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A Sandia National
Laboratories modeling study contradicts a long-held belief of geologists that
pore sizes and chemical compositions are uniform throughout a given strata,
which are horizontal slices of sedimentary rock.
By understanding the
variety of pore sizes and spatial patterns in strata, geologists can help
achieve more production from underground oil reservoirs and water aquifers.
Better understanding also means more efficient use of potential underground
carbon storage sites, and better evaluations of the possible movement of
radionuclides in nuclear waste depositories to determine how well the waste
will be isolated.
“I think our paper
for the first time provides a reasonable explanation for the origin of
patterns,” said lead researcher Yifeng Wang. “We found we could predict the
variations in pores as well as the heterogeneity of a reservoir.”
The analysis,
published in Nature Communications,
was able to match the field observations published in 2006 by second author
David Budd, professor of geological sciences at the University
of Colorado at Boulder.
Budd said Wang put
together a session at the 2010 annual meeting of the Geochemical Society at
which Budd presented field studies of porosity and chemical composition. “He
recognized that the data I showed could be explained by stress-induced chemical
waves. He subsequently developed the numerical model to test his idea. Then we
used the 2006 data set to demonstrate the correspondence between his model’s
outcomes and the field data.”
A chemical wave in
this context relies upon mineral dissolution and precipitation, powered by
geologic stress, to penetrate surrounding material, just as an ocean wave
powered by the moon’s gravitational pull rides up on a beach. Ocean waves shift
sand; Wang found that chemical waves modify the spatial distribution of rock
porosity.
As Wang puts it, a
chemical wave is “like water rippling. The concentration of a chemical species
varies periodically in space (a standing wave) or sometime such variations
propagate through space (a travelling wave).
“The one we revealed
in dolomite (a type of sedimentary rock) may be the largest chemical wave ever
known, because no one has thought to look for chemical waves in strata. It
occurred on the scale of meters to tens of meters and propagated between a
hundred to a thousand years.” Chemical waves are usually observed on much
smaller scales in laboratories.
Using the chemical
wave concept and well-known equations for material stresses, Wang formulated a
mathematical model.
“The remarkable thing
is that the model predictions match very well with many seemingly uncorrelated
observations. The model predictions not only match the observed porosity
patterns, but also match very well with chemical and isotopic signatures. This
is the power of mathematical analysis,” Wang said.
Wang’s model isn’t
large enough yet to derive equations meaningful to an entire reservoir—a
process called upscaling. Still, he said, “Another way to capture this
variability is to use mathematical analysis to derive upscaled flow-transport
equations. This work is on the way.”
The work may help
trounce geologists’ belief that each layer of sedimentary rock, deposited over
eons, is more or less homogenous in porosity and composition. Thus a single
core sample obtained from a given depth was thought to chemically represent the
entire layer.
But Budd’s findings
showed that horizontal variations within a layer of sedimentary rock could be
quite significant—in some cases, as large as vertical variations. This would
affect not only the amount of fluid stored or percolating through a rock but
the amount of pressure needed to shoot liquids to Earth’s surface. No one knew
why these variations occurred, nor had anyone measured their magnitude.
The problem has
always been how to extend horizontally the knowledge gained from vertical bore
holes that may be 1,300 ft apart, Budd said.
Wang’s model also
reveals important information about Earth’s geological changes.
“Even the shape of a
variation may reveal important facts about past times,” he said. “Our work may
have geologists rethinking their method of field sampling and their
interpretation of data about Earth’s evolution.”
