The 7th RPI for September-October 2016

Colin Sayers - Scientific Advisor at Schlumberger, Houston, USA

About Colin Sayers

Colin Sayers was born in the North West of England, and obtained a B.A. in Physics from Lancaster University in 1973. He then moved to Imperial College, London, where he received a D.I.C. (Diploma of Imperial College) in Mathematical Physics and a Ph.D. in Physics in 1976. The title of his thesis was “Correlation effects in the transition metals and the screening of non-transition element impurities in iron and nickel”. Following a two year post-doctorate fellowship at Imperial College, he joined the Materials Physics Division of the Atomic Energy Research Establishment, Harwell, U.K., in 1978, where he first began to work on elastic wave propagation in complex media.

He entered the oil and gas industry in 1986 to join Shell's Exploration and Production Laboratory in Rijswijk, The Netherlands, where he was a member of the Geomechanics group of the General Research Department. There he used ultrasonic waves and acoustic emission to characterize the anisotropic damage that develops in rocks due to applied stresses, and used theoretical and computational methods to understand the results. In 1991, he moved to Schlumberger’s Research Laboratory in Cambridge, U.K. where he was a member of the seismic department. He moved to Houston in 1998, where he is now a Scientific Advisor in the Schlumberger Seismics for Unconventionals Group in Houston. His technical interests include rock physics, exploration seismology, reservoir geomechanics, seismic reservoir characterization, unconventional and fractured reservoirs, seismic anisotropy, borehole/seismic integration, stress-dependent acoustics, and advanced sonic logging.

He is a member of the AGU, EAGE, GSH, SEG, and SPE, a member of the Research Committee of the SEG, and has served on the editorial boards of the International Journal of Rock Mechanics and Mining Science, Geophysical Prospecting, and The Leading Edge. In 2010, he presented the SEG/EAGE Distinguished Instructor Short Course "Geophysics under stress: Geomechanical applications of seismic and borehole acoustic waves", and was chair of the editorial board of The Leading Edge. In 2013, he was awarded Honorary Membership of the Geophysical Society of Houston "In Recognition and Appreciation of Distinguished Contributions to the Geophysical Profession". He also received the award for best paper in The Leading Edge in 2013.

Your pathway for success in becoming a well-known name in the rock physics community

For me, the key to success is to be able to work on interesting and important problems in a challenging multidisciplinary environment with talented colleagues. Attendance at conferences and workshops of an international standard also helps me to remain up-to-date with new developments. I have a wide interest in the physics of complex media, which goes back at least as far as the time I was working on my Ph.D. in Theoretical Solid State Physics at Imperial College, London. I first began to work on elastic wave propagation in complex media when I moved to the Materials Physics Division of the Atomic Energy Research Establishment, Harwell, U.K. This work focused on characterizing the microstructure and stress state of materials using the velocity and attenuation of ultrasonic waves, and is directly related to methods I am using today to characterize the structure and stress state of sedimentary rocks using seismic and borehole acoustic waves. My first serious encounter with rock physics was in 1986 when I joined Shell's Exploration and Production Laboratory in Rijswijk, The Netherlands. Sedimentary rocks are particularly interesting examples of complex media, having structures on a wide variety of length scales and processes that operate on a number of different time scales, and present us with many types of complexity. The challenge is to identify and model those features that are essential to understand the properties of rocks so that we can make appropriate predictions. It is important to realize the limitations of the various models in use in rock physics. Given the complexity of sedimentary rocks, it is impossible to model their properties exactly. For this reason, bounds that are able to take into account the spatial distribution of rock constituents are of particular importance.

Challenges you see in moving rock physics to the next level 

An important challenge is the question of scale dependence. We interrogate rocks with elastic waves of different wavelength, from seismic waves used in exploration to borehole and crosswell seismology to borehole acoustic waves to ultrasonic waves in the laboratory. Sedimentary rocks contain structures on many length scales, from the size of grains and microcracks to fractures, bedding, flow channels and faults, and processes that operate on a wide variety of time scales from creep in salt to flow processes that occur in response to an elastic wave, and that are responsible for much of the attenuation observed in sedimentary rocks. As just one example, the compliance of a fracture varies strongly with fracture size, with the result that large fractures have a much greater effect on elastic wave propagation than do grain boundaries and microcracks. And yet, many laboratory measurements are made on samples carefully chosen not to include compliant features such as fractures that may dominate the large scale response of the reservoir. A major challenge, therefore, is to develop methods of upscaling from the pore to reservoir scale that include the effects of compliant stress-sensitive features such as fractures and faults, and to provide the links between static and dynamic measurements.

Another important challenge is the treatment of uncertainty. The building of earth models requires the integration of data from many sub-surface disciplines, and this requires an understanding of the uncertainty in the different types of data. For example, drilling engineers require an estimate of formation pore pressure so that they can design the mud weight and casing string to avoid kicks and blowouts while drilling the well. In current practice, a seismic velocity to pore pressure transform is often applied with no quantification of uncertainty. This means that expensive drilling decisions are often made without adequate risk assessment. A better approach is to recognize the uncertainty in the prediction and to continually update a rock physics based velocity-to-pore-pressure transform using data acquired while the well is being drilled, so that the best possible prediction with reduced uncertainty is continually provided ahead of the bit.

Advice for early career scientists (rock physicists, geophysicists, etc.)

My advice would be to read widely, immerse yourself in a multidisciplinary environment, and listen carefully to colleagues in your own and related disciplines. Scan journals in related fields, and try to attend conferences and workshops of an international standard. For those in academia, try to spend at least some time in industry to get a feel for the priorities and type of work being done there. For me, the best thing about working in industry has been the opportunity to work on many different topics. Expertise in physics is widely transferable, and experience gained in solving one problem helps to solve others.

For those looking for interesting research problems, one possibility is related to the issue of complexity. There is currently much important work being done on understanding the behavior of sedimentary rocks at the microscale. However, sedimentary basins contain features on a wide variety of length scales from the size of grains and microcracks to fractures, bedding, flow channels and faults. A major challenge is to develop methods of upscaling from the pore to reservoir scale that include the effects of compliant stress-sensitive features, such as fractures and faults, that may dominate the large scale response of the reservoir and surrounding rocks. An important question is how complicated the description should be. Although there are many sophisticated computer programs available, it is best, in my opinion, to start work with a simple, preferably analytically soluble, model in order to understand the physics of the problem before moving onto the full complexity of the earth. To paraphrase Einstein, a model should be as simple as possible, but no simpler; however, it is important also to keep in mind the words of Yogi Berra, and not forget that in theory there is no difference between theory and practice, but in practice there is!

Another important problem is the treatment of uncertainty. It is only because of uncertainty that many of us have a career in the oil and gas industry. If the sub-surface were known with certainty, there would be far fewer jobs. Because of the cost and difficulty of obtaining high resolution data at depth within the earth, most people in the industry operate in a data-limited environment. The treatment of uncertainty requires the integration of different types of data, each with its own degree of uncertainty, into a predictive earth model. The understanding of the uncertainty in the model allows data acquired while drilling, for example, which differ from the prediction of the model, to be analyzed and used to update the model. A practical challenge is how to communicate this uncertainty to the drilling engineer, for example.

Finally, the relation between the dynamic and static properties promises to be an interesting area for future research. Seismic, sonic and ultrasonic waves can be used to measure dynamic properties, but static properties are required for geomechanics. I have greatly enjoyed working with multidisciplinary geomechanics teams in which geomechanicists, rock physicists, geologists, petrophysicists, drilling engineers, completion engineers, and other experts work together to solve problems of geomechanical importance. The interactions between experts in these different disciplines is fascinating, and in common with all multidisciplinary projects, suggest many promising areas for future technical development.

We thank Colin for his continuous contributions to the rock physics community