Fracability Evaluation Research Articles

An integrated multi-scale fracability evaluation method for tight sandstone reservoir

Fracability evaluation of unconventional oil/gas reservoirs has become a general practice during the hydraulic fracturing job design to identify the best candidate pay zone. The brittleness of the reservoir rock was first used to characterize the capability of the reservoir rock to form a complex fracture network. Later, other factors like fracture toughness, in-situ stress, and natural fracture parameters were introduced in combination with brittleness to more comprehensively describe the fracability of the reservoir. However, most of the existing fracability evaluation has been performed for single wells. In contrast, the three-dimensional distribution of fracability within reservoirs was less pursued, though it might be beneficial for guiding the well placement.This study developed an integrated multi-scale fracability evaluation method for tight sandstone reservoirs based on three-dimensional geomechanical analysis. Factors like brittleness, fracture toughness, in-situ stress contrast that influence the hydraulic fracture initiation and propagation at different scales were first identified and analyzed. Then methods of deriving these quantities from well logs and three-dimensional seismic data were summarized. Finally, an integrated index incorporating these quantities was defined to differentiate the so-called engineering sweet-spot zones favorable for stimulation. A typical application of this method is the integrated multi-scale fracability evaluation method applied to a tight sandstone reservoir located in China’s south sea oil field, which demonstrated the effectiveness of the method. In addition, some implications for improving fracturing operations of tight sandstone reservoirs have been provided.

Fracability Evaluation Method and Influencing Factors of the Tight Sandstone Reservoir

Fracability evaluation is the basis of reservoir fracturing and fracturing zone optimization. The tight sandstone reservoir is characterized by low porosity and low permeability, which requires hydraulic fracturing to improve industrial productivity. In this study, a systematic model was proposed for the fracability evaluation of tight sandstone reservoirs. The rock mechanics tests and sonic tests demonstrated that tight sandstone reservoir is characterized by high brittleness, high fracture toughness, and weak development of natural fractures. Numerical simulation was used to analyze the change of reservoir parameters during hydraulic fracturing and the influence of in situ stress on fracture propagation. The results showed that when the horizontal stress anisotropy coefficient is small, natural fractures may lead hydraulic fractures to change direction, and complex fracture networks are easily formed in the reservoir. The horizontal stress anisotropy coefficient ranges from 0.23 to 0.52, and it is easy to produce fracture networks in the reservoir. A new fracability evaluation model was established based on the analytic hierarchy process (AHP). The fracability of tight sandstone reservoir is characterized by the fracability index (FI) and is divided into three levels. Based on the model, this study carried out fracability evaluation and fracturing zone optimization in the study area, and the microseismic monitoring results verified the accuracy of the model.

Novel method and case study of a deep shale fracability evaluation based on the brittleness index

Deep shale has gradually become a focus point for unconventional natural gas exploration and development in China and even the world. The key to deep shale development is the fracability evaluation of a reservoir; conventional evaluation methods cannot provide accurate evaluations of deep shale fracability under high confining pressures. In this study, experimental methods are used to obtain the mechanical parameters, mineral composition, and stress–strain characteristics of shale. The fracture complexity coefficient based on the fractal dimension and fracture angle is defined by analyzing the fracture morphology of shale samples. Using the rock fracturing complexity as an index, the weight coefficients of different factors for the shale fracture morphology are obtained, thus establishing a new evaluation model for the deep shale brittleness index. The results show that the Young's modulus, dilatancy angle, and peak strain are the main factors affecting the complexity of shale fractures. The gray correlation theory is used to calculate the weights of various factors in the model. The weight coefficients of the Young's modulus, dilatancy angle, and peak strain on the rock brittleness index are 0.262, 0.353, and 0.385, respectively. Based on this, a novel method for evaluating the fracability of deep shale using the brittleness index is proposed. The fracability index of the horizontal section of the deep shale gas well in this area was calculated, and the location of the fracturing perforation cluster was optimized accordingly. This technology has been successfully applied in more than 10 deep shale gas wells in the southern Sichuan Basin, effectively reducing the fracturing pressure by 5–10 MPa, increasing the average daily production of a single well by 15.3 × 104 m3. This paper proposes a novel fracability evaluation method for deep shale based on the brittleness index, which provides a reference for optimizing deep shale fracturing design.

Fracability Evaluation Method for Tight Sandstone Oil Reservoirs

Fracability Evaluation Method for Tight Sandstone Oil Reservoirs

Study on a New Rock Fracability Evaluation Model of Shale Gas Reservoir

Shale gas is an important unconventional energy resource that needs large-scale fracturing to form industrial deliverability. The evaluation of reservoir fracability plays a key role in the optimization of the sweet spot, the design of multistage fracturing, and the prediction of economic benefit. Based on volumetric fracturing, the study proceeded from the fracture complexity of the fractured core, and the bursting pressure experiment technology using the constant strain rate method was established. After the core has fractured, the fracture morphology was extracted and the fracture parameters including fracture area ratio and fracture declination dispersion were calculated to construct the fracture complexity of the pressed core. Combined with the core strength, the fracability index of the core was determined to evaluate the reservoir fracability. This method can represent not only the fracturing effect but also the fracturing difficulty. Compared with the monitoring data of hydrofracture-induced microseism of the sample well, the core fracturing index was found to be in good agreement with the actual fracturing effect. This method is more reasonable than the traditional brittleness index method and rock mechanics parameter method.

A NEW EVALUATION PROCEDURE OF ROCK FRACABILITY USING CLUSTER ANALYSIS OF WELL-LOGGED PETROPHYSICAL PROPERTIES OF FACIES

The authors present a new rock fracability evaluation procedure using the cluster analysis of log data on petrophysical properties of facies. The effect of the physical and mechanical properties of rocks on the fracability evaluation results is analyzed in combination with the geophysical log data. The triaxial compression tests of cores are carried out to determine their brittleness indices. An entry-level classification of petrophysical properties of rock facies is implemented by the cluster analysis of the geophysical log curves. A new classification procedure is proposed for the petro facies analysis using the permeability index and brittleness index of rocks, and the profile of rock fracablity index is obtained. Application of the procedure is illustrated using core data from a reservoir in China. The fracability index of cores sampled in a horizontal well correlates well with the calculated profile of fracability index.

Investigating the Meso‐Mechanical Anisotropy and Fracture Surface Roughness of Continental Shale

AbstractContinental shale is abundant with unconventional oil and gas and has considerable prospect for exploitation. However, due to deposited environment differences, continental shale presents unique characteristics in terms of structure and mineralogy compared to marine shale. The mechanical property is elemental parameter for reservoir production, but relevant studies of continental shale are rarely reported. In this study, several groups of mesoscale (0.1–10 mm) uniaxial compression tests were conducted on continental shale with various bedding angles, sourced from the Yanchang Formation in the southern Ordos Basin, China. A new integrated real‐time loading‐observation‐acquisition system, specifically, a miniature tensile instrument‐light microscope (MTI‐LM) system, was utilized, which allowed us to visualize the failure process and simultaneously record the stress‐strain curves, providing snapshots for investigation of fracture initiation, propagation, and fracture surface features. The results demonstrate that as the bedding angle relative to the maximum compression direction increases, the uniaxial compressive strength (σc) and the percentage of bedding‐parallel fractures (Fp) fluctuate reversely, namely, higher σc corresponds to lower Fp. Additionally, a good consistency is revealed among the variations of σc, failure time and fracture surface roughness with bedding angles, suggesting stronger shale having rougher fracture surfaces and longer failure time at a certain loading rate. The meso‐mechanical anisotropy of shales is supposed to have an origin in heterogeneous component and microtexture, as indicated by different bridges connecting the opposite facies of fractures. The findings help reveal the mechanism underlying mechanical anisotropy and facilitate fracability evaluation of shallow shale gas reservoir.

A New Evaluation Method for the Fracability of a Shale Reservoir Based on the Structural Properties

The fracability of shale reservoirs is one of the key indicators used for evaluating whether or not the shale can be used as a “sweet spot zone.” It has been determined that the structural properties of rock have important influences on the evaluation of the fracability of reservoirs. In the current study, five rock quality designation (RQD) calculation methods were compared and analyzed for the purpose of selecting an RQDI for characterizing rock structures. Focused on the lack of structural factors included in the previous fracability evaluation methods, a new model for fracability evaluation based on the combination of the brittleness index, structural index, and fracture toughness was constructed using a linear elastic fracture theory. The model showed that good fracability not only included higher brittleness but also required less energy to produce cracks. Meanwhile, good fracability also required more discontinuous structural planes. In the current study, a formation with a higher fracability index was considered to be a fractureable interval and a formation with a lower fracability index was a fracture barrier. Finally, the reservoir fracability index was modeled using the Xike 2 well in the north of Guizhou Province as a case study. Subsequently, a fracability logging evaluation method based on the fracability index model was determined, which will potentially provide a new technical tool for future fracturing optimization processes.

Modified method for fracability evaluation of tight sandstones based on interval transit time

Fracability is a property that indicates how easy reservoir rocks can be fractured in hydraulic fracturing operations. It is a key parameter for fracturing design and evaluation. In order to utilize continuous logging data to predict fracability, synchronous tests of dynamic and static mechanical parameters of rocks under different confining pressures were conducted on 13 tight sandstone samples derived from the central Junggar Basin, China. A modified formula between dynamic and static mechanical parameters was established. Fracability of the tight reservoir in the Junggar Basin was then evaluated based on brittleness index, fracture toughness, and fracability index. The effectiveness of fracturing was analyzed combined with the oil testing curve after hydraulic fracturing. The results show that: (1) The distribution of oil-bearing formations in the studied area coincides well with stratum of higher fracability index. (2) The critical fracability index is determined to be 0.3, three formations are selected as fracturing candidates, and a thin mudstone interbed is identified in the oil-bearing formation. (3) Well testing curve verifies the reliability of the fracability evaluation method and the accuracy of the modified formula between dynamic and static mechanical parameters. This study provides useful information for improving fracturing operations of tight oil and gas reservoirs.

Fracability Evaluation of Shale of the Wufeng‐Longmaxi Formation in the Changning Area, Sichuan Basin

The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization. It makes much sense to study the mechanical properties and deformation characteristics of shale, due to its close relationship with the fracability of shale gas reservoir. This paper took marine shale in the Changning area, southern Sichuan Basin of China as the research object. Based on field profile and hand specimen observation, we analyzed the development of natural fractures and collected samples from Wufeng Formation and Longmaxi Formation. Combining with the indoor experiment, we investigated the macroscopic and microscopic structural features and the remarkable heterogeneity of shale samples. Then we illustrated the mechanics and deformation characteristics of shale, through uniaxial compression test and direct shear test. The shale has two types of fracture modes, which depend on the angular relation between loading direction and the bedding plane. Besides, the Wufeng shale has a higher value of brittleness index than the Longmaxi shale, which was calculated using two methods, mechanical parameters and mineral composition. Given the above results, we proposed a fracability evaluation model for shale gas reservoir using the analytic hierarchy process. Four influence factors, brittleness index, fracture toughness, natural fractures and cohesive force, are considered. Finally, under the control of normalized value and weight coefficient of each influence factor, the calculations results indicate that the fracability index of the Wufeng Formation is higher than that of the Longmaxi Formation in Changning area, southern Sichuan Basin.

A New Fracability Evaluation Approach for Shale Reservoirs Based on Multivariate Analysis: A Case Study in Zhaotong Shale Gas Demonstration Zone in Sichuan, China

Fracability characterizes the effectiveness of hydraulic fracturing. The existing assessment methods cannot reflect the actual value of the effectiveness due to a lack of comprehensive consideration and neglect of the influences of engineering factors. This study aims to solve this problem by implementing geological static data and production dynamic data in multivariate analysis in Zhaotong shale gas demonstration zone. First, the reservoir quality index (RQI) was introduced to evaluate the exploration potential by integrating the geological parameters with gray relational analysis. Moreover, the differences in fracturing fluid types and proppant sizes were considered, and the operating parameters were normalized on the basis of the equivalence principle. Finally, the general reservoir fracability index (GRFI) was proposed based on a dimensioned processing of the various parameters. A case study was conducted to verify the accuracy and feasibility of this new approach. The results demonstrate that (1) the organic carbon and gas content are adjusted to contribute the most to the calculation of the RQI, while the effective porosity contributes the least; (2) the fracturing scale is the main operating parameter determining the fracability, which has the strongest correlation with the effectiveness of fracking; and (3) the GRFI has a positive correlation with shale gas production, and the lower limit of the GRFI of 2,000 corresponds to a daily production of 50,000 m3/d; this value is defined as the threshold value of a stripper well. The GRFI is consistent with the productivity trend of shale gas wells in the research block, which suggests that the new model is accurate and practical for well candidate selection.

Influencing factors and fracability of lacustrine shale oil reservoirs

Abstract Shale oil reservoirs need fracturing in order to achieve commercial productivity. A key factor influencing the fracturing process is shale fracability, which is strictly dependent on rock fabric. In this study, we focused on the lacustrine shale oil reservoirs located in Dongying Sag and in the Northern Jiangsu Basin. Through X-ray mineral diffraction, rock mechanics, and P- and S-wave velocity tests, we analyzed the mineral components and mechanics of different shale lithofacies, as well as the influence of these factors on shale fracability. Based on the results of these analyses, we proposed a new method for the evaluation of lake shale oil reservoir fracability. The experimental results showed that the macrostructure, mineral content, lithology, and the presence of microfractures in the shale are the main factors affecting its brittleness. Shales with lamellar and sheet-like structures have relatively high Young's modulous or lower Poisson's ratios; under these conditions, netted fractures are easily formed during fracturing. The fracture toughness of carbonate minerals in shales is strong. An appropriate amount of carbonate minerals seemed to increase the shale brittleness; however, an excessive amount increased its cohesiveness, hindering the initiation and propagation of fractures. Felsic minerals are characterized by high brittleness, a characteristic that favors shale fracability; on the other hand, clay minerals and organic matter are highly plastic, a characteristic that hinders fracturing transformations. Due to their macrostructure and composition, lamellar clayey limestones (dolomite), lamellar clayey siltstones, layered calcareous (dolomitic) siltstones, sheet-like clay-containing siltstones, and flaggy carbonate-containing siltstones are expected to be easily fractured. In addition, the presence of microfractures seemed to reduce the overall fracture toughness of the rock: they penetrated artificial fractures, enhancing rock fracability. Taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter, we proposed a new formula for the calculation of the brittleness index, based on the mineral component method. The brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method. Overall, our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture, followed by the calcareous (dolomitic) and clayey or organic-rich lithological successions. This study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs. Especially, the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic-rich shale.

The Study on Brittleness Evaluation of Chang 7 Reservoir in Longdong Area in Ordos Basin

Brittleness is very important in the fracability evaluation and exploration of unconventional oil and gas. In this paper, the rock mechanics parameter method and the mineral composition method are used in order to calculate the brittleness index in Chang 7 reservoir in Longdong area in Ordos basin. The results show that the brittleness index calculated by rock mechanics parameters is 44.9%-62.23% and by mineral composition method is 52.5%. The brittleness indexes calculated by the two different methods are not greatly different, and the overall brittleness index is high, which indicate Chang 7 reservoir in Longdong area has high brittleness and good fracability.

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.

Fracability evaluation of lacustrine shale by integrating brittleness and fracture toughness

Rock brittleness and fracture toughness are important parameters for evaluating rock fracability. The stress-strain curves indicate that the lacustrine shale is strongly brittle. Brittle failure occurs rapidly when the stress of the lacustrine shale reaches its peak value. In addition, the lacustrine shale has different plastic strains before and after peak stress; this can relax the stress concentration of the crack tips. Therefore, the plastic strain that occurs before and after the peak stress can cause decreasing brittleness, which can be used to distinguish the brittleness and fracability in the formation of the lacustrine shale clearly. Moreover, this further enlarges the difference in the brittleness index. Based on the influence of plastic strain on brittleness, we have developed a new brittleness evaluation method that uses the ratio of linear elastic strain energy to the total strain energy before complete rock failure, which can indicate the difference of the lacustrine shale clearly. Fracture toughness is another important parameter that impacts the fracture extension and influences fracability. Based on the impact of brittleness and fracture toughness on the fracability, we have developed a new fracability evaluation method. The brittleness index increases with increases in the quartz content, and it decreases with increases in the albite feldspar and calcite contents. The fracture toughness decreases with increases in the quartz and clay contents, and it increases with increases in the siderite content. In addition, we established an empirical formula that can evaluate the brittleness index and the fracture toughness using mineral contents obtained from elemental logging. Using the new fracability evaluation method to optimize the fracturing stage, the preliminary field test indicates that the new approach was effective in the lacustrine shale formation.

A comprehensive approach for fracability evaluation in naturally fractured sandstone reservoirs based on analytical hierarchy process method

AbstractThe tight sandstone gas reservoir in southern Songliao Basin is naturally fractured and is characterized by its low porosity and permeability. Large‐scale hydraulic fracturing is the most effective way to develop this tight gas reservoir. Quantitative evaluation of fracability is essential for optimizing a fracturing reservoir. In this study, nine fracability‐related factors, particularly mechanical brittleness, unconfined compressive strength (UCS), mineral brittleness, cohesion, internal friction angle (IFA), natural fracture, fracture toughness, horizontal stress difference, and fracture barrier were obtained based on a series of petrophysical and geomechanical experiments. Taking above factors into consideration, a modified comprehensive evaluation model is proposed based on analytic hierarchy process (AHP) method. The UCS and IFA were removed from the AHP model based on the results of factor correlation analysis. The transfer matrix in the weighting procedure was applied to improve the consistency of judgment matrix, and the fuzzy matrix was employed to promote the objectiveness of final decision. The fracability evaluation of four reservoir intervals in Jinshan gas field was analyzed. Field fracturing tests were conducted to verify the feasibility of the proposed evaluation model. Results showed that the tubing pressure curve is more fluctuated in the reservoir interval with more developed natural fractures, and gas production is higher in the reservoir interval with greater fracability coefficient. The field test data coincide with the results of the proposed evaluation model.

Application of catastrophe theory to fracability evaluation of deep shale reservoir

Fracability evaluation is one of the great challenges during the shale gas development, which has become a hotspot for the exploration of unconventional oil and gas. Although much research work was conducted and some evaluation models were developed, most of them could not be applied to practical engineering operations for deep shale gas development. In this paper, a multi-level sub-system was developed based on the analytic hierarchy process, and then, a catastrophe evaluation model, avoiding over-subjective in defining the weighting coefficient of factors, was proposed to evaluate deep shale reservoirs’ fracability by using the catastrophe theory. Finally, the model was verified by using engineering data in five deep shale gas wells. The analytic results showed that the calculated results of the catastrophe evaluation model are in good agreement with the facts compared with the analytic hierarchy process. Moreover, the general expression for the potential function and its normalization formula of the common catastrophe model with arbitrary dimensions could also be derived recursively to meet practical applications. Therefore, the catastrophe evaluation model was simple and suitable for the wide application, which provides new sights for the pre-fracturing prediction and post-fracturing evaluation during deep shale reservoirs development.

A practical fracability evaluation for tight sandstone reservoir with natural interface

Hydraulic fracturing has been widely applied to enhance the conductivity in tight sandstone reservoirs, i.e. reservoirs with low porosity and permeability. The interaction between a hydraulic fracture (HF) and a natural fracture (NF), including crossing, arresting and opening (tensile or dilation), are crucial for controlling the fracability of a reservoir. Previous studies have elucidated that shear dilation is the main mechanism for enhancing the permeability of an unconventional reservoir. Moreover, the brittleness index (BI) is considered another critical parameter that controls the fracability of candidates. However, the fracability of candidates with respect to both shear dilation and BI have not been fully investigated. We performed a practical fracability evaluation by integrating the shear dilation mechanism and BI quantification. We obtained the mechanical parameters from mechanical tests conducted on synthetic tight sandstone samples, and we manufactured specimens with different friction coefficients and shear strengths. Next, we performed scaled hydraulic fracturing experiments on 15 identical 10 cm cubic samples using a true tri-axial stress cell, and the interaction mechanisms between HFs and NFs were investigated. We also evaluated the brittleness of each specimen based on a previous BI model and our own novel BI model. We found that a weak interface cohesion with an interaction (between HF and NF) angle of 60° exhibited a shear dilation (or reactivation) mechanism and a higher BI. We thus conclude that such conditions are more favourable for reservoir stimulation (i.e. hydraulic fracturing) in the field.

Brittleness evaluation of the inter-salt shale oil reservoir in Jianghan Basin in China

Abstract The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of the inter-salt shale from the fourth member of the Qianjiang Formation were tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ condition. An integrated investigation was conducted on the brittleness evaluation of inter-salt shale from the perspective of core characteristics, brittle mineral components, elastic parameters and the complete stress–strain characteristics of rocks. The results indicate that the inter-salt shale exist with bedding planes and glauberite, which leads to high anisotropy and heterogeneity. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, the bedding plane structure and the fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the logging data and the laboratory data in order to obtain the brittleness index profile along the wellbore. The post-peak stress-strain relations indicates the rock has high residual strength at confined stress state, which explains that the rock is damaged through shear failure along the bedding plane and fails to generate complex fractures. This study would provide theoretical basis for fracability evaluation model used in the inter-salt shale oil reservoir.

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.