The 7th RPI for September-October 2016

Colin Sayers - Scientific Advisor at Schlumberger, Houston, USA

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.