Seismic Acquisition for 4D Monitoring
In this chapter, we will discuss how to decide and honor the acquisition requirements for good 4D monitoring. Let us assume that we think we know what production-induced seismic changes we are looking for, that we know what we want to measure, and that we have an estimate of the size of changes we are looking for. We saw some ways of doing this in Chapter 3.
Those requirements should be translated into seismic acquisition specifications. In the early days, the simple and direct approach was to make synthetic seismograms of the changes expected, add noise as measured or judged from previous data in the area, and then see whether we could expect to see those changes. If we thought we would see the expected changes, we could shoot another survey over the field. Just to pass economic hurdles, the second survey often had a dual objective, such as enhanced resolution.
Sometimes this worked and sometimes it did not. We found that poststack matching of dissimilar surveys appeared to work only if the shooting direction of the surveys was the same. If we were meticulous and really tried to repeat the previous survey parameters, we were surprised to find that the 4D differences often were better than expected. After reading Chapter 4, on repeatability, we can understand this. If we repeat the surveys as closely as possible, much of the “noise” repeats and the differences are indeed better because the repeating noise is differenced away.
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.