Time-Lapse Seismic Monitoring: Repeatability Processing Tests
Authors: Ross Christopher P. , Altan Suat 
Published: October 2008

Abstract
The impetus of seismic reservoir monitoring has begun to reach all facets of hydrocarbon production, and much is being expected from this nascent technology. The concept of being able to monitor hydrocarbons, hydrocarbon movement (or lack thereof) and fluidstate change is very alluring to the reservoir engineer, geologist andgeophysicist. However, to be able to trust the results of time-lapse 3D (TUD) seismic surveys for infill drilling and sweep management, one must first trust the repeatability of the system producing the repeat data, to assure that it is free of deleterious artifacts. To this end, zero-time repeatability tests are performed by acquiring and reacquiring the data before any production history has occurred. By comparing the two surveys, a system baseline can be determined and systematic artifacts identified.
System repeatability of data processing is as important as system repeatability of data acquisition for TUD surveys. Results obtained by identical processing of two surveys indicate that processing artifacts or the residual reflector energy can be minimized. The study also demonstrates that approaches traditionally employed for conventional 3D processing may yield artifacts that could be misconstrued as fluid-state change in differential seismic interpretation.

Introduction
Conceptually, repeat or time-lapse seismic monitoring is straightforward. Seismic data sets acquired at different calendar dates arecalibrated, compared and the inter survey differences analyzed and interpreted as fluid-state effects. In ideal petrophysical and porefluid situations, physical changes in the fluid state (or lack thereof) can be interpreted or inferred from repeated data sets, and several papers have been published in a variety of journals demonstrating Time lapse results m cases of fire flooding, steam flooding, gas injection, water injection, aquifer drive, etc.
The published results to date present a very positive and almost euphoric image of what seismic monitoring has done, and what the technology's potential can be, However, we should remember that many of these results are the products of well planned and executed research projects, and time-lapse technology has had limitedcommercial exposure. The phrase limited commercial exposure, is not intended to infer any derogatory connotations, but is a relative, technological assessment at this writing. In time we will perform more of these studies and will become more proficient and wiser about the limitations of time-lapse seismology. Meanwhile, we will encounter some obstacles to successful implementation, and this article addresses one such impediment involving seismic data processing, or rather, uniform processing of time-lapse data sets.

Repeatability
In most companies, the seismic monitoring decision process currently resides between two groups-the geophysical side of R&D, which has developed and is improving the technology application, and production, who's vested interest is efficient cost recovery (which helps pay for the research!) This extraordinary matchup results in many tradeoffs and compromises, with muchfocus on the trade-offs between repeatability and cost. From the research and development perspective, the key to successful seismic monitoring is dominated by repeatability.