2012 Honorary LecturerSponsored by Shell

Middle East & Africa

Rocco Detomo, Jr.

Shell Nigeria Exploration & Production Company, Inc. Lagos, Nigeria

4D time-lapse seismic reservoir monitoring of African reservoirs


Rocco DeTomo When I was asked to be an SEG Honorary Lecturer, the question of my "topic" became paramount. I wanted to discuss a topic for which I was not only very experienced in, but which was temporally current, relevant to the region, and topically interesting. I wanted a topic that would not only stir the imaginations of the burgeoning student population of the region, but also bring state-of-the-art technical information to local industry and government practitioners!

Why lecture on 4D time-lapse seismic monitoring and why for African reservoirs?

4D time-lapse seismic reservoir monitoring is not new, but it has a history of slow application and deployment. Historically, reservoirs are modeled and developed based upon analogs and the limited amount of discovery and appraisal data that are available. Once production begins, these models are updated based upon newly available data and the reservoir's production history. From these refined models, the development is adjusted and steered differently to better optimize the value extracted.  This feedback loop between analysis, action, investment, production, and refinement only works when developments take place over large periods of time or as a continuous process. Sometimes, poor assumptions in our early models are not apparent until late in the development cycle when remedial action may no longer be economic, or when the consequences of nonoptimal choices are unavoidable. In addition, many of our developments today, especially in deep water, do not have the luxury of staging their development over time. Pre-investment decisions, subsea infrastructure, and huge financial commitments may leave limited opportunity to change anything beyond a field's early life.

Since most of our oil and gas reservoirs of today are found and mapped using seismic data, it is natural to ask if the way we change a reservoir impacts the seismic data we measure? If the answer is "yes", then it is a question of relating these changes in the seismic to the reservoir's changing properties, and determining the economics of what this new data would cost versus the value of the information that would be obtained. Old reservoirs where no significant new investment is pending, or reservoirs where no significant change in seismic would have occurred, or reservoirs where acquiring new seismic is impossible or very expensive, may not be good candidates for 4D time-lapse monitoring. This is the case in some parts of the world.  However, fortunately for Africa, many of the large or innovative hydrocarbon developments in the region are just beginning, and the response of the seismic to the reservoir's changes is often significant. This means that there are many opportunities for 4D time-lapse seismic monitoring to add significant value to these developments.

In addition, there is a new, growing need for 4D time-lapse reservoir monitoring in our industry: demonstrating social responsibility. The reputation and social consequences of even a rare, but serious safety, environmental or operational unexpected event, such as an out-of-zone injection or unanticipated fracture propagation, is unacceptable!  4D seismic monitoring may offer early identification of subsurface problems, demonstrate proof of corporate prudence, and insure social challenges can be met with data and facts.

What is different today in the area of 4D time-lapse seismic reservoir monitoring from that which might have been considered in the past?

As is always the case in our industry, technology moves forward quickly. A better understanding of our seismic data, identifying random noise from repeatable "noise" from recognizable signal, has driven us to appreciate the inherent value of source-receiver repeatability. Advances in seismic instrumentation –high channel-count land systems, ocean bottom systems including nodes and cables, ghost-less streamers, downhole optical acoustic sensors – are all rapidly advancing the accessibility, cost-effectiveness, and technical quality of 4D time-lapse seismic data. Recent advances in seismic processing algorithms and soaring compute power are allowing us to uncover increasingly smaller 4D signal levels recorded in the data. And finally, the appreciation of the fact that we are more often surprised by 4D results that are unanticipated, than by those we expect from our models' predictions. The overall result is that there is a growing applicability of 4D seismic monitoring to more and more projects.

What do I hope those that attend will carry away from these lectures?

As I mention in my lecture interview, I hope that those who are new to the idea of 4D time-lapse seismic reservoir monitoring, find an appreciative awareness of its value and an improved understanding of 4D principles. I would hope they will be able to identify the factors that they must consider for a 4D time-lapse seismic survey, and have an idea of what kind of settings and reservoirs are most suitable for the technique. For those who are already familiar with the technique, I hope to expose them to a range of examples, discuss the current issues and look to where the technology is headed.

What does the future of 4D time-lapse seismic reservoir monitoring hold?

I have no "crystal ball!" I can only expect that in an industry that leaves nearly half of our resources in the ground, even a small improvement in our response, efficiency and understanding of our reservoirs can have a large impact. With technology driving a revolution in sensors and data collection, I expect permanent areal and downhole, passive and active seismic monitoring of important reservoirs to become more widespread. But this type of Life-of-Field Seismic (LOFS) will bring an explosion in data volumes and require a level of automation in processing and evaluation that we are only beginning to prepare for. Today's active seismic snapshots are acquired at time intervals driven by business impact, equipment availability, access convenience and cost-effectiveness. However, tomorrow's instrumented fields may provide nearlyinstantaneous "seismic-on demand" for when a reservoir question arises, or perhaps even the installation of permanent active seismic sources.