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1.
以鄂尔多斯盆地X地区长6储层为研究对象,利用多尺度CT成像技术、聚焦离子束扫描电镜技术,结合Avizo软件的强大数据处理和数值模拟功能,对研究区目的层岩石样品进行不同尺度孔喉分维数重构,建立三维超低渗透砂岩储层纳米级孔隙结构模型.研究表明,微米尺度下,超低渗透砂岩储层孔喉形态呈点状、球状和条带状及管状.储集空间类型以溶蚀微孔为主且多孤立分布,局部孔隙为片状,连通性较差.纳米尺度下,超低渗透砂岩储层孔喉系统整体较发育,孔喉形态呈球状、短管状.远离裂隙处多为孤立状,连通性较差,仅具有储集能力.微裂缝、粒间缝发育部位多为短管状,有一定连通性,相当于喉道.微观非均质性较强,岩样整体较致密,局部相互连通,溶蚀孔及裂隙对储集能力、渗滤能力具有控制作用.数值计算求得目的层A、Y、C三个样品的孔隙度分别为6.95%、5.55%、4.44%,渗透率分别为0.828×10^-3μm^2、0.115×10^-3μm^2、0.00065×10^-3μm^2.聚焦离子束扫描电镜与多尺度CT成像技术相结合能够定量表征超低渗透砂岩储层微、纳米级孔隙结构. 相似文献
2.
致密砂岩气藏储层成岩流体演化与致密成因机理——-以四川盆地上三叠统须家河组为例 总被引：15，自引：0，他引：15
四川盆地上三叠统须家河组储层在岩石学上表现为低成分成熟度、低胶结物含量和结构成熟度中等的“两低一中”特征,总体储层物性较差,属低孔低渗和特低孔特低渗储层,局部发育有少量中孔低渗储层．根据各成岩矿物共生组合关系,可以确定成岩矿物由早到晚形成的相对顺序：早期方解石（泥晶菱铁矿）-石英Ⅰ期加大-绿泥石薄膜-长石、岩屑溶解-绿泥石孔隙衬边-石英Ⅱ期加大（加大、剩余粒间孔、粒间溶孔充填石英）-溶蚀作用-石英Ⅲ期加大（粒间溶孔、粒内溶孔中充填石英）-连晶（含铁）方解石-白云石-铁（方解石）白云石-后期溶蚀作用-石英、方解石脉形成．机械压实作用是使本区岩石固结成岩最主要的因素,石英Ⅱ,Ⅲ期加大作用是使本层砂岩致密化的另一个重要原因．长期封闭条件下成岩流体只对长石和部分岩屑进行溶解,而粒间的胶结物如石英、碳酸盐则未发生溶解,使已致密化的须家河组砂岩孔隙度改变不大,这是须家河组砂岩最终保持致密化的又一个原因．有机质大规模生烃发生在第Ⅱ期石英加大后,烃类选择性进入相对有利的储层中形成致密砂岩烃类气藏． 相似文献
3.
《地球物理学进展》2017,(2)
特低渗储层油水分布关系复杂、微观孔喉网络分布模式及油水微观渗流机理复杂多变、水驱效率低、开发矛盾突出.可动流体饱和度是精细评价特低渗储层的关键因素,因此,利用铸体薄片、扫描电镜、X衍射、常规压汞、恒速压汞、核磁共振等实验手段,研究分析甘谷驿油田长6储层可动流体饱和度的分布特征及主控因素.结果表明:研究区长6储层的可动流体饱和度偏小,平均值为37.42%.微观孔隙结构是控制可动流体饱和度大小的主要因素,粘土矿物次之,储层物性的影响最弱.渗透率对可动流体饱和度的敏感性显著强于孔隙度.孔隙连通性好,孔喉比小,喉道半径粗、残余粒间孔保存较好、次生孔隙发育,粘土矿物含量小,可动流体饱和度相对较高.粒间孔的剩余程度、溶孔及喉道的发育程度等对储层的好坏及可动流体饱和度的大小具有至关重要的作用.孔隙特征参数中,喉道半径,孔隙半径,孔喉比、单位体积总有效孔喉体积与可动流体饱和度的关系更为密切. 相似文献
4.
《地球物理学进展》2017,(2)
致密油储层历经多重地质作用,储集体类型多样、尺度不一、分布离散、岩性混杂,常规建模不能有效刻画其内部复杂性.针对其发育不同尺度孔隙、复杂天然/人工缝网的特点,本文提出以致密砂岩储层微观孔隙结构表征、天然/人工裂缝的识别与描述技术为基础,集不同尺度孔隙、不同尺度天然/人工裂缝于一体的地质建模方法,即基于压汞资料,将孔隙型基质储层分级,相控随机模拟储层分布模型;以地震资料为主,结合蚂蚁体等技术,人机交互、确定性模拟大-中尺度天然裂缝;以测井解释小尺度、薄片鉴定微尺度天然裂缝为基础,地震参与随机模拟裂缝发育密度体,采用示性点过程法随机模拟小尺度与微尺度天然裂缝;以微地震资料为主,确定性加随机性综合模拟不同尺度人工压裂缝.以新疆吉木萨尔凹陷某油田开发先导试验区芦二段上甜点为例,按上述方法建立致密油藏模型,突出致密油藏特点,为数值模拟提供了高精度的多重介质油藏模拟静态模型. 相似文献
5.
鄂尔多斯盆地吴起地区上三叠统延长组长2油层组为典型的低孔低渗储集层,研究该区优质储集层的分布规律及形成条件,对于该层位油气勘探具有重要意义.本文通过研究岩芯、铸体薄片、储层物性等资料,探讨了储集层的沉积微相特征、岩石类型、孔隙类型、毛管压力特征、成岩作用特点及其对优质储集层的控制作用,表明吴起地区长2油层组主要发育长石岩屑砂岩,粘土质杂基等填隙物普遍发育,长2{的储集层物性最好,非均质性弱,主要孔隙类型为残余粒间孔和粒内溶孔,毛管排驱压力总体较低;沉积微相和后期成岩作用共同控制了优质储集层的分布. 相似文献
6.
鄂尔多斯盆地陇东地区特低渗透砂岩储层裂缝分布规律及其渗流作用 总被引：4，自引：0，他引：4
为了研究鄂尔多斯盆地上三叠统延长组特低渗透砂岩储层裂缝对开发的影响,利用地表露头、岩心、薄片、测井和实验等资料,对裂缝的成因类型、分布特征及其控制因素进行了分布,并对裂缝的渗流作用进行了讨论.研究区主要发育高角度构造裂缝以及水平层理缝、粒内缝和粒缘缝等成岩裂缝,粒内缝和粒缘缝是沟通储层基质粒间孔和粒内溶孔的重要通道,使特低渗透砂岩储层孔隙的连通性变好.裂缝的形成与分布受古构造应力场以及储层岩性、岩层厚度和岩层非均质性等内外因素的影响,现今应力场影响裂缝的保存状态与渗流作用.在燕山期和喜马拉雅期构造作用下,该区分布有北东向、北西向、近东西向和近南北向4组裂缝,但由于岩层非均质性的影响,在某一部位主要表现为两组近正交的裂缝分布型式.受现今应力场的影响,北东向裂缝的连通性好,张开度大,渗透率最高,开启压力最小,是该区的主渗流裂缝方向.随着油田开发,不同方向裂缝的渗透性还将发生动态变化. 相似文献
7.
国内深层-超深层碳酸盐岩储层普遍经历多阶段、多期次的成岩作用改造,非均质性极强,致使脱离地质背景的岩石物理分析方法难以准确地给出岩石物理变化规律与储层特征之间的联系.本文通过对合川地区震旦系灯影组四段深层白云岩岩石物理性质的测量,并结合岩石学特征分析、多尺度孔隙结构分析及沉积环境的划分,在成岩作用过程框架下分析岩石物理特征的变化规律.研究结果表明,目标层段白云岩差异性成岩过程形成了不同的岩石孔隙结构特征与微观结构特征.丘滩相高能微生物白云岩分别经历同生期微生物白云岩化与渗透回流白云岩化作用、准同生期选择性溶蚀作用、埋深白云岩化作用以及构造-热液白云岩化,形成的晶粒白云岩主要为致密“焊接”型和紧密型白云石晶体接触边界,溶蚀孔隙发育,微裂隙与孔喉共同作为流体渗流通道.滩间海低能硅质、泥质与灰质云岩主要经历弱毛细管浓缩白云岩化作用、强机械压实致密化与埋深白云岩化作用,形成的晶粒白云岩存在致密“焊接”型、黏土型、石英胶结型与方解石型晶体接触边界,孔隙以残余原生粒间或晶间孔为主,以微裂隙作为主要的流体渗流通道.晶粒边界性质与孔隙结构共同控制了样品的宏观物性及地震弹性性质,相同晶粒边界的样品具有一... 相似文献
8.
页岩储层由于其复杂的构造和孔隙特征,目前一般的岩石物理模型无法对其进行精确描述.微纳米孔隙作为页岩气主要的储集空间,对页岩整体弹性参数有较大影响.干酪根作为页岩中重要的有机质矿物,在页岩中的赋存状态随成熟度不同而变化,同时干酪根也是纳米级孔隙的主要发育场所.目前常规的岩石物理建模方法没有体现微纳米孔隙的作用,同时较少考虑不同成熟度下干酪根对页岩储层弹性性质的影响.本文采用一种微纳米孔隙理论描述页岩微纳米孔隙特性,考虑微纳米孔隙和不同成熟度下干酪根的赋存状态,应用上述微纳米孔隙模型、各向异性SCA-DEM模型、各向异性Eshelby-Cheng模型和Brown-Korringa固体替换方程等建立一种新的页岩储层岩石物理模型.利用中国西南某工区页岩气井对该模型进行验证,模型预测的横波速度与测井速度拟合较好.结果表明不同干酪根成熟度的页岩岩石物理建模结果具有一定的差异,据此可大致区分该工区井的干酪根成熟度;最后对微纳米孔参数进行正演分析,结果反映了页岩的纵横波速度随微纳米孔隙参数的变化趋势. 相似文献
9.
以鄂尔多斯盆地J地区下石盒子组致密砂岩气藏储层为研究对象，利用Avizo软件的数据处理和数值模拟功能，处理多尺度三维CT成像技术提供的图像信息.提出核磁阈值分割法，应用该方法对研究区目的层岩石样品进行多尺度三维孔喉分维数重构，建立非常规致密砂岩气藏储层纳米级三维孔喉分布和孔喉球棍模型.利用数字岩心分析模块对孔隙空间进行计算，获得喉道半径、孔隙半径、配位数、喉道长度、喉道形状因子、孔隙形状因子等定量参数，进而应用上述表征参数进行孔喉结构定量评价.研究表明，纳米尺度下致密砂岩储层孔隙呈球状，喉道呈杆状、棍状和管束状分布，孔喉分布随机性强.多尺度三维CT成像技术具有无损、高分辨率等特点，能够在多个分辨率和不同视域获得较为清晰的二维、三维图像.建立的球棍模型，能够直观、定量表征孔喉大小、几何形态、空间分布、连通性等，对微细观尺度储层孔喉研究具有重要意义. 相似文献
10.
针对B区块S油层含泥含钙中低孔特低渗储层渗透率计算精度低的难题,分析岩性、物性、孔隙结构对储层渗透率的影响,明确了孔隙度、泥质含量、钙质含量、孔隙结构是影响B区块S油层特低渗储层渗透率的主要因素,其中,孔隙结构是影响特低渗储层渗透率的关键因素.综合运用压汞曲线、孔喉半径分布特征以及流动单元指数反映特低渗储层孔隙结构变化,将特低渗储层按不同孔隙结构划分成3种类型,建立了特低渗储层类型的判别标准.利用中子测井、密度测井、声波测井、微球形聚焦测井、深浅侧向电阻率测井差值的绝对值等5个储层类型识别的敏感测井响应及参数,使用决策树法、最邻近结点法、BP神经网络法和支持向量机法建立了4种基于机器学习的储层判别方法,储层类型判别准确率依次提高,其中,基于支持向量机的储层类型判别方法判别准确率最高92.2%,且对3种类储层判别效果均很好.针对3类储层分别建立了渗透率计算公式．实际井解释结果表明,基于机器学习储层分类的渗透率模型计算B区块S油层特低渗储层渗透率精度明显高于储层分类前渗透率计算精度,其中,基于支持向量机储层分类计算的渗透率精度最高． 相似文献
11.
利用核磁共振技术对致密砂岩储层不同渗透率级别基质岩心和裂缝基质岩心不同驱替压力下CO2驱油特征进行了研究,简述核磁共振原理及实验方法。表明:致密砂岩储层特低、超低渗透基质岩心在初始CO2驱替压力下,岩心毛细孔隙和微毛细孔隙区间的油不同程度被采出,随着CO2驱替压力增大,特低、超低渗透基质岩心毛细孔隙区间油的采出程度不断增加且累积采出程度不同。裂缝致密砂岩储层岩心,裂缝和毛细孔隙区间的油在初始CO2驱替压力下,岩心裂缝中的油及毛细孔隙中的部分油被驱替出来,CO2驱替压力提高毛细孔隙、微毛细区间油的采出程度和累积采收程度较小。致密砂岩储层特低、超低渗透基质岩心和裂缝致密砂岩储层岩心,随着CO2驱替压力增大毛细孔隙区间的部分剩余油成正比增加进入到微毛细孔隙区间改变储层剩余油分布。核磁共振技术能够深入研究致密砂岩储层CO2不同驱替压力阶段,岩心裂缝、毛细孔隙区间、微毛细孔隙区间油的采出程度和剩余油分布情况,对于研究致密砂岩储层微观驱油机理具有较重要的价值。 相似文献
12.
Quantitative characterization of reservoir space in the Lower Silurian Longmaxi Shale,southern Sichuan,China 总被引：1，自引：0，他引：1
WANG YuMan DONG DaZhong YANG Hua HE Ling WANG ShiQian HUANG JinLiang PU BoLing WANG ShuFang 《中国科学:地球科学(英文版)》2014,57(2):313-322
Based on the drilling data of the Upper Ordovician Wufeng Shale and the Lower Silurian Longmaxi Shale in southern Sichuan Basin,the construction of matrix pores and the development condition of fractures in a marine organic-rich shale are quantitatively evaluated through the establishment of the reservoir petrophysical models and porosity mathematical models.Our studies show that there are four major characteristics of the Longmaxi Shale confirmed by the quantitative characterization:(1)the pore volume of per unit mass is the highest in organic matter,followed in clay minerals,finally in brittle minerals;(2)the porosity of the effective shale reservoir is moderate and equal to that of the Barnett Shale,and the main parts of the shale reservoir spaces are interlayer pores of clay minerals and organic pores;(3)the porosity of the organic-rich shale is closely related to TOC and brittle mineral/clay mineral ratio,and mainly increases with TOC and clay mineral content;(4)fractures are developed in this black shale,and are mainly micro ones and medium-large ones.In the Longmaxi Shale,the fracture density increases from top to bottom,reflecting the characteristics with high brittle mineral content,high Young’s modulus,low Poisson's ratio and high brittleness at its bottom. 相似文献
13.
《中国科学D辑(英文版)》2008,(Z2)
To study the impact of the fractures on development in the ultra-low permeability sandstone reservoir of the Yangchang Formation of the Upper Triassic in the Ordos Basin,data on outcrops,cores,slices,well logging and experiments are utilized to analyze the cause of the formation of the fractures,their distribution rules and the control factors and discuss the seepage flow effect of the fractures. In the studied area developed chiefly high-angle tectonic fractures and horizontal bedding fractures,inter-granular fractures and grain boundary fractures as well. Grain boundary fractures and intragranular fractures serve as vital channels linking intragranular pores and intergranular solution pores in the reservoir matrix,thus providing a good connectivity between the pores in the ultra-low perme-ability sandstone reservoir. The formation of fractures and their distribution are influenced by such external and internal factors as the palaeo-tectonic stress field,the reservoir lithological character,the thickness of the rock layer and the anisotropy of a layer. The present-day stress field influences the preservative state of fractures and their seepage flow effect. Under the tec-tonic effect of both the Yanshan and Himalayan periods,in this region four sets of fractures are distributed,respectively assuming the NE-SW,NW-SE,nearly E-W and nearly S-N orientations,but,due to the effect of the rock anisotropy of the rock formation,in some part of it two groups of nearly orthogonal fractures are chiefly distributed. Under the effect of the present-day stress field,the fractures that assume the NE-SW direction have a good connectivity,big apertures,a high permeability and a minimum starting pressure,all of which are main advantages of the seepage fractures in this region. With the development of oilfields,the permeability of the fractures of dif-ferent directions will have a dynamic change. 相似文献
14.
Diagenetic fluids evolution and genetic mechanism of tight sandstone gas reservoirs in Upper Triassic Xujiahe Formation in Sichuan Basin,China 总被引：4，自引：0，他引：4
RuKai Zhu CaiNeng Zou Nai Zhang XueSong Wang Rong Cheng LiuHong Liu ChuanMin Zhou LiHong Song 《中国科学D辑(英文版)》2008,51(9):1340-1353
The reservoirs of the Upper Triassic Xujiahe Formation in Sichuan Basin have the characteristics of low compositional maturity, low contents of cements and medium textural maturity. The general physical properties of the reservoirs are poor, with low porosity and low permeability, and there are only a few reservoirs with medium porosity and low permeability in local areas. Based on the diagenetic mineral association, a diagenetic sequence of cements is established: early calcites (or micrite siderites) →first quartz overgrowth→chlorite coatings→dissolution of feldspars and debris→chlorite linings→ second quartz overgrowth (quartz widen or filled in remain intergranular pores and solution pores)→dissolution→third quartz overgrowth (quartz filled in intergranular and intragranular solution pores)→intergrowth (ferro) calcites→dolomites→ferro (calcites) dolomites→later dissolution→veins of quartz and calcites formation. Mechanical compaction is the main factor in making the reservoirs tight in the basin, followed by the second and third quartz overgrowth. In a long-term closed system, only feld-spars and some lithic fragments are dissolved by diagenetic fluids, while intergranular cements such as quartz and calcit are not dissolved and thus have little influence on the porosity of the Xujiahe Formation. This is the third factor that may have kept the sandstones of Xujiahe Formation tight finally. The hydrocarbon was extensively generated from organic materials after the second quartz overgrowth, and selectively entered favorable reservoirs to form tight sandstone gas reservoirs. 相似文献
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致密油是重要的非常规石油资源,但由于其一次采收率低,需要开展提高采收率研究。由于致密油储层孔隙结构复杂,不同孔隙对产量的贡献差异很大,需要深入研究不同孔隙中原油的动用情况进而优化开发方式。本文采用核磁共振测试与驱替实验相结合的方法开展研究,直观反映不同注入条件下注入介质与原油的驱替过程,明确了注水和注CO2两种开发方式不同孔隙中原油的动用规律。结果表明:水驱和CO2驱不同孔隙区间原油动用程度随着驱替压差逐渐增大而增大;水驱和CO2驱大孔隙中的原油绝大部分被动用,随着孔隙半径逐渐减小,原油动用的比例逐渐降低;小于0.01μm微小孔隙中CO2驱动用效果明显好于水驱,致密油藏中CO2更易进入小孔隙提高致密油采收率。 相似文献
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A preparative method was developed to sample dissolved organic carbon (DOC) and heavy metals (Ni, Mn) from polluted surface waters. Main focus was set on the preparative production of freeze‐dried samples mainly composed of organic ligands from heavy metal complexes. First characterizations were done in the aqueous phase. Finally, freeze‐dried samples provide stable organic matter for multiple characterizations. Acid mine drainage of the former mining area of Ronneburg (Germany) hold elevated concentrations of heavy metals and low concentrations of DOC. Municipal wastewater, passing the spoil pile, held elevated concentrations of DOC and low concentrations of heavy metals. Dissolved components smaller than 0.45 μm and larger than 1 nm were concentrated by nanofiltration. Organic heavy metal complexes were isolated from this solution by size‐exclusion chromatography (SEC). Different size classes of molecules were collected from the column effluent, depending on their elution time, and were characterized by UV spectroscopy, ICP‐OES, and DOC measurements. All samples held organic heavy metal compounds with different retention times. The organic heavy metal complexes in acid mine drainage had higher retention times than complexes from municipal wastewater. Most interestingly, we found different affinities of heavy metals to different size classes of DOC. This affinity of the heavy metals differs with size and origin of the DOC. 相似文献
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中低温对流型地热资源在华北地区广泛分布,是一种清洁的替代能源.与活动断裂带相关的水热型地热资源是中低温地热系统的重要组成部分.本文基于高精度重力测量、微动测深及钻孔温度测量等数据,从热源、通道、储层和盖层四个方面探讨了南口—孙河断裂带水热系统特征.低重力异常揭示的燕山期花岗二长岩、闪长岩岩体范围为23.8 km~2和14.3 km~2,放射性测井数据计算得到其生热率均值为3.14μW·m~(-3),侏罗系火山岩生热率均值为1.65μW·m~(-3),隐伏岩体和火山岩均难以构成地热系统的附加热源.重力异常显示南口—孙河断裂带宽度约500~800 m,断裂带切割蓟县系雾迷山组白云岩热储层.钻井温度曲线显示断裂带内水热活动强烈,说明该断裂带是导水、导热的重要通道.断裂带南西侧马池口一带第四系松散层与侏罗系火山岩形成了热储盖层,微动测深显示火山岩最大厚度约1500 m.综上源、通、储、盖四个要素分析,该地热系统为热传导一对流复合型,来自京西北山区的大气降水经远距离径流深循环吸收地层热量后沿南口—孙河断裂上移到达裂隙发育的白云岩地层中形成热水.总之,沿南口—孙河断裂带具备了良好的地热地质条件,可达到规模开采的条件. 相似文献
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Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C33 (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C33. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures. 相似文献
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R. Agersborg M. Jakobsen B. O. Ruud T. A. Johansen 《Studia Geophysica et Geodaetica》2007,51(1):89-118
An effective medium model for the stress-dependent seismic properties of fractured reservoirs is developed here on the basis
of a combination of a general theory of viscoelastic waves in rock-like composites with recently published formulae for deformation
of communicating and interacting cavities (interconnected pores/cracks and fractures at finite concentration) under drained
loading. The inclusion-based model operates with spheroidal cavities at two different length scales; namely, the microscopic
scale of the pores and (grain-boundary) cracks, and the mesoscopic scale of the fractures (controlling the flow of fluid).
The different cavity types can in principle have any orientation and aspect ratio, but the microscopic pores/cracks and mesoscopic
fractures were here assumed to be randomly and vertically oriented, respectively. By using three different aspect ratios for
the relatively round pores (representing the stiff part of the pore space) and a distribution of aspect ratios for the relatively
flat cracks (representing the compliant part of the pore space), we obtained a good fit between theoretical predictions and
ultrasonic laboratory measurements on an unfractured rock sample under dry conditions. By using a single aspect ratio for
the mesoscopic fractures, we arrived at a higher-order microstructural model of fractured porous media which represents a
generalization of the first-order model developed by Chapman et al. (2002,2003). The effect of cavity size was here modelled
under the assumption that the characteristic time for wave-induced (squirt) flow at the scale of a particular cavity (pore/crack
vs. fracture) is proportional with the relevant scale-size. In the modelling, we investigate the effect of a decreasing pore
pressure with constant confining pressure (fixed depth), and hence, increasing effective pressure. The analysis shows that
the attenuation-peak due to the mesoscopic fractures in the reservoir will move downward in frequency as the effective pressure
increases. In the range of seismic frequencies, our modelling indicates that the P-wave velocities may change by more than
20% perpendicular to the fractures and close to 10% parallel to the fractures. In comparison, the vertical S-wave velocities
change by only about 5% for both polarization directions (perpendicular and parallel to the fractures) when the effective
pressure increases from 0 to 15 MPa. This change is mainly due to the overall change in porosity with pressure. The weak pressure
dependence is a consequence of the fact that the S waves will only sense if the fractures are open or not, and since all the
fractures have the same aspect ratio, they will close at the same effective pressure (which is outside the analysed interval).
Approximate reflection coefficients were computed for a model consisting of the fractured reservoir embedded as a layer in
an isotropic shale and analysed with respect to variations in Amplitude Versus Offset and aZimuth (AVOZ) properties at seismic
frequencies for increasing effective pressure. For the P-P reflections at the top of the reservoir, it is found that there
is a significant dependence on effective pressure, but that the variations with azimuth and offset are small. The lack of
azimuthal dependence may be explained from the approximate reflection coefficient formula as a result of cancellation of terms
related to the S-wave velocity and the Thomson’s anisotropy parameter δ. For the P-S reflection, the azimuthal dependence
is larger, but the pressure dependence is weaker (due to a single aspect ratio for the fractures).
Finally, using the effective stiffness tensor for the fractured reservoir model with a visco-elastic finite-difference code,
synthetic seismograms and hodograms were computed. From the seismograms, attenuation changes in the P wave reflected at the
bottom of the reservoir can be observed as the effective pressure increases. S waves are not much affected by the fractures
with respect to attenuation, but azimuthal dependence is stronger than for P waves, and S-wave splitting in the bottom reservoir
P-S reflection is clearly seen both in the seismograms and hodograms. From the hodograms, some variation in the P-S reflection
with effective pressure can also be observed. 相似文献