Effect of oil composition on fluid substitution in heavy oil reservoirs

Unlike light oils, heavy oils do not have a well‐established scheme for modelling elastic moduli from dynamic reservoir properties. One of the main challenges in the fluid substitution of heavy oils is their viscoelastic nature, which is controlled by temperature, pressure, and fluid composition. Here, we develop a framework for fluid substitution modelling that is reliable yet practical for a wide range of cold and thermal recovery scenarios in producing heavy oils and that takes into account the reservoir fluid composition, grounded on the effective‐medium theories for estimating elastic moduli of an oil–rock system. We investigate the effect of fluid composition variations on oil–rock elastic moduli with temperature changes. The fluid compositional behaviour is determined by flash calculations. Elastic moduli are then determined using the double‐porosity coherent potential approximation method and the calculated viscosity based on the fluid composition. An increase in temperature imposes two opposing mechanisms on the viscosity behaviour of a heavy‐oil sample: gas liberation, which tends to increase the viscosity, and melting, which decreases the viscosity. We demonstrate that melting dominates gas liberation, and as a result, the viscosity and, consequently, the shear modulus of the heavy oils always decrease with increasing temperature. Furthermore, it turns out that one can disregard the effects of gas in the solution when modelling the elastic moduli of heavy oils. Here, we compare oil–rock elastic moduli when the rock is saturated with fluids that have different viscosity levels. The objective is to characterize a unique relation between the temperature, the frequency, and the elastic moduli of an oil–rock system. We have proposed an approach that takes advantage of this relation to find the temperature and, consequently, the viscosity in different regions of the reservoir.     

基于褶积原理和改进广义S变换的震动波衰减补偿试验研究

为了恢复震动波能量在传播过程中产生的衰减损耗,提出基于褶积原理求取品质因子Q的方法与改进广义S变换相结合的反Q滤波法。通过震动波衰减补偿模型试验,对试验数据进行改进广义S变换的时频特性分析,得出了信号的能量分布情况以及时间频率对应关系;采用基于褶积原理求取品质因子的方法,得到时变Q值;对试验数据进行反Q滤波处理,使震动波能量得到了补偿。结果表明本文提出的反Q滤波法提高了对震动波能量衰减补偿的效果,拓宽了地震资料的频带,提高了地震资料分辨率,有利于进行高分辨率地震勘探、深部信号增强和油气藏预测工作的开展。     

Extraction of permeability from time-lapse seismic data

Two formulae are developed for estimating horizontal permeability directly from maps of 4D seismic signatures. The choice of the formula used depends on whether the seismic is dominated by changes of pressure or saturation. However, pressure derived from time‐lapse seismic, or seismic amplitudes controlled predominantly by pressure are to be preferred for optimal ‘illumination’ of the reservoir. The permeability is predicted to be dependent on porosity but weighted by a 4D term related to the magnitude and spatial gradient of the 4D signature. Tests performed on model‐based synthetic seismic data affirm the validity and accuracy of this approach. Application to field data from the UK continental shelf reveals a large‐scale permeability variation similar to the existing simulation model, but with additional fine‐scale detail. The technique thus has the potential of providing extra information with which to update the simulation model. The resultant permeability estimates have been successfully ground‐truthed against the results of two well tests. As non‐repeatable noise in the time‐lapse seismic data diminishes with improved 4D‐related acquisition, it will become increasingly possible to make robust permeability estimates using this approach.     

Feasibility of borehole ambient noise interferometry for permanent reservoir monitoring

The analysis of seismic ambient noise acquired during temporary or permanent microseismic monitoring campaigns (e.g., improved/enhanced oil recovery monitoring, surveillance of induced seismicity) is potentially well suited for time‐lapse studies based on seismic interferometry. No additional data acquisition required, ambient noise processing can be automatized to a high degree, and seismic interferometry is very sensitive to small medium changes. Thus there is an opportunity for detection and monitoring of velocity variations in a reservoir at negligible additional cost and effort. Data and results are presented from an ambient noise interferometry study applied to two wells in a producing oil field in Romania. Borehole microseismic monitoring on three component geophones was performed for four weeks, concurrent with a water‐flooding phase for improved oil recovery from a reservoir in ca. 1 km depth. Both low‐frequency (2 Hz–50 Hz) P‐ and S‐waves propagating through the vertical borehole arrays were reconstructed from ambient noise by the virtual source method. The obtained interferograms clearly indicate an origin of the ambient seismic energy from above the arrays, thus suggesting surface activities as sources. It is shown that ambient noise from time periods as short as 30 seconds is sufficient to obtain robust interferograms. Sonic log data confirm that the vertical and horizontal components comprise first arrivals of P‐wave and S‐waves, respectively. The consistency and high quality of the interferograms throughout the entire observation period further indicate that the high‐frequency part (up to 100 Hz) represents the scattered wave field. The temporal variation of apparent velocities based on first‐arrival times partly correlates with the water injection rate and occurrence of microseismic events. It is concluded that borehole ambient noise interferometry in production settings is a potentially useful method for permanent reservoir monitoring due to its high sensitivity and robustness.     

An overview of laboratory apparatuses to measure seismic attenuation in reservoir rocks

Intrinsic wave attenuation at seismic frequencies is strongly dependent on rock permeability, fluid properties, and saturation. However, in order to use attenuation as an attribute to extract information on rock/fluid properties from seismic data, experimental studies on attenuation are necessary for a better understanding of physical mechanisms that are dominant at those frequencies. An appropriate laboratory methodology to measure attenuation at seismic frequencies is the forced oscillation method, but technical challenges kept this technique from being widely used. There is a need for the standardization of devices employing this method, and a comparison of existing setups is a step towards it. Here we summarize the apparatuses based on the forced oscillation method that were built in the last 30 years and were used to measure frequency‐dependent attenuation in fluid‐saturated and/or dry reservoir rocks under small strains (10^?8–10^?5). We list and discuss important technical aspects to be taken into account when working with these devices or in the course of designing a new one. We also present a summary of the attenuation measurements in reservoir rock samples performed with these apparatuses so far.     

一种自适应增益限的反Q滤波

地层的Q吸收会造成地震波振幅衰减、相位畸变,分辨率和信噪比明显降低.反Q滤波可消除由于地层Q吸收造成的振幅衰减和相位畸变,从而提高地震资料的分辨率;但反Q滤波振幅补偿的数值不稳定性问题会严重降低地震资料的信噪比,并产生很多假象.截止频率法和稳定因子法反Q滤波振幅补偿方法虽可控制数值非稳定性问题,但振幅补偿函数的增益限为一个时不变的常数,且与地震数据动态范围无关,其经常会压制深层地震波的高频成分,反而降低地震资料的分辨率;因此,本文在研究截止频率法和稳定因子法的基础上,结合地震数据的动态范围对地震记录分辨率的影响,提出了一种自适应增益限的反Q滤波振幅补偿方法,其增益限和稳定因子都是时变的,且都自适应于地震数据有效频带的截止频率.合成数据和实际数据试算表明,本文的自适应增益限的反Q滤波方法可恢复地震信号有效频带范围内的能量,且能较好地控制数值非稳定性问题,最终获得高分辨率和高信噪比的地震数据.     

When 4D seismic is not applicable: Alternative monitoring scenarios for the Arab-D reservoir in the Ghawar Field

Ghawar, the largest oilfield in the world, produces oil from the Upper Jurassic Arab‐D carbonate reservoir. The high rigidity of the limestone–dolomite reservoir rock matrix and the small contrast between the elastic properties of the pore fluids, i.e. oil and water, are responsible for the weak 4D seismic effect due to oil production. A feasibility study was recently completed to quantify the 4D seismic response of reservoir saturation changes as brine replaced oil. The study consisted of analysing reservoir rock physics, petro‐acoustic data and seismic modelling. A seismic model of flow simulation using fluid substitution concluded that time‐lapse surface seismic or conventional 4D seismic is unlikely to detect the floodfront within the repeatability of surface seismic measurements. Thus, an alternative approach to 4D seismic for reservoir fluid monitoring is proposed. Permanent seismic sensors could be installed in a borehole and on the surface for passive monitoring of microseismic activity from reservoir pore‐pressure perturbations. Reservoir production and injection operations create these pressure or stress perturbations. Reservoir heterogeneities affecting the fluid flow could be mapped by recording the distribution of epicentre locations of these microseisms or small earthquakes. The permanent borehole sensors could also record repeated offset vertical seismic profiling surveys using a surface source at a fixed location to ensure repeatability. The repeated vertical seismic profiling could image the change in reservoir properties with production.     

谱比法地震衰减层析反演方法研究

由于地下介质对地震波能量有强烈的吸收作用,降低了地震资料的分辨率以及地震资料处理、解释的精度,因此选择合适的参数对地层吸收衰减情况进行有效描述是对地下构造进行高分辨率、高精度处理及解释的有效途径。目前品质因子Q是对岩石弹性参数进行有效描述的重要参数之一。用品质因子Q来衡量地震波能量在介质中传播的吸收衰减情况,是目前提高地震资料分辨率的有效方法,同时也是提高含油气地层解释精度的有效途径。近些年,许多学者提出了多种计算Q值的方法,其中谱比法是在实际中应用最为广泛的Q值估计方法。本文在前人方法的基础上,对谱比法进行了改进,并结合走时层析方法反演Q值,充分利用谱比法在精度、稳定性等方面的优势以及走时层析方法计算效率高的优点,不仅可以提高反演的精度而且能够保证反演的稳定性,提高计算效率。通过模型试算证明谱比法衰减层析方法能有效估计Q值变化情况,具有较好的发展前景。      

苏里格气田地震资料叠前保幅去噪技术研究

为了满足岩性勘探的需要,苏里格地区开展了高密度地震采集方法.采集技术的进步极大的提高了地震记录的品质,同时对开展以岩性预测为目标的保幅处理提出了更高的要求.而叠前去噪作为室内去噪处理的主战场,在保幅处理中起到重要作用.本文针对苏里格地区地震资料干扰波类型多、背景噪音强的特点,采用多域多方法联合去噪思路,对叠前地震数据上的噪声采用分频压制线性及异常强能量干扰、自适应单频波衰减及高精度反假频去除多次波等保幅去噪方法.在提高信噪比的同时,达到保幅处理效果,提高了处理结果的可靠性,使钻井成功率大大提高,从而大幅度提高了苏里格气田的经济效益.该技术系列对国内其它低孔低渗致密砂岩油气藏研究同样具有借鉴意义.     

Compressional-wave Q estimation from full-waveform sonic data

There is significant evidence that the anelastic loss of seismic energy is linked to petrophysical properties such as porosity, permeability and clay content. Thus, reliable estimation of anelastic attenuation from seismic data can lead to improved methods for the prediction of petrophysical properties. This paper is concerned with methods for the estimation of attenuation at sonic frequencies (5–30 KHz) from in situ data. Two independent methods have been developed and tested for estimating compressional‐wave attenuation from full‐waveform sonic data. A well‐established technique, the logarithm spectral ratio (LSR) method, is compared with a new technique, the instantaneous frequency (IF) method. The LSR method uses the whole spectrum of the seismic pulse whilst the IF method uses a carefully estimated value of instantaneous frequency which is representative of the centre frequency of the pulse. In the former case, attenuation estimation is based on the relative variation of amplitudes at different frequencies, whilst in the latter case it is based on the shift of the centre frequency of the pulse to lower values during anelastic wave propagation. The IF method does not assume frequency independence of Q which is a necessary assumption for the LSR method, and it provides a stable frequency log, the peak instantaneous frequency (PIF) log, which may be used as an indicator for attenuation under certain limitations. The development and implementation of the two methods is aimed at minimizing the effect of secondary arrivals, such as leaky modes, and involved a series of parameter tests. Testing of the two methods using full‐waveform sonic data of variable quality, obtained from a gas‐bearing sandstone reservoir, showed that the IF method is in general more stable and suitable for full‐waveform sonic data compared with the LSR method. This was evident especially in data sets with high background noise levels and wave‐interference effects. For good quality data, the two methods gave results that showed good agreement, whilst comparison with other log types further increased confidence in the results obtained. A significant decrease (approximately 5 KHz) in the PIF values was observed in the transition from an evaporite/shale sequence to the gas‐bearing sandstone. Average Q values of 54 and 51 were obtained using good quality data from a test region within the gas‐saturated sandstone reservoir, using the LSR and IF methods, respectively.     

谐振Q理论井震匹配法及品质因子反演

储层勘测中地震波速度一般低于声波测井速度,撇开观测系统与人为因素造成的误差,岩石本征的黏弹性是造成这一现象的重要原因.本文在井、震匹配问题中引入了黏弹性岩石的杨氏谐振品质因子(Q)模型,对储层岩石进行了井震匹配与Q反演.文章阐述了如何通过谐振Q模型合理的校正声波速度,从而实现井震匹配.在波形匹配的基础上,进行地层品质因子的反演,合理的计算出目的层位的品质因子值.     

Quasi‐continuous reservoir monitoring with surface seismic data

We present an approach that creates the possibility of reservoir monitoring on a quasi‐continuous basis using surface seismic data. Current strategies and logistics for seismic data acquisition impose restrictions on the calendar‐time temporal resolution obtainable for a given surface‐seismic time‐lapse monitoring program. One factor that restricts the implementation of a quasi‐continuous monitoring program using conventional strategies is the time it takes to acquire a complete survey. Here quasi‐continuous monitoring describes the process of reservoir monitoring at short‐time intervals. Our approach circumvents the restriction by requiring only a subset of complete survey data each time an image of the reservoir is needed using surface seismic data. Ideally, the time interval between survey subset acquisitions should be short so that changes in the reservoir properties are small. The accumulated data acquired are used to estimate the unavailable data at the monitor survey time and the combined recorded and estimated data are used to produce an image of the subsurface for monitoring. We will illustrate the effectiveness of our approach using 2D and 3D synthetic seismic data and 3D field seismic data. We will explain the benefits and drawbacks of the proposed approach.     

委内瑞拉奥里诺科重油带J区块新老二维地震资料精细匹配处理及构造特征分析

针对委内瑞拉奥里诺科重油带J区块新老二维地震资料存在的问题,应用精细匹配处理方法,即以精细频率匹配和精细振幅匹配和相位校正为主要手段,结合时间偏移处理以及去噪、混波处理等手段,普遍突出了有效反射信息,提高了不同年度地震剖面之间的闭合精度,大大提高了构造解释的准确性,虽然地震资料很少,但仍解释出了J区块的三层构造图,构造解释结果表明,J区块地层总体是一个北倾单斜,其上发育一些北东和近东西向的断层,油藏类型以岩性油藏为主,断层对油藏主要起复杂化作用,J区块这一构造特征与其所处的构造背景、基底形态和构造应力作用有关.     

稠油热采时移地震监测研究与应用

我国具有丰富的稠油资源,但国内外针对陆相薄互层稠油油藏热采动态的时移地震监测研究还很少.通过分析火烧油层对储层的影响,结合已有的岩石物理测量结果,建立了薄互层油藏火烧油层模型,开展了地震正演模拟研究,并给出了应用实例.结果表明,稠油热采(注蒸汽、火烧油层、注热水)会引起储层岩石和孔隙流体弹性特性的显著改变,从而导致明显的地震属性差异;时移地震可以用来划分油层中高温区范围,跟踪高温前缘等.在陆相薄互层稠油田热采开发中,利用时移地震来监测油田开发的全过程是可行的.     

基于常规测井数据的纵波本征衰减估计

地震波本征衰减反映了地层及其所含流体的一些特性,对油气勘探开发有重要意义.已有的理论研究与实验发现,地震频带内的衰减主要与中观尺度(波长与颗粒尺度之间)的斑状部分饱和、完全饱和岩石弹性非均匀性情况下波诱导的局部流体流有关.这种衰减与岩石骨架、孔隙度及充填流体的性质密切相关.本文着重讨论均匀流体分布、斑状或非均匀流体分布两种情况下部分饱和岩石的纵波模量差异.以经典岩石物理理论和衰减机制认识为基础,通过分析低频松弛状态、高频非松弛状态岩石的弹性模量,讨论储层参数(如孔隙度、泥质含量以及含水饱和度等)与纵波衰减之间的确定性关系.上述方法与模型在陆相砂泥岩地层与海相碳酸盐岩地层中的适用性通过常规测井资料得到了初步验证.     

The bound weighted average method (BWAM) for predicting S-wave velocity

The shear-wave velocity is a very important parameter in oil and gas seismic exploration, and vital in prestack elastic-parameters inversion and seismic attribute analysis. However, shearing-velocity logging is seldom carried out because it is expensive. This paper presents a simple method for predicting S-wave velocity which covers the basic factors that influence seismic wave propagation velocity in rocks. The elastic modulus of a rock is expressed here as a weighted arithmetic average between Voigt and Reuss bounds, where the weighting factor, w, is a measurement of the geometric details of the pore space and mineral grains. The S-wave velocity can be estimated from w, which is derived from the P-wave modulus. The method is applied to process well-logging data for a carbonate reservoir in Sichuan Basin, and shows the predicted S-wave velocities agree well with the measured S-wave velocities.     

叠前纵波和转换波地震资料Q值提取及反Q滤波(英文)

纵波和转换波联合的多波地震勘探技术是解决复杂油气勘探的有效技术,提高转换波的分辨率是其关键问题之一。影响转换波分辨率的主要原因是地层对地震波的吸收,有效地计算地层Q值、消除地层吸收对转换波传播的影响,是提高转换波分辨率的关键。本文提出了从叠前转换波道集中估算横波Q值的方法,并利用沿射线路径的波场延拓,将一种稳定有效的反Q滤波方法应用到叠前共炮点纵波和转换波道集的衰减补偿中。模型资料结果表明,本文提出的估算转换横波Q值的方法精度较高;模型资料和实际资料的吸收补偿结果表明,此稳定全反Q滤波能有效地提高叠前纵波和转换波资料的分辨率。     

The impact of surface‐wave attenuation on 3‐D seismic survey design

Three‐dimensional seismic survey design should provide an acquisition geometry that enables imaging and amplitude‐versus‐offset applications of target reflectors with sufficient data quality under given economical and operational constraints. However, in land or shallow‐water environments, surface waves are often dominant in the seismic data. The effectiveness of surface‐wave separation or attenuation significantly affects the quality of the final result. Therefore, the need for surface‐wave attenuation imposes additional constraints on the acquisition geometry. Recently, we have proposed a method for surface‐wave attenuation that can better deal with aliased seismic data than classic methods such as slowness/velocity‐based filtering. Here, we investigate how surface‐wave attenuation affects the selection of survey parameters and the resulting data quality. To quantify the latter, we introduce a measure that represents the estimated signal‐to‐noise ratio between the desired subsurface signal and the surface waves that are deemed to be noise. In a case study, we applied surface‐wave attenuation and signal‐to‐noise ratio estimation to several data sets with different survey parameters. The spatial sampling intervals of the basic subset are the survey parameters that affect the performance of surface‐wave attenuation methods the most. Finer spatial sampling will reduce aliasing and make surface‐wave attenuation easier, resulting in better data quality until no further improvement is obtained. We observed this behaviour as a main trend that levels off at increasingly denser sampling. With our method, this trend curve lies at a considerably higher signal‐to‐noise ratio than with a classic filtering method. This means that we can obtain a much better data quality for given survey effort or the same data quality as with a conventional method at a lower cost.     

小波尺度域含气储层地震波衰减特征

黏弹性衰减因子Q的可靠估计可通过Q反褶积来提高地震资料的分辨率并有助于振幅分析. 本文从小波理论出发,结合地震波在黏弹性介质中的传播方程,推导出小波尺度域地震波能量衰减公式. 能量衰减公式具有下列性质：（1）Q值越大,能量衰减得越慢;Q值越小,能量衰减越严重;（2）尺度越小,信号中保留的能量越少;（3）对于脉冲源来说在理想的无衰减介质(即Q趋近于∞)中传播时,信号在不同尺度内的能量相同. 利用尺度能量公式,可从反射地震资料中直接估计品质因子Q（即衰减因子）,也可以提取不同尺度的能量衰减剖面作为储层描述的属性参数,用来进行岩性识别和指示气藏,与经典的谱比法相比,避免了谱比法所面临的双时窗问题以及进行谱估计的窗选择问题. 理论模型试验表明了本文方法的正确性和有效性.     

Seismic dynamic monitoring in CO<Subscript>2</Subscript> flooding based on characterization of frequency-dependent velocity factor

The phase velocity of seismic waves varies with the propagation frequency, and thus frequency-dependent phenomena appear when CO₂ gas is injected into a reservoir. By dynamically considering these phenomena with reservoir conditions it is thus feasible to extract the frequency-dependent velocity factor with the aim of monitoring changes in the reservoir both before and after CO₂ injection. In the paper, we derive a quantitative expression for the frequency-dependent factor based on the Robinson seismic convolution model. In addition, an inversion equation with a frequency-dependent velocity factor is constructed, and a procedure is implemented using the following four processing steps: decomposition of the spectrum by generalized S transform, wavelet extraction of cross-well seismic traces, spectrum equalization processing, and an extraction method for frequency-dependent velocity factor based on the damped least-square algorithm. An attenuation layered model is then established based on changes in the Q value of the viscoelastic medium, and spectra of migration profiles from forward modeling are obtained and analyzed. Frequency-dependent factors are extracted and compared, and the effectiveness of the method is then verified using a synthetic data. The frequency-dependent velocity factor is finally applied to target processing and oil displacement monitoring based on real seismic data obtained before and after CO₂ injection in the G89 well block within Shengli oilfield. Profiles and slices of the frequency-dependent factor determine its ability to indicate differences in CO₂ flooding, and the predicting results are highly consistent with those of practical investigations within the well block.