Brittleness characteristics of tight oil siltstones

Rock brittleness directly affects reservoir fracturing and its evaluation is essential for establishing fracturing conditions prior to reservoir reforming. Dynamic and static brittleness data were collected from siltstones of the Qingshankou Formation in Songliao Basin. The brittle–plastic transition was investigated based on the stress–strain relation. The results suggest that the brittleness indices calculated by static elastic parameters are negatively correlated with the stress drop coefficient and the brittleness index B₂, defined as the average of the normalized Young’s modulus and Poisson’s ratio, is strongly correlated with the stress drop. The brittleness index B₂, Young’s modulus, and Poisson’s ratio correlate with the brittle minerals content; that is, quartz, carbonates, and pyrite. We also investigated the correlation between pore fluid and porosity and dynamic brittle characteristic based on index B₂. Pore fluid increases the plasticity of rock and reduces brittleness; moreover, with increasing porosity, rock brittleness decreases. The gas-saturated siltstone brittleness index is higher than that in oil- or water-saturated siltstone; the difference in the brittleness indices of oil- and water-saturated siltstone is very small. By comparing the rock mechanics and ultrasonic experiments, we find that the brittleness index obtained from the rock mechanics experiments is smaller than that obtained from the ultrasonic experiments; nevertheless, both decrease with increasing porosity as well as their differences. Ultrasonic waves propagate through the rock specimens without affecting them, whereas rock mechanics experiments are destructive and induce microcracking and porosity increases; consequently, the brittleness of low-porosity rocks is affected by the formation of internal microcrack systems.     

致密砂岩衰减岩石物理图板分析:储层微裂隙预测

致密砂岩储层普遍具有孔隙度低、微裂隙发育的特点,岩石内部常含有强烈的结构非均质性.致密砂岩发育的微裂隙使储层具有良好的连通性,促成高饱和气的天然气成藏.针对川西某探区须家河组高含气饱和度致密砂岩,本文选取致密砂岩岩心样本,进行了不同围压下的超声波实验测量.考虑储层完全饱气情况下的粒间孔隙、微裂隙双重孔隙结构,采用Biot-Rayleigh双重孔隙方程,构建致密砂岩岩石物理模型,进而分析了裂隙含量对纵波频散和衰减的影响.基于地震波衰减,构建了致密砂岩多尺度岩石物理图板.采用谱比法和改进频移法估算致密砂岩样本及储层衰减,对超声和地震频带下的图板进行校正.将校正后的图板应用到研究工区,选取二维测线和三维区块,进行储层孔隙度和裂隙含量的定量预测.对比实际资料进行分析,结果显示,本文预测的孔隙度和裂隙含量与三口测井的孔隙度曲线和实际产气情况基本吻合,基于孔隙-裂隙衰减岩石物理模型有效地预测了优质储层的分布区域.     

致密砂岩气藏裂隙-孔隙型弹性岩石物理模板研究:以川西坳陷A区为例

相对于常规砂岩,致密砂岩在岩石物理性质、力学性质等方面具有明显差异,并呈现出很强的非均质性.岩石物理模型能将储层参数与地震特性信息联系起来,因此可以作为致密砂岩储层参数与地震特性信息转换的桥梁.常规的岩石物理模型通常只考虑单一因素(例如非均匀性,单一孔隙,单一尺度等),建立的岩石物理模板并不适用于致密砂岩.本文针对高饱和气、微裂隙发育、非均质性强的致密砂岩储层,利用Voigt-Reuss-Hill模型计算混合矿物的弹性模量,采用微分等效介质(DEM)模型描述含裂隙、孔隙岩石的骨架弹性模量,基于Biot-Rayleigh波动方程构建了岩石物理弹性模板,给出了致密砂岩储层弹性参数与物性的关系.基于测井数据和实验数据对岩石物理弹性模板进行校正,并将校正后的岩石物理弹性模板结合叠前地震资料应用于川西地区储层孔隙度与裂隙含量预测.结果显示,反演裂隙含量、孔隙度与储层试气报告、测井孔隙度基本吻合,表明该模板能够较合理地应用于致密砂岩储层孔隙度及裂隙含量解释中.     

基于页岩岩石物理等效模型的地应力预测方法研究

地应力的精确预测是对页岩地层进行水平井钻井轨迹设计和压裂的基础.本文在分析页岩构造特征的基础上,提出了适用于页岩地层的岩石物理等效模型的建立流程,并以此为基础实现了最小水平地应力的有效预测.首先,通过分析页岩地层的矿物、孔隙、流体及各向异性特征,将其等效为具有垂直对称轴的横向各向同性介质,进行了页岩岩石物理等效模型的构建;然后建立了页岩地层纵横波速度经验公式,并将该经验公式与岩石物理等效模型均应用于实际页岩工区的横波速度预测中,二者对比表明,本文中建立的页岩气岩石物理等效模型具有更高的横波预测精度,验证了该模型的适用性;最后,利用该模型计算各弹性刚度张量,进而实现了页岩地层最小水平地应力的预测,与各向同性模型估测结果对比表明,该模型预测的最小水平地应力与地层瞬间闭合压力一致性更高,且储层位置更为明显,具有较高的实用性.     

Amplitude variation with incident angle and azimuth inversion for Young's impedance,Poisson's ratio and fracture weaknesses in shale gas reservoirs

By analogy with P- and S-wave impedances, the product of Young's modulus and density can be termed as Young's impedance, which indicates the rock lithology and brittleness of unconventional hydrocarbon reservoirs. Poisson's ratio is also an effective indicator of rock brittleness and fluid property of unconventional reservoirs, and fracture weaknesses indicate the fracture properties (fracturing intensity and fracture fillings) in fracture-induced unconventional reservoirs. We aim to simultaneously estimate the Young's impedance, Poisson's ratio and fracture weaknesses from wide-azimuth surface seismic data in a fracture-induced shale gas reservoir, and use the horizontal transversely isotropic model to characterize the fractures. First, the linearized PP-wave reflection coefficient in terms of Young's impedance, Poisson's ratio, density and fracture weaknesses is derived for the case of a weak-contrast interface separating two weakly horizontal transversely isotropic media. In addition, an orthorhombic anisotropic case is also discussed in this paper. Then a Bayesian amplitude variation with incident angle and azimuth scheme with a model constraint is used to stably estimate Young's impedance, Poisson's ratio and fracture weaknesses with only PP-wave azimuthal seismic data. The proposed approach is finally demonstrated on both synthetic and real data sets with reasonable results.     

中国南海北部陆坡孔隙度的求取

孔隙度在岩石物理和地震勘探领域都是一个非常重要的参数,水饱和地层的孔隙度可以用于求海底地层的弹性模量进而求取纵横波速度、识别水合物以及估计水合物的储量等.然而在没有打钻的情况下要相对准确地了解海底未固结成岩地层的孔隙度变化趋势是困难的.历史上有学者提出了几个模型,都是基于孔隙度随深度的增加呈现指数下降的趋势,并且需要知道指数模型中的若干参数,因此就产生了两个问题,一是孔隙度是否随深度增加而呈现指数下降,二是指数模型中参数如何选取.本文从另外一个角度,即利用海底介质的压实性从表层开始往下逐层求取水饱和地层的孔隙度,此方法被称之为"体积模量法",它仅需要知道表层孔隙度和地层矿物组分,从而节省了勘探成本.本文将该方法应用于我国南海地区和美国ODP井数据,结果与测井数据以及地震反演数据吻合得较好.     

基于统计岩石物理模型的各向异性页岩储层参数反演

提出了各向异性页岩储层统计岩石物理反演方法.通过统计岩石物理模型建立储层物性参数与弹性参数的定量关系,使用测井数据及井中岩石物理反演结果作为先验信息,将地震阻抗数据定量解释为储层物性参数、各向异性参数的空间分布.反演过程在贝叶斯框架下求得储层参数的后验概率密度函数,并从中得到参数的最优估计值及其不确定性的定量描述.在此过程中综合考虑了岩石物理模型对复杂地下介质的描述偏差和地震数据中噪声对反演不确定性的影响.在求取最大后验概率过程中使用模拟退火优化粒子群算法以提高收敛速度和计算准确性.将统计岩石物理技术应用于龙马溪组页岩气储层,得到储层泥质含量、压实指数、孔隙度、裂缝密度等物性,以及各向异性参数的空间分布及相应的不确定性估计,为页岩气储层的定量描述提供依据.     

Seismic inversion for underground fractures detection based on effective anisotropy and fluid substitution

Underground fractures play an important role in the storage and movement of hydrocarbon fluid. Fracture rock physics has been the useful bridge between fracture parameters and seismic response. In this paper, we aim to use seismic data to predict subsurface fractures based on rock physics. We begin with the construction of fracture rock physics model. Using the model, we may estimate P-wave velocity, S-wave velocity and fracture rock physics parameters. Then we derive a new approximate formula for the analysis of the relationship between fracture rock physics parameters and seismic response, and we also propose the method which uses seismic data to invert the elastic and rock physics parameters of fractured rock. We end with the method verification, which includes using well-logging data to confirm the reliability of fracture rock physics effective model and utilizing real seismic data to validate the applicability of the inversion method. Tests show that the fracture rock physics effective model may be used to estimate velocities and fracture rock physics parameters reliably, and the inversion method is resultful even when the seismic data is added with random noise. Real data test also indicates the inversion method can be applied into the estimation of the elastic and fracture weaknesses parameters in the target area.     

岩石物理模版在储层定量解释中的应用

岩石物理模板采用测井解释的各类岩性矿物骨架点值,选取适合该地区的岩石物理模型,模拟在不同储层组合、不同孔隙及不同饱和度情况下,储层岩石物理参数变化引起的储层测井参数及地球物理响应特征的变化,定量地建立起储层参数同地球物理弹性参数间的解释关系模版.本文根据新场JS42气藏储层参数分析结果,尝试性地将岩石物理解释模板应用于储层定量解释中,对储层高产气区、含水区域进行定量解释,并预测了该气藏的气水界面,该预测结果与实钻井测试情况吻合,证实了该方法的科学性.     

热处理对致密岩石物理性质的影响

致密气藏低孔低渗和超低含水饱和度等特征使其潜在水相圈闭损害严重,致密天然气产出表现为多尺度特征.选取泥页岩、致密砂岩和致密碳酸盐岩岩心,开展了100~600℃高温处理对岩心渗透率、孔隙度、重量、长度、直径和声速的影响实验.实验结果表明,碳酸盐岩、致密砂岩和泥页岩的热开裂阈值分别在300~400℃、300~500℃和500~600℃;高温处理后,岩心重量和密度降低,体积增加,泥页岩岩心孔隙度和渗透率提高幅度最显著,600℃处理后声波时差比常温时岩心声波时差提高了1.3倍.热处理消除了水相圈闭和粘土矿物膨胀损害,提高岩石孔隙度和渗透率,恢复或改善致密储层多尺度传质,有利于致密天然气资源开发,但同时高温使岩石破裂,扩展天然裂缝或产生新裂缝,导致工作液漏失,因此,热致裂给勘探开发致密天然气提出了机遇与挑战.     

有效应力变化对致密砂岩孔隙结构及弹性波响应的影响规律

致密砂岩气藏具有裂缝发育和有效应力高的特征,研究不同有效压力下孔、裂隙介质地震波传播特征,有利于地震解释与地下储层的识别.但是前人的研究较少考虑岩石内部微观孔隙结构特征与孔隙、裂隙间流体流动的关系.本文首先通过选取四川盆地典型致密砂岩岩样,在不同有效压力下对岩石样本进行超声波实验测量.然后基于实验测得的纵、横波速度进行裂隙参数反演,得到不同有效压力下致密砂岩样本的裂隙孔隙度.再将裂隙孔隙度和样本岩石物理参数代入双重孔隙介质模型,模拟得到不同有效压力下饱水致密砂岩样本纵横波速度频散和衰减的变化规律.结果表明模型预测的速度频散曲线与纵波速度实验测量结果能够较好的吻合.最后统计分析了致密砂岩裂隙参数,得到了致密砂岩储层裂隙参数随有效压力及孔隙度变化特征.依据实际岩石物理参数建立模型,其裂隙参数三维拟合结果能够较好描述致密砂岩裂隙结构与孔隙度、应力的关联,可为实际地震勘探中预测储层裂缝性质提供基础依据.     

部分饱和砂岩储层地震物理模拟及含气饱和度预测分析

含气饱和度预测是天然气储层地震解释工作的重要目标.本文将岩石物理分析与地震物理模拟技术相结合,构建了部分;饱和砂岩储层物理模型并进行含气饱和度预测分析.物理模型中设置了高孔渗常规砂岩和低孑孔渗致密砂岩两种模拟储层,每种储层都是由具有不同含水饱和度的气-水双相饱和砂体组成.岩石物理分析结果显示在低孔渗致密砂岩中气-水混合流体更加倾向于非均匀的斑块分布,而结合了Brie等效流体公式的Gassmann流体替换理论可以更准确地描述纵波速度随含水饱和度的变化趋势.对物理模型进行地震资料采集处理后,对比了AVO特征和叠前同步反演结果对两种砂岩储层含气饱和度预测能力的差异.AVO特征结果显示,对于混合流体均匀分布的高孔渗砂岩储层,AVO响应曲线和属性变化很难对含气饱和度进行估算;对于混合流体斑块分布的致密砂岩储层,AVO特征可以定性地分辨出储层是否为高、中、低含气情况.反演结果显示,密度及纵横波速度比分别对高孔渗及致密砂岩储层的含气饱和度有着较好的指示能力.     

Azimuthally pre-stack seismic inversion for orthorhombic anisotropy driven by rock physics

Based on the long-wavelength approximation, a set of parallel vertical fractures embedded in periodic thin interbeds can be regarded as an equivalent orthorhombic medium. Rock physics is the basis for constructing the relationship between fracture parameters and seismic response. Seismic scattering is an effective way to inverse anisotropic parameters. In this study, we propose a reliable method for predicting the Thomsen’s weak anisotropic parameters and fracture weaknesses in an orthorhombic fractured reservoir using azimuthal pre-stack seismic data. First, considering the influence of fluid substitution in mineral matrix, porosity, fractures and anisotropic rocks, we estimate the orthorhombic anisotropic stiffness coefficients by constructing an equivalent rock physics model for fractured rocks. Further, we predict the logging elastic parameters, Thomsen’s weak parameters, and fracture weaknesses to provide the initial model constraints for the seismic inversion. Then, we derive the P-wave reflection coefficient equation for the inversion of Thomsen’s weak anisotropic parameters and fracture weaknesses. Cauchy-sparse and smoothing-model constraint regularization taken into account in a Bayesian framework, we finally develop a method of amplitude variation with angles of incidence and azimuth (AVAZ) inversion for Thomsen’s weak anisotropic parameters and fracture weaknesses, and the model parameters are estimated by using the nonlinear iteratively reweighted least squares (IRLS) strategy. Both synthetic and real examples show that the method can directly estimate the orthorhombic characteristic parameters from the azimuthally pre-stack seismic data, which provides a reliable seismic inversion method for predicting Thomsen’s weak anisotropic parameters and fracture weaknesses.     

Influence of Bedding and Mineral Composition on Mechanical Properties and Its Implication for Hydraulic Fracturing of Shale Oil Reservoirs

The premise of hydraulic fracturing is to have an accurate and detailed understanding of the rock mechanical properties and fracture propagation law of shale reservoirs. In this paper, a comprehensive evaluation of the mechanical properties of the shale oil reservoir in the south of Songliao Basin is carried out. Based on the experiments and the in-situ stress analysis, the fracture propagation law of three types of shale reservoirs is obtained, and the suggestions for fracturing are put forward. The results have shown that the fracture propagation of pure shale and low mature reservoir is easy to open along the bedding plane under compression loading, which is greatly influenced by the bedding. Sand-bearing shale is slightly better, the fractures of which are not easy to open along the bedding plane. The mechanical experimental results show that all the samples have the characteristics of low compressive strength, low Young''s modulus and strong anisotropy, indicating that the shale oil reservoir is certain plastic, which is related to its high clay mineral content and controlled by the bedding development. Compared with pure shale and low mature shale, the sand-bearing shale has less clay content and less developed bedding, which maybe the main reason for its slightly better brittleness. Overall, the expansion of hydraulic fracture is controlled by in-situ stress and bedding. Because of the development of bedding, it is easy to form horizontal fractures. Thus it is not suitable for horizontal well fracturing. Because of the high content of clay minerals, the applicability of conventional slick hydraulic fracturing fluid is poor. It is suggested to use vertical well or directional well to carry out volume fracturing. In this way, the effect of bedding can be effectively used to open and connect the bedding and form a larger fracture network.     

Multiscale rock-physics templates for gas detection in carbonate reservoirs

The heterogeneous distribution of fluids in patchy-saturated rocks generates significant velocity dispersion and attenuation of seismic waves. The mesoscopic Biot–Rayleigh theory is used to investigate the relations between wave responses and reservoir fluids. Multiscale theoretical modeling of rock physics is performed for gas/water saturated carbonate reservoirs. Comparisons with laboratory measurements, log and seismic data validate the rock physics template. Using post-stack and pre-stack seismic inversion, direct estimates of rock porosity and gas saturation of reservoirs are obtained, which are in good agreement with oil production tests of the wells.     

Integrating basin modeling with seismic technology and rock physics

We explore the link between basin modelling and seismic inversion by applying different rock physics models. This study uses the E‐Dragon II data in the Gulf of Mexico. To investigate the impact of different rock physics models on the link between basin modelling and seismic inversion, we first model relationships between seismic velocities and both (1) porosity and (2) effective stress for well‐log data using published rock physics models. Then, we build 1D basin models to predict seismic velocities derived from basin modelling with different rock physics models, in a comparison with average sonic velocities measured in the wells. Finally, we examine how basin modelling outputs can be used to aid seismic inversion by providing constraints for the background low‐frequency model. For this, we run different scenarios of inverting near angle partial stack seismic data into elastic impedances to test the impact of the background model on the quality of the inversion results. The results of the study suggest that the link between basin modelling and seismic technology is a two‐way interaction in terms of potential applications, and the key to refine it is establishing a rock physics models that properly describes changes in seismic signatures reflecting changes in rock properties.     

四川盆地侏罗系致密砂岩弹性特征及岩石物理建模

近年来围绕四川盆地侏罗系陆相致密砂岩已取得了勘探突破,其中川中—川西过渡带具备形成大气田的地质条件,但对该套致密砂岩弹性性质变化规律的研究还较少,致使利用地震方法进行"甜点"储层预测的精度不高.本文利用四川盆地侏罗系沙溪庙组32块样品开展了系统的声学测量,在此基础上,分析了样品弹性性质的变化规律.结合X射线衍射矿物组分分析、扫描电镜、铸体薄片和岩石薄片特征确定了不同成岩作用对岩石储集性能的影响.研究结果表明,研究区致密砂岩储层表现为孔隙型储层,受差异性成岩作用影响,黏土含量、钙质含量和硅质含量的差异以及它们分布特征之间的差异对岩石弹性性质造成了很大的影响.在研究区对岩石物性及弹性性质有明显影响的成岩作用包括早期的钙质胶结作用、压实作用和溶蚀作用,因此针对不同时期的成岩作用对岩石弹性及物性的影响,利用接触-胶结模型、微分等效模量模型和临界孔隙度校正的Hashin-Shtrikman上限模型建立了研究区致密砂岩的岩石物理模型.     

砂岩储层AVO特征影响因素的不确定性研究

传统的地震AVO正演研究多采用参数固定的岩石物理模型,而实际地层属性参数在勘探范围内具有不确定性.本研究以目标地层岩芯样品的实验室测试数据为基础,通过样品孔隙度和干燥状态下纵、横波阻抗的高度线性关系对岩石物理模型进行了简化,并结合实验测量和测井解释建立了主要模型参数的概率密度函数.采用Monte-Carlo随机正演和G...     

致密砂岩气储层的岩石物理模型研究

根据鄂尔多斯盆地苏里格气田以往实测和新测的共17口井51块岩样超声波实验数据,得到304组不同孔隙度和不同含水饱和度下对应的纵横波速度、泊松比等弹性参数.重新优选计算体积模量和泊松比与含气饱和度的关系,表明苏里格气田上古生界二叠系石盒子组盒8致密砂岩储层的模型与Brie模型(e=2)相似度最高.由此建立的苏里格气田储层岩石物理模型,更好的表征了致密岩石储层物理参数随含气饱和度变化规律,为该区储层预测提供了理论依据.致密储层岩石物理模型研究成果应用于苏里格气田多波地震资料气水预测中,实际例子表明该模型适用于该区的储层和含气性预测,并取得了较好的效果.     

Estimation of Dry Fracture Weakness,Porosity, and Fluid Modulus Using Observable Seismic Reflection Data in a Gas-Bearing Reservoir

Fracture detection and fluid identification are important tasks for a fractured reservoir characterization. Our goal is to demonstrate a direct approach to utilize azimuthal seismic data to estimate fluid bulk modulus, porosity, and dry fracture weaknesses, which decreases the uncertainty of fluid identification. Combining Gassmann’s (Vier. der Natur. Gesellschaft Zürich 96:1–23, 1951) equations and linear-slip model, we first establish new simplified expressions of stiffness parameters for a gas-bearing saturated fractured rock with low porosity and small fracture density, and then we derive a novel PP-wave reflection coefficient in terms of dry background rock properties (P-wave and S-wave moduli, and density), fracture (dry fracture weaknesses), porosity, and fluid (fluid bulk modulus). A Bayesian Markov chain Monte Carlo nonlinear inversion method is proposed to estimate fluid bulk modulus, porosity, and fracture weaknesses directly from azimuthal seismic data. The inversion method yields reasonable estimates in the case of synthetic data containing a moderate noise and stable results on real data.