On the use of Data Transformation Filters in Time-Lapse Imagery
Authors: Bemi A.J. , Barros J.C. , Kohli M.J. , Stellman T.A. , Purdy G.E. 
Published: October 2008

The use of time-lapse seismic imagery is rapidly becoming popular. The images are usually derived by forming the difference of two 3-D seismic data sets, one acquired preproduction, the other after several years of hydrocarbon depletion. Many of these (differenced-data) images will have small amplitudes provided the two datasets were acquired and processed in a similar manner. However, in the zones near the wells, fluid substitution (oil to water, oil to gas) can cause large amplitudes in these time-lapse images.

Time-lapse images can be used to show drainage areas, thereby revealing bypassed pay. Other applications would be monitoring gas cap expansion, Carbon Dioxide or water floods, bum fronts, etc. However, experience has shown that seemingly small variations in the data workup and display options can cause significant differences in the interpretable images. Some degree of standardization and a consensus on best practices should improve the situation.

In this paper, we will use two 3-D surveys acquired at South Timbalier 295 to assess the effectiveness of various methods to obtain time-lapse images. These surveys are termed "legacy" data because they were not specifically designed for time-lapse work. Such legacy data have acquisition and processing operations that are not consistent from survey to survey. These inconsistencies can lead to high amplitudes in the time-lapse images that can be misinterpreted. The use of data transformation filters to correct the data prior to differencing is one of the less expensive approaches to the legacy data problem.

The AVS processing package was used extensively in this work. The AVS package includes software modules developed by the Lamont-Doherty Earth Observatory of Columbia University as well as modules developed at Shell's Bellaire Technology Center.

Introduction

The purpose of this paper is to describe data transformation filters (DlT) and to evaluate their effectiveness in improving drainage pattern estimates. Two 3-D surveys acquired at the South Timbalier Block 295 field will be used as a real data example.

The DTF is useful for equalizing wavlets, correcting scaling errors, and removing time-shifts between two data sets. In time-lapse work, the first data set is the preproduction 3-D survey. The other data set is a 3-D survey acquired after significant hydrocarbon production. The purpose of the DTF is to equalize the data prior to the time-lapse analysis.

The basic idea is to derive the filter on parts of the data where you don't expect any changes. Hopefully, the filter then removes waveform differences between the two surveys that represent acquisition and processing artifacts.

Study Area

The South Timbalier Block 295 field is located offshore Louisiana under 300 A of water, 120 miles south of New Orleans. The reservoir geology and production performance have been described by Mason.1 Three major reservoirs have been identified as turbidite sands with good porosity (0.3 1) and high permeability (0.5 Darcy). While the depositional setting varies from mid-fan sheets to channels, all three sands (Kg, K16, and K40) have been produced at high sustained rates.