Processing for Meaningful 4D Differences
Standard 3D acquisition and processing approaches almost never are adequate for 4D monitoring. Two “legacy” surveys optimally imaged for 3D use may be interpreted for large changes, such as gas-cap formation, but almost certainly they will not be suitable for measuring small differences or determining reliable details of waterfloods. “Legacy” is the general euphemism used to describe surveys that have not been acquired and processed carefully for 4D analysis.
If surveys have not been acquired specifically to have repeatable geometry for 4D analysis, the results usually are both late and poor, even after lengthy processing. Trying to perform 4D analysis on poorly repeated acquisition is nearly always a waste of time and money. With properly repeated surveys, the processing can be quick and simple and can yield timely results.
Our inability to equalize surveys with different shooting geometries should tell us that something is incomplete in our industry's current approach. It is not our inability to remove “random” noise that is the problem, nor is it our inability to stack smooth coherent signal in phase. The problem is our inability to remove systematic semicoherent shot-generated effects, such as overburden distortion and multiples. In 4D monitoring, we should aim to repeat those problems and difference them away.
We will not go through all the details of seismic processing. Instead, we will outline a general philosophical approach and then look at some of the unique opportunities available for 4D analysis.
Figures & Tables
The people of Saudi Arabia know about our business. This book is about two camels. One is the 4D-seismic-data camel that should be welcomed into the reservoir-management tent, provided it does not bring all its multifold baggage with it. The other is the financialaccountant camel that is forcing its way into the geoscience tent, demanding to be fed. This camel cannot be satisfied by our hunches and hopes - this camel wants to chew on the current status of our assets and our quantitative predictions of how those assets will perform.
The aim of this book is to help two camels with one throw, so to speak.
Four-dimensional reservoir monitoring enables us to know what is happening to properties in oil-producing reservoirs, in 3D space and time. This knowledge is enormously important and increasingly urgent. Worldwide, the remaining discovered oil reserves are now just about as large as those already consumed. That is a vast amount of oil, but it is being consumed rapidly, and additional conventional reserves are increasingly hard to find. It is imperative for the industry and for consumers that we produce this remaining oil as reliably and efficiently as we can.
If we do not know what is happening in our reservoirs, we cannot hope to produce them optimally. Optimization includes aspects of safety, environmental impact, recovery factor, timeliness and, of course, cost and profit. Four-dimensional seismic data can be a major contributor to the knowledge of what is happening and where it is happening in our reservoirs. We need to work as quickly as possible with those who can use this knowledge, so we all can benefit from 4D surveys. We do not yet know how much recovery improvement ultimately will be possible, but it is proving to be profitable to find out. Currently, the cost of extra oil recovered as a result of 4D knowledge and appropriate action may be as low as $1 per barrel (bbl). That should leave more than $40 per bbl for investing in further improvements.
The simple physical principles of the 4D seismic method are shown in Figure 1-1. If we survey a producing oil or gas field before and during production, we can estimate changes to the reservoir. As hydrocarbons are replaced by water and as pressure changes, the seismic velocity and density of the reservoir change. From 4D surveys, we can measure the effects of those changes and identify where the changes are occurring in the reservoir.