This paper reports a new-generation wireline formation testing tool that quantitatively characterizes reservoir fluid. The tool measures:
fluid composition (C1, C2, C3–C5, C6+, and CO2); gas/oil ratio (GOR) and water fraction by VIS-NIR spectroscopy;
color, fluorescence, and reflectance;
pH if fluid is water;
density by a vibrating sensor;
flowline pressure and temperature; and
water salinity by resistivity measurement.
The first half of the paper presents a schematic description of the tool and explains the spectrometer accuracy improvement over existing commercial offerings. A grating spectrometer is the standard technology used for laboratory spectroscopy, and it was implemented for the first time for this downhole tool. The grating spectrometer has several advantages over existing commercial products for:
being able to measure semi-continuous spectrum in critical hydrocarbon peak region and
better signal baseline stability critical for accurate spectroscopy. More than 45 field test jobs were performed in the field test campaign.
The comparisons made are between measurements performed by a new wireline downhole fluid analysis (DFA) tool and PVT analysis performed in wellsites and laboratories.
The second half of the paper discusses a deepwater appraisal well case example from offshore Nigeria, West Africa. The objective of the formation testing program was to assess the hydrocarbon potential, evaluate the fluid properties and contacts, and determine the presence of any compositional grading with depth across the reservoir section. An advanced wireline formation testing tool equipped with focused sampling and conventional probes and four downhole fluid analyzers including the new-generation tool were used. Pressure pretests were performed in this appraisal well in combination with fluid scanning and fluid sampling at seven depths across the reservoir. The combination of fluid scanning across the main reservoir and real-time measurements of in-situ fluid density also enabled the operator to rapidly assess the fluid composition and locations of fluid contacts during progression of the logging operations. The application of this new sampling and downhole fluid analysis technology provided valuable information to the operator for design of the subsequent drillstem test and results comparison.
Downhole fluid analysis, which started as a means to assure quality fluid sampling for wireline formation testing more than 10 years ago, has evolved into a key technique for characterizing distribution of reservoir fluid properties and determining zonal connectivity across the reservoir. Primarily using visible-to-near-infrared (VIS/NIR) spectroscopic measurements, reflection measurements, and fluorescence measurements, DFA tools have offered increased capabilities over the years: oil/gas/water detection (Smits et al., 1995); oil-based mud (OBM) filtrate contamination quantification (Mullins et al., 2000); gas-to-oil ratio (Mullins et al., 2001; Dong et al., 2006); composition (C1, C2-C5, C6+, and CO2) (Fujisawa et al., 2003); fluorescence measurement (Betancourt et al., 2003); and pH measurement (Raghuraman et al., 2005).
A recent paper (Dong et al., 2007) showed the latest development of the new-generation DFA tool, including the theory of interpretations and test results in both laboratory and fields.