4 Heavy oil 3 Medium oil 2 Condensate 1
OBM filtrate
0 500 Filter array spectrometer
> Optical density of fluids from spectroscopy measurements. The InSitu Fluid Analyzer tool incorporates two optical spectrometers: a filter array spectrometer that covers a frequency range from 400 to 2,100 nm and a grating spectrometer that focuses on a narrow range of 1,600 to 1,800 nm where reservoir fluids have characteristic absorptions that reflect their molecular structures. The frequency of visible light is about 500 nm, and NIR light ranges from 750 to 2,500 nm. Oilfield fluids have specific spectral optical density (OD) characteristics that are functions of the frequency of light passing through them. Visible (Vis) light is best suited for distinguishing relative asphaltene content. The NIR spectrum is useful for water detection, distinguishing water from oil and identifying the type of oil. Optical spectroscopy was originally introduced to determine sample quality, especially the transition from OBM filtrate to reservoir fluids during sampling. OBM filtrates do not contain asphaltenes or significant dissolved gas. Thus, OBM filtrates are differentiated from crude oil using asphaltene concentration determined from OD of visible light measurements. Dissolved gas content from NIR measurements is an additional sample quality indicator.
1,000 Wavelength, nm 1,500 Grating spectrometer 2,000 Light oil Water
and can distinguish between water, gas, crude oil and OBM filtrate (above). Introduced originally to monitor contamination, downhole spectros- copy measurements have undergone a number of advances. The current tool includes two spec- trometers—filter array and grating array. Both spectrometers share the same optical cell, but they cover different wavelength ranges and pro- vide complementary functions. Wavelengths of the 20 channels in the filter array cover the visi- ble and near-infrared spectrum (Vis-NIR) range from 400 to 2,100 nm. These channels indicate the color and molecular vibration absorptions of the fluid and show the main absorption peaks of water and CO2. The sensor also detects color change for the pH measurement. The grating spectrometer has 16 channels that focus on the NIR spectrum of 1,600 to 1,800 nm where reser- voir fluid has characteristic absorptions that reflect molecular structure. For oilfield fluids of interest, much of the information is found in the NIR spectrum.18 Color, ranging from very dark in heavy crudes
to clear or very light for gas condensates, is used to distinguish oil types. The term color should not be confused with hue, such as red, green or blue. These more exotic colors are produced when
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crude oils are observed in background light that induces some fluorescence, and light absorption creates a variety of colors. In fact, a blue crude oil has been produced for many years in the Gulf of Mexico; its blue color is due to strong fluorescence under illumination (next page, top left). Measured properly, crude oils are typically brown, and color- ation refers to degree of brown absorption. One use of coloration is to determine contami-
nation of fluid samples from OBM filtrate, which contains little to no asphaltene and thus has little color. The degree of contamination is determined by monitoring the increase in color over time while the MDT tool pumps fluid from the tested interval through the DFA module. In addition to having little color, OBM filtrate generally has negligible dissolved gas—low GOR—whereas most native oils have appreciable amounts of dissolved gas. During pumpout, sampled fluids transition from low to high GOR, indicating that the level of con- tamination decreases while the percentage of native oil increases. Useful for contamination determination, the GOR measured downhole, before temperature and pressure effects occur, is also an important in situ fluid property.
Oilfield Review Autumn 09 FluidsLab Fig.7
ORWIN09/10-FluidsLab Fig. 7
Sample contamination is only one aspect of the
optical spectroscopy measurement. Molecules interact with electromagnetic waves, such as those in the visible and NIR spectrum, as a func- tion of their complexity. Oils that are high in asphaltenes and resins are darker and more absorptive than simpler hydrocarbons. In the NIR range, light absorption excites
molecular vibration in a manner that is analo- gous to exciting other mechanical oscillators, such as a guitar string. Maximum absorption occurs at characteristic frequencies that are a function of the molecular structure of the hydro- carbon. Methane [CH4]—the simplest hydrocar- bon, with a unique hydrogen/carbon ratio—has a distinct spectral signature. Ethane is composed of two –CH3 groups (the methyl group) and has a different signature. Most hydrocarbon gases are dominated by their –CH3 chemical group. In con- trast, liquid hydrocarbons are dominated by the –CH2– chemical group (the methylene group). The spectral signal is used to differentiate meth- ane and ethane from other gases and liquids. Carbon dioxide [CO2] has its own characteristic frequency of excitation and can be identified from InSitu Fluid Analyzer data.
Oilfield Review
Optical density
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