Petrophysics

     "Petrophysics 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.
     These 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)
     Other 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 means.
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).
    Another example 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.
    To understand 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.

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