EAGE Conference & Exhibition

We-P09-16 Understanding of brittleness and the relative brittleness index construction Z. Liu (China University of Petroleum), S.Z. Sun (China University of Petroleum), N. Dong (Petroleum E&P Research Institute SINOPEC), Y. Sun (China University of Petroleum), Z. Du (China University of Petroleum), Z. Jin (Petroleum E&P Research Institute SINOPEC)

Summary As hydraulic fracturing becomes a routine practice to enhance or enable the recovery of hydrocarbon in tight formation, it is important to estimate the fracturability of subsurface rock formation. The brittleness is the key parameter for rock fracturing evaluation. Since the brittleness of rock is related to mineralogy and mechanical properties, researchers proposed various methods to evaluate brittleness considering the mineral composition and elastic properties. Based on the understanding of brittle and ductile definition, this article proposed two methods to calculate the relative brittleness index. One method uses the mineral composition to construct relative brittleness with the view that different minerals have different brittleness, another one utilizes elastic properties to construct the relative brittleness indexes which are more sensitive than other brittleness indexes presented in the previous study. The evaluation result in a well of Jiannan area in Southern China proves the appropriation of the methods.

76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

Introduction Brittleness is a very important mechanical property of rocks. Nevertheless, there is still a big controversy between different authors as to the definition and measurements of brittleness. Different researchers express and use it differently. In material science, all of brittleness evaluation method is based on measurements or tests, such as tensile strength and compressional strength test, and hardness measurements. The main purpose of these tests is to obtain the stress-strain curve which can be used to determine the brittleness. It is known to all that the modulus of elasticity can be used to determine the stress-strain relationship in the linear elastic portion of the stress-strain curve (Figure 1). So researchers attempts to relate the brittleness to their elastic properties (Altindag, 2002; Rickman et al., 2008; Guo et al., 2012; Liu et al., 2013). Meanwhile, it is an obvious view that the brittleness of rocks is related to its different minerlas, so some petroleum exploration experts attempt to evaluate the brittleness of reservoir using the rock compositions(Mavko’s report). Meanwhile, it is an obvious view that the brittleness of rocks is related to its different material properties, such as grain sizes, pore geometry, and cementation, so some petroleum exploration experts attempt to evaluate the brittleness of reservoir using the rock compositions (Mavko’s report). On the basis of the understandings of physical significance of brittle and ductile, this paper proposes two relative brittleness indexes (RBI), one of them defines the relative brittleness of essential minerals in gas shale and uses it to calculate the RBI of composition of rock. one of them uses the composite of rock, considering the influence of different minerals; While another uses the elastic properties which synthesized to reflect the influence of the different minerals, grain sizes, pore geometry, and cementation. which is more sensitive than other elastic properties brittleness indexes. The logging evaluation result in a shale gas well of Jiannan area in Southern China proves the appropriation and effectiveness of these two methods. Brittle and ductile Figure 1 Graph comparing stress-strain curves for brittle and ductile materials As Figure 1 shows, a material is brittle if, when subjected to stress, it breaks without significant deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Meanwhile, ductility is a solid material’s ability to deform under stress. In elastic region, the relation between the applied stress which is directly proportional and the resulting strain (up to a certain limit) can be explained by a graph in which those two quantities are presented as a straight line （red line）. The slope of this line is known as Young’s modulus (E). E can be used to determine the stress-strain relationship in the linear elastic portion of the stress-strain curve. In plastic region, plastic deformation is retained after the release of the applied stress. Most materials in the linear-elastic category are usually capable of plastic deformation. Brittle materials, like ceramics, do not experience any plastic deformation and will fracture under relatively low stress. In literatures of material science, the brittleness calculating methods mainly use tests results/data of rock strengths (compressional strength & tensile strength) or hardness. Figure 2 shows the correlation of rock elastic properties, UCS (Uniaxial Compressive Strength), hardness, brittleness and minerals. The figure shows that a positive correlation existing among the rock strength, hardness, brittleness and Young’s modulus, while a negative correlation exists between the Poisson’s Ratio and strength, hardness, brittleness. Figure 2 (g) (h) shows that the difference of mineral content will result in the variation of UCS and Hardness. That’s to say, the brittleness of each mineral differs a lot. Relative Brittleness index(RBI) and New Relative brittleness index(RBI) construction


1. Mineral RBI Based on the understanding that each mineral has its own brittleness, researchers defined some RBI by the relative abundance of brittle composite compared to the ductile composite (Mavko’s report). The most famous methods are as equation (1) and (2) show. RBI=Quartz/(Quartz+Carbonate+Clay) RBI=(Quartz+Carbonate)/(Quartz+Carbonate+Clay)

(1) (2)

Where, Quartz, Carbonate, and Clay mean the volume of each mineral in the rock. Obviously, the brittleness of each mineral is considered to be equal in these two equations. And as we’ll see, this can in fact be problematic because different minerals own different brittleness. According to the elastic properties value, this paper defines the brittleness level of each mineral in shale gas reservoir. And then, a new method for brittleness evaluation can be proposed, as equation (3) shows. RBI=ai*fi

(3)

Where, ai means mineral brittleness factor which shown in figure 3, fi means the volume of each mineral. We can evaluate the brittleness using this equation on the basis of core XRD analysis and/or logging multi-mineral analysis. Figure 4 (Panel 6) shows the relative brittleness index evaluation result by equation (1), (2) and (3). The difference of RBI will result in different evaluation results. It is therefore recognized that whether suitable brittleness indexes are selected will influence the effect of fracture. 2. Elastic properties RBI The previously mentioned positive correlation among brittleness, rock strength, hardness and E, and negative correlation among v and strength, hardness. From the real hydraulic fracturing practice results, engineers have established several empirical relationships between fracturability (they called them brittleness, here means RBI) and rock elastic parameters. Currently, the most prevailing one is from Rickman et al. (2008) as equation (4) shows. It is obvious that the RBI defined here has a positive correlation with Young’s modulus and negative correlation with Poisson’s ratio. This means that engineers are looking for stiffer rocks rather than the softer ones to break them. It is believed that v reflects rock’s ability to fail under stress, and E the ability to maintain a fracture once the rock fractures. Low v and high E indicate more brittle shale. Similarly, Guo et al. (2012), Liu et al. (2013) attempt to use equation (5) as the RBI to evaluate the shale gas reservoir. And Guo et al. (2012) used (λ+μ)/ λ as the definition of rock RBI. In our view, the brittleness indicated by E and v is of greater geophysical significance than other elastic parameters, thus, combined with Rickman and Guo’s method, a new formula explaining the difference of brittleness indexes is proposed, as equation (7) shows. RBI=0.5(Y_BRIT+P_BRIT) RBI=(λ+2μ)/λ

(4) (6)

RBI=E/υ RBI=Y_BRIT/P_BRIT

(5) (7)

Where, Y_BRIT= (E-1)/(8-1) ，P_BRIT=(υ-0.4)/(0.15-0.4) Figure 4 illustrates the calculated results of different RBI. Equation (6) (5) (4) (7) are shown in panel 8, panel 9, panel 10, panel 11 respectively. Here the background of red stem and curve denote the RBI (called Mineral RBI) are defined as equation (3) which are regarded as the standard brittleness index in the process of reservoir evaluation. We can see that the new RBI is more agreeable with the Mineral RBI than the other RBI. Figure 5 shows that the value trend of the new RBI is equal to RBI defined by equation (5), which is also similar as equation (4) defines. The trend of all these three RBIs is opposite to RBI defined by equation (6). Figure 6 analyzes the sensitivity of different elastic properties and RBI for siliceous shale （Quartz content=25%） and calcareous shale (Calcite content=25%). Sensitivity here refers to the capacity of 76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

a parameter to differentiate two reservoirs, which can be calculated by equation (8). As can be seen from this figure, the sensitivity of new RBI is higher than the other RBI. Obviously, the new RBI will improve the efficiency of shale gas reservoir prediction and characterization. Sensitivity=|A1-A2|/( A1+A2) Where, A1 and A2 denote the same RBI of siliceous shale and calcareous shale.

(8)

Conclusion Based on the understanding of brittle and ductiles’ definition, this article proposes a new relative brittleness index using the composition of rock which takes the brittleness difference of each mineral into consideration. Meanwhile, a new more sensitive elastic relative brittleness index is proposed for reservoir characterization through the pre-stack methods. Figure 6 The sensitivity analysis of different RBI for Siliceous Shale and Calcareous Shale. RBI1, RBI2, RBI3, and RBI_New are shown in Equation 4.5.6.7 respectively

Figure 2 The relationship among brittle, elastic properties (Young’s modulus (E) and Poisson’s ratio (v)), UCS, Hardness, and Mineral Composition. The data is from different literatures. (a) Brittle versus UCS plot, Brittle=Compressive strength/Ductile strength; (b) Brittle versus E plot; the data in 76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

(a) and (b) comes from Altindag（2002） ;(c) E versus UCS plot; (d) v versus UCS plot; (e) E versus Hardness plot; (f) v versus Hardness plot. In (c)(d)(e)(f), the data of shale2 comes from Chatterjee et al.(2013), and others data come from Yasar and Erdo an(2004); (g) UCS versus the volume of each minerals; (h) Hardness versus the volume of each minerals. In (g) and (h), the data of Quartz2 com from Sabatakakis(2008), others come from Chatterjee et al.(2013).

Figure 3 The brittleness level of each mineral

Figure 4 Brittleness evaluation profile for H well in the lower Jurassic formation in North China. From left to right panels are: depth, logging lithology, gas bearing formation, gas logging curves (Hydrocarbon and CH4), measured TOC and calculated TOC, XRD minerals, RBI based on the XRD minerals, different elastic properties RBI comparison with the XRD minerals RBI respectively.

Figure 5 Comparison of different RBI for each mineral. The Y-axis means the normalized of each RBI. 76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

Reference Altindag, R. [2002] The evaluation of rock brittleness concept on rotary blast hole drills. The Journal of the South African Institute of Mining and Metallurgy, 61-66. Chatterjee, R. Manoharan, K. and Mukhopadhyay, M. [2013] Petrophysical and mechanical properties of cretaceous sedimentary rocks of Cauvery basin, Eastern continental Margin of India. J.Ind. Geophys. Union,17(4): 349-359. Guo, Z. Chapman, M. Li, X. [2012] A shale rock physics model and its application in the prediction of brittleness index, mineralogy, and porosity of the Barnett Shale. 2012 SEG Extended Abstract. Liu, Z. Sun, S. Sun, Y. et al. Formation evaluation and rock physics analysis for shale gas reservoir: a case study from China south. 2013 EAGE annual meeting, London, England. Mavko, G. Rock physics of Shale. Report of Stanford Rock Physics Laboratary. Rickman, R. Mullen, M. Petre, E. et al. [2008] A practical use of shale petrophysics for stimulation design optimization: All shale plays are not clones of the Barnett Shale. SPE115258. Sabatakakis, N. Koukis, G. Tsiambaos, G. and et al. [2008] Index properties and strength variation controlled by microstructure for sedimentary rocks In earth science, ductility refers to the tendency of rock to deform to large strains without macroscopic fracturing. Engineering Geology, 97: 80-90. Yasar, E. and Erdogan, Y. [2004] Estimation of rock engineering properties using hardness tests. Engineering Geology, 71: 281-288.
