is the study of the physical and chemical properties that describe the
occurrence and behavior of rocks, soils and fluids. The earth sciences
(and many other disciplines) describe the earth through the occurrence
and behavior of matter. People live on the surface of the earth,
standing on rock and soil, inside a bubble of gas, growing food in and
from the fluid and solid constituents, and exploiting natural resources
like minerals, water and petroleum. How well the occurrence and behavior
of the physical and chemical properties and processes in rocks, soils and
fluids are understood determines how well buildings and dams are supported
by their foundations (civil engineering), food is grown (agriculture),
resources are developed (petroleum, mining and hydrogeological engineering),
the environment is protected (waste management and environmental remediation),
and energy or data are transmitted (power, electrical engineering and telecommunications).
Geophysical measurements of natural
and artificial fields are interpreted in terms of variations in the physical
and chemical properties and processes within the earth. Gravity,
magnetics, seismic velocity and electrical resistivity are a few examples
of many such measurements. Gravity and magnetic measurements of the
Earth's natural fields and their variation in space and time are used to
interpret properties within the earth like density and iron content as
well as processes like tides and polar wander. Seismic velocity and
electrical resistivity are measurements typically performed with artificial
sources like explosions and injection of electrical current to interpret
properties such as porosity and fluid saturation. Both may also be
performed with natural sources such as earthquakes for seismic velocity
(seismology) and lightning for resistivity (magnetotellurics). Some
measurements (paleomagnetism) allow reconstruction of the time varying
history of the natural fields and geological movement.
measurements of the space and time variations in fields require interpretation
to result in properties and processes of interest. For example, in
exploration and development of petroleum or water resources (or environmental
cleanups), the properties of interest are the porosity, saturation, chemistry
and mobility. These are in pursuit of the questions:
Is there any place in the rocks for fluids to exist? (porosity)
How much of the porosity is fluid filled? (saturation)
What kind of fluids are there? (chemistry)
Can the fluids be moved? (mobility)
disciplines and problems need other properties: foundation engineers and
earthquake hazard investigators want the properties of strength and stress,
agronomists and ecologists want biological activity, and so forth.
None of these can be measured directly deep within the earth by noninvasive
However, measuring variations in
the natural gravity field at the surface of the earth can allow interpretation
of mass density as a function of depth and lateral location. With
assumptions about the zero porosity density and rock type, that gravity
determined density may be converted into a porosity. The measurement
of the fields is fairly straight forward, though complicated by sources
of noise and interference, and requirements to make accurate measurements
to better than parts per million. The most difficult step is the
conversion from what is measured to the desired quantity (from field to
physical property). This step is called interpretation and requires
a model of the relationship between the thing measured and the desired
quantity. For physical and chemical properties and processes in natural
materials, these relationships are embodied in the study of petrophysics
(Tiab and Donaldson, 1996; Schon, 1998).
is use of borehole geophysics in logging petroleum reservoirs. Many
tools are used to make a wide variety of measurements. One uses the
injection of an electrical current and the measurement of the voltage response.
Ohm's Law (Ohm, 1827) relates the ratio of voltage to current as electrical
resistance, which is multiplied by a geometric factor (determined by the
electrode positions in space) to become the material property called electrical
resistivity. Electrical resistivity is the property that describes
the ability of a material to support the process of charge transport.
Archie's Law (Archie, 1942) describes the relationship between electrical
resistivity and porosity, fluid saturation, and fluid type in a rock.
The injection of current and measurement of voltage can result in determination
of porosity, saturation and fluid type. However, the geometric factor
and parameters in Archie's Law have many of built in assumptions.
These include considerations of the rugosity of the borehole wall, properties
of the drilling mud, invasion of the mud into the formation, morphology
of the porosity, connectivity of the pores, wettability of the rock, presence
or absence of clay minerals, and more. Depending upon the choices
made about these assumptions, different interpretations result for porosity,
saturation and fluid type. In petroleum reservoir valuation, these
have significant impacts and consequences for the extraction of oil.
(...and nothing has yet been determined about the mobility of the oil.)
Billions of dollars are wagered every year on the proper interpretation
of these data.
Whether or not
oil is extracted from a well is determined by the technical aspects but
also economic and political factors. To remove oil from the ground
costs money. Costs include the well and pump, energy to run the pump,
a pipeline to carry the oil to a refinery, and so forth. On top of
these are added lease and royalty payments to land owners, extraction taxes,
transport charges, and so forth. When the cost is greater than the
return from selling the oil, the oil is left in the ground. It's
not unusual to leave 25 percent or more oil in the ground. The point
at which the pump is turned off is in great part determined by the technical
interpretation of the borehole geophysical data in the context of the economics.
This same data will also play a role in determining the value of a well
or a reservoir for loans from banks, sharing of costs and revenues among
partners, and buying or selling properties.
For an environmental
spill of the same oil (for example, from a pipeline break), the determining
technical factors are the same as the petroleum reservoir: porosity, saturation,
fluid type, and mobility. However, now a regulatory agency will state
the environmental standards, and that 99.999% of the oil must be removed,
no matter what the economic cost. The chemistry and physics have
not changed, but the requirements for interpretation are now considerably
tightened, and potential litigation requirements may put added cost into
assuring the quality of the measurements and the interpretation.
These same types of considerations also apply to problems in other areas:
agriculture, civil engineering, mining, and so forth.
most of these properties and processes requires an understanding of Euclidean
geometry, Galilean transforms, Newtonian mechanics, and of the motions
and interactions of the electron (Coulomb, Ohm, Faraday, Maxwell, etc.).
However, some properties can only be explained by quantum mechanics (magnetism),
others by particle physics (radioactive decay), and some processes require
the strong, weak and gravitational forces. These will be quickly
reviewed and referenced to the literature to provide the context to proceed
towards an understanding of physical and chemical properties and processes
in rocks, soils and fluids."
Copyright 1998-1999 Gary R. Olhoeft.
All Rights Reserved.