Rock Physics Facilities: An Experimental Geophysics —By Nabarup Ganguly, M.Sc. Dept Of Geography.

Introduction

The Experimental Geophysics Group (EGG) carries out a wide variety of field and laboratory studies. These studies focus on understanding what controls the physical properties of rocks and how these relate to geophysical surface observations. Here, we hope to give some overview of the laboratory facilities that EGG has developed over the years for characterizing porous rock samples and for making ultrasonic P and S wave measurements for both conventional rock property evaluation and for more fundamental studies of wave propagation in porous and anisotropic media. This leads to a summary of some of the recent laboratory results obtained. The motivations for much of this research vary and often it is in support of our research activities in scientific drilling, geomechanics, and stress determination. One portion of ourwork focuses on conventional laboratory rock property measurements, in these we gain knowledge of the properties of the rock under conditions of pressure, temperature, and saturation expected in situ. The results in such cases provideconstraints on geophysical observations, such as providing the difficult-to-obtain dry or drained bulk modulus necessaryfor Gassmann fluid substitution calculations. The second portion concentrates on measurements that allow us to betterunderstand how waves propagate through complex materials. This work is of a more fundamental nature and it seeks to better constrain our interpretations of geophysical data.

Experimental Facilities

Measurements under In Situ Conditions: Rock is a notoriously complex material. One aspect of this is that the P and S wave speeds are highly dependent on in situ conditions, and in particular, the velocities of nearly all rocks depend on the effective pressure they are subject to. As such, making a measurement of a rock property at atmospheric pressure will mislead an interpretation of field data if it is assumed this is representative of the same property at depth in the earth. Consequently, the measurements must usually be made on rocks subject to confining and pore pressures. This significantly complicates carrying out such tests as now one must adequately seal a rock sample to protect it from the fluid transmitting confining pressure. Further, one must be able to send and receive signals from the sensors inside the pressure vessel to the data acquisition systems outside. To carry out such experiments, EGG has constructed two main pressure vessel systems here informally called Blue and Red. In part for reasons of safety, these vessels are capable of reaching relatively high confining pressures of 300 MPa (~45,000 psi or about 12km depth) and 200 MPa (~30,000 psi about 8 km depth), respectively. Red (Fig. 1) is most often used on smaller diameter (~2.54 cm) cylindrical plug samplesin simpler cases that do not require the sample to contain pore fluids. The high confining pressure that can be achieved is much greater than necessary for most sedimentary rock studies (< 50 MPa).

Future Directions

With regards to the materials studied, a good deal of the work in the immediate future will likely focus on continued studies of CO2 saturation on the seismic properties of rocks. We have completed a large study of carbonates associated with the Weyburn CO2 project under various temperatures and pressures. We are currently working on sandstone materials for Carbon Management Canada with the eventual goal of developing seismic models for purposes of time lapse monitoring. We will be broadening this work to better understand the physical properties of CO2 saturated brines themselves and carry this into the rock samples. The system and experimental protocols we have set up to carry out the CO2 studies is somewhat unique and has attracted interest from additional collaborators; so CO2 studies will likely continue for some time. Beginning with Bakhorji [2009] leading from the much earlier work of Schmitt and Li [1995] we have also begun includingpseudo-static strain measurements simultaneously to the dynamic ultrasonic velocity measurements. The strains versus pressure plots yield the static bulk moduli which is useful geomechanical information. This will further be supplemente by strength determinations that are necessary in the design of fracture operations. [Melendez and Schmitt, 2011] Of most interest, however, is our work towards the construction of an instrument to measure the moduli of rocks at seismic frequencies.

Currently, we are working in collaboration with the Australian National University (Canberra) to study the effects of fluid saturation in cracked rocks. High frequency measurements are made in the Rock Physics Laboratory while low-frequency forced-oscillation tests are carried out at ANU on the same quartzite rock samples. We are currently designing a new system to measure the seismic band bulk moduli of the material directly.

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About Nabarup Ganguly

Associate Disciple-Educator, M.Sc. in (Geography & Disaster Management), M.A. in Education, B.Ed, First Class First, Rank holder, Gold Medallist, Author & Life Member; Guide and Counsellor, Inventor (Tripura State Council For Science & Technology), Geographer, Department of Geography, RIO+20, Brazil, South America

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