Lúcia Dillon

A multiscale DHI elastic attributes evaluation

Several analyses have been published in the specialized literature on the use and performance of distinct prestack seismic attributes as direct hydrocarbon indicators (DHI). The main idea is to use combinations of those attributes, in particular the P and S impedances (IP and IS), to minimize the dry rock frame contribution, that cause the increase of the pore fluid discrimination capacity.

Elastic log editing and alternative invasion correction methods

Recent applications of seismic data in reservoir characterization, direct hydrocarbon indication, and production monitoring rely on the accuracy of elastic logs (density and sonic). Trying to develop (semi) quantitative seismic-based studies requires the use of well-to-seismic calibration; the well information is used as a bridge between the geology and the surface seismic data. Nevertheless, we must take into account that well logs are also indirect measurements of rock properties and subject to various sources of errors. Borehole wall rugosity due to washouts (or even threaded well bore) and mud filtrate invasion are the main sources of uncertainty on elastic logs. Velocity dispersion and shale alteration can also be very important. It is commonly believed that the correction of elastic logs is important only in refined studies, but sometimes even a simple acoustic well-to-seismic tie can be drastically improved with such corrections. Figure 1 presents two wavelets extracted from the same well based on raw and invasion-corrected acoustic impedance. Some amplitude and phase differences in the estimated wavelets can be seen. Even though these changes seem subtle, they can be important in quantitative processes like inversion and AVO interpretation. Figure 1. Wavelets extracted with raw (blue) and with invasion-corrected (red) acoustic impedance data. Figure 2 shows synthetic seismograms for four different wells from the same field. The synthetics based on the original logs are blue and those calculated with the corrected logs are red. The synthetics for three wells seem virtually unaffected by the correction but the synthetics for well 2 present significant differences near the objective. Figure 2. Synthetic zero-offset seismograms based on raw logs (blue) and after correction (red). Although elastic-log corrections can be done deterministically, based on rock physics theories and models (e.g., fluid substitution for invasion correction), there are some issues that we must take into consideration. In the case of …

4D integrated technologies for deep-water turbidite reservoirs:from petrophysics to fluid flow simulation

Petrobras has been developing its 4D seismic technological programme since 1998, focused on the Brazilian deep-water fields in the Campos Basin, and considering the technical, operational and economic challenges involved in the development plan and reservoir management in this environment. The first step was to align the objectives of the project with the company goals for the following 15 years, in terms of earnings growth, production growth and reserves replacement. This information guided how the 4D reservoir management should be employed: as hedging technology to ensure that production targets would be achieved in several key fields at once, or as a direct technology investment to increase the production of individual, independent fields. The mission of reservoir management for each field involved was understood and new deep-water seismic technologies were developed to face the global operational and economic targets. 3D seismic reservoir monitoring, or 4D seismic study, was defined as an ‘integration of multidisciplinary technologies that includes the time-lapse monitoring of the drainage efficiency, using cores, well logs, seismic data, production history and pressure management’. Water injection is the preferable recovery method for the deep-water reservoirs in Brazil. Therefore, seismic monitoring should be able to distinguish contrasts of both fluids – injected water and remaining oil – that normally produce small seismic impedance values. This characteristic has brought the first technical challenge: the use of the 3D P-wave surface legacy data from the 1980s and 1990s, when the major fields started production, as 4D base-volumes to be correlated with future recommended 3D seismic data (surface or ocean bottom systems) as 4D monitor-volumes. In addition to all developed seismic technologies for data processing, a general 4D work flow was designed and the concept of the integrated reservoir model was adapted to relate all such technologies to the reservoir engineering needs and to the field economics, generating reliable 4D images for each reservoir study. This paper summarizes the multidisciplinary technical integration, including geological and seismic modelling, petrophysical simulations, seismic processing and interpretation, and reservoir simulation. A 4D methodology was implemented to integrate all such technical development and economic analysis, identifying where, when and how seismic monitoring can contribute to the reservoir management. This methodology has been applied to the Campos Basin deep-water reservoir, Rio de Janeiro State, Brazil.

Petrophysical Seismic Inversion Over an Offshore Carbonate Field

The main objective of this project was to evaluate the ability to derive petrophysical properties like porosity from pre-stack seismic data in a carbonate environment. We apply a direct petrophysical inversion technique to an offshore carbonate reservoir. Starting from an initial geological model in depth and a number of carefully conditioned seismic angle stacks, we derive a detailed 3D model of the porosity and hydrocarbon saturation matching the observed seismic data. We use a wellcalibrated Petro-Elastic Model (PEM) to link the petrophysical properties to the seismic velocities. We compare inversion results obtained using the Xu-Payne and T-matrix PEMs which both account for carbonate pore geometry, lithology, porosity and fluid content but have different elastic sensitivity to fluid saturations. The inverted results provide detailed images of the spatial variations of porosity and fluid content across the reservoir interval. Predicting absolute saturation values is more difficult, as saturation estimation is strongly dependent on the choice of PEM.

Petrophysical Seismic Inversion over an Offshore Carbonate Field

The main objective of this project was to evaluate the ability to derive petrophysical properties like porosity from seismic data in a carbonate environment. A special attention has been given to the possibility of characterizing the geometry of the pore space directly from the pre-stack seismic data. We apply a direct petrophysical inversion technique to a carbonate reservoir offshore Brazil. Starting from an initial geological model in depth and a number of carefully conditioned seismic angle stacks, we derive a detailed 3-D model of the porosity and hydrocarbon saturation matching the observed seismic data. We use a well-calibrated Petro-Elastic Model (PEM) to link the petrophysical properties to the seismic velocities. We compare inversion results obtained using the Xu-Payne and T-matrix PEMs which both account for carbonate pore geometry, lithology, porosity and fluid content but have different elastic sensitivity to fluid saturations. The inverted results provide detailed images of the spatial variations of porosity and fluid content across the reservoir interval. Obtaining estimates of absolute saturations values is more difficult, as saturation estimation is strongly dependent on the choice of PEM.

Automatic invasion correction of elastic logs or “let the tools speak by themselves”

Mud filtrate invasion may be a major issue in quantitative well-to-seismic calibration. Sonic and density log corrections for water and oil based mud invasion is easily implemented through Gassmann fluid substitution, once the real virgin and invaded zone saturations are known. In general, this can be achieved with deep and shallow resistivity logs. Nevertheless, there is not a common place regarding the depth of investigation of sonic tools beyond the borehole wall. In this paper we present a method of invasion correction based only on the elastic logs (compressional and shear sonic and density) that has an advantage of making no assumption on the saturation. In fact, the method extracts the saturation information from the elastic logs.

Methodological issues in facies interpretation using pre-stack inversion attributes

Interpretation of pre-stack data has become a key point for years to understand the heterogeneities of reservoirs in terms of rock and fluid properties.

Dispersion effects and their impact on reservoir geophysics: A Brazilian turbidite field example

Summary In order to better integrate the seismic data into the reservoir characterization and production monitoring process, PETROBRAS have been applying a detailed rock physics evaluation on acoustic logs of some of the major brazilian fields. In this work, we present the results of the evaluation of the dispersion effects on sonic logs and their impact on the seismic inversion for acoustic impedance and feasibility study for production monitoring of an offshore turbidite field. Compressional and Shear velocities of several plugs from well cores were measured in lab and compared with the sonic logs. A log analysis was performed in order to evaluate invasion effects. Biot and Local Fluid Flow theory were used to check the fluid (viscous oil) contribution to the velocity dispersion connected to the seismic and log/lab references respectively. The results indicate that dispersion effects are not negligible and corrections must be applied to the sonic logs in order to generate more accurate constraints for acoustic impedance inversion and more realistic feasibility analysis for 4D time lapse seismic production monitoring.