Influence of sedimentary environments on mechanical properties of clastic rocks

The sedimentary environments are the intrinsic factor controlling the mechanical properties of clastic rocks. Examining the relationship between rock sedimentary environments and rock mechanical properties gives a better understanding of rock deformation and failure mechanisms. In this study, more than 55 samples in coal measures were taken from seven different lithologic formations in eastern China. Using the optical microscope the sedimentary characteristics, such as components of clastic rocks and sizes of clastic grains were quantitatively tested and analyzed. The corresponding mechanical parameters were tested using the servo-controlled testing system. Different lithologic attributes in the sedimentary rocks sampled different stress–strain behaviors and failure characteristics under different confining pressures, mainly due to different compositions and textures. Results demonstrate that clastic rocks have the linear best-fit for Mohr-Coulomb failure criterion. The elastic moduli in clastic rocks are highly dependent upon confining pressures, unlike hard rocks. The envelope lines of the mechanical properties versus the contents of quartz, detritus of the grain diameter of more than 0.03 mm, and grain size in clastic rocks are given. The compressive strength or elastic modulus and the grain diameter have a non-monotonic relation and demonstrate the “grain-diameter softening” effect.     

The effect of decomposition of hydrous minerals on the mechanical properties of rocks at high pressures and temperatures

Extensive experiments have been carried out in which specimens of gypsum, a partially serpentinized peridotite, a serpentinite and a chloritite have been subjected to pressures up to 0.662 GPa together with temperatures up to 780°C and have been deformed at a fixed strain rate of 10⁻⁵/s. The commencement of decomposition of the hydrous minerals is accompanied in sealed specimens by loss of strength, a reduction in sliding friction, and embrittlement of the rocks. Dilatancy-hardening effects are observed. Specimens which are drained to the atmosphere remain strong. In gypsum there is a ten-fold reduction in strength between temperatures of 50°C and 150°C. The partially serpentinized peridotite (40% forsterite, 60% antigorite) which contains 1% of brucite shows a reduction in strength 50% at 300°C, followed by a further ten-fold reduction between 300°C and 700°C. The serpentinite (90% lizardite and chrysotile) shows a ten-fold reduction in strength between 400°C and 600°C. The chloritite (85% ripidolite) shows a reduction of strength by about a half at 300°C; the strength remains approximately constant between 300°C and 600°C, and there is a further five-fold reduction in strength between 600°C and 700°C. The phase changes in the hydrous minerals have been studied by means of differential thermal analysis, X-ray diffraction and optical microscopy, and will be more fully described elsewhere. A detailed discussion is given of the deformation characteristics and mechanisms, with particular emphasis on the role of pore pressure and dilatancy. There is a range of temperature for each of these rocks in which the deformation of sealed samples is most stable, in the sense that brittle faulting accompanied by a stress-drop does not occur. At higher temperatures the rocks become unstable and very weak. Under conditions corresponding to geothermal gradients between 5 K km⁻¹ and 100 K km⁻¹ these rocks would be brittle and weak at shallow depths, and would again become brittle and weak at depths below some level which depends on the rock. Possible implications are discussed in connection with faulting and earthquakes, with syntectonic metamorphism, and with the emplacement of Alpine-type peridotites.     

温度变化对碳酸盐岩储层岩石热物性的影响

以苏北盆地某地热井碳酸盐岩储层岩石为研究对象,对处于20、50、100、150、200 ℃ 5个实时温度点的3组岩石样品进行热物性参数测试,分析导热系数、比热容和热扩散系数随温度变化的规律,拟合趋势曲线和经验公式,探讨影响机制。结果表明：在20～200 ℃的温度区间内,随温度的升高,岩石的导热系数拟合曲线以二次多项式关系呈下降趋势,50 ℃的温度点是岩石导热系数由急速下降到缓速下降的拐点,导热系数平均值减小约31.47%;比热容拟合曲线以二次多项式关系呈上升趋势,比热容平均值增大约23.27%;热扩散系数随温度的变化趋势与导热系数一致,且二者具线性相关,100 ℃温度点是热扩散系数由急速下降到缓速下降的拐点,热扩散系数平均值减小约44.48%。随着温度的升高,比热容增大造成的补偿作用是导热系数下降幅度小于热扩散系数的主要原因。     

Experimental investigation on the carbon isotope fractionation of methane during gas migration by diffusion through sedimentary rocks at elevated temperature and pressure

Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the ¹²CH₄ and ¹³CH₄ species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ¹³C₁ = −39.1‰), a significant depletion of the heavier carbon isotope (¹³C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ¹³C_diff −δ¹³C_source) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the ¹²CH₄ and ¹³CH₄ entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.     

Seismic and mechanical properties of Opalinus Clay: comparison between sandy and shaly facies from Mont Terri (Switzerland)

For the safe disposal of high-level radioactive waste, different host rocks are currently being considered. The favorable properties of clay are low permeability, some retention capacity concerning radionuclides, and the ability to self-seal cracks and fissures, e.g. by swelling or time-dependent compaction creep. In Switzerland, the Jurassic Opalinus Clay is envisaged as a potential host rock which—at Mont Terri—is subdivided into the sandy, shaly, and carbonate-rich facies, the latter being less abundant. For long-term safety assessments, the understanding of the relations of properties (e.g. mineralogical composition and microstructure) and performance (e.g. mechanical behavior) of clays and claystones is essential. In the case of the sandy Opalinus Clay, the mechanical strength increases with increasing carbonate content, because carbonates form the matrix. The mineralogical investigation of a set of sandy facies samples proved a significantly larger carbonate content (20–40 mass %) when compared to the shaly facies (10–20 mass %). The carbonates of the shaly Opalinus Clay, on the other hand, are mostly localized fossils aligned parallel to the bedding, acting as predetermined breaking points. Image analysis of SEM images of polished sections proved the determined microstructural differences. In addition, carbonate particles of the sandy facies are mostly isometric, whereas carbonate particles of the shaly facies cover a greater range of shapes. The mechanical tests were accompanied by investigations of the p- and s-wave velocities, which revealed that the anisotropy of the sandy facies is less pronounced than sedimentological analyses would suggest. The mechanical strength, which, for the first time, presents results of real triaxial tests of the sandy facies. The samples of the sandy facies exhibit a failure strength of σ _eff,B, approximately twice as high as was found for the shaly facies considering the deformation axis parallel to the bedding. Similar values were obtained when measuring perpendicularly to the bedding.     

温度对沥青混凝土力学特性的影响

沥青混凝土是现代水利工程中作为防水心墙的主要材料。通过室内高压温控三轴仪对两种沥青含量的沥青混凝土在不同温度下分别进行力学特性试验,研究温度对其力学性质的影响。试验结果表明：试件的应力-应变特性与温度有密切关系,试验温度为0℃时沥青混凝土出现应力软化现象,但随着温度的升高,应力软化现象消失,应力硬化现象呈现;同一沥青含量下,温度越高,最大偏应力越小;试件的抗剪强度随温度的升高逐渐降低;试件的蠕变随温度的升高而增大。     

不同损伤程度下岩石力学参数变化的声波测试

声波波速变化可有效表征岩体损伤程度。通过对岩样进行人工预损,利用声波检测标定岩样的不同损伤程度,然后通过三轴压缩试验来研究不同损伤程度下岩石的力学特性,建立了声波速度下降幅度与岩体力学参数变化幅度间的关系。研究表明,当岩样的声波速度下降5%～8%时,黏聚力c值降低15%～25%,内摩擦角? 值则升高14%～32%,因此,用声波速度的下降幅度来描述岩体参数的变化是可行的。根据现场实测的岩体损伤区内的声波速度的变化值,可以从一定程度上估计岩体参数的跌落情况,从而更进一步地说明损伤区内岩体的承载力。     

实时高温下加载速率对花岗岩力学特性影响的试验研究

为综合考虑温度、加载速率两个因素对花岗岩力学性质及破坏方式的影响,在实时高温（25～1 000 ℃）作用下利用MTS810电液伺服材料试验系统对岩样进行不同加载速率作用下的单轴压缩试验。研究结果表明：（1）各个温度点,岩样单轴压缩应力-应变曲线大致经历了压密、弹性、屈服、破坏4个阶段。岩样峰后曲线在加载速率为0.001～0.01 mm/s出现台阶型分段跌落状,在加载速率为0.01～0.1 mm/s呈现光滑、陡峭的连续曲线。（2）岩样峰值强度、弹性模量随温度的升高可分为4个阶段：25～200 ℃区间为缓慢上升段;200～600 ℃区间为快速下降段;600～800 ℃区间为缓慢上升段;800～ 1 000 ℃区间为平缓下降段。1 000 ℃时的峰值强度和弹性模量相对于25 ℃时分别降低了53.47%和64.34 %。峰值应变与温度呈现三次多项式拟合关系。（3）岩样峰值强度、弹性模量与加载速率对数均呈现二次多项式增长关系,加载速率为0.1 mm/s时的峰值强度和弹性模量相对于0.001 mm/s时分别提高了38.82%和37.22%。岩样峰值应变与加载速率没有明显的对应关系。（4）单轴压缩状态下,随着温度的升高,花岗岩变形破坏形式由拉剪破裂向锥形破裂并伴随向碎性流动过渡,失稳型式由突发失稳向渐进破坏过渡。同一温度状态下,加载速率对岩样的破裂形式没有明显影响,但失稳型式发生了变化。     

花岗岩力学特性的温度效应试验研究

通过实时高温（常温～850 ℃）加载和高温（常温～1 200 ℃）后冷却再加载两种情况下的单轴压缩试验,对不同高温下花岗岩的力学性质进行了研究,分析了两种情况下单轴抗压强度、弹性模量、纵波波速、剪切滑移应变等随温度的变化规律,并研究了热-力耦合效应。研究结果表明：（1）在实时高温加载作用下单轴抗压强度和弹性模量随着温度升高而发生连续劣化;（2）高温作用冷却后再加载,花岗岩在常温～600 ℃区间峰值强度变化不大,800 ℃左右岩样强度突然降低;（3）纵波波速随加热温度的升高而逐渐降低;（4）剪切滑移应变在800 ℃之前相对较小,且变化不大,之后便迅速增大,表现出明显的塑性;（5）提出了热-力耦合因子的概念,并借助其提出了一维非线性热-力耦合本构模型,模型曲线和试验曲线较吻合。     

温度效应对盐岩力学特性影响的试验研究

对经历不同温度后盐岩的力学性能进行了试验研究,分析了盐岩应力-应变曲线、峰值应力、峰值应变、弹性模量等的变化情况。分析结果表明,温度对盐岩的物理力学特性影响较大,随着温度的升高,盐岩的力学性能劣化明显,峰值应力和弹性模量降低,峰值应变增加;峰值应力可以表示成围压与温度的函数,并从盐岩弹性模量随温度的变化规律入手,导出了热损伤演化方程和一维TM耦合损伤本构方程。     

热处理砂岩微观结构及力学性质试验研究

采用核磁共振、电镜扫描、X射线衍射、单轴压缩等试验手段对某砂岩试样不同温度下(25～900℃)微观结构及力学性质的变化情况进行研究。研究表明:温度对砂岩试样微观结构具有重要影响,试样总孔隙率随温度的升高而升高,在150℃以下,由于中、大孔隙的减少,试样渗透性反而降低;当温度超过600℃时,中、大孔隙快速增加,试样渗透性能大幅增加;温度升高导致砂岩试样弹性模量减小、峰值应变增大以及孔隙压密阶段变长,宏观表现为脆性降低、塑性明显增强;热处理条件下,除微观结构的变化会导致砂岩试样力学强度改变外,试样矿物成分对其力学强度也具有十分重要的影响;450℃以下,由于矿物成分变化较小,试样力学强度主要受到孔隙度增加的影响,表现为强度随温度升高而降低;450～600℃,虽然孔隙率继续增长,但由于主要矿物高岭石发生脱水以及与其他离子形成新相矿物,进而导致试样强度没有随着孔隙增加而降低,反而出现一定的增长;超过600℃后,孔隙尤其是大孔隙的急剧增加,导致强度重新开始降低。     

The uptake and solubility of water in quartz at elevated pressure and temperature

The uptake of water in quartz at 1.5 GPa total pressure, 1173 K and high water fugacity, over times up to 24 h, has been investigated using a newly developed assembly to prevent microcracking. It is found that the uptake is small, and below the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity or the solubility or a combination of both, and is in agreement with the observations of Kronenberg et al. (1986) and Rovetta et al. (1986). It brings into question the interpretation of the early experiments on water weakening by Griggs and Blacic (1964) and the recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985). Rults of a combined T.E.M., light-scattering and infrared-spectroscopy investigation of ‘wet’ synthetic quartz before and after heating at 0.1, 300 and 1500 MPa total pressure and 1173 K, strongly suggest that the water in ‘wet’ quartz is mainly in the form of H₂O in inclusions, consistent with the solubility being low, possibly less than 100 H/10⁶Si. From these observations, water-containing inclusions appear to play a major role in the plasticity of quartz, while any role of water in solid solution remains to be clarified.     

深部复合地层力学性质研究

为了研究深部复合地层在不同围压、不同软硬层厚度比条件下的力学性质,以水泥和纯白高岭土为原料,人工制作复合地层物理模拟样品进行单轴及三轴压缩力学试验,同时结合FLAC~(3D)数值模拟,研究不同围压、软硬层厚度比对复合地层力学性质的影响。研究表明,人工制作的复合地层可以满足强度差异要求;随着围压升高,复合地层会出现应变硬化,之后由于软硬岩不协调变形,应变硬化消失,软岩中的不规则裂纹增多;若不考虑结构面等因素,深部复合地层力学性质由占比多的岩性决定;数值模拟与试验得出的应力–应变曲线具有相似的趋势,都表明深部复合地层的变形主要发生在软岩部分。     

Raman scattering and lattice vibrations of Ni2SiO4 spinel at elevated temperature

Raman spectra of Ni₂SiO₄ spinel (O _h ⁷ Z=8) have been measured in the temperature range from 20 to 600 °C and the Raman active vibrations (A _1g +E _g +3F _2g ) have been assigned. A calculation of the optically active lattice vibrations of this spinel has been made, assuming a potential function which combines general valence and short range force constants. The values of the force constants at 20 and 500 °C have been calculated from the vibrational frequencies of the observed Raman spectra and infrared (IR) spectral data. The Ni spinel at 20 °C has a prominently small Si-O bond stretching force constant of K(SiO)=2.356 ～ 2.680 md/Å and a large Ni-O bond stretching constant of K(NiO)=0.843 ～ 1.062 md/Å and these force constants at 500 °C decrease to K(SiO)=2.327 ～ 2.494 md/Å and K(NiO)=0.861 ～ 0.990 md/Å. The Si-O bond is noticeably weakened at high temperatures, despite the small thermal expantion from 1.657 Å (20 °C) to 1.660 Å (500 °C). These changes of the interatomic force constants of the spinel at high temperatures are in accord with the thermal structure changes observed by X-ray diffraction study. The weakened Si-O bond is consistent with the fact that Si atoms in the spinel lattice can diffuse at significant rates at elevated temperature.     

Correlation analyses of effects of temperature on physical and mechanical properties of clay

In determining the physical and mechanical parameters of clay, it is sometimes necessary to determine them indirectly from other parameters since they cannot be measured directly from laboratory or field tests. In order to determine the effect of temperature on the behavior of clay, an indirect approach is used here by analyzing the changes of mass (\(\Delta m\)), density (\(\rho\)), porosity (\(\phi\)), P-wave velocity (\({v_p}\)), thermal conductivity (\(\lambda\)), specific heat capacity (c), resistivity (R) and uniaxial compressive strength (f) of clay from eastern China for a temperature range between 20 and 800 °C. The results indicate that temperature has a significant effect on these parameters. Comparisons between \(\Delta m\) and \(\rho\), \(\Delta m\) and \({v_p}\), \(\rho\) and \({v_p}\), \(\phi\) and \(\lambda\), \({v_p}\) and f, R and f show a linear change among these parameters,whereas the relationships among \(\Delta m\) and \(\phi\), \(\phi\) and \({v_p}\), \(\phi\) and R, \({v_p}\) and \(\lambda\), \(\phi\) and f are exponential. It is difficult to obtain these relationships by using regression analysis with high levels of accuracy. Further refinement is therefore required.     

渤海湾盆地新生界生油岩系底界面温度分布

依据渤海湾盆地2000余口测温井的地温梯度数据、地层岩性描述、分层数据以及钻井资料,计算了该盆地各生油岩系底界面的温度。统计结果表明:渤海湾盆地沙河街组大部分凹陷区地层底界面温度介于90℃至150℃,目前仍具有大量生油的温度条件,而在隆起或一些凸起地区,该地层组段温度普遍小于90℃,未能达到生油的温度指标。东营组和孔店组地层也仍具有一定的生油温度条件。研究还表明:地层温度与地层界面埋深密切相关,温度随界面埋深的增大而升高,沉积厚度大的凹陷区地层界面温度大于沉积厚度小的凸起区或斜坡地带,说明地层界面埋深是决定地温高低的主要控制因素,而地温梯度对地层界面温度的影响相对较小。