Well Logs and Rock Physics in Seismic Reservoir Characterization
Authors: Walls Joel , Dvorkin Jack , Carr Matt 
Published: October 2008

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.