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High-amplitude pressure pulsing or mechanical excitation of a saturated porousmedium under a pressure gradient increases the flow rate of the liquid alongthe direction of the flow gradient. Experiments show that this occurs forsinglephase liquid systems and two-phase liquid systems (e.g. water- wet, paraffin oil mobile phase) under various conditions and system parameters. Thepresence of free gas in the system leads to a delay of the effect because theexcitation energy is dissipated in compressing the gas.
Experiments have been performed in a wide range of configurations (cylindricalcells and flat plate simulators), grain sizes (30 to 2000 microns), viscosities(I to -10,600 cP), and flow factors (e.g. with and without sand flow). Mobilephase flow rate increases of from 30% to over a thousand percent (in the caseof the most viscous oils) were measured. These beneficial flow rate effectstook place in sand packs without saturation changes, without fabric changes, and under conditions of constant external head They cannot be rationalizedwithin Darcy theory as this theory contains no inertial terms. A new theory hasbeen developed.
The flow enhancement effect requires large strains; seismic amplitude strainsare inadequate. The nature of the excitation is important: high-amplitudenon-seismic pulses dominated by low frequencies are best. This is most easilyachieved in the laboratory and in the field by pressure pulsing, as opposed toother excitation methods.
One of the most interesting series of experiments was related to physical(visual and quantitative) demonstrations that viscous fingering instabilitiescan be suppressed by pulsing applied to the less viscous invading phase. Theseresults have important implications in the execution of water floods in thefield and can lead to methods of converting old water floods tostabilized
Before our experimental activity is defined and results described, it is bestto explain why results do not fall within the "conventional" view of porousmedia mechanics. What is being done during system pulsing is not radical, butcurrently accepted poromechanics models cannot correctly account for suchdynamic effects. A better theory was developed well before the experiments, andit helped guide the testing program. This theory will now be qualitativelydescribed.
Scientists and engineers working in fluid flow have been taught that thequasi-static Darcy flow paradigm (q ? ?p/ ?l), where gradient is amacroscopically defined quantity (?p/ ?l) =(Pl-P2)/l), is asufficient theory for porous media flow over a wide range of conditions.Perhaps some inability to correctly predict flow rates or dispersion behaviorin clays, shales or fractured media is admitted, but otherwise Darcy theory isaccepted uncritically.
Similarly, geophysicists working with porous media wave mechanics have beentaught that Biot-Gassmann theory is sufficient to describe porous media wavepropagation, given a wavelength much greater than the particle size. Neither ofthese "fundamental" theories is complete, although each may be sufficient forpractical purposes under certain restrictive conditions.
Darcy theory is a quasi-static theory, and contains no inertial terms.