Method and apparatus for estimating of fluid contamination downhole

Abstract

The present invention provides method and apparatus for quantifying sample clean up in real time by providing curve-fitting measurements of optical or other physical properties of fluid downhole. Fluid is extracted from the formation surrounding a borehole. As fluid continues to be extracted the composition of the extracted fluid changes, altering the measured values of optical and physical properties of the fluid. Measurements are made of optical or physical properties of the sampled fluid, analysis is performed on the acquired measured data points.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The invention relates generally to a method and apparatus for quantifying fluid contamination as an indication of sample cleanup in real time in a wellbore environment. Specifically, the invention is a method and apparatus for measurement of physical properties of fluid being pumped from a formation surrounding a wellbore by a wireline or monitoring while drilling tool to estimate sample cleanup or to predict the time at which a sample having a desired purity can be obtained. [0003] 2. Summary of the Related Art [0004] In wellbore exploration, typically drilling muds such as oil-based muds and synthetic-based muds or water-based muds are used. The filtrates from these muds generally invade the formation through the borehole wall to an extent, meaning that this filtrate must be removed from the formation in order to access the formation fluids. Open hole sampling is an effective way to acquire representative reservoi...

AI Technical Summary

Benefits of technology

[0010] In a first aspect of the present invention, a method and apparatus are provided that fit fluid measurement data to a non-asymptotic curve. One example of a non-asymptotic curve is a curve (e.g., a power series approximation), which provides an improved fit to the data over the typical pumping time and, which can also be successfully extrapolated to several times that pumping time, but which approaches plus or minus infinity at infinite times. Another example of a non-asymptotic curve is an equation that has an oscillatory component such as a sine wave, which never reaches a fixed limit. The sine wave can be adjusted in frequency, phase and amplitude to provide an improved fit to pulses in the monitored response that are associated with each stroke of the pump. In a third aspect of the invention, a method and apparatus are provided that perform pattern recognition of a straight line to a best fit of the measured data in log-log space.

[0012] The method and apparatus of the present invention can use a data-fitting equation such as log(1-ftp)=(−p)log(t)+log(A1 / A0), which is the equation of a straight line that has no (Y=constant) asymptote, except for the meaningless case of p=0. The method and apparatus perform a series of regressions using different estimates of A0 but do not actually calculate A0, itself. For example, one can start with the current value, A, as the first estimate of A0, then proceed to a slightly higher value of A+ε, then to an even higher value of A+2ε, and so on. The A0 value for which the fit to the measured data is closest to the shape of a straight line (based on the highest coefficient of determination, or R-squared value) then becomes the best estimate of an A0 value. In a third aspect the method and apparatus of the present invention a method and apparatus are provided that fit a differentiable curve to measurement data or physical property data derived from the measurement data. The present invention then estimates ftp from the ratio of (dA / dt) to A. In a fifth aspect of the present invention a method and apparatus are provided that fit an asymptotic curve to difference of two responses such as the difference of two absorbances associated with different wavelengths (optical channels) rather than to an absorbance itself. Using an absorbance difference removes the baseline offsets caused by passing sand particles or bubbles.

Problems solved by technology

Measuring these properties of the fluid therefore provides qualitative insight into a fluid sample's purity but does not provide a quantitative value, fp, for the fluid sample's purity.

Thus, even though a downhole measured property (OD, etc.) has substantially stopped changing, the sample is still not at 100% purity.

On the other hand, if by pumping for a very long time, it is possible to achieve only slightly better contamination level, an operator ends up wasting very expensive rig time and also risks the very costly possibility of allowing a tool to become stuck in the wellbore.

Clearly, an assumption of a sample clean rate of t−5 / 12 can be rigid and inapplicable to real time situations.

Another problem with monitoring sample clean up over time by looking at optical absorption over time is that sand particles and other particulates can cause considerable scattering, which causes the absorption values measured over time to “jump” and appear noisy.

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