We propose a method that allows probing quantitatively the individual saturations of crudeoil/brine/mud-filtrate mixtures during imbibition-drainage experiments on a petroleum rock-system. The experiments have been also performed at different temperature and pressure conditions

Benamsili, Lyès

Hamon, Gérald

Korb, Jean-Pierre

English

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Probing individual saturations of crude-oil/brine/mud-filtrate mixtures confined in rocks Lyès Benamsili1,2, Gérald Hamon1, Jean-Pierre Korb2 1 TOTAL EP, CSTJF, Pau, France 2 Physique de la Matière Condensée, Ecole Polytechnique-CNRS, 91128 Palaiseau, France  Corresponding author: Lyès Benamsili, E-Mail: lyes.benamsili@polytechnique.edu   Abstract We propose a method that allows probing quantitatively the individual saturations of crude-oil/brine/mud-filtrate mixtures during imbibition-drainage experiments on a petroleum rock-system. The experiments have been also performed at different temperature and pressure conditions. Keywords D-T2 experiment, rock, diffusion, relaxation, crude oil, petroleum .1 Introduction 2D nuclear magnetic relaxation measurements are commonly used in well logging for the exploration part of oil recovery. For instance, the diffusion-transverse relaxation (D-T2) correlation spectrum allows discrimination of brine and oil mixtures in pores [1]. However, there are still difficulties in data interpretation. The first difficulty is to probe quantitatively the individual saturations of each fluid in petroleum fluids mixtures. The second difficulty exists, for crude oil containing asphaltenes, in the interpretation of the anomalous relationship observed between the translational diffusion coefficient, D, and the transverse relaxation time, T2 [2]. Here, we propose a new method allowing us to measure quantitatively the different saturations for various petroleum fluids mixtures in a rock-system. The rock samples and petroleum fluids have been properly characterized by standard petrophysical techniques (picnometry, gas flow in steady state conditions for absolute permeability, viscosimeter, SARA...). The 2D NMR experiments have been realized at different in situ successive saturations in monophasic, diphasic and triphasic conditions. For proper measurement of the diffusion coefficients distributions, we used a pulsed field gradient spin–echo PGSE technique with bipolar pulsed gradients to reduce eddy currents and internal gradient effects [3, 4]. The final inverted D-T2 spectrum has been obtained using our own inverse Laplace transform (ILT) program. The experiments of imbibition-drainage on a sandstone sample have been also performed at different temperature and pressure conditions.  .2 Systems and NMR methods We use cylindrical sandstone rock samples 2” in length and 1.5” in diameter whose main characteristics have been measured with standard petrophysical techniques. The average porosity and © 2013, Lyès Benamsili diffusion-fundamentals.org 18 (2013) 6, pp 1-41 permeability are 22.75%, and 2.5 D, respectively. The rock contains less than 5% of clays, 0.13% of iron (Fe) and a few ppm of Mn. X-ray CT shows that there is a homogeneous pore size distribution centered on 25 m [5]. The petroleum fluids (brine/mud-filtrate/crude oil) are selected for their respective viscosities of 1, 2.5, and 25 cp at room temperature. The SARA analysis of the crude oil reveals a composition of saturates 40.3 %(wt), aromatics 44.0%(wt), resins 6.7%(wt) and asphaltenes 9.0%(wt), respectively. The brine is made with NaCl (45g/l) and CaCl2 (5g/l). The oil based mud-filtrate has been synthesized in the laboratory.  We use the standard petroleum industry protocol to saturate an empty rock sample. The drainage/imbibition protocol is not different from the saturation protocol. However, the former depends on the fluids density to avoid gravity effects. First, we saturate the rock sample with a given fluid, then the drainage/imbibition procedure is performed and the volumes are measured during injection-recovery. 1D and 2D NMR measurements have been made with an Oxford 2.5 MHz spectrometer. Data were processed using our own 1D and 2D inverse Laplace transform (ILT). 1D NMR (CPMG) measurements have been performed to probe T2 on three similar samples saturated either with brine, crude oil or mud-filtrate, separately. The three T2 distributions have their main peaks centered at 0.8s for brine, 0.45s for mud-filtrate and 0.05s for crude oil, respectively. The ratio T2,peak,brine/T2,peak,oil≈16 should facilitate an easy separation of brine and crude oil peaks in a mixture. However, we checked that this was not the case for a mixture of almost equal volume fractions of these two fluids. A supplementary dimension was thus necessary. .3 2D NMR measurements  3.1 Confined brine/crude-oil mixture We present in Figure 1 the D-T2 correlation spectra obtained after ILT of a confined brine-crude-oil mixture (SNR≥120). In fact, these spectra correspond to two successive steps of a drainage process. The rock is initially 100% saturated with brine. Then, oil is progressively injected in steady state condition to reach a given saturation. We immediately notice that the brine and oil data are clearly separated on the basis of their individual diffusion coefficient values. We estimate the fluid saturations using these D-T2 results by a localized integration procedure. For instance, in Figure 1a, we obtain a brine saturation of 59.4% with our NMR-method, 58.2% with the injected volumes and 54.8% with the recovered volumes. In Figure 1b, we obtain on the same system and some steps later, a brine saturation of 22.9% by using NMR-method, 22.6% with the injected volumes and 20.2% with the recovered volumes. The latter saturation state is characteristic of the water irreducible saturation state (Swi). The small differences between these volumes, approximately 2-3 % (0.25-0.37cm3), are due to errors during volume measurements in the experimental setting. The anomalous relation, D T2, shown in Figure 1a confirms previous results observed in crude oils containing asphaltenes [2]. However, an interesting and new result is shown in Figure 1b, where a levelling off of this relation for large T2 is observed in the crude oil. This behaviour is generally observed in the wetting fluid phase. This is rather surprising for our rock sample, which is considered to be water-wet. We interpret this seemingly paradoxical behaviour by considering a surface diffusion process of small hydrocarbons (with long T2) at the surface of asphaltene nanoaggregates. NMR dispersion data on crude oils with and without asphaltene confirm this interpretation [6].   3.2 Confined brine/crude-oil/mud-filtrate mixture We present in Figure 2 the D-T2 correlation spectra obtained after ILT of a confined brine/crude-oil/mud-filtrate mixture at two successive steps of an imbibition process. The sample is initially in the final state of Figure 1b with a brine saturation of 22.9% and an oil saturation of 77.1%. Then oil-based filtrate is progressively injected in steady state condition. We notice that the brine, oil and filtrate data are clearly separated on the basis of their individual diffusion coefficient values. For instance, in Figure 2a, we still obtain a brine saturation of 22.7% with our NMR-method while we find 22.6% with the injected volumes. Similarly, oil saturation is 52.7% with NMR and 50.1% with the injected volume. The saturation of filtrate is 24.6% with NMR and 27.3% with injected volume. In Figure 2b, some steps later, we still obtain a brine saturation of 23.0% by using NMR-method. The oil saturation decreases to 17.2% and the filtrate saturation increases to 59.8%. © 2013, Lyès Benamsili diffusion-fundamentals.org 18 (2013) 6, pp 1-42   (a) (b) Figure 1 D-T2 correlation spectra of brine-crude oil mixtures confined in sandstones pores at different steps of the drainage process. (a) Brine saturation of 59.4%, oil saturation of 40.6%. (b) Brine saturation of 22.9%, oil saturation of 77.1%.   (a) (b) Figure 2 D-T2 correlation spectra of crude oil/brine/mud filtrate mixtures confined in sandstones pores at two steps of imbibitions. (a) Saturations of brine (22.7%, Swi), oil (52.7%) and mud-filtrate (24.6%); (b) Saturations of brine (22.9%, Swi) oil (17.2%) and mud-filtrate (59.8%).  .4 D-T2 correlation of confined crude oil at various temperatures and pressures A set of 2D NMR D-T2 measurements are performed on a sandstone sample saturated with crude oil varying the temperature and pressure (Figures 3a, b). The temperature values used are 20, 25, 30 and 40 degrees Celsius and pressure of 1 and 15 bars. After temperature is stabilized, the pressure is stabilized at a given value. We note on Figure 3b the absence of an effect of pressure. On the contrary, there are systematic enhancements of D and T2 values when temperature increases, which is expected for an activated process. We observe again the anomalous relation, D T2 and the levelling off of this relation for © 2013, Lyès Benamsili diffusion-fundamentals.org 18 (2013) 6, pp 1-43 large T2 (Figures 3a, b). The” high diffusivity” anomalies in Figure 3 at 30 and 40°C do not appear at lower temperatures. We verified that these small 2D peaks are not due to ILT or experimental artefacts, but they are not clearly understood at the moment.   (a)  (b) Figure 3 D-T2 correlation spectra of crude oil confined in sandstones at different temperature (20°, 25°,30°, 40°) and two pressures 1bar(a) and 15 bars(b). The continuous lines allow seeing the temperature effects.  .5 Conclusion We proposed 1D and 2D NMR methods allowing quantitative measurements of individual saturations for various petroleum fluids mixtures embedded in a rock-system at various temperature and pressure. The D-T2 experiments have been performed at different in situ saturations (monophasic, diphasic or triphasic). We succeeded in probing quantitatively the individual saturations of the fluids mixtures, during imbibition-drainage experiments in this rock-system. We observed the anomalous relationship, D T2, and a levelling off of this relation for large T2 in the bulk and confined crude oil. These observations persist at different temperature and pressure.  6. References [1] M. D. Hürlimann and L. Venkataramanan, J. Magn. Reson. 157, 31-42 (2002). [2] A. R. Mutina, M. D. Hürlimann, J. Phys. Chem. A112, 3291-3301 (2008). [3] W.S. Price, Concepts in Magnetic Resonance 10, 197-237 (1998). [4]  P.T. Callaghan, Translational dynamics and magnetic resonance, (Oxford University Press 2011). [5] I. Bondino, R. Nguyen, G. Hamon, P.A. Ormehaug, A. Skauge, S. Jouenne, paper SCA2011-18. [6] A. Louis Joseph, L. Benamsili, J.-P. Korb, Microporous and Mesoporous Materials (submitted). © 2013, Lyès Benamsili diffusion-fundamentals.org 18 (2013) 6, pp 1-44

Probing individual saturations of crude-oil/brine/mud-filtrate mixtures confined in rocks

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Probing individual saturations of crude-oil/brine/mud-filtrate mixtures confined in rocks

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