Why are brittleness and fracability not equivalent in designing hydraulic fracturing in tight shale gas reservoirs

Abstract

With respect to brittleness, it is about the type of material and its related strength. In comparison with ductile material under load, brittle material has a relatively shorter plastic deformation and responds dominantly by the elastic deformation. With respect to fracability, it is about the rock failure under the ultimate rock strength in either brittle or ductile formation. In comparison, the higher fracable formation should have smaller formation strength than that of the lower fracable formation. In consequence, it is not certain that the brittle formation is easy to fracture than the ductile formation since brittle formation may have greater strength than ductile formation even though the exceptions may exist.

More complications arise when evaluating the responses of subsurface formation in great depth to the formation types (e.g. brittle formation or ductile formation). Under this condition, the impact of confinement on the fracability cannot be ignored. In general, the formation subject to higher confinement pressure is more difficult to fracture as the formation strength is greater. Conversely, the formation subject to lower confinement pressure is easy to fracture since the formation strength is smaller.

In view of efficient stimulation of tight shale gas reservoirs, it is unclear whether we would choose the brittle interval or the ductile interval to fracture as the strength of either interval is unknown. However, it is apparent that we should choose the formation interval with a higher fracability which is equivalent to the lower formation strength. Under the similar confinements, the lower formation strength may be indicated by the smaller unconfined compressive strength (UCS). As a result, it is advisable that the most fracable interval is the one with lowest UCS.

When evaluating the present technology, the formation brittleness should no longer be the associated subject matter as we are unclear about its role to improve the fracability of the tight formation. Disassociating the brittleness with the fracability enables us to focus on identifying the true mechanisms of efficient fracturing of tight shale gas reservoirs.

With the objective review and sensible definition of brittleness used in the present petro-physical field to identify the desirable fracturing intervals, the paper presents the ambiguities of using the brittleness to define the formation fracability and points out that the formation brittleness can be unrelated to the formation fracability. As an alternative approach, the paper provides an effective method to define the most fracable formation intervals in designing the hydraulic fracturing in tight shale gas formations.

Keywords

Brittleness / Fracability / Rock stiffness / Rock strength / Formation confinement / Horizontal well landing interval

References

[1] S.A. Holditch, Tight gas sands, JPT 58 (6) (2006) 86-93. SPE 103356.

[2] A.T. Zehnder, Fracture mechanics, in: Lecture Notes in Applied and Computational Mechanics, vol. 62, Springer Sci. & Business Media, 2012.

[3] R. Rickman, M. Mullen, E. Petre, B. Grieser, D. Kundert, A practical use of shale petrophysics for stimulation design optimization: all shale plays are not clones of the Barnett shale, in: SPE 115258, SPE ATCE, Denver, CO, USA, Sept. 21-24, 2008.

[4] D.B. Van Dam, P. Papanastasiou, C.J. de Pater, Impact of rock plasticity on hydraulic fracture propagation and closure, SPE 63172, in: SPER ATCE, Dallas, TX, USA, Oct. 1-4, 2002.

[5] H.T. Alassi, R. Holt, O.-M. Nes, S. Pradhan, Realistic geomechanical modeling of hydraulic fracturing in fractured reservoir rock, CSUG/SPE 149375, in: Canadian Unconventional Conf., Alberta, CA, Nov. 15-17, 2011.

[6] R. Romanson, R. Pongratz, L. East, M. Stanojcic, Novel, multistage stimulation processes can help achieve and control branch fracturing and increase stimulated reservoir volume for unconventional reservoirs, SPE 142959, in: SPE/Europec/SPE ATCE, Vienna, Austria, May 23-26, 2011.

[7] R.M. Holt, E. Fjaer, O.M. Nes, H.T. Alassi,A shaly look at brittleness, in:45th US Rock Mechanics/Geomechanics Symposium, ARMA11-366, San Francisco, CA, USA, June 26-29, 2011.

[8] J.L. Miskimins,The impact of mechanical stratigraphy on hydraulic fracture growth and design consideration for horizontal wells, in:Search and Discovery Article #41102, Tulsa Geological Society Lunch Meeting, Nov. 27, 2012.

[9] J.J. Zhang, L.R. Bentley, Factors determining Poisson's ratio, CREWES Res. Rep. 17 (2005).

[10] R. Altindag,Correlation of specific energy with rock brittleness concepts on rock cuttings, J. South Afr. Inst. Min. Metallurgy 103 (3) (2003) 163-172.

[11] H.K. Chiu, L.W. Johnston,The uniaxial properties of Melbourne mudstone, in:Proc. 5th Congr. ISRM, V.1, Melbourne, 1983, pp. A209-A214.

[12] H.I. Inyang, Development of a preliminary rock mass classification scheme for near surface excavation, Int. J. Surf. Min. Reclam. 5 (1991) 65-74.

[13] Y. Niwa, S.H. Kobasayashi,Effect of couple-stresses distribution in specimens of laboratory tests, in:Proc. 3rd Congr. ISRM, V. 2, Denver, USA, 197-201, 1974.

[14] J.B. Walsh, F.A. Brace, A fracture criterion for brittle anisotropic rock, J. Geophys. Res. 69 (1964) 3449-3456.

[15] S. Kahraman, Correlation of TBM and drilling machine performances with rock brittleness, Eng. Geol. 65 (2002) 269-283.

[16] D.M. Jarvie, R.J. Hill, T.E. Ruble, R.M. Pollastro, Unconventional shale-gas systems: the Mississippian barnett shale of north-central Texas as one model for thermogenic shale-gas assessment, AAPG Bull. 91 (4) (2007) 475-499.

[17] X. Jin, S.N. Shah, J.C. Rogiers, Bo Zhang, Fracability evaluation in shale reservoirs e an integrated petrophysics and geomechanics approach, in: SPE Hydraulic Fracturing Technology Conference, Woodlands, TX, USA, 2014. SPE 168589.

[18] H. Honda, Y. Sanada, Hardness of coal, Fuel 35 (1956) 45-461.

[19] M. Protodyakonov, Mechanical properties and drillability of rocks, in: Proc. 5th Symposium on Rock Mech., The Univ. of Minnesota, USA, 1962.

[20] G.E. Andreev, Brittle Failure of Rock Materials: Test Results and Constitutive Models, Taylor & Francis, 1995.

[21] V. Hajiabdolmajid, P. Kaiser, Brittleness of rock and stability assessment in hard rock tunneling, Tunneling Undergr. Space Technol. 18 (1) (2003) 35-48.

[22] A.W. Bishop,Progressive failure with special reference to the mechanism causing it, in:Proc. Geotech. Conf., Oslo, 1967, pp. 142-150.

[23] V. Hucka, B. Das, Brittleness determination of rocks by different methods, Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 11 (10) (1974) 389-392.

[24] B. Lawn, D. Marshall, Hardness, toughness, and brittleness: an indentation analysis, J. Am. Ceram. Soc. 62 (7-8) (1979) 347-350.

[25] J. Sehgal, Y. Nakao, H. Takahashi, S. Ito, Brittleness of glasses by indentation, J. Mater. Sci. Lett. 14 (3) (1995) 167-169.

[26] H. Copur, N. Bilgin, H. Tuncdemir, C. Balci,A set of indices based on indentation tests for assessment of rock cutting performance and rock properties, J. South Afr. Inst. Min. Metallurgy 103 (9) (2003) 589-599.

[27] S. Yagiz, Assessment of brittleness using rock strength and density with punch penetration test, Tunn. Undergr. Space Tech. 24 (1) (2009) 66-74.

[28] J.B. Quinn, G.D. Quinn, Indentation brittleness of ceramics: a fresh approach, J. Mater. Sci. 32 (16) (1997) 4331-4346.

[29] F.P. Wang, J.F. Gale, Screening criteria for shale-gas system, Gulf Coast Asso. Geol. Soc. Trans. 59 (2009) 779-793.

[30] D. Buller, S. Hughes, J. Market, E. Petre, D. Spain, T. Odumosu, Petrophysical evaluation for enhancing hydraulic stimulation in horizontal shale gas wells, SPE 132990, in: SPE Annual Technical Conference and Exhibition Held in Florence, Italy, 19e22 September, 2010.

[31] L. Baron, B. Loguntsov, E. Pozin, Determination of the Properties of Rocks, Gosgortekhizdat, Moscow, 1962.

[32] J. Suppe, Principles of Structural Geology, Prentice-Hall, Eaglewood Cliffs, New Jersey, USA, 1985.

[33] G.M. Ingram, J.L. Urai,Top-seal Leakage Through Faults and Fractures, the Role of Mudrock Properties, vol. 158, Geological Society, London, 1999, pp. 125-135.

[34] H. Sone, M. Zoback,Mechanical properties of shale gas reservoir rocks e part 1: static and dynamic elastic properties and anisotropy, Geophysics 78 (5) (2013) 381-392.

[35] H. Sone, M. Zoback,Mechanical properties of shale gas reservoir rocks e part 2: ductile creep, brittle strength, and their relation to the elastic modulus, Geophysics 78 (5) (2013) 393-402.

[36] Y. Yang, H. Sone, A. Hows, M.D. Zoback, Comparison of brittleness indices in organic-rich shale formations, in: 47th US Rock Mech. Symposium, San Francisco, USA, June 23-26, 2013.

[37] Ticona, Designing with Plastic, The Fundamentals, Ticona, Prod. Info. Services, Florence, KY, USA, 2000.

[38] MPE (Modern Plastics Encyclopedia), The McGraw-Hill Companies, 1996.

[39] J.A. Hudson, J.P. Harrison, Engineering Rock Mechanics e an Introduction to the Principles, Pergamon, Elsevier Science, Amsterdam, Netherland, 1997.

[40] Wickham, Structural Geology, Chapter 5, Rheology, Univ. of Texas at Arlington, USA, 2012. GEOL 3443 course book.

[41] S.J. Lutz, S. Hickman, N. Davatzes, E. Zemach, P. Drakos, A.R. Tait, Rock mechanical testing and petrologic analysis in support of well stimulation activities at the desert peak geothermal field, Nevada, in: Proc. 35th Workshop on Geothermal Reservoir Engineering, Stanford Univ., CA, USA, 2010.

[42] A. Pagoulatos, Evaluation of Multistage Triaxial Testing on Barea Sandstone (M.S. Thesis), Univ. of Oklahoma, Norman, USA, 2004.

[43] F. Ansari, Q. Li, High strength concrete subjected to uniaxial compression, ACID Mater. J. (1998).

[44] X. Lu,Uniaxial and Triaxial Behavior of High Strength Concrete with and without Steel Fibers (Ph.D. Dissertation), New Jersey Institute of Technololgy, USA, 2005.

[45] M. You, Strength and damage of marble in ductile failure, J. Rock Geotech. Eng. 3 (2) (2011) 161-166.

[46] S.H. Kirby, Tectonic stresses in the lithosphere: constrains provided by the experimental deformation of rocks, J. Geophys. Res. 85 (1980) 6353-6363.

[47] M.K. Hubbert, D.G. Willis, Mechanics of hydraulic fracturing, Trans. SPEAIME. 210 (1957) 153-168.

[48] A.M. Raaen, M. Brudy, Pump in/flowback tests reduce the estimate of horizontal in-situ stress significantly, SPE 71367, in: SPE Annual Technical Conference and Exhibition, New Orleans, USA, 30 September-3 October, 2001.

[49] M. Bai, Risk and uncertainties in determining fracture gradient and closure pressure, in: 45th US Rock Mechanics/Geomechanics Symposium, San Francisco, CA, June 26-29, 2011, pp. 11-132.

[50] M. Bai, S. Green, R. Suarez-Rivera, Effect of leakoff variation on fracturing efficiency for tight shale gas reservoirs, in: Proc. 40th U.S. Rock Mechanics Symposium, Anchorage, Alaska, USA, 2005.

[51] P. Papanastasiou, Hydraulic Fracturing: Basic Concepts and Numerical Modeling, Dept. of Civil and Environmental Eng., Univ. of Cyprus, Cyprus, 2002.

[52] P.M. Halleck, R.J. Saucier, L.A. Behrmann, T. Ahrens, Reduction of jet perforator penetration in rock under stress, in: SPE 63th ATCE, SPE 18245, Houston, TX, USA, 1988.

[53] G.E. King, Perforation Basics e How the Perforating Processes Work, 2009. George E. King Engineering.

[54] S.G. Weeks, Formation damage or limited perforation penetration? Test well shooting may give a clue, J. Petroleum Technol. (1974) 979-984.

[55] B. Grove, J. Heiland, I. Walton, D. Atwood, Drilling and completion, SPE J. (2009) 678-685.