Well Logs and Rock Physics in Seismic Reservoir Characterization

Abstract

Seismic Reservoir Characterization, also known as reservoir geophysics, has evolved over the past several years into a multi-disciplinary, business-critical function in most ED&P organizations. Sheriff defines reservoir geophysics as "The use of geophysical methods to assist in delineating or describing a reservoir or monitoring the changes in a reservoir as it is produced." Reservoir geophysics is applied across a wide spectrum of the oilfield life cycle from discovery and early development to tertiary recovery. One critical part of this process is careful analysis and understanding of petrophysical properties from well logs and core data (seismic petrophysics). The purpose of this paper is to illustrate why seismic petrophysics is so important and to show how carefully constructed synthetic models can help the geoscientist interpret acoustic and elastic impedance inversion from seismic data. Introduction Well logs are sometimes viewed by geophysicists as "hard data" and not subjected to the same level of scrutiny as the seismic data. This can be a mistake because well logs are susceptible to errors from a number of sources. In this presentation we will examine some of the processes and procedures that allow well logs to be correctly used in Seismic Reservoir Characterization. The basic steps in seismic petrophysics analysis are:Collect and organize input dataPerform geophysical log interpretation for volume minerals, porosity, and fluidsDetermine fluid properties (oil API, brine salinity, etc.) and reservoir pressuretemperaturePerturb reservoir properties using rock physics effective medium models (pseudo-well modeling)Compute synthetic seismic tracesGenerate trend curves and crossplotsCreate graphics and digital output files. Geophysical Well Log Analysis Well log analysis for geophysics differs in several important ways from standard log analysis. In most cases well logs are obtained for the purpose of estimating recoverable hydrocarbon volumes. Therefore the zone of interest is mainly the producing interval(s). For geophysics, well logs form the basis for relating seismic properties to the reservoir. While we are still concerned about producing intervals, we also need good information about all of the rock through which the seismic waves have passed. Therefore our zone of interest is much larger and encompasses basically everything from the surface to total depth. In all cases the log data will require some editing, normalization, and interpretation before they can be used in a reservoir study. Several specific analysis steps will be followed:De-spike and filter to remove or correct anomalous data pointsNormalize logs from all of the selected wells to determine the appropriate ranges and cutoffs for porosity, clay content, water resistivity, etc.Compute the volumetric curves such as total porosity, Vclay, and SwCalibrate the volumetric curves to core data if availableCorrect sonic and density logs for mud filtrate invasion if neededCompute Vshear on all wells. Missing log curves can often be computed with a reasonable degree of certainty. There are two major ways this is done. The first is through application of modern rock physics principles.

Modelo de física de rocas para la interpretación de propiedades petrofísicas de un campo en el Golfo de México

“…La densidad modelada se ha estimado a partir del registro de resistividad profunda usando como datos de entrada el volumen de arcilla y las fracciones mineralógicas de los componentes de roca estimados. El modelo petrofísico es válido cuando la curva reconstruida de densidad coteja con el registro de densidad medido en las zonas contentivas de agua y en lutitas donde no (1) exista mayor efecto de derrumbes. Ambas densidades no deben coincidir frente a intervalos contentivos de hidrocarburos.…”

“…La disciplina de física de rocas provee el enlace necesario entre la sísmica y las propiedades del subsuelo, su integración con la petrofísica permite identificar las propiedades de rocas necesarias para construir un modelo del yacimiento [1,2,3], cuyo objetivo final es lograr representar con la menor incertidumbre posible lo que se encuentra en ellos, siendo necesario integrar todos los datos disponibles en su cuantificación. Sin embargo la información no siempre está disponible, como es el caso de los registros sónicos, esenciales para caracterizar un yacimiento a partir de las propiedades elásticas y acústicas de los pozos.…”

From qualitative to quantitative interpretation: An interpreter’s guide to fluid prediction in Pliocene to Turonian deepwater turbidites from West Africa to Asia Pacific

“…In practice, this requires that all seismic gathers have been properly conditioned (Singleton, 2009) for amplitude preservation, correct phase, statics, NMO stretch, residual move-out, and multiple suppression. In the case of well data, erroneous or missing wireline curve data introduced by bore hole breakouts, washouts, cycle skips, bed corrections, and mud-filtrate invasion must be successfully corrected or estimated (Walls et al, 2004).…”

Modeling Investigation of Correlations between Elastic Properties Predicted by Rock Physics Model with Seismic Data (Case Study of an Iranian Oil Field)

“…A synthetic models was constructed to illustrate why seismic petrophysics is so important as well as to show how carefully help the geoscientist can interpret acoustic and elastic impedance inversion from seismic data [9].…”