Determination of Hydrocarbon Properties by Optical Analysis During Wireline Fluid Sampling
- Alexandra Van Dusen (Schlumberger) | Stephen Williams (Norsk Hydro) | Finn Hallstein Fadnes (Norsk Hydro) | Jamie Irvine-Fortescue (Norsk Hydro)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2003
- Document Type
- Journal Paper
- 286 - 292
- 2003. Society of Petroleum Engineers
- 1.6 Drilling Operations, 5.2.2 Fluid Modeling, Equations of State, 5.2 Fluid Characterization, 1.11 Drilling Fluids and Materials, 5.6.4 Drillstem/Well Testing, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 7.5.5 Communities of Practice, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 4.3.3 Aspaltenes, 5.5.11 Formation Testing (e.g., Wireline, LWD)
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Recent innovations in wireline fluid sampling have allowed the expedient recovery of high-quality oil samples. The use of the Optical Fluid Analyzer*** (Mark of Schlumberger) (OFA), a part of the Modular Formation Dynamics Tester (Mark of Schlumberger) (MDT), to predict sample fluid type and quality has now become an essential part of the sampling process. The ability to use the optical data from the OFA in a more quantitative manner has been under development for several years. This paper demonstrates that the analysis of the optical spectra measured downhole can be used to give a quantitative indication of some hydrocarbon properties before or during sampling. Clearly, there are many potential benefits to be gained from using these optical spectra while sampling to assist in reservoir characterization and determination of oil type.
Optical data gathered while taking oil samples in 21 wells (drilled with water-based mud) offshore Norway were correlated with the pressure/volume/temperature (PVT) properties of those samples. The samples were selected to ensure a wide range of oil types to establish correlations between the fluid and optical properties. Correlations have been found between the optical response and some fluid properties, as determined from PVT laboratory measurements of the samples. Oil density, saturation pressure (pb), oil compressibility (co), formation volume factor (Bo), and gas/oil ratio (GOR) gave good correlations. Weaker correlations were found with other properties. The GOR and the optical responses could become measured variables with the possible future addition of a GOR measurement sensor to the MDT. Improved correlations are demonstrated when this feature is simulated. The results of this study show that the optical data measured during wireline fluid sampling can help determine key in-situ hydrocarbon properties.
The quality expected of a wireline fluid sample was improved significantly with the evolution of the MDT by Schlumberger in the early 1990s. An essential part of the MDT is the OFA, which distinguishes not only between liquid and gas but also between water and oil. The OFA thus allows identification of fluids before taking a sample, which optimizes the quality and quantity of the samples taken.
The functionality of the OFA (Fig. 1) has been described in detail in other work.1 The focus of this study is the use of the visible-light range in the OFA spectrometer (Fig. 2) to discern between different types of crude oils. Since the earliest days of sampling with the MDT, researchers have been investigating the possibility of using the OFA's spectrometer section for more extensive fluid characterization while sampling.2-4 While the primary emphasis of recent research involving the OFA has been the aim of determining downhole oil-based-mud contamination of oil samples,2,4,5 many researchers have noticed strong correlations between the OFA visible-light-range responses and laboratory-derived fluid characterization, or PVT properties.2,3 While quantification of PVT properties from OFA light-range responses has been preliminarily investigated elsewhere,2,3 this work extends the research significantly. The goal of this work is to systematically explore correlations between different OFA signals and PVT properties over a range of wells to demonstrate whether robust quantification of PVT properties can be made in real time during well logging.
The possibility of obtaining primary fluid-characterization results in real time offers the chance to improve the success of wireline fluid sampling. More specifically, this technique provides for more extensive reservoir characterization, optimal sample quality, more efficient sampling, earlier knowledge of reservoir parameters, and (if necessary) optimal well-test design. Some examples of this are the ability to accurately estimate saturation pressure from the optical responses, allowing optimal drawdown while sampling; analysis of fluids from multiple zones without necessarily sampling; and increased knowledge of fluids before sampling.
The results of this study demonstrate correlations between several PVT properties and OFA optical-density data from the visiblelight range. Furthermore, a broader range of properties can be ascertained in real time using the correlations among PVT properties, as demonstrated in other work.6 To verify the results, a series of successful blind tests were also conducted.
The preliminary results from the research indicate that the optical density of oil, as measured by the OFA, can be used to provide estimates of fluid properties in real time downhole. While significant work is needed to make this a commercial reality, the research results suggest an exciting advance in the information available through wireline logging.
In an attempt to determine if correlations could be established between OFA responses and fluid-characterization analyses while sampling, a database was compiled of all Norsk Hydro (North Sea) MDT sampling jobs involving the OFA over the past 7 years. This research focuses only on oil samples obtained in a water-basedmud environment. This affords us the opportunity to analyze only the crude-oil response in the absence of any oil-based mud or synthetic oil-based mud filtrate, which could potentially influence the optical response. Because some mud systems contain glycol, laboratory experiments were also done to ensure that glycol in the mud would not affect the oil response or change the color analyses. These revealed that the presence of glycol would have no effect on the oil response in either the visible- or near-infrared-light ranges.
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