Lfa Brochure

LFA Live Fluid Analyzer

Confidence in sampling

Applications■ Downhole fluid identification

■ Fluid sampling

■ Oil and gas condensate sampling in oil-based mud(OBM) environments

Benefits■ Monitors OBM filtrate

contamination for fluid sampling

■ Allows informed decision-making for timely sampleacquisition

■ Increases and optimizes sampling efficiency

■ Assures single-phase fluidsampling

Features■ Independent, real-time,

quantitative contaminationmonitoring using color andmethane content

■ Predictable cleanup periodfor quality sample collection

■ Reliable discriminationbetween water, oil and gas

■ Differentiation between various reservoir hydrocarbons

■ Dependable identification of free gas using two inde-pendent detectors

■ Timely detection of phasechange onset

LFA overviewSince 1991, Schlumberger has usedoptics for downhole fluid identification inthe MDT* Modular Formation DynamicsTester. The LFA* Live Fluid Analyzer is anew MDT module that utilizes new down-hole optical techniques to analyze fluidsas they flow through the MDT tool. Theterm “live fluid” is commonly used to referto pressurized reservoir fluid samples thatremain in single phase. The LFA fluidanalyzer builds on and improves existingoptical fluid analysis with its unique abilityto detect and measure dissolved methanein live fluids. Oils of different types can bedifferentiated based on both their methanecontent and color.

Importance of samples Reservoir fluid samples are normallyevaluated in the laboratory to measuretheir physical and chemical properties.The accurate determination of theseproperties is critical, not only to charac-terize and produce a reservoir, but also to design well completions, subsea tie-backs and topside facilities. Error in thesemeasurements can be significant evenwith relatively small levels of misciblecontamination. To acquire a representa-tive downhole fluid sample, the unwanteddrilling fluids that have invaded the

formation have to be removed by extract-ing fluid until the level of contaminant isacceptable. Then the fluid sample can beacquired.

Today, many wells are drilled with oil-based muds (OBM), which are misciblewith formation hydrocarbons. The use of this type of OBM, or synthetic OBM,creates the need to quantify and monitorthe level of oil-based filtrate that is mixedwith the reservoir hydrocarbons as itflows through the MDT tool. The abilityto analyze and monitor the flowing fluidallows real-time decisions to be madeduring the operation.

Optics and reservoir fluidsThe LFA module employs an absorptionspectrometer that utilizes visible and nearinfrared light to quantify the amount ofreservoir and drilling fluids that are in theflowline. Light is transmitted through thefluid and measured as it flows past theLFA spectrometer. The amount of lightabsorbed by the fluid depends on the com-position of the fluid. Water and oil arereliably detected by their unique absorp-tion spectra. A second sensor in the LFAmodule is the gas refractometer, whichcan be used to differentiate between gasand liquid.

Gas detector Lamp

Flowline

Liquid detector

Oil Gas

Fluid flow

The LFA spectrometer uses light in the visible and near infrared range to characterize the fluid flowing through the flowline. The refractometer provides discrimination between the liquid phase and the gas phase.

OBM filtrate and sample contamination Since methane is present in all naturallyoccurring hydrocarbons and is absentin drilling fluids, any methane detectedis due to reservoir hydrocarbons andnot filtrate. By detecting the methane,the LFA module can ensure that a qualitysample can be captured once the oil-based filtrate contamination is reducedto a level specified by the customer. As a result, high-quality fluid samples,which meet customer requirements, areacquired efficiently and cost effectively.

Bubblepoint and samplingQuantitative monitoring of OBM filtratecontamination is not sufficient to ensurethat the sample will be valid. During thesampling process, care must be taken to ensure the flowing pressure remainsabove the bubblepoint. Initially, the fluidflowing from the reservoir will be largelycontaminated with OBM filtrate, whichwill tend to lower the bubblepoint of themixed fluid. As the flow from the reser-voir continues, the contamination levelof the produced fluid is reduced, and thebubblepoint will increase to that of thevirgin reservoir fluid. If the bubblepointunknowingly rises above the flowingpressure, the sample collected will notbe representative of the reservoir fluid.The LFA module includes a sensitiveand reliable gas detection system thatmonitors the fluid as it flows from thereservoir. This ensures that a single-phasefluid sample is captured by adjusting theflowing pressure as needed.

The LFA gas detector, in combinationwith the methane measurement capabil-ity of the LFA module, is a unique systemthat can determine if the bubblepoint ofthe produced fluid rises above the flow-ing pressure. The methane measurementresponds to the entire fluid volumewithin the MDT flowline and improvesthe sensitivity to free gas.

Pressure

Time

Capture sampleFlowing pressure

Bubblepoint offlowing fluid

Increase flowingpressure

LFA module detectsfree gas (bubblepoint)

50

40

30

20

10

0

ContaminationOptical density

Time

Contamination(%)

1.0

0.5

0

Opticaldensity

Valid samplecan be acquired

Acceptablecontamination level

s fluid flows from the formation, the miscible oil-based mud filtrate contamination is reduced. The LFA module can monitor current contamination levels and predict when a specified acceptablelevel of contamination can be achieved.

The increased sensitivity of the LFA module to free gas ensures that a single-phase sample is collected downhole.

Oil samples from around the world have been analyzed by the LFA module.

Principles of methane measurementAt certain wavelengths of near-infraredlight, the molecular bonds that are specif-ically associated with a hydrocarbonfluid will vibrate. This vibration resultsin an absorption of light. Measuring thelight absorption at these wavelengthscan identify a fluid as a hydrocarbon.

The dead oil spectrum (black) at theright, which is hydrocarbon with nomethane, has its strongest absorptionpeak at 1720 nm. This wavelength isspecific to carbon compounds with twohydrogen atoms for every carbon atom.The methane spectrum (red) has itsstrongest absorption peak at 1670 nm.This wavelength is specific to methane,which is the only molecule with fourhydrogen atoms for each carbon atom.The live oil spectrum (green), which is amixture of methane with dead oil, exhi-bits absorption peaks at both 1670 nmand 1720 nm.

The optical sensor in the LFA tool usesthis principle to measure the absorptionof light at both 1670 nm and 1720 nm.The resulting graph accurately identifieslive oil for quality sampling.

Opticaldensity

Wavelength (nm)

0.8

0.6

0.4

0.2

01600 1650 1700 1750 1800 1850 1900 1950

Live crude oilMethaneDead oil

The graph below illustrate the optical density (OD) of methane, dead oil and live crude oil. An OD of 0 meansthere is full transmission (no absorption) of light. An OD of 1 means that 10% of the light is transmitted, and90% is absorbed. Methane and dead oil peaks are prominently shown in the live crude oil spectrum.

Opticaldensity

Wavelength (nm)

500 1000 1500 2000

4

3

2

1

0

Crude B

Oil-basemud filtrate

Diesel

Condensate

Crude A

Water

Optical density spectra can be used to uniquely identify different fluids.

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