Full 3-D numerical modeling of borehole electric image logging and the evaluation model of fracture

A full 3-D finite element method numerical modeling program is written based on the principle and technical specification of borehole electric image well logging tool. The response of well logging is computed in the formation media model with a single fracture. The effect of changing fracture aperture and resistivity ratio to the logging response is discussed. The identification ability for two parallel fractures is also present. A quantitative evaluation formula of fracture aperture from borehole electric image logging data is set up. A case study of the model well is done to verify the accuracy of the for-mula. The result indicates that the formula is more accurate than the foreign one.     

Estimation of the apertures of water-saturated fractures by nuclear magnetic resonance well logging

Fracture aperture is an important transport property in subsurface hydrology because it influences well productivity and the volume of the water resource. Nuclear magnetic resonance (NMR) well logging measures the hydrogen‐bearing fluid molecules in porous or fractured strata, and the NMR signal intensity increases with the amount of fluid in the sensed region of the NMR sonde. Fluid confined in a large fracture of >>0.2 mm in aperture has T2 (i.e. spin‐spin relaxation time) values as long as those of the bulk fluid. The bulk‐fluid porosity (i.e. porosity calculated using this long T2 component in a T2 histogram data) increases linearly with aperture. Therefore, NMR logging enables quantitative estimation of fracture apertures of >>0.2 mm using the bulk‐fluid porosity data if the calibration of the NMR sonde is performed adequately. We applied NMR logging to a borehole in a Holocene andesite lava at Sumikawa, Japan, to estimate the aperture of open fractures within the lava. A test well of 100 m depth and 20 cm diameter, filled with bentonite drilling mud, was scanned with an NMR sonde to obtain a profile of the porosity and the T2 histogram of the andesite. The bulk‐fluid porosity was calculated from the T2 histogram data, as the porosity at which the T2 value is larger than or equal to a threshold T2 of bulk bentonite mud. The bulk‐fluid porosity of a specific inclined fracture responsible for the total loss of circulation at 61.2 m depth during drilling was calculated assuming a threshold or T2 cut‐off of 33 ms, and again for a cut‐off of 100 ms. Calibration of the NMR sensor in a laboratory and measurement of the fracture dip angle by electrical microimaging logging enabled us to estimate the fracture aperture as 1.7 cm, assuming a T2 cut‐off of 33 ms, or 1.6 cm for a T2 cut‐off of 100 ms. The method of aperture determination described in this study is independent of fluid species and lithology, and is applicable to various hydrogen‐bearing borehole fluids (clean water, mud and oil) and geological settings.     

含有倾斜薄裂缝孔隙地层中的井孔声场

应用三维交错网格应力-速度有限差分方法,数值模拟了含有倾斜裂缝孔隙介质地层中点声源所激发的井孔声场问题.为满足薄裂缝计算需求,开发了不均匀网格有限差分算法,提高了计算精度及计算速度.利用将孔隙介质方程参数取为流体极限的办法来处理裂缝中的流体,实现了流体-孔隙介质界面处的差分方程统一,使界面处的计算更加灵活方便.在验证了方法正确性的基础上,分别考察了单裂缝宽度、裂缝带宽度、裂缝倾斜角度以及孔隙介质渗透率等参数的变化对井轴上阵列波形的影响并进行了分析.结果表明,声波经过裂缝时可能产生反射横波及斯通利波,后者随裂缝宽度的减小而减小,而前者随裂缝宽度的改变,变化不大,在裂缝很小(20μm)时依然存在;裂缝带的宽度、密度越大,反射斯通利波越强;当裂缝(裂缝带)倾斜时,反射横波消失,但反射斯通利波受裂缝倾斜角度的影响较小;渗透率的改变对斯通利波的衰减影响较为明显.     

成像测井解释模式在基岩油气藏裂缝性储层的应用研究

基岩油气藏裂缝性储层具有复杂的储集空间和储层非均质性,为了实现对基岩油气藏储层的精细评价,以地层微电阻率扫描成像测井和井周声波成像测井资料为核心,通过岩心资料标定,结合录井、常规测井、试油、地质等实际资料,系统建立了基岩油气藏变质岩储层的成像测井解释模式.根据成像测井模式的识别实现了对基岩油气藏特征的认识、准确的裂缝分析和现今地应力场分析.分析结果表明,研究区基底变质岩地层中基本以基岩内幕油气藏为主;裂缝以中高角度缝、网状裂缝为主,其主要走向与井旁断层走向大致平行,属纵裂缝;裂缝主要发育在东西两侧靠近断层、近源的构造陡坡上;现今最大水平主应力方向主要呈NE-SW和NEE-SWW.成像测井解释结果与地质情况吻合较好.     

Acoustic reflection well logging modeling using the frequency-domain finite-element method with a hybrid PML

In this paper, we propose a hybrid PML (H-PML) combining the normal absorption factor of convolutional PML (C-PML) with tangential absorption factor of Mutiaxial PML (M-PML). The H-PML boundary conditions can better suppress the numerical instability in some extreme models. and the computational speed of finite-element method and the dynamic range are greatly increased using this HPML. We use the finiteelement method with a hybrid PML to model the acoustic reflection of the interface when wireline and well logging while drilling (LWD), in a formation with a reflector outside the borehole. The simulation results suggests that the PS- and SP- reflected waves arrive at the same time when the inclination between the well and the outer interface is zero, and the difference in arrival times increases with increasing dip angle. When there are fractures outside the well, the reflection signal is clearer in the subsequent reflection waves and may be used to identify the fractured zone. The difference between the dominant wavelength and the model scale shows that LWD reflection logging data are of higher resolution and quality than wireline acoustic reflection logging.     

裂缝性地层方位侧向测井响应数值模拟

为研究井周裂缝发育特征,本文提出一种新型方位侧向测井方法,利用三维有限元法,模拟裂缝的方位侧向测井响应.结果显示,深浅侧向电阻率幅度差异受裂缝倾角的控制,低角度缝为负差异,高角度缝为正差异;倾斜裂缝张开度的增大使测井响应值减小,方位电阻率差异增大;井周方位电阻率可反映裂缝方位产状,单一缝或裂缝密度较小时,沿裂缝走向的方位电阻率小,沿裂缝倾向的方位电阻率大;裂缝发育地层的测井响应显示宏观各向异性特征,但方位电阻率的差异显示发生反转现象,即沿裂缝走向/层理方向的方位电阻率大,沿裂缝倾向/垂直层理方向的方位电阻率小;对方位电阻率测井响应进行井周成像,直观显示了裂缝的产状和发育特征.     

裂隙充填型天然气水合物的各向异性分析及饱和度估算——以印度东海岸NGHP01-10D井为例

沿裂隙发育的天然气水合物是印度深水盆地细粒沉积物中水合物的重要产出方式,水合物以结核状或脉状充填在高角度裂隙中.天然气水合物主要沿着构造主应力方向生成,由于裂隙的存在,含水合物的沉积物层呈现各向异性.利用孔隙介质中水合物呈均匀分布的速度模型计算的NGHP01-10D井水合物饱和度高达40%,而压力取芯表明水合物饱和度占孔隙空间的20%左右.为了研究水合物饱和度差异,基于层状介质的各向异性模型计算了裂隙充填型水合物的饱和度.在垂直井孔中,由于波入射角与裂隙倾角有关,考虑裂隙倾角变化,利用纵波和横波速度同时反演水合物饱和度和裂隙倾角.利用层状介质模型计算的水合物占孔隙空间的15%～25%,裂隙的倾角在60°～90°,多为高角度裂隙.在NGHP01-10D井中,纵横波速度联合计算的饱和度与压力取芯结果吻合更好.     

利用钻孔雷达探测地下含水裂缝

钻孔雷达是地质雷达的一种特殊模式.和通常的地质雷达比起来,它有好几个显著的特点.比如,利用钻孔接近地下的某个区域,天线可以相对接近所要测量的地质异常体或目标,从而导致在较深的范围对目标的精确测量.实验场地位于北京西部的一个石灰岩的小山上.该处有一组钻孔,并被大量的裂缝所切割.在这些钻孔中,我们进行了单孔反射测量和跨孔测量.测量的数据经过处理和解释表明,在该场地雷达的径向探测范围可达30米,地下裂缝的分布可以形成清晰的图像.我们可以看到很多裂缝,它们距钻孔的距离及倾角都可以确定下来.在有些情况下,确定裂缝分布的方位是可能的.     

声波电视在刚果(金)铜钴矿勘察中的应用

声波电视是利用声波走时和振幅参数进行钻孔壁成像的一种地球物理测井系统。由于采用数字技术和配置功能完善的软件,系统具有图像分辨率高、测井速度快、可在混浊的井液中工作、数据处理和分析过程相对简单的特性,是研究岩层结构面的有效手段。在刚果（金）某铜钴矿岩体稳定性评价中,采用声波测井方法,获取岩体的优势结构面,为确定采坑边坡角、岩体抗剪强度指标提供了可靠数据。本文介绍RG声波电视测井的原理及方法,以DK02孔为例,对所揭露岩层结构面、裂隙发育信息进行分析。工程实践说明,声波井下电视在进行深部岩体的原位勘察方面具有明显优势。      

Influence of Pressure Change During Hydraulic Tests on Fracture Aperture

In a series of field experiments, we evaluate the influence of a small water pressure change on fracture aperture during a hydraulic test. An experimental borehole is instrumented at the Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT). The target fracture for testing was found from the analyses of borehole logging and hydraulic tests. A double packer system was developed and installed in the test borehole to directly observe the aperture change due to water pressure change. Using this packer system, both aperture and flow rate are directly observed under various water pressures. Results indicate a slight change in fracture hydraulic head leads to an observable change in aperture. This suggests that aperture change should be considered when analyzing hydraulic test data from a sparsely fractured rock aquifer.     

垂直井眼中三维阵列感应测井资料自适应井眼校正

三维感应测井受井眼环境影响很大,如何消除这些不利影响是有效获取各向异性地层电导率真值并提高资料处理和解释质量的一项重要工作.本文以一维柱状各向异性地层模型为基础,通过最小平方反演技术从实际测量资料中快速反演井径、偏心距、泥浆电导率、地层水平电导率和各向异性系数等模型参数,设法减小井眼环境对测量结果的影响.首先,利用仪器偏心情况下不同旋转角电导率张量的关系方程,建立旋转角提取与校正方法,获取无旋转角三维感应测井数据.在此基础上,根据事先计算好的井眼校正库并结合多维非线性有限元逼近技术,建立快速计算三维感应测井响应以及Jacobi导数矩阵的插值公式,并利用自适应正则化迭代反演技术不断修改模型参数,实现理论合成资料与输入资料的最佳拟合.最后,通过反演出的模型参数计算三维感应测井资料所有分量的校正量,实现三维感应资料的井眼校正.理论模型和实际资料的处理结果均证实,在低阻井眼泥浆情况下,一维柱状模型中的反演方法仍然可以提取出较可靠的地层水平和垂直电导率,且井眼环境校正对于受井眼环境影响相对较小的测井曲线具有非常好的校正效果.     

用传播矩阵法研究层状交错地层中的多分量感应测井

为了实现交错沉积等复杂环境中的电磁场数值模拟,本文在常规横向同性模型的基础上引入了电导率主轴坐标系相对地层坐标系的层理方位角和倾角,建立了交错地层模型.并利用传播矩阵法建立了一维层状交错地层模型中的多分量感应测井仪器响应的正演模拟算法.首先将频率-波数域中的电磁场分解为上行和下行模式波,给出了任意朝向的磁偶极子在无限大地层中模式波的解析解.进一步通过引入地层界面上的透射、局部反射以及广义反射系数矩阵,推导了一维层状地层中的模式波表达式.在此基础上,利用二维Gauss-Legendre积分实现了Fourier逆变换,得到了可用于多分量感应测井模拟的频率-空间域磁场并矢格林函数.最后,通过多个数值模拟结果考察了井眼倾角、层理方位角和倾角变化对多分量感应测井响应的影响.     

Study on the simulation of acoustic logging measurements in horizontal and deviated wells

The conventional acoustic logging interpretation method, which is based on vertical wells that penetrate isotropic formations, is not suitable for horizontal and deviated wells penetrating anisotropic formations. This unsuitability is because during horizontal and deviated well drilling, cuttings will splash on the well wall or fall into the borehole bottom and form a thin bed of cuttings. In addition, the high velocity layers at different depths and intrinsic anisotropy may affect acoustic logging measurements. In this study, we examine how these factors affect the acoustic wave slowness measured in horizontal and deviated wells that are surrounded by an anisotropic medium using numerical simulation. We use the staggered-grid finite difference method in time domain (FDTD) combined with hybrid-PML. First, we acquire the acoustic slowness using a simulated array logging system, and then, we analyze how various factors affect acoustic slowness measurements and the differences between the effects of these factors. The factors considered are high-velocity layers, thin beds of cuttings, dipping angle, formation thickness, and anisotropy. The simulation results show that these factors affect acoustic wave slowness measurements differently. We observe that when the wavelength is much smaller than the distance between the borehole wall and high velocity layer, the true slowness of the formation could be acquired. When the wavelengths are of the same order (i.e., in the near-field scenarios), the geometrical acoustics theory is no longer applicable. Furthermore, when a thin bed of cuttings exists at the bottom of the borehole, Fermat's principle is still applicable, and true slowness can be acquired. In anisotropic formations, the measured slowness changes with increments in the dipping angle. Finally, for a measurement system with specific spacing, the slowness of a thin target layer can be acquired when the distance covered by the logging tool is sufficiently long. Based on systematical simulations with different dipping angles and anisotropy in homogenous TI media, slowness estimation charts are established to quantitatively determine the slowness at any dipping angle and for any value of the anisotropic ratio. Synthetic examples with different acoustic logging tools and different elastic parameters demonstrate that the acoustic slowness estimation method can be conveniently applied to horizontal and deviated wells in TI formations with high accuracy.     

基于小波高频属性的泥页岩裂缝测井识别方法研究

近几年受地震属性分析技术的启发,出现了利用测井属性分析泥页岩裂缝油气藏的新方法.为了避免常规测井信号受到噪声干扰影响裂缝识别效果,利用小波变换的高频属性对裂缝识别展开研究.以贵州岑巩地区下寒武统牛蹄塘组的泥页岩裂缝性油气藏为例,观察和分析野外黑色页岩露头、岩心薄片及镜下照片,总结了该地区裂缝发育特征和成因;然后着重利用研究区的测井资料,运用小波阈值去噪和小波高频属性方法对裂缝进行了识别.研究结果表明:研究区主要发育高角度剪性缝、张剪性缝、低角度滑脱缝及层间缝,采用的小波阈值去噪方法具有高信噪比和低均方差特点,在保留原始信号特点的基础上得到更加真实的信号;通过小波高频属性方法提取了声波时差信号中的高频信号,识别出了常规测井方法无法识别的泥页岩裂缝,该套方法成功在贵州岑巩页岩气裂缝识别中得到成功应用,并具有一定的借鉴意义.     

井旁裂缝的双侧向测井物理模拟及特征分析

为深入认识裂缝性地层的双侧向测井响应特征,指导裂缝识别及裂缝参数表征,基于数值仿真开展了井旁裂缝的缩小比例物理模拟.在物理模拟平台构建的基础上,用不同浓度的氯化钠溶液模拟基岩,用不同宽度的镀锌板来模拟高导裂缝,实现测量过程的可视化.通过实验分析井旁裂缝的规模、发育位置和产状等因素对深、浅侧向电阻率的影响.研究表明:在仪器有效探测范围内,垂直缝延伸长度对深浅侧向电阻率的数值影响较小;裂缝离井壁越远,双侧向电阻率明显增大,但深浅侧向电阻率值的降低幅度不同;随着裂缝倾角增大,深侧向电阻率增大,浅侧向电阻率无明显变化,深浅侧向电阻率从负幅度差向正幅度差过渡,但临界角的观测十分困难,应用双侧向测井判断地层产状时,需特别注意其适用条件.     

APPLICATION OF BOREHOLE GEOPHYSICS AT AN EXPERIMENTAL WASTE STORAGE SITE*

A suite of electrical, radiation, and mechanical borehole probes were run in a 76-mm-diameter borehole drilled to a slant depth of 380 m in leptite and granite. The hole is located in Precambrian bedrock in central Sweden where a site is dedicated to in-situ experiments pertaining to the disposal of radioactive wastes. The challenge to borehole logging methods for such site investigations is to resolve geological features and fluid flow parameters in geological sites which are initially chosen for their homogeneity, low porosity, and minimal fracturing. The Stripa borehole is characterized by high electrical resistivity values in the 20–100 kΩm range, by acoustic velocities around 5800 m s^-1 (which is close to laboratory values on intact specimens), and by total porosity of around one volume percent. In this context, probe resolution was adequate to produce interpretable information on almost all of the logs. Two principal rock types were encountered in the hole: granite, of quartz monzonitic composition, and leptite. The granite and leptite intercepts are subdivided into units characterized by mafic mineral content, sulfide mineral content, and electrical and radiation properties. Iron-rich zones in the leptite are highly anomalous on the gamma-gamma and neutron logs; thin mafic zones in the granite can also be distinguished. Occurrences of a few percent pyrite are detected by the electrical, gamma-gamma, and neutron logs. Although overall porosity is quite low throughout the hole, analysis of the resistivity and neutron logs indicates the porosity increases by a few volume percent at fracture zones. The differential resistance and caliper probes detect borehole diameter roughness of less than 1 mm, helping to confirm acoustic waveform anomalies which are indicative of fracture zones. Compres-sional wave transit time and shear-wave interference patterns usually occur coincident with open fractures observed in core, the correlation being especially good at major fracture zones.     

An effective data processing flow for the acoustic reflection image logging

Recent studies have revealed the great potential of acoustic reflection logging in detecting near borehole fractures and vugs. The new design of acoustic reflection imaging tool with a closest spacing of 10.6m and a certain degree of phase steering makes it easier to extract the reflection signals from the borehole mode waves. For field applications of the tool, we had developed the corresponding processing software: Acoustic Reflection Imaging. In this paper, we have further developed an effective data processing flow by employing multi‐scale slowness‐time‐coherence for reflection wave extraction and incorporating reverse time migration for imaging complicated subtle structures with the strong effects of borehole environment. Applications of the processing flow to synthetic data of acoustic reflection logging in a fractured formation model and interface model with fluid filled borehole generated by 2D finite difference method, and to the physical modelling data from a laboratory water tank, as well as to the field data from two wells in a western Chinese oil field, demonstrate the validity and capability of our multi‐scale slowness‐time‐coherence and reverse time migration algorithms.     

Hydraulic Conductivity Calibration of Logging NMR in a Granite Aquifer,Laramie Range,Wyoming

In granite aquifers, fractures can provide both storage volume and conduits for groundwater. Characterization of fracture hydraulic conductivity (K) in such aquifers is important for predicting flow rate and calibrating models. Nuclear magnetic resonance (NMR) well logging is a method to quickly obtain near-borehole hydraulic conductivity (i.e., K_NMR) at high-vertical resolution. On the other hand, FLUTe flexible liner technology can produce a K profile at comparable resolution but requires a fluid driving force between borehole and formation. For three boreholes completed in a fractured granite, we jointly interpreted logging NMR data and FLUTe K estimates to calibrate an empirical equation for translating borehole NMR data to K estimates. For over 90% of the depth intervals investigated from these boreholes, the estimated K_NMR are within one order of magnitude of K_FLUTe. The empirical parameters obtained from calibrating the NMR data suggest that “intermediate diffusion” and/or “slow diffusion” during the NMR relaxation time may occur in the flowing fractures when hydraulic aperture are sufficiently large. For each borehole, “intermediate diffusion” dominates the relaxation time, therefore assuming “fast diffusion” in the interpretation of NMR data from fractured rock may lead to inaccurate K_NMR estimates. We also compare calibrations using inexpensive slug tests that suggest reliable K_NMR estimates for fractured rock may be achieved using limited calibration against borehole hydraulic measurements.     

一种基于圆弧片状压电振子的相控圆弧阵随钻声波测井辐射器

随钻三维反射声波成像测井技术可以实时地对井周围的地层构造和地质体进行成像,为地质导向钻井提供必要的信息,是下一代声波测井技术的发展方向.针对该项技术,提出了一种基于圆弧片状压电振子的相控圆弧阵声波测井辐射器,推导了该辐射器的声学性能在波数-频率域的数学描述,并采用了实轴积分的方法对该辐射器在无限大液体、井旁地层中产生的波场进行了求解.研究结果显示,无论在无限大液体中还是在充液井孔内,该声源均可以向任意方位定向辐射能量,其水平指向性图主瓣明显,旁瓣级低,具有较高的方位分辨率;相控阵技术能够使得即使在较低的频率下,该声源辐射的声场仍具有较好的方位特征;与传统的单极子反射成像技术相比,有希望利用该声源发展一种具有较好的方位分辨能力和更深径向探测深度的随钻反射成像测井方法;与偶极子反射成像技术相比,采用该声源可以在周向上360°范围内确定反射体的方位,能够消除井旁地层界面方位测量的多解性.     

Study on 3D simulation of wave fields in acoustic reflection image logging

The borehole acoustic reflection imaging logging is a newly developed acoustic logging method that has attracted many interests. These converted and reflected waves for imaging are usually mixed up with borehole guided waves and therefore difficult to be clearly identified. To improve the downhole tool design and develop more sophisticate data processing and interpretation algorithms,studies on precisely numerical modeling of the wave fields in the acoustic reflection imaging logging are neces-sary and critical. This paper developed a parallelized scheme of 3D finite difference (3DFD) with non-uniform staggered grid and PML absorbing boundary to simulate the acoustic wave fields in isotropic and anisotropic formations. Applications of this scheme to the typical cases of isotropic and anisot-ropic formations and comparison with the results from published analytical solutions have demon-strated the validation and efficiency of the scheme. Higher accuracy and lower computation cost (3.5 times faster than the conventional schemes) have been achieved with this scheme for modeling such a complex wave fields of 60 dB dynamic range with higher frequency (10 kHz). This simulating program provides a quantitative analytical means for studying acoustic reflection imaging tool and development of the data processing and interpretation methods.