Regular Article
Measurement of interfacial area from NMR time dependent diffusion and relaxation measurements

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Abstract

The interfacial area between two immiscible phases in porous media is an important parameter for describing and predicting 2 phase flow. Although present in several models, experimental investigations are sparse due to the lack of appropriate measurement techniques. We propose two NMR techniques for the measurement of oil-water interfacial area: (i) a time dependent NMR diffusion technique applicable in static conditions, similar to those used for the measurement of the solid specific surface of a porous media, and (ii) a fast relaxation technique applicable in dynamic conditions while flowing, based on an interfacial relaxation mechanism induced by the inclusion of paramagnetic salts in the water phase. For dodecane relaxing on doped water, we found an oil interfacial relaxivity of 1.8 μm/s, large enough to permit the measurement of specific interfacial surface as small as 1000 cm2/cm3. We demonstrate both NMR techniques in drainage followed by imbibition, in a model porous media with a narrow pore size distribution. While flowing, we observe that the interfacial area is larger in imbibition than in drainage, implying a different organization of the oil phase. In a carbonate sample with a wide pore size distribution, we evidence the gradual invasion of the smallest pores as the oil-water pressure difference is increased.

Introduction

Taking into account the interfacial area may be a new way of understanding and predicting two-phase flow in porous media, especially in the context of enhanced oil recovery where predicting the behavior of trapped phases is crucial. For example, capillary effects are intimately linked to interfacial phenomena from a thermo-dynamical point of view [1], [2], [3]. During forced displacement of water by oil in a water-wet porous media, a fraction of the energy injected into the system is dissipated due to the multiple restrictions in the pore network system. The other fraction, reversible, is used to create an interfacial area between the two phases. These thermo-dynamical aspects have been considered as early as 1970 [1]; today, based on recent high resolution 3D tomography, it is estimated that about 40% of the energy is reversible [4], [5].

Despite its importance, the interfacial area parameter has not been really studied due to measurement difficulties. For air-water systems in the context of hydrogeology, tracer techniques have been used successfully [6], [7], [8], [9], [10], [11] but they require a flow of the tracer through the porous media; therefore, they can only measure the interfacial area of connected gas phases (and not trapped). In a more general way, recent progress in micro-tomography both in terms of resolution and acquisition speed [5], [12] allows the determination of interfacial area [13], [14], [15].

We will first recall two NMR techniques that can be used for the measurement of solid and interfacial area: the time dependent diffusion and relaxation techniques. Then we first describe the results obtained on a reference material to show that the diffusion and gas adsorption techniques give similar values. Then we evidence and calibrate an interfacial relaxivity mechanism allowing the measurement of interfacial area from simple and fast NMR transverse T2 relaxation. We use this new observation to determine the variation of interfacial area during flooding in an artificial porous media (Aerolith). Then, we describe qualitatively the distribution of the oil phase in a carbonate sample with a large pore size distribution.

Section snippets

Principles of surface area measurement

We explain below how surface area can be measured from NMR data. The first technique uses the reduction of diffusion coefficient due to the confinement, at diffusion length shorter than the length scale of the investigated porous structure. The second uses a measure of the magnetization decay to deduce the surface to volume of the investigated liquid phase.

Materials and experimental set-up

We performed diffusion and relaxation measurements on a grain pack made with grains of known specific surface area (0.10 m2/g) fully saturated with water. The material (BCR169 alpha alumina, LGC Promochem) is commonly used as a reference for adsorption apparatus. The grains were packed in a small tube of inner diameter 12 mm and saturated with brine (20 g/l). The porosity of the pack measured using NMR profiles is 0.42 ± 0.01.

Interfacial area measurements at various saturation were performed on

Surface measurement, single phase saturation

Firstly, we want to demonstrate that the time dependent diffusion gives similar values of specific surface as gas adsorption techniques. For this purpose, we used a reference material described in Section 3. For the grain pack considered, we found V/S = 1.8 μm using the interpolation technique (Eq. (6)); in this case and given the dead time of the NMR sequence, the use of an interpolation technique is critical to obtain correct values (Fig. 3) because a sharp decrease is occurring during the dead

Conclusions

We propose two NMR techniques to determine the interfacial area between oil and water in porous media. Firstly, we adapted the time dependent diffusion technique already known in single phase saturation [16] to measure interfacial area in oil-water 2 phase situation. By using paramagnetic water solutions to reduce the bulk relaxation time around 1 ms, the NMR diffusion sequence becomes only sensitive to oil. For more accurate results, the surface to volume ratio related to the slope of the

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