Recent innovations in wireline fluid sampling have allowed the expedient recovery of high quality oil samples. The use of the Optical Fluid Analyzer* (OFA), a part of the Modular Formation Dynamics Tester* (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 prior to or during sampling. Clearly, there are many potential benefits from using these optical spectra whilst 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 PVT properties of those samples. These samples were selected to ensure a wide range of oil types in order 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. GOR as well as 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.
Introduction
The quality expected of a wireline fluid sample was significantly improved 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 differentiates water from oil. The OFA thus allows identification of fluids prior to taking a sample, which optimises the quality and quantity of the samples taken.
The functionality of the OFA, Fig. 1, has been described in detail in other work1. 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 sampling2,3,4. While the primary emphasis of recent research involving the OFA has been with the aim of determining downhole oil based mud contamination of oil samples2,4,5 , many researchers have noticed strong correlations between the OFA visible light range responses and laboratory derived fluid characterization, or PVT properties2,3.
The possibility to obtain primary fluid characterization results 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.
