The 17th RPI for May - June 2018

Rune M Holt - Professor in Geophysics at the Geoscience & Petroleum Department of NTNU, Trondheim, Norway

About Dr. Rune M Holt

As a high school student in the 60’ies, Rune was fascinated by the ongoing space exploration as well as atomic scale physics. His favourite topic was math, and through his physics teacher he discovered that mathematics was not only a subject in itself, but also could be used for something meaningful. Besides, without knowing exactly what he wanted to be, he decided to study physics at the Norwegian Institute of Technology (now NTNU), since people told him that “if you have studied physics, you can do anything”. His MSc and PhD studies (also at NTNU) were in solid state physics, where he specialized in laboratory experiments using time-reversed ultrasound (so-called electroacoustic echoes, generated by higher order piezo-coupling) to investigate phase transitions in crystals, under inspiring supervision by Professor Kristian Fossheim. At this time, around 1980, the oil and gas activities were developing at their peak rate offshore Norway, and when Rune joined SINTEF (and IKU, which became the petroleum division of SINTEF) as a researcher, project funding was readily available for petroleum related work. In the beginning, his work was dedicated to acoustic classification of the sea-floor, including beach-scale measurements of surface waves in sand. With time, he went deeper, into hydrocarbon reservoirs and overburden shales. His research has addressed rock mechanics of borehole stability during drilling in shales, and of reservoir compaction due to depletion. The importance of measuring key rock mechanical parameters (like static moduli and strength) from logs and cores made way for research on the impact of stress release during coring on laboratory core measurements. In this work, controlled experiments were performed on synthetic sandstone that was cemented under stress, enabling systematic comparison between a material that was undergoing “simulated coring” and an identically formed material that was deformed under “in situ” conditions. The experimental work was followed up by numerical simulations using a bonded discrete particle model, contributing further to understanding of the mechanisms involved. This also led to substantial research on “digital rock mechanics”. 

In 1993, Rune was appointed a professorship within petroleum engineering and applied geophysics at NTNU, and he has combined his university work with work for SINTEF since then, currently as a scientific adviser. He is one of the authors of the textbook “Petroleum Related Rock Mechanics” by Fjær, Holt, Horsrud, Raaen & Risnes. During the last decade, Rune’s main research interest has been in shale rock physics and mainly related to 4D seismic interpretation of the overburden response. Looking back, his carrier has led him somewhere in the middle (on a logarithmic scale) between nuclear and astrophysics, namely to rock physics. 

Pathways or recipes for your success in becoming a well-known name in the rock physics community

Life is to some extent a random walk – the path is influenced by strong and weak forces of different nature, be it your dreams, your colleagues and friends, or political and financial incentives. For me, I believe all of these have been part of the pathway: A passion for exploring and understanding nature, work environments where I have been surrounded by clever and motivating people, and of course the ability to attract funding from industry and governmental bodies. To cite a few of the other rock physics influencers: First of all, it has been fun!

I believe one of my benefits has been to work in between purely academic and industrially applicable research. Being in the formation physics team in IKU / SINTEF and at NTNU with dedicated people has been an important ingredient in building and maintaining the passion. In daily discussions with my colleague Erling Fjær over more than 30 years, we have solved (and created) a number of scientific problems (and solved a few global problems in addition (- unpublished…)). 

Having literally to sell your ideas to industry from time to time, and then receive guidance and insight from industry into what is relevant in the field, has enriched the work I have been involved with. Some very clever customers have helped a lot, like Leon Thomsen (then with Amoco) who helped us to create synthetic rocks with controlled crack patterns for calibration of anisotropic rock physics theories (including his own, which at the time gave the best fit to our data!), and like Cor Kenter (Shell), who guided us along the core quality path mentioned above, further to digital rocks, and finally showed us the potential and challenges of rock physics and geomechanics for overburden 4D seismic. 

One of the things that keeps surprising me at work is that even if we do our experiments with small samples, that may not be representative for the big field problems, we can very often see the same effects in the lab as people see in the field. For instance, how shales respond to different brines may be directly translated into drilling practices to avoid borehole instabilities. Or, the 4D effects in the overburden can be seen in laboratory specimens. Even the synthetic rocks tell us some truth that we can recognize in the field, and controlling the specimens helps us to understand their behavior. There is of course no direct translation – there are scale issues and so on – but the principles of physics and chemistry are scale invariant – they work the same way in the lab as in the field. We need to use the laboratory to learn about mechanisms, so that we develop and use models that contain the right physics. But, the experiments need to be performed under the right conditions, and I believe all tests that address in situ rock properties and rock behavior should be done as close to the relevant in situ conditions as possible. I believe this has been and is a key to success in rock physics research for field applications. 

Challenges you see in taking rock physics to the next level 

The growth of rock physics over the last 20-30 years has been driven by improvements in seismic acquisition and processing. Better imaging calls for rock physics models to improve quantitative subsurface characterization, and 4D interpretation needs rock physics and geomechanics to fully utilize its potential for surveillance of reservoir behaviour. During the 4IWRP in Trondheim last year, Leon Thomsen raised a question on the future of rock physics: He claimed that unconventional resources have been found largely without seismic as a main part of exploration, and he added that rock physicists will have to look for new areas to use their expertise. One obvious application is to try to quantify rock engineering parameters (strength, static moduli, stresses etc) from seismic data. This is indeed a favourite topic of our team, since static mechanical behaviour and acoustic wave propagation always were handled together, both in our laboratory work and in several modelling efforts.

Rock physicists should struggle to get even closer to in situ conditions in the laboratory. For instance, we do not know how important true triaxial stress state is. The role of applying in situ temperature is uncertain – most experiments are performed at room temperature only. We can handle seismic frequencies through the forced oscillator technique and ultrasonics under controlled stress, but data from the sonic logging interval is often lacking. Large dispersion is observed in shale by several laboratories where seismic and ultrasonic frequencies are explored, including ourselves, but we lack the complete physical understanding. Does this dispersion manifest itself in field data, and what determines the borderline between low and high frequency behaviour?

It is somewhat surprising that petrophysics and rock physics seem to be two different disciplines. Log analysis is about understanding the physics of rocks in a broad sense, but common rock physics models often appear unknown in log interpretation. Remembering back to my solid state physics textbook, it dealt with a broad spectrum of physical properties, including electromagnetic, elastic, and thermal properties plus topics like light and neutron scattering plus nuclear magnetic resonance. In the physics of rocks we could add fluid flow and fluid-solid interactions, and we could aim for a “grand unified” theory.

Anisotropy found its way into geophysics and rock physics 30 years ago, and is now well established. As a physics student, I learned the theory of elastic anisotropy, and it is indeed one of the few tools that I enjoy working with every day as a rock physicist! However, isotropic models and concepts are still widely used, even more in rock mechanics than in rock physics, even for cases which clearly require anisotropic treatment – like shales. Even if the anisotropy parameters may be hard to determine, assuming that they are all zero is clearly wrong and can have unwanted consequences.

The upscaling issue is the “holy grail”, I believe it can only be solved more and more accurately by more and more detailed rock mass description and numerical modelling. This is maybe a “Big Data” issue, but I am admittedly a bit scared about machine learning based on empirical observations only. Will we just have more sophisticated correlations, or maybe that will be good enough for several practical purposes. However, from a scientific perspective: Where have all the fractals gone?

Last, but not least: I believe we should seek more applications for or joint rock physics knowledge outside the petroleum industry: Geotechnical, geothermal, seismological, mining, tunnelling - or maybe to porous media that are not rocks - indeed, if you search papers on Biot theory, you find quite a number of articles in biophysics.

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

First of all, you have to study fundamental topics, learn mathematics, physics (and maybe even some chemistry) by heart. Just too many of us (including myself) lack in depth knowledge of geology – I think it is a good idea to learn more about the materials that rock physicists try to describe.

Second, follow your dreams, but remember that hard work is necessary. What you learn in the studies, may not be what you will work with 20-30 years from now, but you will need to develop your ability to search the literature and learn from others, maybe in a different field of science.

Find an attractive work environment, preferably one where you can develop your individual skills and at the same time work in a team. Read relevant publications, and learn how to write them!

Go to conferences – for me the first conferences I went to after I started to study rocks were called “Physics & Chemistry of Porous Media” – there were two of them, both attracting people from industry, research institutes – and a few Nobel price category physicists – it was fantastic! Today: Well, for the IARP web page, I will of course recommend the upcoming 5IWRP and those to follow…

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