Time-lapse Seismic Interpretation Research Articles

Effects of patch size changes on the time-lapse seismic reflections of highly attenuating reservoirs

Reservoir fluid depletion and injection inevitably induce changes in pore pressure and saturation, leading to a varying fluid patch size and hence perturbations in seismic velocities and observable time-lapse seismic attributes. The patch size, which may exhibit spatial variations, can possess a characteristic size at each scenario, and therefore another variable impacting time-lapse seismic signatures. In this study, we extend previous research efforts of time-lapse seismic interpretation by considering the fluid patch size effect on time-lapse seismic signals to investigate uncertainties in time-lapse seismic observations. We predicted and analyzed the influence of reservoir permeability and gas saturation on seismic acoustic properties, i.e., attenuation and velocity dispersion, reflectivity and phase, on the basis of a generalized White's spherical model for varied patch sizes on millimeters to centimeters. A stratified model consisting of a fluid-saturated sandstone reservoir showing viscoelastic properties sandwiched between two elastic half-spaces is employed, so as to give insights into the impacts of fluid extraction induced changes of velocity dispersion and attenuation, as well as acoustic impedance contrast, on time-lapse seismic reflections. Analysis of time-lapse seismic travel-time and reflection amplitude differences indicates that ignoring the time-dependent patch size behaviour can give rise to underestimation or overestimation of the reservoir fluid saturation and permeability change.

Qualitative time-lapse seismic interpretation of Norne Field to assess challenges of 4D seismic attributes

Interpretation of time-lapse (or 4D) seismic data in terms of reservoir changes due to production posed many challenges in the Norne Field as the field experienced intense production activity from 1997 to 2006. For some segments within the field, fluid movement and pressure changes have approximately the same degree of impact and possibly opposite effects on the seismic data. Moreover, hardening anomalies could be caused by the increase in water saturation or gas going back to solution, while softening anomalies could be related to the increase in pore pressure or the decrease in fluid bulk modulus following the injection of gas. Therefore, for time-lapse seismic analysis to be most effective and less erroneous, different seismic attributes must be addressed to infer reservoir changes caused by production activity such as seismic amplitude and impedance derived by seismic inversion. In the present work, we analyze the challenges of 4D seismic interpretation in the Norne benchmark case. Our study indicates that acoustic impedance differences derived by a 4D model-based inversion provide an increase in vertical resolution compared to standard seismic amplitude differences. We also present a comparison between results of 4D model-based and colored inversions to evaluate the confidence of inversion anomalies. As this is a benchmark case, this study can be considered to enrich the discussions over qualitative and quantitative time-lapse seismic interpretation and to improve reservoir characterization.

Using simulation and production data to resolve ambiguity in interpreting 4D seismic inverted impedance in the Norne Field

The Norne Field started production in 1997 and up to 2006 the field experienced intense production activity, making the Norne benchmark case an ideal candidate to explore the challenges in interpreting complex time-lapse seismic data. Seismic amplitude changes and time-shifts are used as the first level approach to interpret the time-lapse differences and to update reservoir models. A common alternative is to invert the seismic data and obtain acoustic impedance variations caused by production activity, and to evaluate their possible interpretations. For this case study, we use a 4D inversion approach to invert the base (2001) and monitor (2006) seismic surveys in order to provide field-wide insights for the Norne benchmark case. We extensively interpret the observed 4D inversion anomalies and decouple, as much as possible, the effects of fluid and pressure variations, supported by production and reservoir engineering data. Moreover, we compare the inversion results with the simulation model from the Norne benchmark case to suggest areas of future modification to the simulation model. This research is intended as a resource to improve the quality of history matching or other 4D inversion methods applied to the Norne benchmark case, and to demonstrate a detailed time-lapse seismic interpretation within the reservoir segments of the Norne Field. Supplementary material: Well-history data of six producer and injector wells is available at https://doi.org/10.6084/m9.figshare.c.3890251

Prediction of residual oil saturation by using the ratio of amplitude attributes of time-lapse seismic data

Subtraction of baseline and monitoring seismic data is a common step in highlighting reservoir changes in time-lapse seismic interpretation. However, ambiguity exists in the interpretation of the amplitude difference, which is controlled by fluid change and reservoir thickness. To estimate the residual oil saturation quantitatively, we have developed a time-lapse seismic interpretation method that uses the ratio of amplitude attributes extracted from the baseline and monitoring seismic data. The relationship between impedance change and the ratio of the baseline and monitoring amplitude attributes is determined to avoid the influence of reservoir thickness. Subsequently, the fluid saturation is calculated from the impedance change by using a proper petrophysical relationship. We have tested our new method on a real time-lapse seismic data set from a water-flooded reservoir in the deepwater area of West Africa. The water-flooded area determined from the amplitude difference does not completely match the production logs because of the influence of variations in the reservoir thickness. However, the residual oil distribution calculated with the proposed method matches the production logs well. The connectivity of sandstone bodies is also evaluated based on an integrated interpretation of estimated oil saturation. With its simple principles and easy accessibility, our method improves the accuracy of time-lapse seismic data interpretation in water-flooded oil reservoirs. Furthermore, the quantitative interpretation of fluid change enables the time-lapse seismic technology to guide reservoir development directly.

Sensitivity analysis of effective fluid and rock bulk modulus due to changes in pore pressure, temperature and saturation

Fluid substitution plays a vital role in time-lapse seismic modeling and interpretation. It is, therefore, very important to quantify as exactly as possible the changes in fluid bulk modulus due to changes in reservoir parameters. In this paper, we analyze the sensitivities in effective fluid bulk modulus due to changes in reservoir parameters like saturation, pore-pressure and temperature. The sensitivities are analyzed for two extreme bounds, i.e. the Voigt average and the Reuss average, for various fluid combinations (i.e. oil–water, gas–water and gas–oil). We quantify that the effects of pore-pressure and saturation changes are highest in the case of gas–water combination, while the effect of temperature is highest for oil–gas combination. Our results show that sensitivities vary with the bounds, even for same amount of changes in any reservoir parameter. In 4D rock physics studies, we often neglect the effects of pore-pressure or temperature changes assuming that those effects are negligible compare to the effect due to saturation change. Our analysis shows that pore-pressure and temperature changes can be vital and sometimes higher than the effect of saturation change. We investigate these effects on saturated rock bulk modulus. We first compute frame bulk modulus using the Modified Hashin Shtrikman (MHS) model for carbonate rocks and then perform fluid substitution using the Gassmann equation. We consider upper bound of the MHS as elastic behavior for stiffer rocks and lower bound of the MHS as elastic behavior for softer rocks. We then investigate four various combinations: stiff rock with upper bound (the Voigt bound) as effective fluid modulus, stiff rock with lower bound (Reuss bound) as effective fluid modulus, soft rock with upper bound as effective fluid modulus and soft rock with lower bound as effective fluid modulus. Our results show that the effect of any reservoir parameter change is highest for soft rock and lower bound combination and lowest for stiff rock and upper bound combination.

Time-lapse seismic monitoring of hydraulic fracture stimulations within the Niobrara-Codell Reservoir Wattenberg Field, Colorado

Time-lapse seismic interpretation is demonstrated as a viable method for observing physical modification of a tight reservoir resulting from the hydraulic fracturing process. Compressional seismic data sets were acquired before and after completions of 11 horizontal wells within a section (one-square mile) of Wattenberg Field, Colorado. The time-lapse seismic data sets, after crossequalization, showed an acoustic impedance anomaly within the reservoir interval consistent with pressurization of the reservoir from surface operations. The spatial location of the anomaly is interpreted such that pressure compartmentalization exists due to faulting within the reservoir. A decrease in the acoustic P-impedance of up to 7% was observed from the baseline to the monitor survey in the reservoir interval. An increase in pore pressure owing to hydraulic fracturing was interpreted as the physical mechanism causing the greatest change in P-impedance. Hydraulic fracturing is highly dependent on local geology, and the integration of geoscience with parameter design is necessary for optimization. Time-lapse seismic reservoir monitoring can assist in documenting fault-related reservoir compartmentalization. The Colorado School of Mines Reservoir Characterization Project (RCP) undertook a monitoring project with Anadarko Petroleum Corporation (APC) in Wattenberg Field, Colorado. The Wishbone section is a one-square mile area containing 11 horizontal wells. Each well was fractured with multiple stages and completion design was varied from stage-to-stage. The completions parameters that varied were: well spacing, targeted formation, hydraulic fracturing volumes, pressure, rates, completion type, and number of stages.

岩石物理模板在时移地震定量研究中的应用

针对时移地震定量研究难题，提出了一套岩石物理模板构建技术，并应用于时移地震可行性分析和剩余油分布定量预测。该技术是在井资料的约束下，通过Hertz-Mindlin接触模型结合Hashin-Shtrikman界限模型得到与地质特征匹配的岩石物理模板，并通过改变孔隙度、泥质含量、流体性质、温度和孔隙压力等参数模拟各种条件下含油饱和度变化后各种弹性参数的变化规律，进而用于时移地震定量研究。S油田的实际应用表明该技术的预测结果与实钻结果有较高的吻合程度。 In this paper, a technique of rock physics template construction is proposed and applied in time-lapse seismic feasibility analysis and quantitative prediction of remaining oil distribution. Based on well data, the Hertz-Mindlin contact model and the Hashin-Shtrikman lower bound model, forward modeling with different parameters such as porosity, shale content, fluid properties, oil saturation, and temperature and pore pressure was carried out to get data for rock physics template. With the rock physics template matched with geological features, we can get quantitative regulation between elastic parameters and oil saturation under various conditions, which can be used for time-lapse seismic interpretation. The practical application shows that the time-lapse seismic prediction is in good agreement with the well drilling results in S oilfield.

Analysis of time-lapse travel-time and amplitude changes to assess reservoir compartmentalization

Fluid depletion within a compacting reservoir can lead to significant stress and strain changes and potentially severe geomechanical issues, both inside and outside the reservoir. We extend previous research of time-lapse seismic interpretation by incorporating synthetic near-offset and full-offset common-midpoint reflection data using anisotropic ray tracing to investigate uncertainties in time-lapse seismic observations. The time-lapse seismic simulations use dynamic elasticity models built from hydro-geomechanical simulation output and a stress-dependent rock physics model. The reservoir model is a conceptual two-fault graben reservoir, where we allow the fault fluid-flow transmissibility to vary from high to low to simulate non-compartmentalized and compartmentalized reservoirs, respectively. The results indicate time-lapse seismic amplitude changes and travel-time shifts can be used to qualitatively identify reservoir compartmentalization. Due to the high repeatability and good quality of the time-lapse synthetic dataset, the estimated travel-time shifts and amplitude changes for near-offset data match the true model subsurface changes with minimal errors. A 1D velocity–strain relation was used to estimate the vertical velocity change for the reservoir bottom interface by applying zero-offset time shifts from both the near-offset and full-offset measurements. For near-offset data, the estimated P-wave velocity changes were within 10% of the true value. However, for full-offset data, time-lapse attributes are quantitatively reliable using standard time-lapse seismic methods when an updated velocity model is used rather than the baseline model.

A review on time-lapse seismic data processing and interpretation

Time-lapse (TL) seismic or 4D seismic has great potential in monitoring and interpreting time-varying variations in reservoir fluid properties during hydrocarbon production. The TL surveys interpret dynamic changes in reservoir parameters by detecting differences in seismic responses from different vintages, which are obtained by repeated seismic surveys over the same area. For a proper and more precise quantitative interpretation of the TL seismic responses, TL seismic processing enhances repeatability of the TL-data sets, and thus reduces artificial differences that come from unavoidable differences in repeated-seismic-survey environments. This study aims to bring an insight to the state of the art in both TL seismic data processing and interpretation by reviewing previously-published researches. TL data processing is a sequential process conducting 4D binning and simultaneous 4D pre-stack processing; 4D binning improves spatial repeatability while “simultaneous 4D pre-stack processing” derives common processing operators employing cross-equalization (XEQ) as a core technique. TL seismic interpretation is categorized by the characteristics of data being interpreted into two strategies: interpretation of data sets from all TL vintages and interpretation of differences in seismic data between vintages.

Multicomponent time-lapse seismic interpretation of Rulison Field, Colorado using spectral-decomposition attributes

The most common applications of spectral decomposition in seismic interpretation have been to delineate and visualize stratigraphic features and as a direct hydrocarbon indicator. In this study, we applied spectral decomposition to time-lapse seismic interpretation. We used the multi-component seismic data acquired by the Reservoir Char-acterization Project of Colorado School of Mines at Rulison Field in 2003, 2004, and 2006.

Study on attribute characterization for reservoir dynamic monitoring by seismic

Study on characterizing reservoir parameters dynamic variations by time-lapse seismic attributes is the theoretical basis for effectively distinguishing reservoir parameters variations and conducting time-lapse seismic interpretation, and it is also a key step for time-lapse seismic application in real oil fields. Based on the rock physical model of unconsolidated sandstone, the different effects of oil saturation and effective pressure variations on seismic P-wave and S-wave velocities are calculated and analyzed. Using numerical simulation on decoupled wave equations, the responses of seismic amplitude with different offsets to reservoir oil saturation variations are analyzed, pre-stack time-lapse seismic attributes differences for oil saturation and effective pressure variations of P-P wave and P-S converted wave are calculated, and time-lapse seismic AVO (Amplitude Versus Offset) response rules of P-P wave and P-S converted wave to effective pressure and oil saturation variations are compared. The theoretical modeling study shows that it is feasible to distinguish different reservoir parameters dynamic variations by pre-stack time-lapse seismic information, including pre-stack time-lapse seismic attributes and AVO information, which has great potential in improving time-lapse seismic interpretation precision. It also shows that the time-lapse seismic response mechanism study on objective oil fields is especially important in establishing effective time-lapse seismic data process and interpretation scheme.

Core Damage: can We Calibrate Pressure Response with Lab Data?

Time-lapse (4D) seismic quantitative interpretation is based mainly on measurements of how saturation and pressure changes affect seismic velocities. Effect of saturation can be modeled using Gassmann equations. Pressure effect is usually obtained by laboratory measurements, which can be affected by core damage. In order to assess the adequacy of the core measurements to the properties of the intact reservoir rocks, it is necessary to compare them to in situ measurements. We present a study to assess the adequacy of ultrasonic measurements on core samples by comparing measured ultrasonic velocities at reservoir pressures with sonic log data from two wells located in an oil field in Campos Basin, offshore Brazil. The analysis is performed for these densely cored wells: more than 50 samples were extracted from a turbidite reservoir. We use Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. Comparisons of these computed velocities with sonic logs measurements show very good agreement. This confirms that for those particular regions the effect of core damage on ultrasonic measurements is below the measurement error. Consequently stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.

Time-lapse seismic inversion for pressure and saturation in Foinaven field, west of Shetland

Christophe Ribeiro and Colin MacBeth present a new petro-elastic-based approach to independently estimate reservoir pressure and saturation from 4D seismic data obtained at BP’s Foinaven field, west of Shetland. In the last decade, the use of seismic monitoring (4D) has greatly increased, and oil companies have shown their commitment to this technology (Marsh et al., 2003; De Waal and Calvert, 2003), as confirmed by its more systematic application. Time-lapse seismic is now an integrated part of reservoir management strategy, and has proved its ability to improve reservoir understanding. In the past, the applications of seismic monitoring were predominantly focused on the tracking of fluid contacts and movements (i.e. gas-cap expansion, water sweep); discrimination between lithology and fluid effects; and identification of pressure compartment and reservoir connectivity. The growth of seismic reservoir monitoring has brought new technical challenges to the industry, in order to improve and enlarge the existing portfolio of this technology. In fact, the advance in acquisition (i.e. steerable streamers, full-wave field recording devices, permanent sensors), survey design (i.e. 4D dedicated acquisition), seismic processing (i.e. specific 4D workflow) and integration between contractors and oil companies (i.e. in-house processing and R&D teams) have improved the repeatability and quality of the data, and paved the way for new applications. Research has now moved towards the more quantitative aspects of 4D, attempting to estimate hydrocarbon-production-related changes directly from seismic data. However, in most cases, seismic amplitude changes are not only due to variation in fluid saturation, pore pressure, or even compaction, but to a combination of these effects. The goal of distinguishing between pore pressure and fluid saturation effects is a challenge in quantitative time-lapse seismic interpretation, and has been the subject of many papers. The separation of pressure and saturation effects has mainly been tackled by means of rock-physics-based techniques (Brevik, 1999; Tura and Lumley, 1999; Landrø, 2001), and recently engineering approaches using production, pressure and PVT data, have been developed (He et al., 2004; MacBeth et al., 2004). The independent estimation of pore pressure and fluid saturation, which are also two products from the reservoir flow simulation, allows a direct comparison between attributes derived from the seismic and engineering domains. These production attributes could be used in a 4D seismic history matching workflow in order to directly constrain the predictions from the fluid flow simulator with the seismically derived dynamic properties (Gosselin and Menezes, 2006; Stephen and MacBeth, 2006). Modifications of the reservoir model could also be made to improve the agreement between the flow simulation results and the seismically derived production estimates, and ultimately to increase reservoir performance and oil recovery.

A Study on Multiple Time-Lapse Seismic AVO Inversion

The seismic responses caused by different reservoir parameter variations are numerically simulated, and then the feasibility of discriminating different reservoir parameters and realizing quantitative interpretation using time-lapse seismic AVO technique is ensured. Based on Aki and Richards’ simplified AVO equation, the formula of P-P wave and P-S wave for time-lapse seismic AVO was derived in details. According to the rock physical model of S oil field and the formula acquired, the multiple time-lapse seismic AVO inversion equations are achieved to discriminate the changes of oil saturation and effective pressure. It is shown by simulated data experiment that the time-lapse seismic AVO inversion is feasible, and the formula derived in this paper is effective to discriminate the changes of oil saturation and effective pressure, and to improve the precision of time-lapse seismic interpretation.