Laboratory study on the mechanical behaviors of an anisotropic shale rock

Shale gas is becoming an important energy source worldwide. The geomechanical properties of shalerocks can have a major impact on the efficiency of shale gas exploration. This paper studied themineralogical and mechanical characteristics of a typical gas shale in Ohio, USA. Scanning electron microscope（SEM） with energy dispersive X-ray （EDX） analyses was employed to measure the microstructureand material composition of the shale rock. The anisotropic behaviors of shale rock, includingcompressive and tensile strengths, were experimentally measured. The characteristics of shale rock werealso studied by nondestructive wave speed measurements. The shale demonstrated strong anisotropicbehaviors with the tensile strengths perpendicular to the bedding plane around 300e360 times of thatparallel to bedding plane. Results of ultrasonic tests indicated that both compression and shear wavevelocities show strong anisotropic patterns. The compression wave speed was the smallest in the directionperpendicular to the bedding plane; while the shear wave speed was the smallest in the directionparallel to the bedding plane. The ratio of wave speed anisotropy is around 1.3e1.4 for compressionwave; the ratio of shear wave speed anisotropy is larger and more diverse compared with thecompression wave anisotropy. This might be related to the larger variability in the frictional adhesivestrength along bedding plane than the compressive adhesive strength. 2015 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

A review on experimental techniques and their applications in the effects of mineral content on geomechanical properties of reservoir shale rock

Understanding the effects of mineral composition on geomechanical characteristics is critical in order to design and optimize the hydraulic fracturing necessary for shale gas reservoir production.Fundamental information is still missing in effects of mineral content and the experimental methodologies used.This paper provided an in-depth assessment of the various experimental methodologies and their applications in the relationship between the mineralogical and geomechanical features of the shale formation.The results revealed that more brittle minerals increase their strength,but chemical reaction that creats pores decrease their strength.High content of carbonate or quartz increases a rock's brittleness,while a high content of clay increases a rock's plasticity and decreases its brittleness.As phyllosilicate content increases,the uniaxial compressive strength decreases,and this could be because phyllosilicate minerals have a weakening effect on the mineral bond.Young's modulus often climb as clay minerals decline and as silica with carbonate concentration rises,however Poisson's ratio increases in relation to an increase in clay minerals,which also increases the ductility of the reservoir shale rock.However,compared to minerals and matrix,does not significantly impact the strength of shale rock.Besides,the benefits and drawbacks of using uniaxial and triaxial compression,ultrasonic testing,and nano-indentation techniques in unconventional reservoirs were described.The findings suggest that,because of the possibility for experimental testing repeatability for increased accuracy,ultrasonic testing is the most appropriate experimental approach in the scenes of assessing static and dynamic geomechanical properties of reservoir shale rock.We suggested that numerically-based simulation of experimental techniques used for shale geomechanical evaluations and numer-ical modeling of heterogeneous shale rock samples will be necessary in light of the limitations faced in the ap-plications of experimental techniques for shale geomechanical evaluation.

Impact of hydraulic perforation on fracture initiation and propagation in shale rocks

To enhance the oil and gas recovery rate, hydraulic fracturing techniques have been widely adopted for stimulation of low-permeability reservoirs. Pioneering work indicates that hydraulic perforation and layout could significantly affect fracture initiation and propagation in low-permeability reservoir rocks subjected to complex in-situ stresses. This paper reports on a novel numerical method that incorporates fracture mechanics principles and the numerical tools FRANC3D and ANSYS to investigate the three-dimensional initiation and propagation behavior of hydro-fracturing cracks in shale rock. Considering the transverse isotropic property of shale rocks, the mechanical parameters of reservoir rocks attained from laboratory tests were adopted in the simulation. The influence of perforation layouts on the 3D initiation of hydro-fracturing fractures in reservoir rocks under geo-stresses was quantitatively illuminated. The propagation and growth of fractures in three dimensions in different perforating azimuth values were illustrated. The results indicate that： 1） the optimal perforation direction should be parallel to the maximum horizontal principal stress, 2） the crack plane gradually turns toward the direction of the maximum horizontal principal stress when they are not in parallel, 3） compared with the linear and symmetric pattern, the staggered perforation is the optimal one, 4） the proper perforation density is four to six holes per meter, 5） the optimal perforation diameter in this model is 30 ram, and 6） the influence of the perforation depth on the fracture initiation pressure is low.

Evaluation on the anisotropic brittleness index of shale rock using geophysical logging

The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs.Various brittleness indices have been proposed to characterize the brittleness of shale rocks,but almost all of them ignored the anisotropy of the brittleness index.Therefore,uniaxial compression testing integrated with geophysical logging was used to provide insights into the anisotropy of the brittleness index for Longmaxi shale,the presented method was utilized to assess brittleness index of Longmaxi shale formation for the interval of 3155e3175 m in CW-1 well.The results indicated that the brittleness index of Longmaxi shale showed a distinct anisotropy,and it achieved the minimum value at β=45°-60°.As the bedding angle increased,the observed brittleness index(BI_(2_β))decreased firstly and increased then,it achieved the lowest value at β=40°-60°,and it is consistent with the uniaxial compression testing results.Compared to the isotropic brittleness index(β=0°),the deviation of the anisotropic brittleness index ranged from 10%to 66.7%,in other words,the anisotropy of brittleness index cannot be ignored for Longmaxi shale.Organic matter content is one of the main intrinsic causes of shale anisotropy,and the anisotropy degree of the brittleness index generally increases with the increase in organic matter content.The present work is valuable for the assessment of anisotropic brittleness for hydraulic fracturing design and wellbore stability analysis.

Sliding friction of shale rock on dry quartz sand particles

The sliding friction of rock, involving all kinds of particles at the contact surface, is relevant to many problems, ranging from those in artificial engineering to earthquake dynamics. In this work, the frictional performance of the shale rock–dry quartz sand contact was investigated using a self-developed testing device. The study showed that the coefficient of friction of the contact increases with nominal stress and that the corresponding friction force increases approximately linearly with nominal stress, which is directly related to the contact stress between each single sand particle and rock shale. An overall dynamic coefficient, γ, reflecting the response of friction force to nominal stress, first decreases and then increases with area ratio, which is determined by not only the contact stress but also the interparticle friction force. These have important repercussions for a preliminary understanding of the frictional properties of the shale rock–dry quartz sand contact in hydraulic fracturing and related industrial applications.

Occurrence of Highly Mature Organic Matter in Marine Black Shale Petroleum Source Rocks of Basal Cambrian from Northern Tarim Basin, China

More and more evidence indicates that organic matter (OM) in immature organic-rich sediments and sedimentary rocks is chemically adsorbed onto the outer surfaces of minerals and into interlayer (inner) surfaces of smectitic clay minerals in the form of amorphous molecular-scale carbon. But there have been few reports about the occurrence of highly mature OM in marine black shales (petroleum source rocks). The occurrence of highly mature OM in the black shales of basal Cambrian from northern Tarim Basin is studied in this paper. Based on the comprehensive analyses of total organic carbon contents (TOC), maximum thermolysis temperatures (T-max) of OM, mineral surface areas (MSA), and scanning electronic microscopic (SEM) and transmission electronic microscopic (TEM) observations of the black shales, it is concluded that the highly mature OM in the marine black shales of the basal Cambrian from northern Tarim Basin occurs in particulates ranging in size from 1 to 5 μm in diameter. Through the contrast of the occurrence of the highly mature OM in the black shales with that of the immature ones in modern marine continental margin sediments, some scientific problems are proposed, which are worth to study further in detail.

A rock physics model for the characterization of organic-rich shale from elastic properties

Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and kerogen porosity using the Kuster and Toks6z theory and the selfconsistent approximation method. Rock physics modeling results show that with the increase of kerogen content and kerogen-related porosity, the velocity and density of shales decrease, and the effect of kerogen porosity becomes more obvious only for higher kerogen content. We also find that the Poisson＇s ratio of the shale is not sensitive to kerogen porosity for the case of gas saturation. Finally, for the seismic reflection responses of an organic-rich shale layer, forward modeling results indicate the fifth type AVO re- sponses which correspond to a negative intercept and a positive gradient. The absolute values of intercept and gradient increase with kerogen content and kerogen porosity, and present predictable variations associated with velocities and density.

Characteristics and Evaluation of the High-Quality Source Rocks of Cretaceous Continental Shale Oil in Tonghua Basin, China

The presence of shale oil in the Cretaceous Hengtongshan Formation in the Tonghua Basin, drilled by the well TD-01, has been discussed in this geological investigation for the first time. To evaluate the high-quality source rocks of Cretaceous continental shale oil, the distribution characteristics and the evolution of the ancient environment, samples of shale were systematically analyzed in terms of sedimentary facies, organic geochemistry, and organic carbon isotopic composition. The results demonstrate that a TOC value of 1.5% represents the lower-limit TOC value of the high-quality source rocks. Source rocks have an aggregate thickness of 211 m and contain abundant organic matter, with TOC values of 2.69% on average and a maximum value over 5.44%. The original hydrocarbon-generative potential value(S_1+S_2) is between 0.18 mg/g and 6.13 mg/g, and the Ro is between 0.97% and 1.40%. The thermal maturation of the source rocks is relatively mature to highly mature. The δ^(13)C value range is between -34.75‰ and -26.53‰. The ratio of saturated hydrocarbons to aromatic hydrocarbons is 1.55 to 5.24, with an average of 2.85, which is greater than 1.6. The organic types are mainly type Ⅱ_1, followed by type Ⅰ. The organic carbon source was C_3 plants and hydrophytes. The paleoclimate of the Hengtongshan Formation can be characterized as hot and dry to humid, and these conditions were conducive to the development of high-quality source rocks. A favorable paleoenvironment and abundant organic carbon sources provide a solid hydrocarbon generation base for the formation and accumulation of oil and gas in the shale of the Tonghua Basin.

Construction of a novel brittleness index equation and analysis of anisotropic brittleness characteristics for unconventional shale formations

The brittleness prediction of shale formations is of interest to researchers nowadays.Conventional methods of brittleness prediction are usually based on isotropic models while shale is anisotropic.In order to obtain a better prediction of shale brittleness,our study firstly proposed a novel brittleness index equation based on the Voigt–Reuss–Hill average,which combines two classical isotropic methods.The proposed method introduces upper and lower brittleness bounds,which take the uncertainty of brittleness prediction into consideration.In addition,this method can give us acceptable predictions by using limited input values.Secondly,an anisotropic rock physics model was constructed.Two parameters were introduced into our model,which can be used to simulate the lamination of clay minerals and the dip angle of formation.In addition,rock physics templates have been built to analyze the sensitivity of brittleness parameters.Finally,the effects of kerogen,pore structure,clay lamination and shale formation dip have been investigated in terms of anisotropy.The prediction shows that the vertical/horizontal Young’s modulus is always below one while the vertical/horizontal Poisson’s ratio(PR)can be either greater or less than 1.Our study finds different degrees of shale lamination may be the explanation for the random distribution of Vani(the ratio of vertical PR to horizontal PR).

Multiple-stacked Hybrid Plays of lacustrine source rock intervals：Case studies from lacustrine basins in China

Hydrocarbon-producing lacustrine basins are widely developed in the world, and China has a large number of lacustrine basins that have developed since the early Permian. The organic-rich shale-dominated heterogeneous source rock intervals in Chinese lacustrine basins generally contain frequent thin interbeds of stratigraphically associated sandstone, siltstone, marl, dolomite, and limestone. The concept of ‘‘Hybrid Plays＇ ＇ as put forth in this article recognizes this pattern of alternating organicrich shale and organic-lean interbeds and existence of mixed unconventional and conventional plays. Hybrid Plays in lacustrine source rock intervals present a unique closed petroleum system hosting continuous hydrocarbons.The interbedded organic-lean siliciclastic and/or carbonateplays are efficiently charged with hydrocarbons via short migration pathways from the adjacent organic-rich shale that is often also a self-sourced play. We assert ‘‘Hybrid Plays＇ ＇ provide the most realistic exploration model for targeting multiple-stacked and genetically related very tight shale, tight and conventional plays together in the entire source rock interval rather than individual plays only. The Hybrid Play model has been proven and works for a wide variety of lacustrine rift, sag and foreland basins in China.

Estimation of the Effect of Anisotropy on Young’s Moduli and Poisson’s Ratios of Sedimentary Rocks Using Core Samples in Western and Central Part of Tripura, India

The velocity anisotropy parameters and elastic constants play a very important role to estimate Young’s modulus and Poisson’s ratios accurately. For geomechanics applications such as hydraulic fracturing design, analysis of wellbore stability and rock failure, determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress, Young’s modulus and Poisson’s ratios play a very important role. Four rock samples were collected from four different wells situated in study area. The ultrasonic transmission method has been used to measure P-wave, Sh-wave and Sv-wave travel times as a function of orientation and confining pressure. The five independent stiffnesses constants, Young’s moduli, Poisson’s ratios and Bulk moduli of the samples were estimated. The Poisson’s ratios ( and ) are varying as the confining pressure is changed. The axial strain is larger than the lateral strain, resulting . For shales, the Young’s modulus measured parallel to bedding E1 is usually greater than the Young’s modulus measured perpendicular to bedding E3. Through this study it has been observed that, there is a strong effect of anisotropy parameters on Young’s modulus and Poisson’s ratio.

Investigation of mechanical properties of bedded shale by nanoindentation tests: A case study on Lower Silurian Longmaxi Formation of Youyang area in southeast Chongqing, China

The mechanical properties such as Young's modulus, hardness and fracture toughness of Lower Silurian Longmaxi shale samples from Youyang area in southeast Chongqing, China were investigated using dot matrix nanoindentation measurements. With the help of field emission scanning electron microscope(FESEM) and energy dispersive X-ray fluorescence spectroscopy(EDS), the indentation morphology and mineral composition in indentation area were quantitatively analyzed. According to mechanical strength classification, a micromechanical model with three components was introduced and the Mori-Tanaka model was used to upscale mechanical parameters from nano-scale to centimeter-size scale, which were further compared with uniaxial compression results. The experimental results show that there is a positive linear correlation between Young's modulus and hardness and between the Young's modulus and the fracture toughness under nano-scale; the Young's modulus, hardness and fracture toughness perpendicular to the bedding are slightly lower than those parallel with the bedding. According to data statistics, the mechanical properties at the nano-scale follow Weibull distribution feature and the dispersion degree of hardness results is the highest, which is mainly due to shale anisotropy and nanoindentation projection uncertainty. Comparing the results from nanoindentation test, with those from upscaling model and uniaxial compression test shows that the mechanical parameters at the nano-scale are higher than those from upscaling model and uniaxial compression test, which proves mechanical parameters at different scales have differences. It's because the larger the core, the more pores and internal weakness it contains, the less accurate the interpreted results of mechanical parameters will be.

Upper Jurassic Source Rock in Gangba Basin,Southern Tibetan Plateau

The Tibetan Plateau is located in the east part of the Tethys realm which is the largest oil and gas accumulation region of the world.The Gangba Basin, located on the south of Tibetan Plateau,is a Mesozoic marine petroliferous basin covering an area of 7000 km2.The basin has been received little hydrocarbon interest for its severe climate and life conditions.The Upper Jurassic siliciclastics in Gangba Basin

Correction of source-rock permeability measurements owing to slip flow and Knudsen diffusion: a method and its evaluation

Source-rock permeability is a key parameter that controls the gas production rate from unconventional reservoirs. Measured source-rock permeability in the laboratory, however, is not an intrinsic property of a rock sample, but depends on pore pressure and temperature as a result of the relative importance of slip flow and diffusion in gas flow in lowpermeability media. To estimate the intrinsic permeability which is required to determine effective permeability values for the reservoir conditions, this study presents a simple approach to correct the laboratory permeability measurements based on the theory of gas flow in a micro/nano-tube that includes effects of viscous flow, slip flow and Knudsen diffusion under different pore pressure and temperature conditions. The approach has been verified using published shale laboratory data.The ＇＇corrected＇＇（or intrinsic） permeability is considerably smaller than the measured permeability. A larger measured permeability generally corresponds to a smaller relative difference between measured and corrected permeability values. A plot based on our approach is presented to describe the relationships between measured and corrected permeability for typical Gas Research Institute permeability test conditions. The developed approach also allows estimating the effective permeability in reservoir conditions from a laboratory permeability measurement.

大庆油田裸眼井测井技术进展与展望

为了提高大庆油田裸眼井测井技术支撑能力和研究成果领先水平,全面回顾了大庆油田测井采集系列及解释技术的创新发展历程,系统总结了特高含水期剩余油解释、火山岩等复杂岩性测井评价、碎屑岩储量参数解释、非常规油气“甜点”分类、缝洞型碳酸盐岩储层测井评价等油田勘探开发测井评价技术。在客观分析大庆油田勘探开发测井解释评价需求和面临瓶颈问题的基础上,结合当前油田测井评价对象规模小、物性差、埋藏深、地层结构复杂、非均质性强的特点。指明了测井解释评价核心技术主攻方向。围绕新阶段测井采集及解释评价技术体系完善与建立,对高分辨率和成像测井采集、后油藏时期和非常规测井解释评价、新一代智能解释技术体系等未来发展进行了展望。

Response analysis of shale bedding structure to ultrasonic characteristics and its application

Based on the wave theory, different bedding structure models for shales in Lower Silurian Longmaxi Formation of southeastern Chongqing area were established, numerical simulations of responses of different bedding structures of shale to ultrasonic wave were carried out by using the second order in time and fourth order in space grid finite difference method, based on the grey system theory, sensitive factors of acoustic parameters of bedding structure were selected, and the dynamic mechanical parameter model of bedded shale was established, which was verified by the ultrasonic transmission experiment results on core down Well ZY1 and YY1 and the logging data of Well ZY2. The results show that:(1) The correlation coefficient between analog and experimental waveforms is greater than 80%, indicating that the numerical simulation method can effectively simulate ultrasonic transmission experiment.(2) Acoustic velocity is a conventional sensitive factor used to characterize shale bedding structure, whereas the attenuation coefficient is sensitive to the change of bedding thickness, with correlation coefficient of 0.89, therefore, using the normalized results of attenuation coefficient to comprehensively describe the shale bedding can make the results more accurate.(3) The correlation between the dynamic and static parameters calculated by the model is better than that of the traditional model; and the predicted values of rock mechanics obtained by using the model and logging data inversion are in good agreement with the experimental values. The research results lay the foundation for further accurate prediction of rock mechanic parameters using sonic logging data.

鄂尔多斯盆地南缘华北探区页岩油储层岩石力学特性实验研究

针对鄂尔多斯盆地南部延长组7段页岩储层钻井过程中发生钻头适应性差和井壁垮塌等井下复杂状况,开展了岩石组构特征分析、力学特征分析和可钻性参数等力学特征分析。结果表明,鄂尔多斯盆地南缘页岩油储层中黏土以伊利石和伊/蒙混层为主,岩石结构及力学特性非均质性强。岩石表面层理结构明显,微裂缝和孔隙发育。页岩抗张强度为6.58~18.27 MPa,单轴抗压强度为76.79~169.29 MPa,高温下的内摩擦角为14.67°~36.71°,泊松比为0.167~0.352,主要集中在0.21附近,弹性模量为4 100~17 344 MPa。最小水平主应力梯度为1.89~2.14 MPa/100 m,最大水平主应力梯度为2.37~3.04 MPa/100 m,其中洛河地区最大、最小地应力梯度差值较大。地层可钻性级值为4.36~6.11,主要分布在5.5以下,属于软-中硬度地层,平均地层倾角小,在钻至水平段时,钻进方向与层理相同,与岩石接触面多互层,钻头磨损加重。基于页岩储层岩石力学特性认识,对于鄂南页岩油安全、高效地钻探具有指导意义。

Organic Geochemical Evaluation of Shale Units of Bokh Formation, Ogaden Basin, Ethiopia

Shale sample from Bokh Formation, Ogaden basin, Ethiopia was assessed for their petroleum potential by Rock-Eval pyrolysis. The Total Organic Carbon (TOC) values range from 0.76 to 0.86 wt%. These values exceed the minimum value of 0.5 wt% required for potential petroleum source rocks. The genetic potential (GP) and hydrogen index (HI) values range from 0.09 to 0.5 mg/g and 8 - 32 mgHC/gTOC, respectively. These values are generally lower than the minimal 2 mg/g GP and 200 mgHC/g HI required for a potential source rock. Several plots from the Rock-Eval pyrolysis data classify the samples as type IV kerogen with no potential to generate hydrocarbons. The Tmax and Production Index (PI) values range from 388°C to 453°C and 0.28 to 0.46, respectively. These values indicate that the shales are thermally matured. These results show that the shales from the Bokh Formation in Ogaden basin have no potential to generate hydrocarbons mainly crude oil, but are thermally matured and may generate natural gas.

川东南页岩气储层的岩石物理特征分析

四川盆地志留系龙马溪组页岩因富含有机质、埋藏深度适中的特点,已成为国内页岩气勘探的主要层位之一。页岩气储层非均质性强,物性参数所表现出的各向异性及脆性特征会受到多种因素的影响,不利于勘探中的测井解释和地震“甜点”的预测。基于以上原因,在四川长宁双河、重庆华蓥山三百梯等野外地质露头剖面采集页岩样品,利用X射线衍射分析页岩的矿物成分及含量,在实验条件下研究围压和温度的变化对地震速度的影响,确定地震速度与矿物成分的关系,以及页岩各向异性参数的变化规律。研究成果在一定程度上揭示了川东南地区含气储层黏土含量对页岩各向异性和脆性的控制作用,并总结出各向异性参数变化的经验关系,从而为岩石物理建模、各向异性参数反演以及脆性评价等提供地质依据。