Brittle Shale Research Articles

Quantitative seismic interpretation of rock brittleness based on statistical rock physics

Rock brittleness is one of the important properties for fracability evaluation, and it can be represented by different physical properties. The mineralogy-based brittleness index (BIM) builds a simple relationship between mineralogy and brittleness, but it may be ambiguous for rocks with a complex microstructure; whereas the elastic moduli-based brittleness index (BIE) is applicable in the field, but BIE interpretation needs to be constrained by lithofacies information. We have developed a new workflow for quantitative seismic interpretation of rock brittleness: Lithofacies are defined by a criterion combining BIM and BIE for comprehensive brittleness evaluation; statistical rock-physics methods are applied for quantitative interpretation by using inverted elastic parameters; acoustic impedance and elastic impedance are selected as the optimized pair of attributes for lithofacies classification. To improve the continuity and accuracy of the interpreted results, a Markov random field is applied in the Bayesian rule as the spatial constraint. A 2D synthetic test demonstrates the feasibility of the Bayesian classification with a Markov random field. This new interpretation framework is also applied to a shale reservoir formation from China. Comparison analysis indicates that brittle shale sections can be efficiently discriminated from ductile shale sections and tight sand sections by using the inverted elastic parameters.

Mineral composition and brittleness evaluation of shale gas reservoir in the early Cambrian Qiongzhusi formation of Huize area, Eastern Yunnan Province

The brittleness of shale gas reservoir determines the effect of fracturing reconstruction, which is of great significance for improving shale productivity. The study of shale brittleness evaluation can provide basic data for fracturing design. The mineral composition characteristics of shale gas reservoir in the early Cambrian Qiongzhusi formation of Huize area, Eastern Yunnan Province are reported in this paper, and the brittleness is evaluated according to the proportion of brittle minerals in total minerals. The results show that the mineral composition of the reservoir is complex. After comparing the brittleness index calculated by different formulas, it is considered that the minerals with high young’s modulus and low poisson’s ratio should be considered as brittle minerals, which is conducive to the universality of the brittleness index formula and the objectivity of reservoir brittleness evaluation. Shale gas reservoir of Qiongzhusi formation is obviously divided into two sections: the layers of 1 to 7 with a large brittleness index (77.60% on average) on the lower part are beneficial to reservoir fracturing reconstruction, while the layers of 8 to 14 with a small brittleness index (37.78% on average) on the upper part are not conducive to reservoir fracturing reconstruction. This study is conducive to guiding further exploration and development of shale gas in the research area.

Experimental investigation on rock structure and chemical properties of hard brittle shale under different drilling fluids

The physical and chemical properties of hard brittle shale and its mechanical properties under different drilling fluid systems are of great practical significance to the in-depth study of shale borehole stability. Microscopic geological characteristics of hard brittle shale and the changes of physical and chemical properties after soaking different drilling fluids are revealed. After soaking in different drilling fluid systems, the mechanical properties have changed. After soaking in distilled water, water expands along micro-cracks due to the structural composition of rocks, eventually lead to cracking along the structural plane. After soaking in polysulfonate drilling fluid, the sample is destroyed and the strength of rock sample is weakened. After soaking in organic salt drilling fluid, the strength of rock sample weakens, drilling fluid fills the fracture surface to a certain extent. After soaking in oil-based drilling fluid, the strength of rock sample weakens, and oil-based drilling fluid fills and closes the fracture surface. The research results are of great significance to the analysis of borehole instability mechanism of hard brittle shale and drilling fluid optimization of shale gas drilling.

An Evaluation Method of Brittleness Characteristics of Shale Based on the Unloading Experiment

Shale reservoir has an initial unloading effect during the natural uplift and erosion process, which causes the shale brittleness to change, affecting the design of the fracturing scheme. To consider this, the axial compression loading and confining pressure unloading experiment of shale is carried out, and then the influence of unloading rate on the mechanical parameters, failure characteristics, and the brittleness of rock are analyzed. What is more, a new evaluation method of brittleness characteristics that take the unloading effect into consideration is proposed. The conclusions are as follows: (1) The unloading rate has a weakening effect on the mechanical parameters, such as the destructive confining pressure and the residual strength of the samples. (2) The failure characteristics of shale specimens are a single shear failure in an oblique section under low unloading rate, and multiple shear zones accompanied with bedding fracture under high unloading rate. (3) The brittleness of shale samples is well verified by the brittleness index B d 1 and B d 2 during the loading path; nevertheless, it has shortage at the unloading path. This paper proposes a new brittleness evaluation method which can consider the influence of the different unloading rates and unloading points. Furthermore, there is a nice characterization between the brittleness damage and this method.

Key Drilling Fluid Technology in the Ultra Deep Section of Well Ying-1 in the Shunbei Oil and Gas Field

Well Ying-1 is an ultra-deep key investigative risk management pre-exploration well deployed by Sinopec in the Shunbei Oil and Gas Field with designed well depth of 9 016.85 m (TVD 8 603.00 m). Drilling the welln encountered downhole problems including wellbore instability, well leakage and borehole wall sloughing. They also easily occur in drilling hard brittle mudstone formations, such as the Silurian Kepingtage Formation and the Ordovician Sangtamu Formation. For this reason, laboratory studies have been carried out to analyze mechanisms contributing to the instability of large section of hard brittle mudstone shale. As such, the reasons for leakage in the Silurian high pressure sensitive fissured formation, and the reasons for instability in Ordovician fractured formations. Based on the basic theory of multivariate synergistic” wellbore stability, the drilling fluid system SMHP-1 with strong inhibition and sealing capacity was constructed, and the technical measures of mud loss and borehole wall collapse prevention were worked out. The well successfully drilled through a large section of hard brittle mudstone and broken formation to a total depth of 8 588 m without borehole wall instability and drilling fluid loss, setting a record of the deepest onshore well depth in Asia. The field application showed that the drilling fluid system SMHP-1 could effectively solve the problems of wellbore stability and leakage in deep mudstone and broken formations by strong inhibition and sealing capacity, and provide best practices for safe drilling of deep or ultra-deep wells at home and abroad.

Modelling of Time-Dependent Wellbore Collapse in Hard Brittle Shale Formation under Underbalanced Drilling Condition

In recent years, the lithologic traps in a mid-depth formation are the focus of oil or gas exploration and development for eastern oilfields in China. The Shahejie Formation develops thick hard brittle shale, and the wellbore instability problem is prominent due to obvious hydration effect for long immersion time during drilling. Through the analysis of laboratory tests and field test results of physical and chemical properties and microstructure and mechanical properties of hard brittle shale, the instability mechanism is discussed for the wellbore in the shale formation. To simulate the whole process of progressive collapse of a wellbore in a hard brittle shale formation, a coupled hydraulic-mechanical-chemical (HMC) model is developed and this model is compiled with ABAQUS software as the solver. Then the coupled HMC model is applied to simulate the progressive evolution process of wellbore collapse in a hard brittle shale formation, and the influence of different parameters on the progressive failure of the wellbore is analysed. The results show that the wellbore enlargement rate increases with the drilling fluid immersion time and the influence of different parameters on the wellbore enlargement rate is different. The water absorption diffusion coefficient and the activity of the drilling fluid have the most obvious influence on the expansion of the wellbore, and the sensitivity is strong. The permeability of shale has little effect on the wellbore enlargement rate. The calculated progressive failure process of the wellbore is basically consistent with that of the actual drilling.

A fully coupled simulation model for water spontaneous imbibition into brittle shale

Water spontaneous imbibition (WSI) induces complicated multi-physical phenomena in shale gas reservoir after the hydraulic fracturing, including the shale matrix swelling, the micro-crack initiation, and the changes of Young's modulus and permeability. Since these phenomena were not fully described in most of the previous numerical models, in this study, a numerical simulation model coupled of WSI, clay-swelling-induced shale deformation and micro-crack initiation was established, solved through a proposed calculation procedure, and validated with the experimental data on shale samples from Lower Silurian Longmaxi formation, Sichuan Basin, China. In addition, the effects of WSI on shale properties and shale gas production were quantitatively studied through four numerical cases. The results show that our proposed model is able to describe the micro-crack generation around the interface between the clay and non-clay minerals due to their different swelling behaviours. Moreover, there are great descrepancies in the evolutions of Young's modulus and permeability between the laboratory and in-situ conditions during the WSI, due to the different boundary conditions, direction of the maximum principle stress and swelling strain, resulting in a large difference in shale gas recovery rate. Consequently, the difference between the laboratory and in-situ conditions should be taken into account to analyse the effect of WSI on shale gas production.

Loading rate effect on the mechanical behavior of brittle longmaxi shale in nanoindentation

Understanding the properties of sedimentary rock shale is critical for wellbore stability and hydraulic fracturing design in unconventional shale gas extraction. The mechanical properties of shale are a function of various environmental factors, such as stress, temperature, saturation and loading rate. Thus, in this study, the loading rate effect on the mechanical behavior of shale using nanoindentation was investigated. Nanoindentation tests were performed at a peak load of 500 mN and loading rates of 5–30 mN/s. In addition, X-ray diffraction (XRD) analysis was performed to identify the composition of the selected shales, and reveal the target sample is brittle. Scanning electron microscopy (SEM) analyses were performed to determine the surface morphology and demonstrate the existence of pile up and sink-in events. The results of the nanoindentation study indicate that the material behavior of rock depends on the loading rate at the nanoscale: with the increase in the loading rate, the contact hardness, Young's modulus, yield stress and indentation shear stress gradually increase. However, final penetration depths, the contact penetration depths, the maximum penetration depths and the contact stiffness decrease with increasing loading rates. Pop-ins were imperceptible at both the low and high loading rates, reflecting that the shale is brittle. Comparison of the pressure hardening coefficient in the elastic–plastic yield criterion model and the strain hardening coefficient in the strain rate sensitivity model indicates that shale is more sensitive to pressure hardening than strain hardening. A discussion regarding the uncertainty of evaluating the mechanical properties via nanoindentation indicates that the contact surface determination and the loading rate control are both critical to obtain a meaningful value for the mechanical parameters.

Design and application of green drilling fluid of strong inhibition

Borehole instability problem is always common when intersecting the brittle shale formations using either water-based mud or oil-based mud. An extensive wellbore stability drilling fluid was carried out mainly focused on integrating a variety of anti-collapse technology and the rigorous environmental protection. In this paper, a set of the environmental friendly drilling fluid system of strong inhibition was designed and developed based on the multi-element concept of inhibition, plugging, flocculating and environmental protection, combined with the characteristics and brittle shale and the special environmental requirements. In this system, KCOOH and SIAT are taken as the inhibitors, PAC-LV as the fluid loss agent, KPAM as the flocculant and nano-barite as the weighting and plugging material to improve the quality of mud cake and reduce the solid content of the system.

Study on micro-scale properties of cohesive zone in shale

Cohesive fracture model is well known for obtaining a concise interpretation on the underlying mechanism of failure in the near-crack-tip field, especially in concrete and rock materials. In this study, the micro-scale properties of the cohesive zone in brittle shale are investigated. Micro-scale measuring method is designed to capture the crack opening displacement. A small apparatus is developed to induce the ongoing subcritical crack within double-cantilever-beam specimen in environmental scanning electron microscope, by which successive in-situ images of crack propagation are acquired. Micro-scale crack opening displacement of Mode I fracture is either directly measured or measured by digital image correlation method. By comparing experimental results and analytical solutions of micro-scale crack opening displacement, the fitting of cohesive zone models is performed. Based on numerous observations of results, the concept of cohesive-zone-model unit is introduced to provide a better understanding of the essence of cohesive stress. The mechanical properties of cohesive-zone-model unit is simulated using a two-dimensional particle flow code. The simulation results conform to the fitting results. Taking both the fitting and the simulation results into consideration, the traction-separation curve based on cubic polynomial law provides the best interpretation of the micro-mechanical properties of cohesive zone in shale. The introduction of cohesive-zone-model unit gives a better interpretation on the essence of cohesive stress in brittle material with small-scale flaws.

Triaxial tests on weak planes damage of hard brittle shale of Longmaxi formation in South Sichuan Basin, China

Triaxial tests on weak planes damage of hard brittle shale of Longmaxi formation in South Sichuan Basin, China

Triaxial tests on weak planes damage of hard brittle shale of Longmaxi formation in South Sichuan Basin, China

In petroleum field, rock mechanical property is a significant parameter for design of drilling operation. Shale formation has high risk of wellbore instability during drilling. To reduce risk in drilling, more investigations on shale should be conducted. Therefore, in this paper, by using X-ray diffraction and electron microscope scanning test, the mineral composition and microstructure of brittle shale of Longmaxi formation have been analysed. Based on the triaxial compression tests, rock mechanical parameters in variable time have been discussed when shale is in the external force condition. In particular, considering shale failure along weak plane, changes in shale mechanical properties, like stress, strain, structure integrity and elastic parameters, have been analysed. This work offers comprehensive exploration on shale mechanical properties. Mechanical parameters of shale in this study can provide reference for engineering design, and more importantly, establish the foundation for investigation of rock damage in the future.

Authigenic embrittlement of marine shale in the process of diagenesis

Studies on the origin of shale brittleness are of great significance to understanding shale gas accumulation laws. The current studies, however, mostly focus on the improvement of shale brittleness by biological quartz enrichment, but rarely on the recrystallization of quartz in the process of diagenetic evolution. In this paper, a series of researches were carried out on the Lower Silurian Longmaxi Fm marine shale in the Changning block of the Sichuan Basin. Firstly, shale pore, mineral component and bedding characteristics were observed by means of cathodoluminescence (CL), scanning electron microscope (SEM) and so on. Then, the control effects of quartz origin, overpressure environment and diagenetic evolution on shale brittleness were investigated by analyzing the mineral composition and siliceous composition of shale microscopically. Finally, the authigenic embrittlement model of marine shale was put forward and the geological significance of authigenic microcrystalline quartz to the improvement of shale brittleness in the process of diagenesis was illustrated. The following results were obtained. First, during the sedimentation, burial and diagenetic evolution, shale experiences diagenetic changes which help increase quartz content and form concealed fracture networks, thus benefiting the formation of complex flow pathways in later stimulation. Second, organic matters play a positive role in improving shale brittleness in the process of geological evolution. It promotes the formation of authigenic microcrystalline quartz and improves the brittleness of shale. In conclusion, due to the authigenic embrittlement of shale in the process of diagenesis, rock structure is changed, rock strength is enhanced and shale porosity is preserved. Consequently, reservoir space in shale is improved and the enrichment state of shale gas is modified. Therefore, the preservation condition under the control of tectonic movement factors is vital to geological evaluation on shale gas. In addition, marine shale is certainly well brittle after authigenic embrittlement in the process of diagenesis as long as its organic matter content is abundant.

Validation of shale brittleness index calculation from wireline log of well BETRO-001 by using XRD test results and uniaxial test as parameters for determining potential of shale hydrocarbon - brown shale of Pematang Group Formation, Central Sumatra Basin, Bengkalis Trough

The parameter of the shale brittleness index is the key to success in hydrocarbon shale stimulation, as in Barnett Shale, Texas, USA, where brittleness is controlled by quartz content. Based on the mineral content of the shale, the ductility (opposite of brittleness) is controlled by the content of clay and calcite. The Brittleness index determines the value of ductile and brittle a rock formation, based on its mineral content. Brittleness of hydrocarbon shale, needed for initiation and propagation of hydraulic fracturing and fracture re-opening. Shale fractability can be evaluated based on geomechanical and mineralogical evaluations for optimization of stimulation planning. Laboratory tests on shale samples from the Brown Shale Formation, Pematang Group in the Central Sumatra Basin, were carried out to obtain elastic properties of rocks, such as Young’s Modulus, Poisson’s ratio, and unconfined compressive strength. Furthermore, mineralogical analysis is carried out to predict brittleness based on the results of the XRD test, which produces mineral content in rocks. Brittleness analysis is very important for hydraulic fracturing stimulation planning, and to get a valid calculation result from BETRO-001 well log data which is dynamic data, it must be validated using static data, namely from uniaxial compressive tests on in-situ rock cores, but because of the difficulty of obtaining core data from BETRO-001 well drilling and other wells in the research location, a coring approach from analog outcrops is considered to represent the target formation. From the results of prospect criteria according to Matt McKeon (2013), it can be concluded that the Brown Shale Formation in Bengkalis Trough has good hydrocarbon shale potential and can be carried out for further research, because oil production from conventional Indonesian reservoirs in general has experienced a significant decline.

Drill-in fluid loss mechanisms in brittle gas shale: A case study in the Longmaxi Formation, Sichuan Basin, China

Brittle gas shale consists of a high content of brittle mineral and developed natural fractures and has become the focus of petroleum industry in recent years. Lost circulation may easily occur due to the long soaking time and an overbalanced pressure difference during the drill-in process of the horizontal long section. The dynamic fracture width simulation induced by drill-in fluid invasion, plugging zone pressure containment test, friction coefficient test between lost circulation materials and shale surface, oil based drill-in fluid filtrate immersion test were conducted to investigate the drill-in fluid loss mechanisms in the brittle Longmaxi Shale in the Sichuan Basin of China. Results show that the fracture width at the fracture mouth increased from 2 μm to 239 μm for the two mutually perpendicular fractures connecting to the wellbore due to drill-in fluid invasion. Effective plugging for dynamic fractures was failed to be achieved for the original drill-in fluid. Fracture width increase induced by drill-in fluid invasion is the major cause of plugging failure of brittle gas shale. Instability of the brittle shale induced by oil phase lubrication and high pH drill-in fluid erosion further exacerbates lost circulation problems. Optimized LCMs considering the dynamic apertures of fractures should be selected and added into drill-in fluid to rapidly seal pre-exiting fractures and induced fractures for lost circulation control in brittle shale. The results serve as a guide for the drill-in fluid design of shale gas in brittle gas shale formation.

Experimental study of the effect of liquid nitrogen pretreatment on shale fracability

When cryogenic liquid nitrogen (LN2) comes into contact with deep reservoirs, it will bring about serious thermal damage to rock pore structure. Therefore, LN2 pretreatment on reservoirs can contribute to further improving the stimulated reservoir volume (SRV) during large-scale hydraulic fracturing. To investigate the effect of LN2 pretreatment on the SRV of shale reservoirs, a series of physical and mechanical experiments related to shale reservoir fracability were conducted. A quantitative fracability evaluation model suitable for different confining pressures was developed from the perspective of fracturing mechanisms. The changes in shale fracability due to LN2 pretreatment were discussed in detail. The responses in P-wave velocities and T2 spectrum curves demonstrate that shale pore structure is subjected to serious damage after freeze-thaw cycles, and some micro-pores are aggregated to form micro-cracks or even macro-cracks. The decreases in tensile, compressive and shear strength and elastic modulus show that the mechanical properties of shales are deteriorated due to cryogenic damage effect. The changes in physical and mechanical properties of shales reveal respectively that LN2 pretreatment can lead to a slight reduction in shale brittleness, a significant improvement in micro-crack development, a moderate decrease in stability of the bedding planes, a relative increase in the propagation ability of fractures. Based on the result of the analytic hierarchy process, it can be concluded that shale fracability is efficiently enhanced due to LN2 pretreatment, especially under high confining pressure condition. Finally, the hydraulic fracturing experiments under tri-axial stress were carried out to further validate the feasibility of LN2 pretreatment to improve the SRV of shale reservoirs. Compared with the experimental results without LN2 pretreatment, it is found that the initiation pressure and initiation time decline by about 54% and 60% respectively. Both the size and complexity of the SRV of shales are enhanced obviously.

Formation and occurrence of organic matter-hosted porosity in shales

Porosity within organic matter (OM) is considered to be the main site for gas storage (via fluid phase saturation of the pores and sorption on the pore walls) and thus its presence or absence is very critical for in-place gas assessment for organic-rich rocks. Numerous workers have suggested that OM-hosted porosity increases with thermal maturity mainly related to the process of bituminized organic matter cracking to gas. Comprehensive reviews of published literature enable us to conclude that organic porosity dominantly develops within bituminized organic matter (i.e., that portion that is petrographically identified, mainly based on its morphology, as solid bitumen (Mastalerz et al., 2018)) and primary (i.e, structured or amorphous) organic matter (kerogen) is mostly deficient in porosity. We show that in the same shale sample, structured kerogen shows no porosity whereas solid bitumen contains abundant porosity. It has been previously reported that sample preparation for SEM by ion milling may alter the organic matter. In this study, some adverse effects of ion milling have been observed by comparing SEM-visible pores of the same sample prepared by mechanical grind polish (MGP) and ion milling (IM) methods. Since solid bitumen is more labile, and probably more prone to chemical/physical alterations, we recommend that SEM observations of ion milled samples to be conducted with more attentiveness.We show that 1) MGP is a suitable sample preparation method for SEM observations, 2) the overwhelming majority of SEM-visible organic matter-hosted pores develop within solid bitumen and not in structured organic matter (kerogen), and 3) we reaffirm that shale mineral content is a key factor in preservation or destruction of solid bitumen-hosted porosity.Finally, we propose a conceptual model for the formation, preservation and destruction of OM-hosted porosity for shales. Accordingly, we suggest that, all other factors being equal, a quartz-rich brittle shale will preserve more SEM-visible solid bitumen-hosted (meso and micro) porosity compared to that of a clay-rich ductile shale. Therefore, gas storage capacity of a quartz-rich shale will be relatively high compared to that of a clay-rich shale.

Latest research progresses in network fracturing theories and technologies for shale gas reservoirs

In view of the bottleneck of shale gas development technologies, the latest progresses in such theories on the fields like fracture network fracability evaluation, fracture network propagation mechanism, stimulated reservoir volume (SRV) evaluation and fracturing fluid research & development were analyzed comprehensively in reference to the existing achievements and experiences in North American shale gas reservoir development and based on the probes, practices and cognitions of shale gas development technologies in China since 2005. First, the study on the shale brittleness develops from the definition of mineral and mechanical semi-quantitative threshold measurement to a comprehensive evaluation which integrates rock constituents, elastic mechanics and development characteristics of natural fractures together. Second, the study on the fracture network development develops from the directional extension theory to the simulation of fracture network formation in the stochastic distribution pattern of natural fractures. Third, the study on SRV develops from the micro-seismic monitoring based instrument and technology to the mathematical theory evaluation method dependent on the discrete fracture network and extended finite element. And fourth, the study on fracturing fluids develops from the wide application of slick water (drag reducing water) and a linear gel fracturing fluid system to the development and application of novel fracturing fluids with little or no water. Then, the theoretical and technological challenges were pointed out, including the comprehensive fracability evaluation of reservoir fracture networks, the deep seated shale gas fracturing, the operation curve diagnosis of shale gas fracturing, the shale gas refracturing theory, and the research & development and flowback control of novel fracturing fluids. Finally, the development trend of related technologies was predicted so as to provide a theoretical and technical guidance for the efficient shale gas development in China in the future.

Mechanical Properties and Brittleness of Shale with Different Degrees of Fracturing-Fluid Saturation

The mechanical characteristics of Longmaxi Formation shale with different degrees of fracturing-fluid saturation were characterized by applying triaxial compression tests at a confining pressure of 50 MPa. The test samples were collected from fresh outcrop shale in Dayou, Chongqing, China and the shale brittleness was evaluated based on brittleness-drop coefficient, stress decrease coefficient and softening modulus. The weakening of related rock parameters of shale specimens being immersed in fracturing-fluid for different time periods was studied and discussed. The degree of deterioration of the peak strengths, elastic and softening moduli and brittleness were significant and varied exponentially when the samples were soaked in fracturing-fluid. The samples were found to fail by shear on the whole accompanied by varying degrees of bedding plane cracking. With increase of sample immersion time, the number of shear failure surfaces changes from one to two and finally to more than three. The length and number of cracks parallel to bedding planes increased gradually, however, no cracks were formed perpendicular to the bedding plane even when the shale was soaked for a long time. The weakening of the brittleness and mechanical parameters with sample fracturing-fluid saturation are mainly related to change of stress state at the crack tips caused by hydration swelling, the dissolution caused by alkaline fracturing-fluid and the formation of liquid film on the surface of shale particles, all of which are the results of mechanical–physical–chemical coupling.

An improved evaluation method for the brittleness index of shale and its application — A case study from the southern north China basin

The brittleness index (BI) is an important indicator used to characterize the brittleness and fracability of shale reservoirs. The BI calculation methods based on the brittle mineral and mechanical parameters content of shale are widely used, but there are still some limitations, such as the separation of minerals and mechanics, the inconsistent viewpoints regarding brittle minerals of different scholars, and the equivalent treatments of the brittleness of each mineral. By taking the Permian Shanxi and Taiyuan Formation in the Southern North China Basin (SNCB) as an example, this paper improved the BI calculation method, then examined and applied it. The results show that the weight coefficients of brittleness contributions of various minerals are different because of their marked differences in mechanical properties. By multiplying and summing the mineral contents and their weight coefficients, an improved evaluation method for a shale's BI is proposed. The new BI has more reasonable physical significance and avoids the equivalent treatment of various minerals on the rock brittleness. Applications indicate that when compared with the original mineral BI, most of the new BI decrease, which reduces the artificial optimization of the shale brittleness. The new BI of limestone reduces significantly, which reveals that the brittleness and fracability of limestone should be less than those of shale and sandstone. The roof and floor limestone of shale will prevent induced fractures from propagating across the interfaces between the limestone and shale, and can thus be used as an effective barrier bedding to help the fracturing and formation of the complex fractures in shale sections. The improved BI method is able to evaluate shale brittleness and selecting a favorable fracturing shale section more accurately.