WO2001020322A1

WO2001020322A1 - Method and device for determining asphaltene precipitation onset pressure

Info

Publication number: WO2001020322A1
Application number: PCT/US2000/024166
Authority: WO
Inventors: Oliver C. Mullins; Xu Wu; Henning Groenzin
Original assignee: Schlumberger Limited; Schlumberger Canada Limited; Schlumberger Technology B.V.; Services Petroliers Schlumberger; Schlumberger Holdings Limited; Schlumberger Overseas S.A.; Prad Research And Development N.V.; Schlumberger Surenco, S.A.
Priority date: 1999-09-14
Filing date: 2000-09-05
Publication date: 2001-03-22
Also published as: AU7106700A; BR0013936A; MXPA02002696A

Abstract

A process is provided for determining asphaltene precipitation onset pressure in formation fluid. In one embodiment the process includes the steps of: (a) isolating a downhole sample of formation fluid; (b) illuminating the sample with UV light; (c) measuring fluorescence energy emanating from the sample; (d) reducing pressure on the sample; (e) repeating steps (b) to (d); and (f) setting asphaltene precipitation onset pressure equal to pressure on the sample when an abrupt change in fluorescence energy is detected. A downhole optical analyzer includes a UV lamp; a piston adapted to decrease pressure of the fluid in an optical cell region by increasing the volume of an isolated sample of formation fluid; and a pressure sensor for sensing pressure of the isolated sample.

Description

METHOD AND DEVICE FOR DETERMINING ASPHALTENE PRECIPITATION ONSET PRESSURE

The present invention is related to co-owned U.S. Patent Nos. 3,780,575 and 3,859,851 to Urbanos y, co-owned U.S. Patent Nos. 4,860,581 and 4,936,139 to Zimmerman et al., co-owned U.S. Patents No. 4,994,671 to Safinya et al . , and co-owned U.S. Patent Nos. 5,266,800 and 5,859,430 to Mullins. The invention is also related to co-owned, copending U.S. application Serial Number 09/015,812, filed January 29, 1998.

Field of the Invention

The invention relates generally to open hole logging of a borehole to gather data for designing oil well production facilities and for avoiding plugging of the oil well.

BACKGROUND OF THE INVENTION

Asphaltene Precipitation

One of the problems encountered in crude oil production is asphaltene plugging of an oil well. Asphaltenes are components of crude oil that are often found in colloidal suspension in the formation fluid. If for any reason the colloidal suspension becomes unstable, the colloidal particles will precipitate, stick together and plug the well. Asphaltene precipitation during production causes severe problems. Plugging of tubing and surface facilities disrupts production and adds cost. Plugging of the formation itself is very difficult and expensive to reverse, especially for a deep water well.

Asphaltenes can precipitate from crude oils during production of the crude oil due to a drop in pressure. Crude oils which are somewhat compressible are particularly susceptible to this effect because the reduction in dielectric constant per unit volume which accompanies fluid expansion causes the asphaltene suspension to become unstable. Asphaltenes are colloidally suspended in crude oils in micelles which are approximately 5 nm in diameter. (See "Asphaltenes, Fundamentals and Applications," E.Y. Sheu, O.C. Mullins, Eds., Plenum Pub. Co. New York, NY, 1995). With pressure reduction or addition of light hydrocarbons, the suspension can become unstable; the colloidal asphaltene particles stick together and flocculate or precipitate out of the solution. For production of crude oil, it is desirable to know accurately at what pressure the asphaltenes will flocculate (or precipitate) and, in a given formation with a given temperature. This pressure is referred to as "asphaltene precipitation onset pressure" . It is known that when asphaltenes precipitate, they induce significant optical scattering of light. This is because prior to flocculation, the asphaltenes are smaller than a wavelength of light and thus are in the Rayleigh optical scattering limit, yielding very low cross sections.

When they flocculate, the particles are large compared to a wavelength of light, thereby greatly increasing the optical scattering.

Predicting Asphaltene Precipitation

The onset of asphaltene precipitation is difficult to predict. When asphaltene plugging happens, it usually happens unexpectedly. Advance warning of asphaltene precipitation based on laboratory testing of formation fluid samples is not reliable for a number of reasons discussed further herein below.

It is known to detect asphaltene precipitation in the laboratory by measuring optical transmission through a sample of crude oil as a function of pressure. Near- infrared light is preferred because optical attenuation due to absorption is less. Asphaltene precipitation is detected as a sharp reduction of transmitted light. Stirring is necessary to keep the floes suspended, otherwise the increased scattering is transient. Oil producers currently use this method to test whether asphaltene plugging is likely to be a problem by obtaining a bottom hole sample and performing laboratory analyses .

Laboratory systems designed to detect asphaltene plugging typically use a sight glass with monitoring light (or laser) transmission. The optical transmission of a sample of crude oil is determined at reservoir pressure (and temperature) . The pressure of the sample is then reduced. Asphaltene precipitation is accompanied by a large increase in the light scattering strength of the sample. Asphaltenes suspended in crude oil are in micelles about 5 nm in diameter. (See "Asphaltenes, Fundamentals and Applications" as referenced above) . Thus, the interaction of light with the asphaltenes particles is in the Rayleigh limit. The ratio R_σ of the Rayleigh cross section divided by the geometric size goes as (D/λ)⁴ where λ is the wavelength of light (about 500 nm) .

R_σ = 1/6 (Dk)⁴ [(ε-l)/(ε+2)]^: Eq. (1)

where k = 2π/λ and ε is the ratio of dielectric constants for the discrete and continuous phases.

Upon flocculation, the aggregates are larger than the wavelength of light so the cross section is geometric. For basic aggregates of 1 micron size (which then cluster with a fractal dimension probably in the diffusion limited regime) , the increase in scattering cross section per unit mass of asphaltene is roughly a factor of 10⁴. (See J. Phys . Chem, 99, 9576 (1995), M.A. Anisimov, I.K. Yudin, V. Nikitin, G. Nikolaenko, A. Chernoutsan, H. Toulhoat, D. Frot, Y. Briolant) .

Although such laboratory systems are in widespread use, predicting asphaltene precipitation onset pressure based on retrieving a bottom hole sample followed by laboratory testing does not provide a reliable method of avoiding asphaltene plugging of an oil well. Objects of the Invention

The inventors recognized the several problems that contribute to unreliability when predicting asphaltene precipitation onset pressure using the laboratory testing approach: (1) It is necessary to maintain pressure on the sample because low pressure can cause the asphaltene to precipitate, an irreversible process at reasonable time scales. (2) Sample transfer for analysis can yield an asphaltene free crude oil which is not representative of the formation fluid when undetected loss of pressure results in asphaltene precipitation. (3) Non-asphaltene factors such as clay, formation fines and emulsions can cause optical scattering which reduces the robustness of the transmission measurement. (4) It is necessary to stir the solution to maintain scattering and prevent solids from settling. This also reduces the robustness of the transmission measurement.

These difficulties contribute to the unreliability of the laboratory method of predicting asphaltene precipitation onset pressure.

Accordingly, it is a first object of the invention to provide a reliable method of predicting asphaltene precipitation onset pressure.

It is a second object of the invention to provide a method of predicting asphaltene precipitation onset pressure that does not require retrieving a bottom hole sample for laboratory analysis.

It is a third object of the invention to provide a method of predicting asphaltene precipitation onset pressure that is substantially independent of other factors that can cause optical scattering.

SUMMARY OF THE INVENTION

A preferred embodiment of the process for determining asphaltene precipitation onset pressure in formation fluid, includes the steps of: (a) isolating a downhole sample of formation fluid; (b) illuminating the sample with UV light; (c) measuring optical energy emanating from the sample; (d) reducing pressure on the sample; (e) repeating steps (b) to (d) ; and (f) setting asphaltene precipitation onset pressure equal to pressure on the sample when an abrupt change in optical energy is detected. In one embodiment of the process the abrupt change is an abrupt change in value of a parameter that is a function of intensity of fluorescence at one wavelength and intensity of fluorescence at another wavelength, the two wavelengths being approximately 425nm and 550nm, respectively.

In the preferred embodiment of the process, the abrupt change is a calculated value selected from a set of calculated values, each of which is a function of intensity from at least one of said multiple wavelengths.

In one alternative embodiment of the process, the abrupt change is a change in fluorescence spectrum.

Another alternative embodiment of the process includes illuminating the sample with light and measuring intensity of transmitted light.

The preferred embodiment of the apparatus for determining asphaltene precipitation onset pressure in formation fluid includes a flow line for accepting a flow of formation fluid. The flow line includes isolation means for isolating a sample of the fluid from formation pressure, an optical cell region for downhole optical analysis of the sample, and a piston adapted to decrease pressure of the fluid in the optical cell region by increasing the volume of the isolated sample, and a pressure sensor for sensing pressure of the isolated sample. The preferred embodiment further includes an optics system. The optics system includes means for illuminating the sample, and a detector for detecting intensity of optical energy emanating from the isolated sample. The preferred embodiment further includes processor means for detecting an abrupt change of a value derived from at least one measured intensity. The preferred embodiment of the apparatus further includes a UV lamp, a total fluorescence detector for detecting total fluorescence intensity, a shorter-wavelength fluorescence detector for detecting fluorescence intensity at a shorter wavelength, and a longer-wavelength fluorescence detector for detecting fluorescence intensity at a longer wavelength.

An alternative embodiment borehole apparatus includes a lamp, an optical transmission path through the sample, and an optical transmission detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a first preferred embodiment of the invention.

FIG. 2A is a graph simulating determination of asphaltene precipitation onset pressure based on change in fluorescence intensity.

FIG. 2B is a graph simulating determination of asphaltene precipitation onset pressure based on change in the ratio of green fluorescence intensity to total fluorescence intensity, indicative of fluorescence spectral blue-shift.

FIG. 3 displays the asphaltenes fluorescence spectrum.

FIG. 4A illustrates the large increase in fluorescence intensity following asphaltene precipitation.

FIG. 4B illustrates the large spectral blue-shift following asphaltene precipitation.

FIG. 5 is a flow chart illustrating the process for determining asphaltene precipitation onset pressure in downhole fluid based on total fluorescence intensity.

FIG. 6 is a flow chart illustrating the process for determining asphaltene precipitation onset pressure in downhole fluid based on ratio of fluorescence intensity at a first wavelength to the sum of fluorescence intensities at a first and second wavelength.

FIG. 7 is a flow chart illustrating the process for determining asphaltene precipitation onset pressure in downhole fluid based on measuring fluorescence intensity at multiple frequencies, processing intensity data by multiple algorithms to produce multiple output signals and selecting one output signal to determine asphaltine precipitation onset pressure.

FIG. 8A is a schematic representation of an alternative embodiment of the invention based on light transmission. FIG. 8B shows the change in optical transmission indicative of asphaltene precipitation pressure on a curve produced by the embodiment of FIG 8A.

FIG. 8C is a flow chart illustrating the process for determining asphaltene precipitation onset pressure in downhole fluid based on light transmission, using the apparatus of FIG. 8A.

FIG. 9 locates the asphaltene precipitation onset pressure detection module in a wireline system tool.

DETAILED DESCRIPTION OF THE INVENTION

General

The inventors recognized a determination of asphaltene precipitation onset pressure, based on optical analysis, would be significantly more reliable if the optical analysis were performed downhole instead of continuing the current practice of bringing samples of formation fluid to the surface for analysis. The inventors further recognized that this could be done by isolating a sample of formation fluid downhole and reducing pressure on the sample downhole by defining an isolation chamber in a flowline and expanding the isolation chamber using a piston to permit optical analysis downhole under conditions of stepwise reducing pressure.

The inventors further recognized that measuring UV- stimulated fluorescence intensity would provide a more robust method for determining asphaltene precipitation onset pressure than methods which rely on measuring simple optical transmission. Accordingly, in a preferred embodiment, the invention provides a process and apparatus for determining asphaltene precipitation onset pressure in downhole formation fluid based on measuring fluorescence of maltene chromophores .

It is known that UV and short wavelength visible light can induce strong fluorescence in crude oils from small light absorbing molecules. However, when the asphaltenes are colloidally dispersed, diffusion with collision between small UV absorbing molecules and asphaltenes causes collisional quenching, greatly reducing the intensity of fluorescence. In addition, these molecular collisions transfer electronic excitation energy from small molecules to asphaltenes, yielding the very red fluorescence of asphaltenes. (See "Structures and Dynamics of Asphaltenes," O.C, Mullins, E.Y. Sheu, Eds., Plenum Pub. Co. New York, NY (1998) ) .

The inventors recognized that following asphaltene precipitation, the small molecules cannot collide with asphaltene molecules because they are too far away; and that the asphaltene precipitation onset pressure can be determined by measuring intensity of fluorescence at one or more wavelengths and detecting a change either in intensity or in spectral shift of intensities across the spectrum of the fluorescence. The inventors also recognized that there are very few other factors which can cause comparable effects so fluorescence methods are likely to be more robust than prior art optical transmission methods. They also recognized that fluorescence does not require transmission of light through the sample, which presents problems when attempting to measure light transmission through heavy crude oils .

The present invention therefore, in a first preferred embodiment, provides a process and apparatus for determining asphaltene precipitation onset pressure in downhole formation fluid based on measuring fluorescence intensity of maltene chromophores . Total Fluorescence Measurement

The first preferred embodiment of apparatus in accordance with the present invention is illustrated in FIG. 1. This embodiment uses a measurement of total fluorescence and measurement of fluorescence at two wavelengths.

FIG. 1 shows an isolated downhole sample 20 captured within flowline 21. A portion of the flowline defines optical cell region 23. The flowline includes a piston 25 for altering the effective volume of the isolated downhole sample and a pressure sensor 27 for measuring the pressure of the isolated sample. Sample 20 is illuminated by UV light from UV lamp 32 passing via fiber optic illumination line 33, fiber optic bundle 34 and window 35. Fluorescence from sample 20 emanating from window 35 passes to total fluorescence detector 41 via fiber optic bundle 34, fiber optic line 43 and UV filter 42. The UV filter filters out the UV source light component and allows substantially all of the total fluorescence to pass.

Fluorescence from sample 20 also passes to shorter wavelength detector 51 via fiber optic bundle 34, fiber optic line 53 and filter 52. Filter 52 is a band-pass filter passing a narrow spectrum of light around wavelength S. Wavelength S in the preferred embodiment is approximately 425nm (blue fluorescence) .

Fluorescence from sample 20 also passes to longer wavelength detector 61 via fiber optic bundle 34, fiber optic line 63 and filter 62. Filter 62 is a band-pass filter passing a narrow spectrum of light around wavelength L. Wavelength L is longer than wavelength S and in the preferred embodiment is approximately 550nm (green fluorescence) .

FIG. 2A is a simulated intensity vs. pressure curve, that shows how total fluorescence intensity (axis I) changes abruptly (at line A--A) when the pressure, which is decreasing from left to right along axis P, reaches the asphaltene precipitation onset pressure. The asphaltene precipitation onset pressure is indicated by an abrupt increase of fluorescence intensity at line A—A. The simulated curve of FIG. 2A is derived from FIGS. 3 and 4A. FIG. 3 shows the asphaltenes fluorescence spectrum. FIG. 4A illustrates the large increase in intensity of fluorescence across the spectrum following asphaltene precipitation.

FIG. 2B is a simulated fluorescence ratio vs. pressure curve that shows the ratio of short-wavelength fluorescent emission (blue fluorescence at wavelength S, 425nm) to long- wavelength fluorescence emission (green fluorescence at wavelength L, 550nm) changing abruptly (at line B—B) when the pressure, which is decreasing from left to right along axis P, reaches the asphaltene precipitation onset pressure the asphaltene precipitation onset pressure is indicated by an abrupt spectral blue shift at line B—B. The simulated curve of FIG. 2B is derived from FIGS. 3 and 4B. FIG. 4A illustrates the large spectral blue shift following asphaltene precipitation.

FIG. 5 illustrates a first process associated with the embodiment of FIG. 1. This process is based on change in total fluorescence intensity following asphaltene precipitation, as illustrated in FIGS. 2A and 4A.

FIG. 6 illustrates a second process associated with the embodiment of FIG. 1. This process is based on spectral blue shift following asphaltene precipitation, as illustrated by FIGS. 2B and 4B.

FIG. 7 is a flow chart illustrating the process for determining asphaltene precipitation onset pressure in downhole fluid based on measuring fluorescence intensity at multiple frequencies, processing intensity data by multiple algorithms and selecting a single output on the basis of largest change to determine asphaltene precipitation onset pressure .

An example of this technique is measuring fluorescence at three frequencies (instead of the two shown in FIG. 1) ; taking the frequencies in pairs to measure three ratios; selecting the ratio having the largest change; and using the selected ratio as a parameter to determine asphaltene precipitation onset pressure. By using more than three frequencies, more than three ratios could be measured. Also, more complex algorithms than a simple ratio could be used.

This technique enables automatic selection of the most robust of several ratio measurements or several parameter calculation algorithms, thereby broadening the range of borehole types for which the method will be effective.

Optical Transmission Measurement

Another embodiment of apparatus for determining asphaltene precipitation onset pressure in downhole formation fluid uses optical transmission measurement. This embodiment is shown in FIG. 8A.

FIG. 8A shows an isolated downhole sample 20 captured within flowline 68, a portion of which defines optical cell region 69. Light from lamp 70 is transmitted through sample 20 via entry window 71, exit window 72 and fiber optic connectors 75 and 76. Light that passes through the sample is detected by light detector 77.

FIG. 8B is a simulated optical transmission vs. pressure curve that shows how the optical transmission changes abruptly at line C—C when the pressure, which is decreasing from left to right along axis P, reaches the asphaltene precipitation onset pressure. The asphaltene precipitation onset pressure is indicated by an abrupt decline in optical transmission at line C—C.

FIG. 8C illustrates the process associated with the embodiment of FIG. 8A.

FIG. 9 locates the asphaltene precipitation onset pressure detection module in a wireline system tool. A wireline tool such as shown in FIG. 9 (but without the asphaltene precipitation onset detector module of the present invention) is discussed in US Patent No. 4,860, 581, issued August 29, 1989, to Zimmerman et al .

Claims

What is claimed is:

1. A method for determining asphaltene precipitation onset pressure in formation fluid, comprising the steps of:

(a) isolating a sample of formation fluid downhole;

(b) illuminating the sample downhole;

(c) measuring optical energy emanating from the sample;

(d) reducing pressure on the sample;

(e) repeating steps (b) to (d) ; and

(f) setting asphaltene precipitation onset pressure equal to pressure on the sample when an abrupt change in optical energy is detected.

2. A method according to claim 1, wherein the abrupt change is an abrupt change in intensity.

3. A method according to claim 1, wherein the optical energy is fluorescence.

4. A method according to claim 3, wherein the abrupt change is a change in value of a parameter that is a function of at least two fluorescence intensities.

5. A method according to claim 4, wherein the abrupt change is a change in value of a parameter that is a function of intensity of fluorescence at a first wavelength, and of total fluorescence intensity.

6. A method according to claim 5, wherein the function includes ratio of intensity at said first wavelength to total intensity.

7. A method according to claim 6, wherein said first wavelength is approximately 425nm.

8. A method according to claim 4, wherein the abrupt change is a change in value of a parameter that is a function of intensity of fluorescence at a second wavelength and intensity of fluorescence at a third wavelength.

9. A method according to claim 8, wherein said second and third wavelengths are approximately 425nm and 550nm, respectively .

10. A method according to claim 3, wherein measuring optical energy includes measuring intensity of fluorescence at multiple wavelengths, and wherein the abrupt change is a calculated value selected from a set of calculated values, calculated value being a function of fluorescence intensity having a wavelength that is one of said multiple wavelengths .

11. A method according to claim 3, wherein the abrupt change is a change in fluorescence spectrum.

12. A method according to claim 3 , wherein illuminating the sample includes illuminating with UV light.

13. A method according to claim 2, wherein illuminating the sample includes illuminating the sample with light; and measuring intensity includes measuring intensity of transmitted light.

14. A method according to claim 1, wherein reducing the pressure includes expanding the isolation chamber.

15. A borehole apparatus for determining asphaltene precipitation onset pressure in a fluid obtained from a formation, comprising: a flow line for accepting a flow of formation fluid, the flow line including isolation means for isolating a sample of the fluid from formation pressure having an optical cell region for downhole optical analysis of the sample, a piston adapted to decrease pressure of the fluid in the optical cell region by increasing the volume of the isolated sample, and a pressure sensor for sensing pressure of the isolated sample; an optics system having means for illuminating the sample, and a detector for detecting intensity of optical energy emanating from the isolated sample; and means for detecting an abrupt change of a value derived from at least one measured intensity.

16. A borehole apparatus according to claim 15, wherein the optics system includes a UV lamp and at least one fluorescence detector.

17. A borehole apparatus according to claim 16, wherein the at least one detector includes a total fluorescence detector for detecting total fluorescence intensity, and a first fluorescence detector for detecting fluorescence intensity at a first wavelength.

18. A borehole apparatus according to claim 16, wherein the at least one detector includes a second fluorescence detector for detecting fluorescence intensity at a second wavelength and a third fluorescence detector for detecting fluorescence intensity at a third wavelength.

19. A borehole apparatus according to claim 16, wherein the optics system includes a lamp, an optical transmission path through the sample, and at least one optical transmission detector.