Guilherme Vasquez

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.

Experimental determination of the effective pressure coefficients for Brazilian limestones and sandstones

Ha varios esforcos no sentido de obter informacoes sobre mudancas de saturacao e pressao, devidas a producao dos reservatorios, a partir de dados sismicos com lapso de tempo. A despeito de alguns bons exemplos existentes na literatura especializada, varias aproximacoes sao feitas para a realizacao destes estudos. Geralmente, e assumido que as propriedades sismicas sao funcoes da saturacao e da pressao diferencial Pd (pressao de soterramento menos pressao de fluido). Na verdade, a tensao que determina o comportamento sismico e a tensao efetiva Pef, que nao e exatamente igual a Pd. De fato, esta e igual a tensao de soterramento menos n vezes a pressao de fluido. Tal coeficiente n e conhecido como coeficiente de tensao efetiva. Eventualmente, a depender das circunstâncias, este pode ser identificado com o coeficiente de Biot-Willis considerando o modulo de compressao volumetrica do meio poroso. Neste trabalho serao apresentados resultados de medidas em laboratorio procurando quantificar os valores de n para calcarios e arenitos fechados brasileiros. E importante salientar que este e o primeiro esforco de realizacao deste tipo de medidas em rochas brasileiras. Os resultados mostram que as aproximacoes mencionadas podem introduzir erros nas estimativas de pressaoa partir de dados de monitoramento sismico mesmo para rochas muito porosas.

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 …

Stress history of producing reservoirs and 4D seismic studies: Some often-forgotten aspects

In Brazil, even with severe repeatability issues due to weather, tide, and marine current conditions, seismic monitoring of hydrocarbon production has often yielded good results.

Some relaxed constraints on the stress history of producing reservoirs and 4D seismic studies

Numerous successful applications of seismic monitoring of hydrocarbon production, in different types of reservoirs, have been published in the last decades. Even quantitative evaluations of saturation and pressure evolution due to reservoir production were illustrated in some case histories. Every 4D study, however, involves some restrictive assumptions related to rock and fluid behavior. This paper discusses some shortcomings related to the stress boundary conditions considered in time-lapse feasibility and interpretation studies and yields an insight on the possible errors involved.

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.