CDP mapping and image reconstruction by using offset VSP data

Because zero-offset VSP (Vertical Seismic Profile) data can only provide the information of rock properties and structure in the area around the Fresnel zone within the well, the scheme of VSP with offset was developed to acquire the reflection information away from the borehole in order to widen the range of VSP survey and to improve the precision of imaging.In this paper, we present a new CDP (Common Depth Point) mapping approach to image the reflecting structure by using offset VSP data. For the processing of offset VSP data, we firstly separated the up-going and down-going wave-fields from VSP data by means of F-K filtering technique, and we can calculate the mapping conditions (position and reflecting traveltime for CDP point) in homogeneous media, and then reconstruct the inner structure of the earth.This method is tested by using the offset VSP data which are used to simulate the case of super-deep borehole by means of finite-difference method. The imaged structure matches the real model very well. The results show that the method present here could accurately image the inner structure of the earth if the deviation of initial velocity model from the true model is less than 10%. Finally, we presented the imaged results for the real offset data by using this method.     

Inversion of ocean-bottom seismometer (OBS) waveforms for oceanic crust structure: a synthetic study

The waveform inversion method is applied—using synthetic ocean-bottom seismometer(OBS) data—to study oceanic crust structure. A niching genetic algorithm(NGA) is used to implement the inversion for the thickness and P-wave velocity of each layer, and to update the model by minimizing the objective function, which consists of the misfit and cross-correlation of observed and synthetic waveforms. The influence of specific NGA method parameters is discussed, and suitable values are presented.The NGA method works well for various observation systems, such as those with irregular and sparse distribution of receivers as well as single receiver systems. A strategy is proposed to accelerate the convergence rate by a factor of five with no increase in computational complexity; this is achieved using a first inversion with several generations to impose a restriction on the preset range of each parameter and then conducting a second inversion with the new range. Despite the successes of this method,its usage is limited. A shallow water layer is not favored because the direct wave in water will suppress the useful reflection signals from the crust. A more precise calculation of the air-gun source signal should be considered in order to better simulate waveforms generated in realistic situations; further studies are required to investigate this issue.     

Study of the Reservoir Damage during Cyclic Injection and Extraction Process in the Hutubi Gas Storage

During the injection and extraction process in gas storage,cyclic loading stress is applied to the reservoir pore structure,affecting the capacity and service life of gas storage. To study the influence of cyclic injection and extraction on reservoir physical property,a finite-element model of reservoir elastoplastic damage under the effect of pore pressure is established based on elastoplastic mechanics and fatigue-damage mechanics. Considering the mechanical property of the Hutubi reservoir and the injection-extraction mode of gas storage,the finite-element model is used to study the pore pressure and porosity change,fault safety,and ground surface deformation. The results show that the change in pore pressure and porosity,the deformation of the Hutubi fault,as well as the uplift and sink of the grounds surface are controlled by the fatigue damage of the reservoir pore structure during the injection-extraction process. After a 6-year service life, the cumulative deformation of the reservoir pore reaches its limit,causing pore structure collapse and porosity decrease. As a result,storage capacity declines and the ground surface starts to sink. Through this study,the safety of the Hutubi gas storage is evaluated,and some suggestions for field operations are offered.     

Optimization of the Sefid-Roud Dam desiltation process using a sophisticated one-dimensional numerical model

Although water and soil conservation activities reduce reservoir sedimentation, it is inevitable that reservoirs fed by rivers transporting high amounts of sediment will experience sedimentation. The Ghezel-Ozan and Shah-Roud rivers, which flow to the Sefld-Roud reservoir dam, are both highly sediment-laden and transport significant amounts of sediment in both bed load and suspended load forms to the reservoir. Hence, it seems that the only practical way to remove the sediment from the reservoir is to flush it out using the Chasse method. In the present paper, field measurements of Chasse operation characteristics taken in previous years are presented, and a numerical model that simulates this process is introduced. After calibrating the model using field measured data, the calculated results （for reservoir pressure flushing and released sediment volume） of the numerical model were compared with other measured data for the same Chasse operation and the results agree well. Finally, using the numerical simulation results, the best approaches to ensure highly effective flushing while conserving reservoir water are presented （at least for the Sefid-Roud dam）. The operation of the bottom outlet gates, the shape of the output hydrograph, and the reservoir water level variation during flushing were optimized. In addition, the numerical model and related parameters, which need to be calibrated, are discussed.     

Seismic Signatures of Rock Pore Structure

Rock pore structure is one of the important parameters in controlling both seismic wave velocity and permeability in sandstones and carbonate rocks. For a given porosity of two similar rocks with different pore structures, their acoustic wave speeds can differ 2 km/s, and permeability can span nearly six orders of magnitude from 0.01 mD to 20 D in both sandstone and limestone. In this paper, we summarize a two-parameter elastic velocity model reduced from a general poroelastic theory, to characterize the effect of pore structures on seismic wave propagation. For a given mineralogy and fluid type of a reservoir, this velocity model is defined by porosity and a frame flexibility factor, which can be used in seismic inversion and reservoir characterization to improve estimation of porosity and reserves. The frame flexibility factor can be used for quantitative classification of rock pore structure types (PST) and may be related to pore connectivity and permeability, using both poststack and prestack seismic data. This study also helps explain why amplitude versus offset analysis (AVO) in some cases fails for the purpose of fluid detection: pore structure effect on seismic waves can mask all the fluid effects, especially in carbonate rocks.     

The Analysis of Seismic Data Structure and Oil and Gas Prediction

In this paper, a new concept called numerical structure of seismic data is introduced and the difference between numerical structure and numerical value of seismic data is explained. Our study shows that the numerical seismic structure is closely related to oil and gas-bearing reservoir,so it is very useful for a geologist or a geophysicist to precisely interpret the oil-bearing layers from the seismic data. This technology can be applied to any exploration or production stage. The new method has been tested on a series of exploratory or development wells and proved to be reliable in China. Hydrocarbon-detection with this new method for 39 exploration wells on 25 structures indicates a success ratio of over 80 percent. The new method of hydrocarbon prediction can be applied for: (1) depositional environment of reservoirs with marine facies, delta, or non-marine facies (including fluvial facies, lacustrine facies); (2) sedimentary rocks of reservoirs that are non-marine clastic rocks and carbonate rock; and (3) burial depths range from 300 m to 7000 m, and the minimum thickness of these reservoirs is over 8 m (main frequency is about 50 Hz).     

An explicit four-dimensional variational data assimilation method

A new data assimilation method called the explicit four-dimensional variational (4DVAR) method is proposed. In this method, the singular value decomposition (SVD) is used to construct the orthogonal basis vectors from a forecast ensemble in a 4D space. The basis vectors represent not only the spatial structure of the analysis variables but also the temporal evolution. After the analysis variables are ex-pressed by a truncated expansion of the basis vectors in the 4D space, the control variables in the cost function appear explicitly, so that the adjoint model, which is used to derive the gradient of cost func-tion with respect to the control variables, is no longer needed. The new technique significantly simpli-fies the data assimilation process. The advantage of the proposed method is demonstrated by several experiments using a shallow water numerical model and the results are compared with those of the conventional 4DVAR. It is shown that when the observation points are very dense, the conventional 4DVAR is better than the proposed method. However, when the observation points are sparse, the proposed method performs better. The sensitivity of the proposed method with respect to errors in the observations and the numerical model is lower than that of the conventional method.     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.     

Behavior of large post-liquefaction deformation in saturated Nanjing fine sand

Laboratory tests on the large post-liquefaction deformation of saturated Nanjing fine sand were performed by using a hollow cylinder apparatus. The stress-strain responses and the characteristics of excess pore water pressure after liquefaction were studied. It was found that the relationship between deviatoric stress and axial strain presented a sigmoid curve, and there was a good linearity relationship between normalized pore water pressure and deviatoric stress. On this basis, a constitutive model of stress-strain responses and a dissipation model of excess pore water pressure were established. It was found that the results predicted by the two models were in good agreement with the experimental data. The influence of relative densities and confi ning pressure on the characteristics of liquef ied soil were studied. The results showed the relative densities and initial effective confining pressure all had an important influence on the stress-strain responses of liquefi ed saturated Nanjing fine sand. However, the dissipation model of excess pore water pressure after liquefaction was only affected by the confining pressure.     

Teleseismic receiver function using stacking and smoothing of multi seismic-records at a single station

Receiver functions(RFs)obtained using teleseismic wave records at a seismic station and synthetic seismograms indicate that RF with a single teleseismic wave record is related to the selection of record section and to the calculating parameters of the RF.The scatter noise contained in the seismogram also affects the quality of RF.A new method for calculating receiver function,stacking and smoothing multi-seismic records in a single station,is presented in this paper.The RF results using some records and some synthetic seismograms with different noises indicate prominent mantle discontinuity and thus prove that the method is effective and satisfied.     

核磁共振T2分布评价岩石孔径分布的改进方法

岩芯核磁共振（NMR）T2分布和毛管压力分析数据均在一定程度上反映了岩石的孔隙结构,理论分析表明,这两组数据具有相关性.应用NMR T2分布研究岩石孔径分布，关键是在分析两者的相关性的基础上，从T2分布构造出可靠的毛管压力曲线.但以前用饱和水T2分布构造的毛管压力曲线的方法与实际毛管压力曲线匹配性差.事实上，薄膜束缚水部分的存在引起T2分布反映的孔隙空间与毛管压力曲线反映的孔隙空间有差异.本文提出一种改进方法，在消除薄膜束缚水对T2分布的贡献后，用自由水T2分布构造毛管压力曲线.应用本文方法,对24块岩芯数据自由水T2分布构造的毛管力曲线及其孔喉半径分布与隔板毛管压力分析结果进行了对比.结果表明，改进方法对于毛管压力曲线的构造精度有明显改进，从而为NMR T2分布研究孔隙结构提供了可靠的理论和方法上的支持.     

低孔渗砂岩孔隙结构分类方法研究

低孔渗砂岩储层孔隙结构复杂,在二维与三维空间中具有较强非均质性,对储集空间的测井响应有极大影响。而压汞曲线、核磁T₂谱、铸体薄片3种资料在一定程度上各自反映了孔隙结构的不同特征,本文分别提取3种资料的微观孔隙结构参数进行相关性分析,最终分别利用主成分分析-模糊均值聚类方法实现孔隙结构分类,其分类结果压汞曲线分形法所得一致。      

基于核磁共振测井的低渗透砂岩孔隙结构定量评价方法——以东营凹陷南斜坡沙四段为例

低渗透砂岩油气藏已成为油气增储生产的重要勘探开发目标,但孔隙结构复杂使得储层及其有效性难以准确识别.笔者利用物性、压汞、核磁等资料,对东营凹陷南坡沙四段(Es4)低渗透砂岩孔隙结构进行分析,划分出了3种类型.核磁T2谱与毛管压力曲线都在一定程度上反映孔喉分布,但常规方法利用T2谱重构伪毛管压力曲线所得到的孔隙半径与压汞孔喉半径有较大误差,而岩石孔隙自由流体T2与压汞孔喉分布对应关系更好,以此建立了不同孔隙结构类型二者之间不同孔喉尺度对应的关系式(大尺度:线性;小尺度:分段幂函数),可在井筒剖面上通过识别孔隙结构类型,进而利用核磁共振测井(NML)定量反演孔径分布,省去了构建伪毛管曲线环节,为低渗透砂岩储层有效性评价提供了直接依据,也是测井用于定量反演储层微观孔隙结构信息的有益探索.     

Establishment and application of logging saturation interpretation equation in vuggy reservoirs

Vuggy reservoirs are the most common, albeit important heterogeneous carbonate reservoirs in China. However, saturation calculations using logging data are not well developed, whereas Archie method is more common. In this study, electrical conduction in a vuggy reservoir is theoretically analyzed to establish a new saturation equation for vuggy reservoirs. We found that vugs have a greater effect on saturation than resistivity, which causes inflection in the rock-electricity curve. Using single-variable experiments, we evaluated the effects of vug size, vug number, and vug distribution on the rock-electricity relation. Based on the general saturation model, a saturation equation for vuggy reservoirs is derived, and the physical significance of the equation parameters is discussed based on the seepage-electricity similarity. The equation parameters depend on the pore structure, and vugs and matrix pore size distribution. Furthermore, a method for calculating the equation parameters is proposed, which uses nuclear magnetic resonance (NMR) data to calculate the capillary pressure curve. Field application of the proposed equation and parameter derivation method shows good match between calculated and experimental results, with an average absolute error of 5.8%.     

近五年核磁共振测井在储集层孔隙结构评价中的若干进展

近期核磁共振测井技术在定量评价储集层孔隙结构方面发展迅速,为解决复杂油气藏的勘探开发问题提供了新的思路和方法.本文对近五年来利用核磁共振测井资料定量评价储集层孔隙结构方面的最新进展进行述评.重点分析了各种方法在实际储集层评价中的优、缺点及适用条件.同时针对目前各种方法在实际应用中存在的问题,提出了利用核磁共振测井资料定量评价储集层孔隙的未来发展方向在于对含烃储集层的核磁共振测井T₂谱的形态进行校正.     

POROSITY AND PORE STRUCTURE FROM ACOUSTIC WELL LOGGING DATA1

Wyllie's time-average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional (P)-wave velocity (or its reciprocal, transit time) recorded on a sonic log. This model, while simple, needs to be more convincingly explained in theory and improved in practice, particularly by making use of shear (S)-wave velocity. One of the most important, although often ignored, factors affecting elastic velocities in a rock is pore structure, which is also a controlling factor for transport properties of a rock. Now that S-wave information can be obtained from the sonic log, it may be used with P-waves to provide a better understanding of pore structure. A new acoustic velocities-to-porosity transform based on an elastic velocity model developed by Kuster and Toksöz is proposed. Employing an approximation to an equivalent pore aspect ratio spectrum, pore structure for reservoir rocks is taken into account, in addition to total pore volume. Equidimensional pores are approximated by spheres and rounded spheroids, while grain boundary pores and flat pores are approximated by low aspect ratio cracks. An equivalent pore aspect ratio spectrum is characterized by a power function which is determined by compressional-and shear-wave velocities, as well as by matrix and inclusion properties. As a result of this more sophisticated elastic model of porous rocks and a stricter theory of elastic wave propagation, the new method leads to a more satisfactory interpretation and fuller use of seismic and sonic log data. Calculations using the new transform on data for sedimentary rocks, obtained from published literature and laboratory measurements, are presented and compared at atmospheric pressure with those estimated from the time-average equation. Results demonstrate that, to compensate for additional complexity, the new method provides more detailed information on pore volume and pore structure of reservoir rocks. Examples are presented using a realistic self-consistent averaging scheme to consider interactions between pores, and the possibility of extending the method to complex lithologies and shaly rocks is discussed.     

基于储层砂岩微观孔隙结构特征的弹性波频散响应分析

储层砂岩微观孔隙结构特征不仅影响干燥岩石的弹性波传播速度,也决定了岩石介质中与流体流动相关的速度频散与衰减作用.依据储层砂岩微观结构特征及速度随有效压力变化的非线性特征,将其孔隙体系理想化为不同形状的硬孔隙(纵横比α0.01)与软孔隙(纵横比α0.01)的组合(双孔隙结构).基于孔弹性理论,给出软孔隙最小初始纵横比值(一定压力下所有未闭合软孔隙在零压力时的纵横比最小值)的解析表达式,并在此基础上利用岩石速度-压力实验观测结果给出求取介质中两类孔隙纵横比及其含量分布特征的方法.通过逐步迭代加入软孔隙的方法对基于特征纵横比的"喷射流"(squirt fluid)模型进行了扩展,以考虑复杂孔隙分布特征对岩石喷射流作用的影响及其可能引起的速度频散特征.相较于典型的喷射流作用速度频散模式,对于岩石中软孔隙纵横比及其对应含量在较宽的范围呈谱分布的一般情况,其速度频散曲线不存在明显的低频段和中间频段,速度随频率的增大呈递增趋势直至高频极限.这说明即使在地震频段,微观尺度下的喷射流作用仍起一定作用,同样会造成流体饱和岩石介质的地震速度与Gassmann方程预测结果有不可忽略的差异.本文是对现有喷射流模型的重要补充,也为利用实验数据建立不同频段间岩石弹性波传播速度的可能联系提供了理论依据.     

致密砂岩储层CO2驱油特征的核磁共振实验研究

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。      

储层流体识别的新方法

利用小波变换方法可以把原始信号分解为一系列载有不同信息的子信号,每种子信号载有储层的特定信息,包括流体、岩性和孔隙结构等。我们开发出了基于小波变换的谱分析方法,利用该方法可以从这些子信号中提取出反映地层流体特性的子信号,通过对该子信号的处理识别出地层所含流体的特性。本文利用该流体识别新方法对某油田的实际测井资料进行了处理,处理结果与试油结果符合得很好,进一步证明了本方法的可靠性和实用性。     

碳酸盐岩孔隙结构参数构建与储层参数反演(英文)

碳酸盐岩储层孔隙结构相对碎屑岩更复杂,常用的岩石物理模型不能较好的描述其孔隙结构的变化规律,且岩石孔隙结构的差异较大程度上会影响岩石的弹性性质。本文首先利用岩石薄片分析了碳酸盐岩的微观孔隙结构。然后基于Gassmann方程和Eshelby-Walsh椭球包体裂缝理论,在合理的假设前提下给出了一种新的岩石物理建模方法,并且从中提取了一个参数来表征孔隙结构的变化规律。最后,基于全波列测井数据,我们利用该方法计算了单井的孔隙度,并与用常规方法预测的结果进行了比较,同时进行了地震储层参数反演。研究结果表明,孔隙结构对岩石的弹性性质的影响较大,且新的建模方法预测的孔隙度误差仅为0.74%。因此,该方法可有效的减小孔隙结构对计算各岩石弹性参数的影响并提高孔隙度的预测精度。