Schlumberger

Technical Paper: Multichannel Oil-Base Mud Contamination Monitoring Using Downhole Optical Spectrometer

Society: SPWLA
Paper Number: SPWLA_2008_ultichannel_oil-base_mud
Presentation Date: 2008
 

Abstract

Contamination of formation fluid is caused by the filtrate of drilling mud that invades the formation surrounding the borehole during and after drilling. Consequently, a fluid sample taken by a downhole formation tester tool is inevitably a mixture of the mud filtrate and virgin fluid. Contamination can largely reduce the sample quality and make the subsequent pressure/volume/temperature (PVT) analysis unreliable. The objective of real-time contamination monitoring is to continuously analyze the fluid pumped from the formation through the flowline using a downhole optical spectrometer until an acceptable contamination level is measured and then to capture a sample of the fluid.

In this paper, we describe a new multichannel oil-base mud (OBM) contamination monitoring technique using optical density measurements recorded at different wavelength channels by an optical spectrometer. Compared to the conventional single-channel approach, the multichannel method has several advantages. First, it can produce a consistent contamination estimate among all the channels of the spectrometer. Second, rather than assuming that the filtrate optical density is zero, the multichannel approach computes the filtrate optical density by exploiting the linear relationship among all the channels. Third, it provides a confidence measure that quantifies the statistical variation of the contamination estimate among all the channels. Finally, the multichannel approach can also estimate the spectra of filtrate and formation oil.

To demonstrate the capability of the new method, we apply it to field data from recent field tests and compare the results with the contamination estimate obtained from the laboratory PVT analysis of captured samples. We find very good agreement between the contamination estimates of the multichannel approach and the PVT analysis.

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