser, S. 2014. Time-Dependent Fracture-Interference Effects in Pad Wells. SPE Prod & Oper 29 (4): 274–287. SPE-164534-PA. https://doi.org/10.2118/164534-PA.
Marsala, A. F., Loermans, T., Shen, S. et al. 2012. Portable Energy-Dispersive X-Ray Fluorescence Integrates Mineralogy and Chemostratigraphy Into Real-Time Formation Evaluation. Petrophysics 53 (2): 102–109. SPWLA-2012-v53n2a3.
Mayerhofer, M. J., Lolon, E. P., and Warpinski, N. R. 2010. What Is Stimulated Reservoir Volume? SPE Prod & Oper 25 (1): 16–18. SPE-119890-PA. https://doi.org/10.2118/119890-PA.
McClure, M. W. 2014. The Potential Effect of Network Complexity on Recovery of Injected Fluid Following Hydraulic Fracturing. Presented at the SPE Unconventional Resources Conference, The Woodlands, Texas, USA, 1–3 April. SPE-168991-MS. https://doi.org/10.2118/168991-MS.
Montemagno, C. D. and Pyrak-Nolte, L. J. 1999. Fracture Network Versus Single Fractures: Measurement of Fracture Geometry With X-Ray Tomography. Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy 24 (7): 575–579. https://doi.org/10.1016/s1464-1895(99)00082-4.
Nierode, D. and Kruk, K. 1973. An Evaluation of Acid Fluid Loss Additives Retarded Acids and Acidized Fracture Conductivity. Presented at the Fall Meeting of the Society of Petroleum Engineers of AIME, Las Vegas, Nevada, USA, 30 September–3 October. SPE-4549-MS. https://doi.org/10.2118/4549-MS.
Oliver, W. C. and Pharr, G. M. 2004. Measurement of Hardness and Elastic Modulus by Instrumented Indentation: Advances in Understanding and Refinements to Methodology. Journal of Materials Research 19 (1): 3–20. https://doi.org/10.1557/jmr.2004.0002.
Ouchi, H., Agrawal, S., Foster, J. T. et al. 2017. Effect of Small-Scale Heterogeneity on the Growth of Hydraulic Fractures. Presented at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 24–26 January. SPE-184873-MS. https://doi.org/10.2118/184873-MS.
Palisch, T., Duenckel, R., Bazan, L. et al. 2007. Determining Realistic Fracture Conductivity and Understanding its Impact on Well Performance—Theory and Field Examples. Presented at the SPE Hydraulic Fracturing Technology Conference, College Station, Texas, USA, 29–31 January. SPE-106301-MS. https://doi.org/10.2118/106301-MS.
Pedlow, J. and Sharma, M. 2014. Changes in Shale Fracture Conductivity due to Interactions With Water-Based Fluids. Presented at the SPE Hydraulic Fracturing Technology Conference, Woodlands, Texas, USA, 4–6 February. SPE-168586-MS. https://doi.org/10.2118/168586-MS.
Potocki, D. J. 2012. Understanding Induced Fracture Complexity in Different Geological Settings Using DFIT Net Fracture Pressure. Presented at the SPE Canadian Unconventional Resources Conference, Calgary, 30 October–1 November. SPE-162814-MS. https://doi.org/10.2118/162814-MS.
Pyrak-Nolte, L. J. and Morris, J. P. 2000. Single Fractures Under Normal Stress: The Relation Between Fracture Specific Stiffness and Fluid Flow. International Journal of Rock Mechanics and Mining Sciences 37 (1): 245–262. https://doi.org/10.1016/s1365-1609(99)00104-5.
Robinson, R. A. and Stokes, R. H. 1959. Electrolyte Solutions, second edition. London: Butterworths Scientific Publications.
Rowe, H., Hughes, N., and Robinson, K. 2012. The Quantification and Application of Handheld Energy-Dispersive X-Ray Fluorescence in Mudrock Chemostratigraphy and Geochemistry. Chem. Geol. 324: 122–131. https://doi.org/10.1016/j.chemgeo.2011.12.023.
Ruffet, C., Fery, J. J., and Onaisi, A. 1998. Acid Fracturing Treatment: A Surface-Topography Analysis of Acid-Etched Fractures to Determine Residual Conductivity. SPE J. 3 (2): 155–162. SPE-38175-PA. https://doi.org/10.2118/38175-PA.
Sandler, S. I. 2006. Chemical, Biochemical, and Engineering Thermodynamics, fourth edition. Hoboken, New Jersey: John Wiley & Sons.
Sharma, M. M. and Yortsos, Y. C. 1987. Fines Migration in Porous Media. AIChE Journal 33 (10): 1654–1662. https://doi.org/10.1002/aic.690331009.
Sharma, M. M. and Manchanda, R. 2015. The Role of Induced Unpropped (IU) Fractures in Unconventional Oil and Gas Wells. Presented at the SPE Annual Technical Conference and Exhibition, Houston, 28–30 September. SPE-174946-MS. https://doi.org/10.2118/174946-MS.
Soliman, M. Y., Miranda, C., and Wang, H. M. 2010. Application of After-Closure Analysis to a Dual-Porosity Formation, to CBM, and to a Fractured Horizontal Well. SPE Prod & Oper 25 (4): 472–483. SPE-124135-PA. https://doi.org/10.2118/124135-PA.
Tripathi, D. and Pournik, M. 2014. Effect of Acid on Productivity of Fractured Shale Reservoirs. Presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Denver, 25–27 August. URTEC-1922960-MS. https://doi.org/10.15530/urtec-2014-1922960.
Vaidya, R. N. and Fogler, H. S. 1992. Fines Migration and Formation Damage: Influence of pH and Ion Exchange. SPE Prod Eng 7 (4): 325–330. SPE-19413-PA. https://doi.org/10.2118/19413-PA.
Verwey, E. J. W. and Overbeek, J. T. G. 1955. Theory of the Stability of Lyophobic Colloids. Journal of Colloid Science 10 (2): 224–225.
Wang, L. and Cardenas, M. B. 2016. Development of an Empirical Model Relating Permeability and Specific Stiffness for Rough Fractures From Numerical Deformation Experiments. Journal of Geophysical Research: Solid Earth 121 (7): 4977–4989. https://doi.org/10.1002/ 2016jb013004.
Wang, H., Yi, S. and Sharma, M. M. 2018. A Computationally Efficient Approach to Modeling Contact Problems and Fracture Closure Using Superposition Method. Theoretical and Applied Fracture Mechanics 93: 276–287. https://doi.org/10.1016/j.tafmec.2017.09.009.
Watanabe, N., Ishibashi, T., Hirano, N. et al. 2011. Precise 3D Numerical Modeling of Fracture Flow Coupled With X-Ray Computed Tomography for Reservoir Core Samples. SPE J. 16 (3): 683–691. SPE-146643-PA. https://doi.org/10.2118/146643-PA.
Wu, W. 2017. Unpropped Fractures in Shale: Surface Topography, Mechanical Properties, and Hydraulic Conductivity. PhD dissertation, The University of Texas at Austin, Austin, Texas (December 2017).
Wu, W. and Sharma, M. M. 2017a. Acid Fracturing Shales: Effect of Dilute Acid on Properties and Pore Structure of Shale. SPE Prod & Oper 32 (1): 51–63. SPE-173390-PA. https://doi.org/10.2118/173390-PA.
Wu, W. and Sharma, M. M. 2017b. A Model for the Conductivity and Compliance of Unpropped and Natural Fractures. SPE J. 22 (6):1893–1914. SPE-184852-PA. https://doi.org/10.2118/184852-PA.
Xu, T., Lindsay, G., Baihly, J. et al. 2017. Proposed Refracturing Methodology in the Haynesville Shale. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 9–11 October. SPE-187236-MS. https://doi.org/10.2118/187236-MS.
Yasuhara, H., Polak, A., Mitani, Y. et al. 2006. Evolution of Fracture Permeability Through Fluid-Rock Reaction Under Hydrothermal Conditions. Earth and Planetary Science Letters 244 (1): 186–200. https://doi.org/10.1016/j.epsl.2006.01.046.
Ye, Z., Janis, M., Ghassemi, A. et al. 2017. Laboratory Investigation of Fluid Flow and Permeability Evolution Through Shale Fractures. Presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Austin, Texas, USA, 24–26 July. URTEC-2674846-MS. https://doi.org/10.15530/urtec-2017-2674846.
Zhang, J., Al-Bazali, T., Chenevert, M. et al. 2006. Compressive Strength and Acoustic Properties Changes in Shale With Exposure to Water-Based Fluids. Presented at the 41st US Symposium on Rock Mechanics (USRMS), Golden, Colorado, USA, 17–21 June. ARMA-06-900.
Zhang, J., Kamenov, A., Zhu, D. et al. 2014. Laboratory Measurement of Hydraulic Fracture Conductivities in the Barnett Shale. SPE Prod & Oper 29 (3): 216–227. SPE-163839-PA. https://doi.org/10.2118/163839-PA.
Zhang, J., Zhu, D., and Hill, A. D. 2015. Water-Induced Damage to Propped-Fracture Conductivity in Shale Formations. SPE Prod & Oper 31 (2): 147–156. SPE-173346-PA. https://doi.org/10.2118/173346-PA.
Zhou, J. 2015. Interactions of Organic-Rich Shale With Water-Based Fluids. PhD dissertation, The University of Texas at Austin, Austin, Texas (August 2015).
Zoback, M. D., Kohli, A., Das, I. et al. 2012. The Importance of Slow Slip on Faults During Hydraulic Fracturing Stimulation of Shale Gas Reservoirs. Presented at the SPE Americas Unconventional Resources Conference, Pittsburgh, Pennsylvania, USA, 5–7 June. SPE-155476-MS. https://doi.org/10.2118/155476-MS.