A microwave attenuation technique has beendeveloped to measure water saturations in laboratoryporous-media models. Microwave energy is absorbed by the water molecule dipole in its losing struggleto stay aligned with the oscillating electric held.Energy is dissipated as beat, but not enough toraise the temperature noticeably. The resultingelectrical-field exponential decay, as radiationtraverses the absorbing medium, leads directly to the Beer-Lambert absorption law. Other modelcomponents - oil, gas, consolidated rock, epoxypaint - are nearly transparent compared with water,
The experimental setup is a noncontact deviceconsisting of a microwave generator (klystron) andsending born that focuses the radiation on the model(1-in.-thick sandstone slab). The receiving born anddetector are aligned on the opposite side of themodel. The detector signal is processed through alogarithmic amplifier, resulting in a voltageproportional to water saturation. The assembly ismounted on a "trolley" so that a position-vs-saturation "snapshot" can be obtained at any time during theflooding experiment.
Use of this tool is illustrated by a 1-in. x 4-in. x4-ft Berea slab that was saturated with water, thenoilflooded, then waterflooded. Saturation snapshotsshow, the typical Buckley-Leverett saturationbehavior for both oilflood and waterflood. A sharpflood front progresses through the rock;and, behindthe flood front, the saturation of the displacing phaseincreases quite slowly.
The potential of this tool is in studyingmechanisms of a variety of secondary and tertiaryrecovery processes.
Oil recovery experiments in porous media can beperformed with varying degrees of sophisticationand, hence, of understanding. The simplest type ofinstrumentation measures only total oil recovery.The ultimate experiment would generate data onflow behavior and distribution of phases andcomponents as functions of both location and time.Such detailed data would aid in understandingdisplacement mechanisms, scaling laws, effects ofheterogeneities, etc.
Various techniques, including resistivity, pressure, X-ray, radioisotopes, and dissection techniques, have been used to arrive at some of this detail.
Although each has been somewhat useful, eachhas drawbacks. Resistivity and pressure-dropmeasurements see some kind of integrated averagebetween two fixed points (electrodes or pressuretaps). The X-ray and radioisotope techniquesintroduce a foreign substance into the fluid system.Dissection gives data for only one point in time.These methods can also be combined.
This report deals with the use of microwaves tomonitor saturation distributions continuously.Microwave radiation is absorbed strongly by watercompared with many other materials. This waterselectivity has been used to monitor the moisturecontent of such diverse materials as plastics, soaps, detergents, grains, animal feeds, dairyproducts, meat, clay, paper, wood products, andtextiles. However, as far as we know, the microwave technique has not been used before to define thetransient anatomy of laboratory flooding experiments.
Microwaves are electromagnetic radiationoccupying the spectrum between infrared and radiowaves. The microwave frequency range, 0.3 to 300gigahertz(GHz), corresponds to wave lengths of 1meter to 0.1 cm. Radar and the microwave ovens forhome and fast-food-service commercial use are thebest known applications for microwaves.
Absorption of energy as any electomagneticradiation travels through some material can bedescribed in terms of either dielectric or optical phenomenon. Since the materials we are concernedwith are nonmagnetic, the discussion relates onlyto the electric field-medium interactions. The theoretical discussion is condensed from standardtexts such as Refs. 2 through 4.
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