Page 32 - Summer2019

Peculiar Acoustics ot Rocks
_ _ usually named for the places where they are first commonly
E;::mW found; Berea is a town in Ohio) is often used for research
. , . as a representative oil- and gas-bearing rock It was formed
_ _ y 1., about 400 million years ago from river sands deposited i.n
it ' .‘i, what is now the US Midwest from an ancient river delta
. 7 ,.v_ ( up in what is now Canada. It is perhaps worth noting that
W Q, G - some buildings are constructed of Berea sandstone (e.g,,
"Q?" -L -’ the Iohnson County Courthouse in Iowa; Rossman, 1975).
5 . W‘ What is especially notable about this sandstone is that the
l constituent grains are cemented together in such a way that
they also contain extensive pore space. These pore spaces
lTl contain remaining fluids, occasionally some fine clays and
mu silts, and in an oil and gas reservoir the remains of plants
and animals i.n the form of oil.
Figure 1. Thin section Hf n Bzreu sandstone sample under cross-
polarizing filters. oifilenmt colors highlight aifimnt grain orientations. Figure 1 shows a thin section ofBerea sandstone, with the light
Black areas are vuids, and snme of the pore space between grains through it observed through cross polarizers to best show off
contains clays and silt in this sample. See text for further details. the crystalline orientation of the grains. The dark material in
between the grains is the amorphous quartz and clays holding
the rock together. From a mechanical and acoustical point of
aid and interpret the underlying structurebeneath the surface View, this imperfect grain-to-grain bonding, with distributions
(go.l.ias:t.gov/2Fe9KRP; Smrekar et al., 2019). of stress concentrations via various grains and the ability for
grains to rotate in and out of pore spaces under stress, make
Finally, the question of where to store high-level radioactive for an interesting solid.
waste may find an answer in rocks too, such as in the Waste
Isolation Pilot Plant (WIPP; wippenergygov), a deep geolog- How does a stress change or acoustic pulse get transmitted
ical repository in New Mexico. For the WIPP, understanding through such a collection of grains? Unlike a homogeneous
rock properties (e.g., fluid and gas migration in the salt, how solid, forces are not distributed smoothly through a rock
the rock was formed, mechanical behavior) in this case, and Each individual grain is cemented to its neighbors in various
really in all cases above, is random ways. The particular collection of grains that partici-
pate, and not all do, in transferring a force or acoustic wave
Properties of Rocks through a sandstone is called a force chain. Many hundreds
To better understand the mechanical and acoustical proper- of these force chains might participate in wave propagation
ties of rocks, it is instructive first to know something about or maybe just a few. You can imagine that building a (finite-
how they are formed. Most of the rocks of financial inter- element) model of such a rock is no easy task. Add trapped
est are sedimentary, Indeed, sedimentary rocks are a focus fluids to that matrix, and you have a recipe for fascinating
of commercial oil and gas companies, The Oflice of Basic and challenging material behavior.
Energy Sciences, US Department of Energy, supports such
research as well. Ideal Solld Versus the Behavior of a Rock
So how does such an interesting solid behave? The mechani-
The primary building blocks of sandstones are weathered cal quasi-static behavior will be examined first. Imagine
quartz grains that are cemented together under millions fi.rst a spring that obeys Hooke’s law, F = ~k;r, where F is the
of years of pressure and fluid flow. These cementation pro- required force to extend or compress a spring, kis the spring
cesses are called lithificntion and/or diagenesis, and books consmnt, and xis the distance to be moved. (Of course, such
are written about these processes and especially about those a spring cannot be compressed tau much lest the coils of the
sandstones that are relevant to oil and gas exploration (Burley spring will come in contact with each other.) Put the spring
and Worden, 2009). For example, Berea sandstone (rocks are in a stress-strain (load-frame) machine, compress and release
30 | Aam.—Hn£‘I'bdfly | Summer2Dl9