Cassiane Nunes

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 …

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.