Time-lapse (4D) seismic technology is a key enabler for improved hydrocarbon recovery and more cost-effective field operations. By analyzing differences of multiple seismic surveys acquired over a producing reservoir and by integrating with conventional reservoir monitoring data, 4D seismic data provides valuable insight on dynamic reservoir properties such as fluid saturation, pressure and temperature. Changes in these reservoir properties that occur during hydrocarbon production are identified and quantified by 4D analysis and used to identify areas of bypassed and undrained pay and to improve geological and engineering models. The purpose of this course is to provide an overview of the fundamentals of 4D seismic technology, starting from its role in field lifecycle planning and then through seismic acquisition, processing, and analysis. However, a primary focus of the course is interpretation and data integration. Case study examples will be used to demonstrate key concepts and will be drawn upon to demonstrate the range of interpretation methods currently employed by the industry and the diversity of geological settings and production scenarios where 4D is making a difference.
The classical meaning of the word dispersion is frequency-dependent velocity. Here we take a more general definition that includes not just wave speed but also interference, attenuation, anisotropy, reflection characteristics, and other aspects of seismic waves that show frequency dependence. At first impression, the topic seems self-evident: Of course everything is frequency dependent. Much of classical seismology and wave theory is nondispersive: the theory of P- and S-waves, Rayleigh waves in a half-space, geometric spreading, reflection and transmission coefficients, head waves, and so forth. Yet when we look at real data, strong dispersion abounds. This course is a survey of selected frequency-dependent phenomena that routinely are encountered in reflection-seismic data.
The following topics will be addressed in the course:
- Introduction to 4D seismic technology: A review of reservoir management concepts and the incentives for seismic reservoir monitoring, key 4D concepts, technical issues, success factors, and the role of 4D in field lifecycle planning.
- Reservoir engineering fundamentals: Describes how different reservoir depletion mechanisms influence fluid and pressure distributions in hydrocarbon reservoirs and how 4D seismic might be used to monitor them. Hydrocarbon fluid systems and conventional reservoir surveillance tools are also discussed.
- The petrophysical basis for 4D: Understanding the rock physics link between the geological and engineering properties of a reservoir and the elastic properties is essential to 4D interpretation. This section presents a review of the acoustic properties of fluids and how seismic velocities and density depend on rock properties, stress, temperature, and fluid saturation.
- 4D seismic modeling and feasibility studies: Where and when can 4D seismic methods be successfully applied? Time lapse seismic modeling is taken from simple spreadsheet approaches to well-log fluid substitution and then to seismic models derived from reservoir flow simulation. Approaches to estimate the business impact of 4D data are also discussed.
- Seismic acquisition and repeatability: The reliability of 4D seismic data is determined in large part by the similarity of repeated seismic surveys. Measures of repeatability and causes of non-repeatability are discussed along with strategies for acquiring repeatable seismic data in both marine and land settings.
- 4D seismic processing, data analysis and QC: The objectives of 4D processing are to maximize repeatability, preserve and resolve differences associated with production, and retain true relative amplitudes. Critical factors in 4D processing are discussed along with cross-equalization and data QC methodologies.
- Interpretation and data integration: What time-lapse seismic attributes are most effective for interpretation? How is production data used to validate 4D interpretation? When can map-based or volume-based interpretation methods be used? How can 4D inversion add value? How can 4D data be used to update geological and reservoir flow simulation models? What are the pitfalls in 4D interpretation? These issues and others will be discussed in the context of case studies that demonstrate 4D seismic application for water and gas sweep, pressure depletion and compaction, steam and CO2 floods, and CO2 sequestration.
- The future for geophysical monitoring: Other geophysical methods such as gravity and microseismic monitoring are seeing increased application within the industry. In addition, 4D seismic technology is moving from qualitative interpretation to quantitative analysis, enabled in part by the development of permanent monitoring systems and the instrumented oil field.
At the end of this course, the student should have an understanding of the fundamental principles of time-lapse 4D seismic monitoring applied to reservoir surveillance of saturation and pressure changes. The student will know:
- In which geological settings, reservoir and fluid property conditions, and production scenarios 4D monitoring may be appropriate.
- How seismic acquisition and processing of seismic data can impact the ability to detect reservoir changes and what can be used to measure and maximize repeatability.
- The basic concepts and workflows for time-lapse seismic interpretation and integration with geological and production data.
- How 4D seismic data can be used to impact reservoir management.
- How 4D seismic data have been applied in a number of case studies.
- Recent advances in time-lapse geophysical technology.
Who should attend
4D seismic interpretation is inherently integrative, drawing upon geophysical, geological, and reservoir engineering data and concepts. As a result, this course is appropriate for individuals from all subsurface disciplines. The presentations will focus on fundamental principles and applications, emphasizing case studies and minimizing mathematics. As a result, attendees do not need a theoretical background in either geophysics or engineering. For those who would like to explore 4D seismic technology in more detail, the course book will provide additional material and references.
David H. Johnston is geophysics coordinator for ExxonMobil Production Company in Houston, Texas. He earned a Bachelor of Science degree in earth sciences from the Massachusetts Institute of Technology in 1973 and a Ph.D. in geophysics in 1978, also from MIT. After graduation, Johnston joined Exxon Production Research Co. (later ExxonMobil Upstream Research) and held assignments in rock physics research, velocity analysis and interpretation, and seismic reservoir characterization and monitoring. He moved to ExxonMobil Exploration Co. in 2002 where he was responsible for the worldwide application of time-lapse seismic technology. In 2008 Johnston began his current assignment where he provides technical and business stewardship of ExxonMobil's global production geophysics activity, including 4D seismic.
Johnston is a member of SEG, SPE, AAPG, EAGE, and AGU. He served as Secretary/Treasurer of the SEG in 1990 and has chaired the SEG Development and Production and Interpretation Committees. He was an editor of Seismic Wave Attenuation (1981), Reservoir Geophysics (1992), and Methods and Applications in Reservoir Geophysics (2010), all published by the SEG. He is an editorial board member for Interpretation, the new SEG journal.
Johnston was awarded Best Presentation by the SEG in 1993, honorable mention for Best Presentation in 2010, and Best Paper in The Leading Edge in 2005. He was an SPE Distinguished Lecturer from 1992 to 1993, the SEG Distinguished Lecturer in 1999, and an AAPG Distinguished Lecturer in 2008. In 2003 he received Honorary Membership in the Geophysical Society of Houston and in 2004 he was awarded Life Membership in the SEG. In 2007 Dr. Johnston was the first recipient of ExxonMobil's Peter Vail award for distinguished technical achievement.