考虑软矿物纵横比的页岩岩石物理建模及其应用
A petrophysical model of shales considering soft-mineral aspect ratios and its application
责任编辑: 叶佩
收稿日期: 2022-11-25 修回日期: 2023-09-8
基金资助: |
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Received: 2022-11-25 Revised: 2023-09-8
作者简介 About authors
杨骐羽(1999-),博士在读,专业为地质资源与地质工程,研究重点为地震反演与储层预测、页岩气“甜点”预测。Email:
以往针对页岩储层的岩石物理建模常忽略孔隙类型与软矿物纵横比对弹性模量的影响。本文同时考虑孔隙类型、孔隙形状与软矿物纵横比,建立横向各向同性页岩岩石物理模型:将固体矿物分为硬矿物与软矿物两类,考虑软矿物的各向异性特征与形状变化;基于储层实际情况将孔隙分为粒内孔、有机孔与粒间孔三类,孔隙形状分为硬孔隙与软孔隙两种;最后,利用粒子群法反演输入参数,进一步计算纵横波速度、各向异性参数与岩石力学参数。结合实际资料应用,用已知测井横波速度与各向同性岩石力学计算结果进行对比,结果表明该模型的应用效果较好。
关键词:
Previous petrophysical modeling of shale reservoirs often ignored the influence of pore types and soft-mineral aspect ratios on the elastic modulus.This study built a petrophysical model for transversely isotropic shales considering pore types and shapes,and soft-mineral aspect ratios.In this study,solid minerals were divided into hard and soft minerals,and soft minerals'anisotropic characteristics and shape changes were considered.According to the actual conditions of reservoirs, pores were categorized into intragranular,organic,and intergranular pores,and they were classified into stiff and soft pores based on their shapes.Finally,the input parameters were inverted using the particle swarm optimization algorithm to further calculate compressional and shear wave velocities,anisotropy parameters,and rock mechanical parameters.Combined with the actual data application,the results of this study were compared with the known results of shear wave velocity and isotropic rock mechanical calculation,suggesting that the model in this study is effective.
Keywords:
本文引用格式
杨骐羽, 李景叶, 吴凡, 李文瑾, 崔津铭.
YANG Qi-Yu, LI Jing-Ye, WU Fan, LI Wen-Jin, CUI Jin-Ming.
0 引言
建立更精确的页岩岩石物理模型,将有助于分析储层物性参数与弹性参数的关系,寻找敏感性参数,开展“甜点”预测等工作。1981年,Jones和Wang[1]提出页岩内部垂直定向排列的裂缝所引起的HTI(horizontal transverse isotropy)各向异性程度远小于由颗粒定向排列导致的VTI(vetical transverse isotropy)各向异性。对于垂向裂缝不发育的页岩储层,通常建立横向各向同性页岩岩石物理模型。
1994年,Hornby等[2]将自相容模型(self compatible model,SCA)和微分等效介质模型(differential equivalent medium model,DEM)推广至各向异性,为各向异性储层岩石物理建模提供理论基础。2007年,原宏壮[3]对Xu-White模型进行改进,将孔隙分为黏土孔隙与砂岩孔隙,建立了各向同性双重孔隙泥质砂岩有效介质模型。Bandyopadhyay[4]、Wu等[5] 与胡起等[6-7]分别考虑到页岩储层中有机质的各向异性与孔隙形状对弹性模量的影响,建立页岩岩石物理模型,但均忽略了黏土的各向异性及其形状的变化。2015年,王璞等[8]认为孔隙形状与矿物形状共同影响着岩石的纵横波速度,建立适用于各向同性介质的S-S模型。2019年,张琦斌等[9]基于各向同性假设,将孔隙分为干酪根中存在的有机孔与混合物中的粒间孔,建立了一种考虑含混合流体干酪根的页岩岩石物理模型,该模型的前提假设限制了模型精度。Ruiz等[10-11]通过实验室测量,证明了岩石基质中存在软孔隙(低孔隙纵横比),分别讨论了软孔隙与单一孔隙纵横比对岩石物理建模的影响,研究说明考虑页岩储层中的孔隙变化是有必要的。此后,桂俊川等[12]、张益明等[13]、刘致水等[14]以不同方法精细描述孔隙形状,建立了不同的岩石物理模型。
考虑到页岩储层的各向异性的特征,“甜点”区的孔隙度一般在0.01附近,且矿物形状与孔隙形状共同影响着储层的弹性模量[3]。本文基于页岩储层的实际地质情况,建立了同时考虑孔隙类型、孔隙形状与软矿物形状的横向各向同性的页岩岩石物理模型,基于粒子群法反演输入参数,开展纵横波速度、各向异性参数与岩石力学参数计算的工作,并结合实际资料进行验证。
1 页岩岩石物理建模及参数反演
1.1 储层特征分析
图1为研究工区储层测井曲线,其中,CAL为井径曲线、DEN为密度曲线、CNL为中子孔隙度曲线、GR为自然伽马曲线、RT为电阻率曲线;TOC为干酪根含量曲线;VSH为泥质含量曲线、POR为孔隙度曲线、SW为含水饱和度曲线;图中箭头所指区域为测井曲线指示的“甜点”区。本文选取黑色线框(2 250~2 290 m)区域为研究区进行岩石物理建模。在深度为2 269 m和2 290 m处有明显的扩径现象,可能导致预测精度的降低。目标层内干酪根含量基本高于0.02,具有工业开发价值;孔隙度整体较小,大致在1%附近,含水饱和度较高,大致在99%附近,且变化趋势不大。页岩在微观尺度下黏土、干酪根以及不为球形孔隙的定向排列,使得页岩具有较强的各向异性特征,且页岩储层内部矿物成分和孔隙类型较为复杂。
图1
1.2 岩石物理建模框架及其原理
研究区具有孔隙度低、干酪根含量高、泥质含量高的特征,扁平状孔隙、黏土与干酪根的定向排列均使得页岩具有较强的各向异性特征。若考虑过多因素进行岩石物理建模,必将使得模型的输入参数增加,给正演模拟带来一定困难。
页岩储层矿物成分复杂,本文将固体矿物分为脆性矿物(石英与长石)与软矿物(黏土、干酪根)两类。认为脆性矿物表现为各向同性特征,基于VRH平均模量(式1)计算脆性矿物混合物的弹性模量:
式中:
式中:
建立矿物混合物后,将储层孔隙按其类型分为脆性矿物中存在的粒内孔、介质I中存在的有机孔以及所有矿物混合物中的粒间孔,并利用各向异性DEM模型添加不同种类的孔隙。最后,基于Brown和Korringa[20]提出的各向异性流体替换模型(式6),得到饱和流体页岩岩石物理模型:
式中:
图2
1)选取长石的体积模量为37.5 GPa,剪切模量为15 GPa;石英的体积模量为37 GPa,剪切模量为4 GPa。工区内缺少脆性矿物含量,考虑到石英与长石模型差异不大,基于工区地质背景认为工区内4脆性矿物占比为常数(石英在混合矿物中占比为75%)。利用VRH模型计算得到混合矿物的等效模量,并基于各向同性DEM模型在混合矿物中添加粒内孔将其作为背景介质。
2)选取黏土的体积模量为21 GPa,剪切模量为7 GPa;干酪根的体积模量为2.9 GPa,剪切模量为2.7 GPa。干酪根的弹性模量较小,且实际储层中黏土与干酪根是相互嵌合的。考虑软矿物的各向异性特征,基于各向异性SCA+DEM模型建立介质I,并基于各向异性DEM模型中加入有机孔建立介质II。
3)基于各向异性DEM模型,先后将介质II与粒间孔逐步加入到步骤1)所建立的背景介质中,建立干页岩。
4)考虑工区内含水饱和度较高,基于地质背景认为孔隙内的混合流体为水和天然气。基于Reuss等应力平均模型计算混合流体的弹性模量。最后,利用各向异性流体替换模型(BK)进行流体替换,建立饱和流体页岩岩石物理模型。
1.3 模型参数的确定及其反演
岩石物理模型的输入参数可以分为各组分含量、各组分模量及描述孔隙与矿物几何形状的三类参数,其中各组分含量(干酪根含量、黏土含量、孔隙度等)可由工区测井曲线获得,各组分模量可通过查阅获得(如表1)。因此,本文所建立的岩石物理模型仅剩下3个参数:软矿物纵横比、孔隙纵横比和不同种类孔隙占比,其中,软矿物纵横比与孔隙纵横比均为地层压力、地应力等共同作用的结果,不易通过选取某一常数等效模拟。
组分 | 体积模量/GPa | 剪切模量/GPa | |
---|---|---|---|
矿物 | 石英 | 37 | 44 |
长石 | 37.5 | 15 | |
黏土 | 2.9 | 2.7 | |
干酪根 | 21 | 7 | |
流体 | 天然气 | 0.01 | 0 |
水 | 2.2 | 0 |
本文将孔隙细分为粒内孔、粒间孔和有机孔三类。将泥质含量设定为0.4,干酪根含量设定位0.05,含水饱和度设定为0.9,密度设定为2.2 g/cm3,分析仅加入粒间孔与加入三类孔隙对速度的影响,如图3所示。图3中黑线和红线分别为仅加入粒间孔得到的纵波速度曲线与横波速度曲线;带星号的黑线和红线分别为本文所提出的将孔隙细分为粒内孔、有机孔与粒间孔得到的纵波速度与横波速度,且三类孔隙占比为1∶1∶1。将孔隙细分后的速度明显降低;因此,有必要将孔隙按照储层实际情况进行细分。三类孔隙占比对速度的影响如图4所示,其中黑线表示粒内孔、有机孔、粒间孔的占比为1∶1∶1,蓝色星线表示占比为3∶1∶1,红色点线表示占比为1∶3∶1,绿色圈线表示占比为1∶1∶3。当孔隙度小于0.05时,纵波速度与横波速度并未受到孔隙占比的变化,其值基本没有变化;当孔隙度大于0.05时,纵波速度与横波速度随着孔隙占比的变化略微波动,但并不明显。基于上述分析,本文参考Xu-White模型,将各类孔隙按矿物在总矿物中的占比进行分配。
图3
图3
单一孔与三类孔对速度的影响
Fig.3
Effect of single hole and three kinds of hole on velocity
图4
设定与上文相同参数,分析与形状相关的参数对速度和各向异性参数的影响。三类孔隙的孔隙纵横比均由硬孔隙(孔隙纵横比为1)与软孔隙(孔隙纵横比为0.01)加权得到:
式中:
图5
图6
图6
软孔隙占比对Thomsen各向异性参数的影响
Fig.6
Influence of soft pore ratio on Thomsen anisotropy parameters
图7
图8
图8
软矿物纵横比对Thomsen各向异性参数的影响
Fig.8
Influence of soft mineral aspect ratio on Thomsen anisotropy parameters
图9
2 模型测试与应用
选取工区长7的井资料用于验证岩石物理模型的准确性与适用性。基于粒子群法的模型参数反演结果如图10a、b所示。参考Thomsen 给出的横向各向同性介质弹性参数的表达式(式 8)[23]计算纵横比速度,其结果如图10c~e所示,其中黑色实线与红色点线分别为模型得到的速度曲线和测井得到的速度曲线,图10e中黑色实线与黑色点线分别为模型得到的纵波速度与实际纵波速度的比值和模型得到的横波速度与实际横波速度的比值。由于纵波速度参与构造最佳适应度,模型得到的纵波速度与测井得到的纵波速度基本吻合,其相对误差介于[-0.03,0.03]之间,模型得到的横波速度与测井得到的横波速度吻合度低于纵波速度,但仍具有一定的精度,其相对误差介于[-0.09,0.09]之间。基于Thomsen给出的各向异性参数与等效刚度系数的关系式[20],其结果如图11a~c所示;在测井指示的“甜点”各向异性参数
式中:
图10
图10
反演曲线与模型得到的速度曲线
Fig.10
Inversion curve and velocity curve obtained by the model
图11
图11
各向异性参数与岩石力学参数计算结果
Fig.11
Anisotropy parameter and rock mechanics parameter calculation results
3 结论
本文同时考虑页岩储层孔隙类型、孔隙形状与软矿物形状,利用粒子群法反演主要参数,建立了一种更精确描述页岩储层的横向各向同性岩石物理模型,开展了基于该模型的横波速度预测、各向异性参数与岩石力学参数计算的工作。结合研究区实际测井资料和常规各向同性岩石力学参数计算结果对比验证,模型输出的波速度具有较高精度,各向异性参数与岩石力学参数具有一定的参考价值,本文所提出的模型对页岩储层具有一定的应用性。
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[本文引用: 1]
Three single-scattering approximations for coefficients in Biot’s equations of poroelasticity are considered: the average T-matrix approximation (ATA), the coherent potential approximation (CPA), and the differential effective medium (DEM). The scattering coefficients used here are exact results obtained previously for scattering from a spherical inclusion of one Biot material imbedded in another otherwise homogeneous Biot material. The CPA has been shown previously to guarantee that, if the coefficients for the scattering materials satisfy Gassmann’s equation, then the effective coefficients for the composite medium satisfy Brown and Korringa’s generalization of Gassmann’s equation. A collection of similar results is obtained here showing that the coefficients derived from ATA, CPA, or DEM all satisfy the required conditions for consistency. It is also shown that Gassmann’s equation will result from any of these single-scattering approximations if the collection of scatterers includes only spheres of fluid and of a single type of elastic solid.
On the dependence of the elastic properties of a porous rock on the compressibility of the pore fluid
[J].
DOI:10.1190/1.1440551
URL
[本文引用: 2]
An equation is derived for the dependence of the elastic properties of a porous material on the compressibility of the pore fluid. More generally, the elastic properties of a container of arbitrary shape are related to the compressibility of the fluid filling a cavity in the container. If the pore system or cavity under consideration is filled with a fluid of compressibility [Formula: see text], the compressibility κ* of the closed container is given by [Formula: see text] Here [Formula: see text] is the compressibility of the container with the fluid pressure held constant in the interconnected pore system or cavity. Fluids in other pores or cavities not connected with the one in question contribute to the value of [Formula: see text]. ϕ is the porosity, i.e., the volume fraction corresponding to the pore system or cavity in question. The equation contains two distinct effective compressibilities, [Formula: see text] and [Formula: see text], of the material exclusive of the pore fluid. When this material is homogeneous, one has [Formula: see text], and the equation reduces to a well‐known relation by Gassmann. For the other elastic properties, we also obtain expressions which generalize Gassmann’s work and which also differ from it only in the appearance of [Formula: see text] instead of [Formula: see text] in one term. Our result is intimately related to the reciprocity theorem of elasticity. Special cases are discussed.
正交各向异性岩石弹性参数的空间展布
[J].
The spatial distribution of elastic parameters of Orthotropic Rocks
[J].
A shale rock physics model and its application in the prediction of brittleness index,mineralogy,and porosity of the Barnet Shale
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