Patent EA-005261-B1 (EA)
Title: Method and apparatus for downhole fluids analysis
Inventor: MULLINS OLIVER C
IPC: G01N33/24
Filed: 2001-09-12
Abstract: 1. A method of determining gas-oil ratio of a formation fluid comprising: a) subjecting the fluid to spectroscopic analysis at least two wavelengths, one of which is sensitive to the presence of gas and the other of which is sensitive to the presence of oil and generating response data; b) determining a response matrix from the response of gas at the two wavelengths and the response of oil at the two wavelengths; c) determining a signal response vector at the two wavelengths; d) calculating a mass fraction vector of a gas-oil mixture according to the relationship and e) determining the gas-oil ratio from the mass fraction vector. 2. A method as claimed in claim 1, wherein the response matrix comprises a first column comprising the spectroscopic response of gas at each of the two wavelengths, and a second column comprising the spectroscopic response of oil at each of the two wavelengths. 3. A method as claimed in claim 2, further comprising solving the equation to derive mass fractions of gas m1 and oil m2 according to m1=D1/D and m2=D2/D; wherein D is the determinant of m1=D1/D and m2=D2/D , D1 is the determinant obtained from the response matrix with the first column replaced by , and D2 is the determinant of the response matrix with the second column replaced by . 4. A method as claimed in claim 1, wherein the gas-oil ratio is determined according to the relationship GOR=c1(m1/(m2-c2m1)), wherein c1 and c2 are constants, m1 is the mass fraction of gas and m2 is the mass fraction of oil. 5. A method as claimed in claim 2, wherein the response matrix is derived from a series of measurements made on synthetic mixtures of gas and a hydrocarbon. 6. A method as claimed in claim 5, wherein a correction factor is applied to the response matrix so derived when applying it to measurements from real formation fluids. 7. A method as claimed in claim 1, wherein one of the wavelengths is approximately 1671 nm and the other is approximately 1725 nm. 8. A method as claimed in claim 1, wherein the spectroscopic analysis is performed in a predetermined spectroscopy apparatus, the method comprising determining a response matrix for that predetermined spectroscopy apparatus. 9. A method as claimed in claim 2, wherein the elements of the matrix comprise the slopes of the integrated and average spectrometer OD values obtained over specified wavelength windows for gas and oil. 10. A method as claimed in claim 1, wherein the spectroscopic analysis of the fluid is performed in a tool located in a borehole and in communication with a formation from which the fluid is obtained. 11. Apparatus for determining gas-oil ratio of a fluid obtained from a formation surrounding a borehole comprising: a) a tool body which can be located in the borehole and establish fluid communication with the formation so as to withdraw a sample of fluid therefrom; b) a spectroscopy module located in the tool body for subjecting the fluid sample to spectroscopic analysis at least two wavelengths, one of which is sensitive to the presence of gas and the other of which is sensitive to the presence of oil and generating response data; c) means for determining the gas-oil ratio of the sample which calculates a mass fraction vector of a gas-oil mixture according to the relationship , wherein is a signal response vector at the two wavelengths, is a response matrix formed from the response of gas at the two wavelengths and the response of oil at the two wavelengths; and determines the gas-oil ratio from the mass fraction vector . 12. Apparatus as claimed in claim 11, wherein the spectroscopy module includes a broadband light source for illuminating the fluid sample and detectors which include bandpass filters with pass bands including one or other of the two wavelengths. 13. Apparatus as claimed in claim 12, wherein the pass band including the wavelength responsive to gas includes 1671 nm, and the pass band including the wavelength responsive to oil includes 1725 nm. 14. Apparatus as claimed in claim 13, wherein the pass band including 1671 nm wavelength is defined by a filter having a pass band of about 1660 nm to about 1675 nm. 15. Apparatus as claimed in claim 13, wherein the pass band including 1671 nm wavelength is defined by a filter having pass band of about 1640 nm to about 1675 nm. 16. Apparatus as claimed in claim 13, wherein the pass band including 1671 nm wavelength also includes 1650 nm. 17. Apparatus as claimed in claim 13, wherein the pass band including 1725 nm wavelength is defined by a filter having a pass band of about 1715 nm to about 1730 nm.