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Production Studies Using NMR and High-Frequency Dielectric Permittivity

Abstract

Mohammad Reza Taherian, Andrea Valori, Ahmad AlZoukani, and Farhan Ali Schlumberger Dhahran Carbonate Research Center The movement of free and bound fluid and its effect on production has been examined using nuclear magnetic resonance (NMR) and high-frequency dielectric spectroscopy. Fully water-saturated plugs were centrifuged with air using small capillary-pressure steps at 1-psi increments from 0 to 54 psi. At each pressure step the low-frequency (2-MHz) NMR and high-frequency (0.001–1 GHz) dielectric permittivity were measured. These fine-scale experiments replace the water with air (permittivity of 80 and 1 respectively). As a result the dielectric permittivity signal decreases with centrifugal pressure since water is replaced with air. The dielectric permittivity data show a gradual depletion of free water from the pore space as the centrifugal pressure increases from 0 to 5 psi. Above 5 psi the rate of fluid production decreases as the pressure increases, suggesting that the produced water is bound to the pore surface. Plotting the dielectric permittivity versus centrifugal pressure shows at least two different production regimes that can be attributed to the free and bound water production. The frequency dependent permittivity fits a sum of three Cole-Cole terms very well. The three Cole-Cole terms are centered at frequencies below 1 GHz which is lower that for the free water (20 GHz) suggesting the signal is originating from water molecules that are not free to reorient and follow the electric field as easily as free water is. As a result the part of the signal represented by the three Cole-Cole terms are attributed to the bound waters that are hindered to different extent by their interaction with the pore wall. The NMR signal also decreases as the centrifugal pressure increases because the centrifugal pressure replaces water with air which has no NMR signal. The NMR T2 distribution confirms the production of free water at centrifugal pressures of 0 to 5 psi. As with the permittivity data the NMR signal above 5 psi can be attributed to waters which are bound to the pore surface to varying extent.