Method for validating a downhole connate water sample

• US 6,729,400 B2
• Filed: 11/27/2002
• Issued: 05/04/2004
• Est. Priority Date: 11/28/2001
• Status: Expired due to Term

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: M-I LLC (SLB), Schlumberger Technology Corporation (SLB)
• Inventors: Ayan, Cosan, Hodder, Michael, Rabbito, Phillip, Zhu, Yifu, Mullins, Oliver C.
• Primary Examiner(s): Tsay, Frank
• Application Number:
  US10/305,878
• Publication Number:
  US 20030145988A1
• Time in Patent Office: 524 Days
• Field of Search: 166/165, 166/166, 166/65.1, 166/373, 166/264, 166/66, 166/250.01
• US Class Current: 166/264
• CPC Class Codes: E21B 49/08 Obtaining fluid samples or testing fluids, in boreholes or wells
• CPC Class Codes: E21B 49/0875 determining specific fluid parameters

Abstract

A downhole connate water sample drawn from the formation surrounding a well is validated when mud filtrate concentration is acceptably low. A preferred method includes drilling the well with a water-based drilling fluid, or more generally a water-based mud (WBM), containing a water-soluble dye. The dye acts as a tracer to distinguish connate water from WBM filtrate in a downhole sample of formation fluid contaminated by mud filtrate from the water-based mud. Preferably, an optical analyzer in a sampling tool measures light transmitted through the downhole sample to produce optical density data indicative of dye concentration. Preferably, optical density is measured at a first wavelength to obtain a first optical density, and at a second wavelength, close in wavelength to the first wavelength, to obtain a second optical density. First and second optical density data are transmitted to the surface. At the surface, in a data processor, the second optical density is subtracted from the first optical density to produce a third optical density that is substantially free of scattering error. The data processor validates each sample that has an acceptably low third optical density. The invention also provides a method of determining when to collect a sample of downhole fluid drawn over a period of time from a formation surrounding a well.

Claims

1. A method for validating a downhole connate water sample drawn from formation surrounding a well, comprising:

drilling the well with a water-based mud containing a water-soluble dye;

obtaining a sample of formation fluid downhole;

measuring optical density of the sample downhole; and

validating the sample if sample optical density is acceptably low.

2. A method according to claim 1, further repeating said act of obtaining a sample of formation fluid downhole and said act of measuring optical density of the sample downhole to obtain optical density from each of a series of samples.

3. A method according to claim 1, wherein said water-soluble dye is a blue dye.

4. A method according to claim 1, wherein said water-soluble dye is a dye selected from a group of dyes, the group consisting of Acid Blue #1 (EMI-600) and Acid Blue 9, alphazurine FG.

5. A method according to claim 1, wherein said water-soluble dye is a dye that is active in the ultraviolet region of the spectrum.

6. A method according to claim 1, wherein said water-soluble dye is a fluorescent dye.

7. A method according to claim 1, wherein said water-soluble dye is added to said water-based mud to produce a concentration within the range 0.2-2.0 kg/m³ (200-2000 mg/L).

8. A method according to claim 1, wherein measuring optical density includes measuring optical density at a first wavelength to obtain a first optical density, measuring optical density at a second wavelength to obtain a second optical density, and subtracting said second optical density from said first optical density.

9. A method according to claim 8, wherein said first wavelength and said second wavelength are close in wavelength.

10. A method according to claim 1, further comprising:

determining scattering from a series of optical density values; and

validating a sample if the scattering is acceptably low.

11. A method according to claim 1, further comprising:

calculating from a series of optical density values an asymptotic value indicative of water-based mud filtrate fraction; and

validating a sample if the asymptotic value is stable.

13. A method according to claim 11, wherein said at least one measured characteristic is optical density.

14. A method according to claim 11, wherein said at least one measured characteristic is fluorescence emission, ion concentration, or relative ion concentration.

15. A method according to claim 11, wherein said water-based mud contains a predetermined salt concentration, and wherein said at least one measured characteristic is conductivity or resistivity.

12. A method for validating a downhole connate water sample drawn from formation surrounding a well, comprising:

drilling the well with a water-based mud;

obtaining a sample of formulation fluid downhole;

measuring at least one characteristic of downhole fluid indicative of water-based mud filtrate contamination levels in the sample; and

;

validating the sample if the at least one measured characteristic is acceptably low.

16. A method for determining when to collect a sample of downhole fluid drawn from a formation surrounding a well, comprising:

measuring at least one characteristic of downhole fluid indicative of water-based mud filtrate contamination levels in downhole fluid drawn from a formation surrounding the well over a period of time; and

using said measurements to determine when to collect a sample of said downhole fluid.

17. A method according to claim 16, wherein said characteristic is optical density, fluorescence emission, conductivity, resistivity, ion concentration, or relative ion concentration.

18. A method according to claim 16, wherein said water-based mud filtrate contains a water-soluble dye.