FR2864241A1 - Determining at least one characteristic parameter of a hydrocarbon/water emulsion using nuclear magnetic resonance to measure the distribution of relaxation times - Google Patents

Abstract

The method involves using nuclear magnetic resonance to measure the distributions of relaxation times T2 of the emulsion, analysing a magnetic signal from a CPMG type sequence. From these distributions obtained respectively for oil and water, at least one of the following is determined: water/oil volume ratio; average size of droplets; polydispersity of droplets : The emulsion is made of a hydrocarbon in a continuous aqueous phase, the hydrocarbon having a viscosity notably different to that of the aqueous phase. The distribution of relaxation times T2 of the continuous aqueous phase of the emulsion is analysed from the following relationship: (1/T2) = (1/T2B) + rho 2(S/V), where S is the surface of tehs space between the oil droplets; V the volume of this space; T2B the reference relaxation time of water not in emulsion and rho 2 a parameter which represents the intensity of the water-oil interactions. The average size of droplets and the coefficient rho 2 are obtained by determining the ratio S/V by measuring the diffusion coefficient Deff of the aqueous phase as a function of time. A comparison is made between the distributions T2 with the distributions obtained from a sample of emulsion in which the continuous phase has been replaced by a deuterium aqueous phase. An independent claim is made for a device to characterise an emulsified effluent, comprising means of taking a sample of the emulsion circulating in a transport pipe; means of carrying out the method described above, and means of controlling the quality of the emulsion as a function of the parameter obtained by the above method.

Description

10

  Because of their resources currently identified at 1000 Gb, heavy crude hydrocarbons (defined by a density less than 20 API) constitute a considerable hydrocarbon reserve almost identical to that of conventional oils. Nevertheless, because of their high viscosity, the exploitation of these petroleum products remains technically difficult. In reservoir conditions, these crudes generally have viscosities of at least greater than 100 centipoise. Marginal today, their production represents only 6 to 8% of global activity. But heavy crudes are strategic hydrocarbon reserves that will be brought to be widely exploited in the years to come. By heavy crudes, it also includes extra heavy crudes, including bitumens. The present invention is also applicable to aqueous emulsions of residues that can be obtained after refining, for example residues of atmospheric or vacuum distillation.

  Processes are known for transporting these viscous crudes which consist in fluidifying the crude by heating, or mixing it with a fluidifying product. Another technique currently used to facilitate the transport of heavy oils is their emulsification. The direct emulsification of a heavy crude consists in dispersing it in the form of droplets in water in order to reduce its viscosity. It is an effective technique to reduce the viscosity of these petroleum products and to make it compatible with in-line transport specifications [Rimmer D., Greogoli A., Hamshar J., Yildirim E., Pipeline emulsion transportation for heavy oils , in LL Schramm (Ed.), Emulsions Fundamentals and Applications in the Petroleum Industry, American Chemical Society, Washington DC, chapter 8, 295-312, 1992]. The possibilities of formulation are not only varied (addition or creation in situ of surfactants ...) but also reliable. They result in low viscosity and very stable emulsions which generally contain up to 70% by volume of crude.

  Although already used for some deposits in Indonesia, Venezuela and Canada, the emulsification of heavy crudes is a new technology in full development. It involves many parameters whose interactions are complex. The size of the drops is the key parameter to control during the emulsification of heavy crudes.

  On the one hand, it is preferable to make emulsions with bulky drops to reduce the viscosity and facilitate their transport. On the other hand, large drops coalesce more easily than small ones and make these systems more unstable. . It has already been attempted to obtain information on emulsions of crude oil (water in oil) by NMR (Nuclear Magnetic Resonance). But, this can not apply to viscous oil emulsions in water.

  Despite the importance of knowing precisely the particle size of such an emulsion, it is very difficult to correctly characterize the size of the droplets of heavy crudes emulsified in water. This is because these emulsions are concentrated emulsions (generally greater than 60% by volume of crude dispersed in water), relatively viscous and black. They can also be very polydisperse. Conventional techniques such as light scattering or microscopy are here very little effective to characterize the size distribution of these concentrated aqueous emulsions of heavy crudes. To illustrate these difficulties, reference can be made to FIG. 1 which represents a SEM photo of a "Venezuelan" crude 70/30 emulsion at pH = 12. It is clear that the characterization of this emulsion is difficult from the microscopy images, due to the polydispersity of the drop sizes, the high concentration of crude, and the low contrast of the images.

  Using conventional NMR techniques, the measurement of the diffusion coefficient of a liquid contained in a drop makes it possible to determine the size thereof. This can be applied in principle in the case of an aqueous emulsion of oil. In fact, the NMR technique generally makes it possible to measure the self-diffusion coefficient of a liquid as a function of time, that is to say as a function of the average distance traveled by the molecules. If the fluid is not confined in a space such as a drop, the measured diffusion coefficient is time independent; on the other hand, in a confined space, the measured (or effective) diffusion coefficient decreases sharply when the average free path reaches the size of the drop and it is this information that is used. In the case of a viscous oil aqueous emulsion, the application of the standard method described briefly above becomes very difficult. First, the measurement of the coefficient of a highly viscous liquid poses serious technical problems because the molecules are not very mobile and the NMR signal decreases too rapidly to allow a diffusion measurement for sufficiently long times or distances. Secondly, the water signal contributes significantly and must be removed, for example by using deuterated water. Such a process therefore poses practically insoluble problems.

  In the context of the present invention, the aqueous emulsions of heavy crude are constituted by a continuous water phase, in which the heavy crude is in droplets. In a more general manner, the aqueous emulsion according to the invention consists of hydrocarbons having a relatively high viscosity value compared with that of the aqueous phase, that is to say having a viscosity of at least greater than 100 centipoise at room temperature, for example at 25 C.

  Thus, the present invention relates to a method for determining at least one characteristic parameter of a hydrocarbon emulsion in an aqueous continuous phase, said hydrocarbon having a viscosity significantly different from that of said aqueous phase, in which method is used. nuclear magnetic resonance for measuring the T2 relaxation time distributions of said emulsion by analysis of a magnetization signal resulting from a CPMG type sequence, and that from the distributions respectively obtained for the water and the oil, at least one parameter is determined according to one of the characteristics of the emulsion included in the following list: water / oil volume ratio; - average size of the drops; - polydispersity of the drops.

  According to the method, the distribution of the T2 relaxation times of the aqueous continuous phase of said emulsion can be analyzed from the following relation: ## EQU1 ## where: S is the area of space between drops of oil, V the volume of this space, T. the reference relaxation time of the water excluding emulsion, p2 (m / s) parameter which reflects the intensity of the water-oil interactions.

  The average size of the drops and the coefficient p2 can be obtained by determining the ratio S2./V, by measuring the diffusion coefficient Deff of the aqueous phase as a function of time.

  These T2 distributions can be compared with the distributions obtained from an emulsion sample in which the continuous phase has been replaced by a deuterated aqueous phase.

  The invention also relates to a device for characterizing an emulsified effluent comprising: means for sampling an emulsion sample circulating in a transport pipe, means for implementing the method described above. means for controlling the quality of the emulsion as a function of the parameter obtained by said method.

  The present method is therefore based on the analysis of the relaxation and diffusion signal of the continuous water phase. In fact, nuclear magnetic relaxation provides a solution to the problem of determining the droplet size of heavy crude emulsions.

  The present invention will be better understood and its advantages will appear more clearly, in the light of the examples of implementations hereinafter described, in no way limiting, illustrated by the appended figures in which: FIG. 1 represents a photograph under a microscope a heavy crude emulsion in water; FIG. 2 illustrates a stimulated echo sequence for the measurement of diffusion coefficients; FIG. 3 represents the measurement of the relaxation times on water, of the viscous oil, and on an oil-in-water emulsion; FIG. 4 shows the calibration of the size distributions by the measurement of the diffusion coefficient; FIG. 5 shows an example of drop size distribution after calibration of the water / oil interactions by the diffusion measurement; - Figure 6 shows the measurement of the relaxation times on different emulsions.

  The present method according to the invention has been tested on different emulsions in order to validate it. The emulsions were prepared on a laboratory scale with the following procedure: Stirring speed: 13500 rpm. Stirring time: 5 min. ^ Temperature: 60-70 ° C A homogenizer was used for the emulsion manufacturing process. This is an Ultra-Turrax T25 device, the principle of which is explained below. Due to the high speed of the rotor, the two liquids to be emulsified are drawn axially into the dispersing head of the apparatus, then radially in the rotor-stator air gap. The presence of notches leads to significant turbulence that ensures optimal mixing. The volume of emulsion prepared is 150 ml, respecting the oil / water volume ratio = 70/30. This volume allows to properly immerse the dispersing rod of the Ultra-Turrax.

  The emulsions were made with two different heavy crudes: a heavy Venezuelan crude and a heavy Canadian crude. In a natural way, a mixture of crude and water forms a water emulsion in the crude (inverse emulsion). It is indeed established that the polar compounds contained in the heavy crudes confer to them this emulsifying, dispersing and stabilizing power of the water inside the crude. To form a concentrated emulsion of crude in water can be used two ways: the addition of bases or surfactants.

  The water used is water milli Q pH adjusted to 12.5 with a solution of soda (30%). A basic pH makes it possible to saponify the acids present in the crude, thus creating natural surfactants making it possible to form the crude emulsion in water.

  The influence of adding a surfactant was also tested. For this, we chose a neutral surfactant, Triton X405 (HLB = 18.6) (marketed by FLUKA), and an ionic surfactant (anionic), Sodium Dodecyl Sulfate SDS (HLB = 40). Each surfactant is introduced at a concentration of 1% by weight in water. We can thus study the influence of the charges present at the interfaces thanks to the ionic surfactant.

  The influence of the volume fraction in oil is also observed by preparing an emulsion containing 50% oil and 50% water by volume.

  Finally, the influence of the stirring speed of the emulsion was tested using the process described above, while increasing the stirring speed to 24,000 rpm.

  The following table summarizes the various tests carried out. Table 1 N tests Continuous phase Scattered phase Ratio Ultra Turax 1 Water at pH = 12 Venezuelan 70/30 13000 rpm 2 Water + 1% Venezuelan Triton 70/30 13000 rpm 3 Water + 1% SDS Venezuelan 70/30 13000 t / min 4 Water at pH = 12 Canadian 70/30 13000 rpm Water at pH = 12 Venezuelan 50/50 13000 rpm 6 Water at pH = 12 Venezuelan 70/30 24000 rpm The proposed NMR method is based on measuring the distribution lo of the relaxation times of the water and the oil constituting the aqueous emulsion of heavy crude. Several characteristics can be extracted: - the volume ratio water / oil, - the average particle size, - and an indication on the polydispersity of the droplets.

  The NMR interactions of water on the surface of oil drops for a type of crude are evaluated using a diffusion experiment such as that described above but whose interpretation is very different.

  An example of measurement on emulsion 1 (Table 1) is shown in FIG. 3. The three T2 relaxation time distributions were obtained by multiexponential analysis of the magnetization signal measured using a standard CPMG sequence. . The high relaxation times are characteristic of water (reference 1), while the low relaxation times are characteristic of the oil (referenced 2). The relaxation times of the water (1) and the oil (2) measured separately before the manufacture of the emulsion are shown for comparison (line 3 of FIG. 3).

  For viscous oil, the relaxation time is dominated by inter and intramolecular interactions. The average relaxation time depends on the temperature T and the viscosity r) (Dunn KJ, Bergman DJ and GA La Torraca, "Nuclear Magnetic Resonance Petrophysical and Logging Applications", Seismic Exploration Vol 32, Pergamon, 2002.): 1 T T2 27 and the observed distribution reflects the complexity of the oil in terms of molecular weight. This signal is used to calculate the volume fraction water / oil (ratio of areas under the respective curves of water and oil).

  For the water of the emulsion (reference 3), it can be seen that its relaxation time greatly decreased compared to its value before manufacture of the emulsion (from 2717 to 797 ms in FIG. 3). This indicates an interaction of water on the surface of the oil drops. Indeed, by diffusion, the protons of water interact with the protons at the water-oil interfaces. For a low-viscosity oil, the water-oil interactions are not different from the interactions between water molecules. On the other hand, it has been found that this is not true for viscous crudes because the oil can be considered as a solid and contains polar components. The relaxation of the water is analyzed by means of the following fundamental relation: ## EQU1 ## where: S is the surface of the space between the drops of oil, V the volume of this space , T2B the reference relaxation time of the water excluding emulsion, p2 (m / s) parameter which reflects the intensity of the water-oil interactions. (3) (4)

  Relation 4, used in porous media (Godefroy, S., Korb, J.-P., Fleury M., Bryant, RG, "Surface nuclear magnetic relaxation and dynamics of water and oil in macroporous media", Phys. E, 64 2001), is applied according to the invention to a concentrated emulsion. It should be noted that the relation 4 also applies to the longitudinal relaxation time Ti, but it is preferred to use T2 in practice since its measurement is much faster and adapted to the analysis of the relaxation time distributions. Essentially, molecular diffusion allows the proton to. the water to interact with the surface of the oil drops during the measurement time (this is the fast diffusion regime), but it does not allow the transport of this proton in the neighboring space. The T2 relaxation time distribution, corrected by the T2B value, then reflects the S / V ratio distribution of the oil drops. These being spherical, it therefore directly translates the distribution of drop sizes (S / V = 6 / D), in the case where they are substantially monodisperse.

  A major advantage of the CPMG measurement is that it is very fast and robust (of the order of a few minutes). We can therefore easily study the evolution over time of emulsions.

  The second step is to calibrate the T2 relaxation time distribution by determining the interaction coefficient p2. For this, a measurement independent of the S / V ratio is obtained by measuring the variation of the diffusion coefficient Deff of the aqueous phase (and not of the dispersed phase) as a function of time.

  A measurement of self-diffusion of the liquid around the drops is carried out using, for example, the NMR sequence indicated in FIG. 2. Without going into the details of these sequences (reference may be made to: Price WS, "Pulsed- field gradient nuclear magnetic resonance as a tool for studying translational diffusion ", Part I and II, Concept in Magnetic Resonance, Vol 10 (4), 1998), the method essentially consists in coding the nuclear magnetic moment with a first gradient pulse. (duration 8, amplitude g, Figure 2), to let the system evolve for a time A, then to identically decode the nuclear spin system and record the echo. The molecules carrying the nuclear magnetic moment of the studied species (proton) move in space and thus causes an attenuation of the echo.

  The effective diffusion coefficient Deff can be calculated by the relation: ln (E (k, A)) = A- Deff k2 F (A - 8/3) (1) with: A: time between gradients (diffusion time) k yg 8 (wavenumber) F: shape factor of the pulse A: relaxation-dependent constant E: echo intensity Deff: diffusion coefficient y: gyromagnetic ratio of the proton (26752.02 radJs / Gauss) g gradient intensity (Gauss / cm) 8: gradient duration In general, we vary k by acting on the value g to determine by linear regression Deff at a fixed value A of the diffusion time.

  This Deff measurement is used to deduce the total area ST of the oil drops present in the divided system by the total volume of water VT interacting with these drops. For a confined liquid, it has been shown (Mitra, PP, Sen, PN, Schwartz, LM, and Doussal, P., 1992, "Diffusion propagator as a probe of the structure of porous media" Physical Review Letters, v. 65 3555) that the short time variations of the diffusion coefficient are related to the ST / VT ratio by the following relation: ## EQU1 ## measuring the Deff (t) curve makes it possible to determine the ratio S., JVT, the average size of the oil drops. More generally, it can be shown that the diffusion curve as a function of time can be approximated by the following relation: ## EQU1 ## a) + CLT L + (1 a) i LD

VT (6) with:

  Dm is the molecular diffusion coefficient (without confinement), LD is the diffusion length (6Dmt) the 2, a is the asymptotic value of the diffusion coefficient (when the average free path of the molecules exceeds several spaces between drops), Lc is a characteristic length of the system.

  It is preferable to use the relation (6) because the effects of order 2 in (5) are not negligible at the diffusion time realizable in practice. The relation (6) does not come from a physical model but comes from a mathematical interpolation using the method of Pade where we suppose that the slope at the origin (t = 0) is related to the ratio S ,, JVT , (relation 5) and the asymptotic value is a. The is a parameter acting on the curvature.

  The curve Deff (t) was measured on the emulsion 1 (FIG. 4) and adjusted according to the relation (6) to extract the parameters ST / V T, Le and a. We find: ST / VT = 0.42 m1, ie D = 14.2 m This result is consistent with the size of the drops observed in the photos taken with a scanning electron microscope (Figure 1). (5)

  with C = To show the sensitivity of the measurement, we drew the curve corresponding to an average diameter of 10 m (Figure 4), the other parameters Lc and a remain fixed. There is a good sensitivity. Finally, we can calculate the interaction parameter p2 (relation 4) to express the T2 relaxation times in terms of drop sizes: 1 1 vc-i P2 = - II = 2.lum / s T2 T2B JIVT with: T2 = 0.797 s, T2B = 2.713 s (Figure 3) S, IVT = 0.42 micron. By extracting the water signal from the measured distribution on emulsion 1, the size distribution can be plotted. drops (Figure 5).

Examples

  Relaxation measurements were carried out on all the emulsions of Table 1. Figure 6 gives the distribution of the T2 times (abscissa) as a function of the emulsions 1 to 6. In each case, a very high value of the oil-water fraction is obtained. close to that used initially in the manufacture of emulsions. 0.759, 0.666, 0.661, 0.718, 0.475, 0.639, respectively, are obtained for emulsions 1, 2, 3, 4, 5 and 6.

  With regard to the drop size, the influence of a high homogenizer speed can be verified to produce smaller drops, compared to the measurement of T2 = 391 ms (emulsion 6), relative to the emulsion 1 (T2 = 815 ms).

  It can also be seen that with an initial fraction of 50/50, for the same nature of oil and water, larger drops are obtained (compare emulsions 1 and 5). (7)

  This method also makes it possible to assess the effect of adding an additive, for example a surfactant (emulsions 2 and 3) with respect to emulsion 1, without additive.

  The calculated distributions for emulsions 5 and 6 show the presence of a secondary peak located between the dominant peak of water and that of oil. This is, for the emulsion 5 at a high water level (50%), for the emulsion 6 at a very important stirring. This peak can be interpreted in two ways: first, it can be associated with the presence of drops of water of a size of the order of one micron within the drops of oil, secondly it can also be spaces between very small drops reflecting a very important polydispersity of oil drops. To be able to choose one or the other interpretation, it suffices to replace the continuous phase by an aqueous phase of the same nature but deuterated, which does not produce a relaxation signal. For this, we can for example slowly inject this phase through the emulsion with a water-permeable filter to retain the oil phase. The persistence or not of the secondary peak then makes it possible to decide the existence of drops of water within the drops of oils.

  The examples also show that the method applies to different types of crudes. Emulsion 4 made from a Canadian heavy crude produces a usable relaxation signal.

  The type of surfactant also influences the measured relaxation time distribution and thus the drop size. However, in these cases, one approaches the limit value T2b and the method makes it possible to determine only the average size. The distribution becomes inaccurate because the difference 1 / T2-1 / T2B becomes sensitive to measurement errors. One solution is to work at a higher temperature (60 C instead of 30 C for example) which has the effect of increasing the value T2B and thus the measuring range of the spaces between drops.

  The method according to the invention is advantageously implemented by a measuring apparatus comprising an NMR device which can take, automatically or not, a sample of an effluent transported by pipeline according to the emulsification technique of a crude. The analysis provided by this apparatus is used to adjust, or control, the emulsification process for transport, for example by flow rate settings, additives, or other parameters.

  This apparatus can be arranged close to the transport pipe and in communication with the emulsion by bypass ducts.

Claims (6)

  1) Method for determining at least one characteristic parameter of a hydrocarbon emulsion in an aqueous continuous phase, said hydrocarbon having a viscosity significantly different from that of said aqueous phase, method in which nuclear magnetic resonance is used for measuring the distributions of the T2 relaxation times of said emulsion by analysis of a magnetization signal resulting from a CPMG type sequence, and that from the distributions obtained respectively for water and oil, determines at least one parameter that is a function of one of the characteristics of the emulsion included in the following list: water / oil volume ratio; - average size of the drops; - polydispersity of the drops.   2) Method according to claim 1, wherein the distribution of the T2 relaxation times of the aqueous continuous phase of said emulsion is analyzed from the following relation: ## EQU1 ## where: S is the surface of the space between the drops of oil, V the volume of this space, T. the reference relaxation time of the water excluding emulsion, p2 (m / s) parameter which reflects the intensity of the water-oil interactions.   3) Method according to one of the preceding claims, wherein one obtains the average size of the drops and the coefficient p2 by determining the ratio S ,, JVT by measuring the De diffusion coefficient of the aqueous phase as a function of time.   4) Method according to one of the preceding claims, wherein a comparison of said T2 distributions with the distributions obtained from an emulsion sample in which the continuous phase has been replaced by a deuterated aqueous phase.   5) Device for characterizing an emulsified effluent comprising: means for sampling an emulsion sample circulating in a transport pipe, means for implementing the method according to one of claims 1 to 4, means for controlling the quality of the emulsion as a function of the parameter obtained by said method.   6) Application of the method according to one of claims 1 to 4, the characterization of aqueous emulsions of heavy crudes, extra heavy, or distillation residues.