Methods and apparatus for determining precipitation onset pressure of asphaltenes

• US 6,501,072 B2
• Filed: 01/29/2001
• Issued: 12/31/2002
• Est. Priority Date: 01/29/2001
• Status: Expired due to Term

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Jamaluddin, Abul, Joshi, Nikhil B., Mullins, Oliver C.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): MORAN, TIMOTHY J
• Application Number:
  US09/772,209
• Publication Number:
  US 20020139929A1
• Time in Patent Office: 701 Days
• Field of Search: 250/256, 250/269.1, 250/301, 250/339.07, 250/255, 356/441, 356/442, 356/335, 356/336
• US Class Current: 250/256
• CPC Class Codes: E21B 47/113 using electrical indications; using light radiations
• CPC Class Codes: E21B 49/088 combined with sampling

Abstract

The optical density of an oil sample at a plurality of wavelengths over a plurality of different (typically decreasing) pressures is monitored and used to find the size of agglomerated asphaltene particles which are precipitating from the oil sample. The optical density information used in finding the particle size is preferably optical density information relating to the scattering of light due to the asphaltene particles only. Thus, baseline optical density information of the oil sample at a high pressure is subtracted from optical density information obtained at test pressures at each wavelength of interest. Asphaltene particles of a radius of one micron and smaller were found to be powdery, while asphaltene particles of a radius of three microns and larger were found to include paving resins. The precipitation of asphaltenes is reversible by increasing the pressure under certain circumstances.

Claims

1. A method of determining the size of asphaltene particles precipitating in a sample of oil obtained from a formation, comprising the steps of:

a) illuminating the sample with light at first and second different wavelengths at at least one intensity;

b) measuring optical energies at said first and second different wavelengths of light transmitted through the sample;

c) changing pressure on the sample to cause precipitation of asphaltene particles;

e) repeating steps a) and b) at the changed pressure; and

f) determining the size of the asphaltene particles precipitating from the sample as a function of the measured optical energies.

2. A method according to claim 1, wherein:

said function is also a function of said first and second wavelengths.

3. A method according to claim 2, wherein:

said function is also a function of a ratio of the indices of refraction of the asphaltene particles and the oil.

4. A method according to claim 2, wherein:

said function is also a function of the intensity of said illuminating at said first and second different wavelengths.

5. A method according to claim 1, wherein:

said determining comprises finding baseline optical densities of said oil sample at said first and second wavelengths, finding test optical densities of said oil sample at said first and second wavelengths at the changed pressure, relating a wavelength dependence of scattering of light to a function of said first and second wavelengths, and said baseline and test optical densities of said oil sample, and relating said wavelength dependence to said asphaltene particle size.

6. A method according to claim 5, wherein:

said wavelength dependence of scattering of light is related to said first and second wavelengths according to $g=\frac{\ln <br><br>(\frac{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{1test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>1<br><br>\mathrm{baseline}}}{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2baseline}}})}{\ln <br><br>\frac{\mathrm{\lambda <br><br>}<br><br>1}{\mathrm{\lambda <br><br>}<br><br>2}}$

where g is said wavelength dependence of scattering of light, λ

1 and λ

2 are said first and second different wavelenghs, and OD is the optical density for its stated subscript.

7. A method according to claim 6, wherein:

said wavelength dependence is related to said asphaltene particle size according to $g=4+\frac{12<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}{5<br><br>\left(n^2+2\right)+6<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}$

when n is a ratio of the indices of refraction of the asphaltene particles and the oil, and $\mathrm{\alpha <br><br>}=\frac{2<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{nr}}{{\mathrm{\lambda <br><br>}}_{\mathrm{ave}}},$

with λ

_ave being the average of wavelengths λ

1 and λ

2, and r being the radius of the asphaltene particles.

8. A method according to claim 7, wherein:

n is selected to be approximately 1.2.

9. A method according to claim 1, wherein:

said first and second wavelengths are chosen in the near infrared spectrum.

10. A method according to claim 9, wherein:

said first and second wavelengths are chosen from a group of wavelengths including approximately 1115 nm, approximately 1310 nm, approximately 1580 nm, and approximately 1900 nm to approximately 2100 nm.

11. A method according to claim 1, wherein:

said first and second wavelengths are chosen to be within an order of magnitude of the radius of the particle size being measured.

12. A method according to claim 1, wherein:

said illumination is conducted in a borehole or wellbore of the formation.

13. A method according to claim 12, further comprising:

prior to said step of illuminating, obtaining said sample of formation oil with a borehole tool which is movable in a borehole or wellbore in the formation.

14. A method according to claim 12, further comprising:

prior to said step of illuminating, isolating said sample of formation oil in a fixed cell in a wellbore in the formation.

15. A method according to claim 1, wherein:

said illumination is conducted uphole out of the formation.

16. A method according to claim 1, wherein:

said step of illuminating comprises illuminating at at least three different wavelengths,

said step of measuring comprises measuring optical energies at said at least three different wavelengths, and

said step of determining comprises making a plurality of determinations of the size of the asphaltene particles precipitating from the sample, each of said plurality of determinations being made as a function of two different measured optical energies.

17. A method according to claim 1, wherein:

said function is also a function of a density of said asphaltene particles, a density of said oil, and a viscosity of said oil.

18. A method according to claim 17, wherein:

said function is also a function of the intensity of said illuminating at said first and second different wavelengths.

19. A method according to claim 1, wherein:

said determining comprises using said optical energies at said first and second different wavelengths to find a velocity of said asphaltene particles precipitating in said oil sample, and relating said velocity to the size of the asphaltene particles.

20. A method according to claim 19, wherein:

said velocity (V) is related to said size of the asphaltene particles (r) according to $V=\frac{2<br><br>r^2<br><br>\left(\mathrm{\rho <br><br>}-{\mathrm{\rho <br><br>}}_s\right)<br><br>a}{9<br><br>\mathrm{\eta <br><br>}},$

where a is the gravity constant, η

is the viscosity of the oil, ρ

is the density of the asphaltene particle, and ρ

_s is the density of the oil.

21. A method according to claim 19, wherein:

said velocity is determined by repeating steps a) and b) at the changed pressure a plurality of times and finding how long it takes for an indication of said optical energies to change a certain amount, and dividing a dimension of a cell in which said sample is located by that length of time.

22. A method according to claim 21, wherein:

said length of time is the length of time it takes for the optical energy to increase from a measured minimum value which represents a maximum optical density after said pressure is changed at step c), to a threshold value.

23. A method according to claim 22, wherein:

said threshold value is a fraction of a difference between said maximum optical density and a baseline optical density.

24. A method of finding the precipitation onset pressure of asphaltene particles of a desired size in an oil sample, comprising the steps of:

a) illuminating the sample with light at first and second different wavelengths at at least one intensity;

b) measuring optical energies at said first and second different wavelengths of light transmitted through the sample;

c) changing pressure on the sample to cause precipitation of asphaltene particles;

e) repeating steps a) and b) at the changed pressure;

f) determining the size of the asphaltene particles precipitating from the sample as a function of the measured optical energies; and

g) repeating steps a) through f) until the size determined at step f) is said desired size.

25. A method according to claim 24, further comprising:

prior to said step of illuminating, isolating said oil sample downhole in a borehole or wellbore of a formation.

26. A method according to claim 25, further comprising:

after step g), isolating another oil sample and repeating steps a) through g) for said another oil sample.

27. A method according to claim 24, wherein:

said function is also a function of said first and second wavelengths, a ratio of the indices of refraction of the asphaltene particles and the oil, and the intensity of said illuminating at said first and second different wavelengths.

28. A method according to claim 24, wherein:

said determining comprises finding baseline optical densities of said oil sample at said first and second wavelengths, finding test optical densities of said oil sample at said first and second wavelengths at the changed pressure, relating a wavelength dependence of scattering of light to a function of said first and second wavelengths, and said baseline and test optical densities of said oil sample, and relating said wavelength dependence to said asphaltene particle size.

29. A method according to claim 28, wherein:

said wavelength dependence of scattering of light is related to said first and second wavelengths according to $g=\frac{\ln <br><br>(\frac{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{1test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>1<br><br>\mathrm{baseline}}}{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2baseline}}})}{\ln <br><br>\frac{\mathrm{\lambda <br><br>}<br><br>1}{\mathrm{\lambda <br><br>}<br><br>2}}$

where g is said wavelength dependence of scattering of light λ

1 and λ

2 are said first and second different wavelengths, and OD is the optical density for its stated subscript.

30. A method according to claim 29, wherein:

said wavelength dependence is related to said asphaltene particle size according to $g=4+\frac{12<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}{5<br><br>\left(n^2+2\right)+6<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}$

where n is a ratio of the indices of refraction of the asphaltene particles and the oil, and $\mathrm{\alpha <br><br>}=\frac{2<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{nr}}{{\mathrm{\lambda <br><br>}}_{\mathrm{ave}}},$

with λ

_ave being the average of wavelengths λ

1 and λ

2, and r being the radius of the asphaltene particles.

31. A method according to claim 24, wherein:

said first and second wavelengths are chosen in the near infrared spectrum from a group of wavelengths including approximately 1115 nm, approximately 1310 nm, approximately 1580 nm, and approximately 1900 nm to approximately 2100 nm.

32. A method according to claim 24, wherein:

said function is also a function of a density of said asphaltene particles, a density of said oil, a viscosity of said oil, and the intensity of said illuminating at said first and second different wavelengths.

33. A method according to claim 24, wherein:

said determining comprises using said optical energies at said first and second different wavelengths to find a velocity of said asphaltene particles precipitating in said oil sample, and relating said velocity to the size of the asphaltene particles.

34. A method according to claim 33, wherein:

said velocity (V) is related to said size of the asphaltene particles (r) according to $V=\frac{2<br><br>r^2<br><br>\left(\mathrm{\rho <br><br>}-{\mathrm{\rho <br><br>}}_s\right)<br><br>a}{9<br><br>\mathrm{\eta <br><br>}},$

where a is the gravity constant, η

is the viscosity of the oil, ρ

is the density of the asphaltene particle, and ρ

_s is the density of the oil.

35. A method according to claim 33, wherein:

said velocity is determined by repeating steps a) and b) at the changed pressure a plurality of times and finding how long it takes for an indication of said optical energies to change a certain amount, and dividing a dimension of a cell in which said sample is located by that length of time.

36. A method according to claim 35, wherein:

said length of time is the length of time it takes for the optical energy to increase from a measured minimum value which represents a maximum optical density after said pressure is changed at step c), to a threshold value.

37. A method according to claim 36, wherein:

said threshold value is a fraction of a difference between said maximum optical density and a baseline optical density.

38. A method of determining the size of asphaltene particles precipitating in a sample of oil obtained from a formation, comprising the steps of:

a) illuminating the sample with light at at least a first wavelength at a first intensity;

b) measuring optical energy at said first wavelength of light transmitted through the sample;

c) changing pressure on the sample to cause precipitation of asphaltene particles;

e) repeating steps a) and b) at the changed pressure; and

f) determining the size of the asphaltene particles precipitating from the sample as a function of the measured optical energies at said first wavelength by using said measured optical energies at said first wavelength to find a velocity of said asphaltene particles precipitating in said oil sample, and relating said velocity to the size of the asphaltene particles.

39. A method according to claim 38, wherein:

said function is also a function of a density of said asphaltene particles, a density of said oil, and a viscosity of said oil.

40. A method according to claim 38, wherein:

said velocity (V) is related to said size of the asphaltene particles (r) according to $V=\frac{2<br><br>r^2<br><br>\left(\mathrm{\rho <br><br>}-{\mathrm{\rho <br><br>}}_s\right)<br><br>a}{9<br><br>\mathrm{\eta <br><br>}},$

where a is the gravity constant, η

is the viscosity of the oil, ρ

is the density of the asphaltene particle, and ρ

_s is the density of the oil.

41. A method according to claim 38, wherein:

said velocity is determined by repeating steps a) and b) at the changed pressure a plurality of times and finding how long it takes for an indication of said optical energy to change a certain amount, and dividing a dimension of a cell in which said sample is located by that length of time.

42. A method according to claim 41, wherein:

said length of time is the length of time it takes for the optical energy to increase from a measured minimum value which represents a maximum optical density after said pressure is changed at step c), to a threshold value.

43. A method according to claim 42, wherein:

said threshold value is a fraction of a difference between said maximum optical density and a baseline optical density.

44. An apparatus for determining the size of asphaltene particles precipitating in a sample of oil obtained from a formation, comprising:

a) an optical cell for holding the sample of oil;

b) means optically coupled to said optical cell for illuminating the sample with light at first and second different wavelengths at at least one intensity;

c) means optically coupled to said optical cell for measuring optical energies at said first and second different wavelengths of light transmitted through the sample;

d) means fluidly coupled to said optical cell for changing pressure on the sample of oil to cause precipitation of asphaltene particles; and

e) means for determining the size of the asphaltene particles precipitating from the sample as a function of the measured optical energies.

45. An apparatus according to claim 44, wherein:

said means for changing pressure is adapted to change pressure multiple times at least until said means for determining the size of the asphaltene particles precipitating from the sample determines that the size of said asphaltene particles is a desired size.

46. An apparatus according to claim 44, further comprising:

e) means for isolating the oil sample downhole in a borehole or wellbore of a formation.

47. An apparatus according to claim 44, wherein:

said function is also a function of said first and second wavelengths, a ratio of the indices of refraction of the asphaltene particles and the oil, and the intensity of said illuminating at said first and second different wavelengths.

48. An apparatus according to claim 46, wherein:

said means for determining comprises means for finding baseline optical densities of said oil sample at said first and second wavelengths, for finding test optical densities of said oil sample at said first and second wavelengths at the changed pressure, for relating a wavelength dependence of scattering of light to a function of said first and second wavelengths, and said baseline and test optical densities of said oil sample, and for relating said wavelength dependence to said asphaltene particle size.

49. An apparatus according to claim 48, wherein:

said wavelength dependence of scattering of light is related to said first and second wavelengths according to $g=\frac{\ln <br><br>(\frac{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{1test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>1<br><br>\mathrm{baseline}}}{{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2test}}-{\mathrm{OD}}_{\mathrm{\lambda <br><br>}<br><br>\mathrm{2baseline}}})}{\ln <br><br>\frac{\mathrm{\lambda <br><br>}<br><br>1}{\mathrm{\lambda <br><br>}<br><br>2}}$

where g is said wavelength dependence of scattering of light, λ

1 and λ

2 are said first and second different wavelengths, and OD is the optical density for its stated subscript.

50. An apparatus according to claim 49, wherein:

said wavelength dependence is related to said asphaltene particle size according to $g=4+\frac{12<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}{5<br><br>\left(n^2+2\right)+6<br><br>\left(n^2-2\right)<br><br>{\mathrm{\alpha <br><br>}}^2}$

where n is a ratio of the indices of refraction of the asphaltene particles and the oil, and $\mathrm{\alpha <br><br>}=\frac{2<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{nr}}{{\mathrm{\lambda <br><br>}}_{\mathrm{ave}}},$

with λ

_ave being the average of wavelengths λ

1 and λ

2, and r the radius of the asphaltene particles.

51. An apparatus according to claim 44, wherein:

said first and second wavelengths are chosen in the near infrared spectrum from a group of wavelengths including approximately 1115 nm, approximately 1310 nm, approximately 1580 nm, and approximately 1900 nm to approximately 2100 nm.

52. An apparatus according to claim 44, wherein:

said function is also a function of a density of said asphaltene particles, a density of said oil, a viscosity of said oil, and the intensity of said illuminating at said first and second different wavelengths.

53. An apparatus according to claim 44, wherein:

said means for determining comprises means for using said optical energies at said first and second different wavelengths to find a velocity of said asphaltene particles precipitating in said oil sample, and for relating said velocity to the size of the asphaltene particles.

54. An apparatus according to claim 53, wherein:

said means for relating relates said velocity (V) to said size of the asphaltene particles (r) according to $V=\frac{2<br><br>r^2<br><br>\left(\mathrm{\rho <br><br>}-{\mathrm{\rho <br><br>}}_s\right)<br><br>a}{9<br><br>\mathrm{\eta <br><br>}},$

where a is the gravity constant, η

is the viscosity of the oil, ρ

is the density of the asphaltene particle, and ρ

_s is the density of the oil.

55. An apparatus according to claim 52, wherein:

said means for determining includes means for timing a length of time it takes for an indication of said optical energies to change a certain amount, and dividing a dimension of said cell by that length of time.

56. An apparatus according to claim 55, wherein:

said length of time is the length of time it takes for the optical energy to increase from a measured minimum value which represents a maximum optical density after said pressure is changed by said means for changing pressure to a threshold value.

57. An apparatus according to claim 56, wherein:

said threshold value is a fraction of a difference between said maximum optical density and a baseline optical density.

58. An apparatus for determining the size of asphaltene particles precipitating in a sample of oil obtained from a formation, comprising:

a) an optical cell for holding the sample of oil;

b) means optically coupled to said optical cell for illuminating the sample with light at at least a first wavelength at a first intensity;

c) means optically coupled to said optical cell for measuring optical energy at said first wavelength of light transmitted through the sample;

d) means fluidly coupled to said optical cell for changing pressure on the sample of oil to cause precipitation of asphaltene particles; and

e) means for determining the size of the asphaltene particles precipitating from the sample as a function of the measured optical energies at said first wavelength at different pressures by using said measured optical energies at said first wavelength to find a velocity of said asphaltene particles precipitating in said oil sample, and for relating said velocity to the size of the asphaltene particles.

59. An apparatus according to claim 58, wherein:

said function is also a function of a density of said asphaltene particles, a density of said oil, and a viscosity of said oil.

60. An apparatus according to claim 58, wherein:

said means for relating relates said velocity (V) to said size of the asphaltene particles (r) according to $V=\frac{2<br><br>r^2<br><br>\left(\mathrm{\rho <br><br>}-{\mathrm{\rho <br><br>}}_s\right)<br><br>a}{9<br><br>\mathrm{\eta <br><br>}},$

where a is the gravity constant, η

is the viscosity of the oil, ρ

is the density of the asphaltene particle, and ρ

_s is the density of the oil.

61. An apparatus according to claim 58, wherein:

said means for determining includes means for timing a length of time it takes for an indication of said optical energies to change a certain amount, and dividing a dimension of said cell by that length of time.

62. An apparatus according to claim 61, wherein:

said length of time is the length of time it takes for the optical energy to increase from a measured minimum value which represents a maximum optical density after said pressure is changed by said means for changing pressure to a threshold value.

63. An apparatus according to claim 62, wherein:

said threshold value is a fraction of a difference between said maximum optical density and a baseline optical density.