Ben Clennell is a research stream leader in the Wealth from Oceans National Research Flagship, attached to CSIRO Petroleum in Perth, Western Australia. Ben graduated in geology from the University of Oxford in 1988, and gained a Ph.D. in geology from the University of London in 1992. From 1992 to 1993 he worked at the University of Birmingham on experimental sediment deformation. At this time, he became involved in the Ocean Drilling Program research into active margins. These interests continued at the University of Leeds, where he was also attached to the Rock Deformation Research Group (RDR). Further work with the Ocean Drilling Program in Costa Rica prompted a long spell working on natural gas hydrates, from a sediment physical properties point of view. Involvement with RDR had, however, provided an introduction to the oil and gas industry and led to a gradual transition from geology to petrophysics. The focus on oil and gas research, and work in the crossover between petrophysics and geophysics was invigorated by a move to the Federal University of Bahia (UFBa), Brazil in 1998. Ben worked as a visiting professor and visiting researcher in the areas of petrophysics, petroleum geology, and hydrogeology at the Centro de Pesquisa em Geofisica e Geologia (CPGG), at UFBa until 2003, under the expert guidance of Prof. Olivar Lima. During his time in Brazil, he was fortunate to meet many luminaries of the geophysics world and joined SEG in 2002. In 2003, the opportunity came to move to Australia and work for CSIRO as a full-time research petrophysicist. Since arriving in Perth, Ben has served on the committee of the local SPWLA chapter, FESAUS. His research continues in the areas of petrophysics/rock physics and rock deformation, and he has very much enjoyed working with Tony Siggins and Matthew Josh to develop a world-class rock electrical properties laboratory at CSIRO.
2010 SEG Honorary Lecturer, Pacific South
Electrical properties of sedimentary rocks from DC to dielectric frequencies
Electromagnetic phenomena underlie many of the methods used for finding and characterizing oil, minerals, and gas in the earth and are of vital interest to geophysicists. Applications span an enormous range of frequencies, from millihertz magnetotelluric readings to radar investigations in the hundreds of megahertz range. For a petrophysicist evaluating subsurface formations, the resistivity log has long been the main tool to discrimate oil- and gas- bearing zones from water-bearing zones. Dielectric tools, operating up to 1 GHz in frequency have been used in the past and are now making a comeback for specialist applications. One problem with dielectric logging was the generally poor understanding of the physics involved and the major differences between dielectric responses and standard electrical responses made at low frequency (i.e., a frequency much less than 1 MHz).
The popularity of resistivity for oil detection in petrophysics has in the past decade or so been taken up with renewed vigor in the search for oil and gas, with controlled source electromagnetics (CSEM) being applied widely, especially in frontier exploration settings. The need to monitor and verify safe, long-term disposal of CO2 through geosequestration has also led to increased interest on onshore CSEM surveys, both from the surface and crosswell. The resurgence in EM methods has led many geophysicists with a predominantly seismic background to seek a better understanding of rock electrical properties and how they are measured. Petrophysicists are being asked tricky questions like "does this downhole log from my offset well give a good representation of the resistivity structure in my basin that will be seen in a CSEM survey?
The aim of the lecture is to outline the fundamentals of rock electrical behavior. Topics to be covered are:
- Introduction: High and low frequency realms of electromagnetic behavior and the crossover area between them
- A primer on dielectrics
- Making valid laboratory measurements of electrical properties
- Galvanic, inductive, and capacitive coupling into the earth
- Downhole "resistivity" tools and what they measure: i.e., galvanic, induction, and propagation type tools
- Log versus survey: some effects of scale and anisotropy
The focus is on sediments and sedimentary rocks, though many of the basic principles apply also to "hard rock" situations.