Formation testing and sampling are essential for effective reservoir management and production optimisation. Advanced technologies, such as the evolution of real-time optical fluid analysers have significantly improved the accuracy and efficiency of these processes by addressing the key challenges of contamination, environmental conditions, and effective monitoring of clean-up. These advancements enable better decision-making and more effective operations.
In this weeks Tool Tuesday we take a closer look at what downhole measurements can be made to identify fluid type and characteristics whilst pumping from the formation, plus a review of some of the key technologies available.
What is downhole fluid analysis?
Generally, the primary objectives of formation testing and sampling include:
1. Pressure Measurement: Determining formation pressure to assess reservoir properties and drive mechanisms.
2. Fluid Sampling: Collecting representative fluid samples for laboratory analysis to understand fluid composition, PVT properties, and potential production issues.
3. In-Situ Fluid Analysis: Performing real-time fluid analysis to obtain immediate data on fluid properties such as gas/oil ratio (GOR), fluid density, viscosity, and contamination levels
Downhole fluid analysis relates to point 3 above and describes the downhole assessment of formation fluid to evaluate the properties and behaviour of those fluids within a controlled enviornmnet (typically a flowline within a wireline (or LWD) tool.
What are the measurements made downhole which can provide downhole fluid analysis?
Various sensors are positioned in sections of the wireline toolstring which can deliver some or all of the following real-time measurement to surface:
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Pressure and Temperature Measurements: Basic but essential measurements that form the foundation for other fluid property evaluations. These are typically captured using standard downhole tools;
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Fluid Resistivity and Capacitance Sensors: Primarily used for distinguishing between water and hydrocarbons. These sensors measure the resistivity and capacitance of the fluid to identify its type;
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Fluid Analysers: These tools use near-infrared (NIR) and visible spectroscopy to analyse fluid composition. They can distinguish between water, oil, and gas based on their unique absorption spectra;
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i) Absorption Spectrometers: Measure the absorption of light across the fluid, helping identify and quantify different hydrocarbons and contaminants like oil-based mud (OBM) filtrate;
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ii) Refraction Spectrometers: Sometimes referred to as gas detectors, these measure the refraction of light to identify gas or liquid phases;
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Density and Viscosity Sensors: These measure fluid density and viscosity in real-time. The most common sensor is the vibrating rod or tube, which measures changes in vibration frequency and damping to determine these properties. These measurements are critical for evaluating fluid mobility and producibility;
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Fluorescence Sensors: Used to detect the presence of hydrocarbons and identify phase separations. These sensors measure the intensity of fluorescent light emitted by the fluid, which helps in identifying fluid types and monitoring phase behavior during sampling;
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Sound Speed Sensors: Measure the speed of sound in the fluid, which correlates with fluid composition, especially the gas-oil ratio (GOR). This method provides qualitative and quantitative data about fluid.
How does a spectrometer work?
A spectrometer is an essential module in most modern wireline sampling devices. It is an analytical instrument used to measure and analyze the properties of light. A spectrometer in its simplest form works by separating light into its component wavelengths (spectrum) and measuring the intensity of each wavelength. This allows for the identification and quantification of different substances (fluids) based on their light absorption, emission, or reflection characteristics (ref)
Light Source: The spectrometer begins with a light source that emits a broad spectrum of light. For visible to near-infrared (VIS/NIR) spectrometers, a tungsten halogen lamp is commonly used.
Sample Interaction: The light from the source passes through or reflects off a sample. As it interacts with the sample, specific wavelengths of light are absorbed or emitted depending on the sample’s properties.
Optical Components: The light then enters the spectrometer’s optical system, which typically includes lenses, mirrors, and optical fibers to direct the light towards a dispersive element.
Dispersive Element: This element (such as a prism or diffraction grating) separates the light into its component wavelengths. In some designs, bandpass filters are used to select specific wavelength ranges.
Detector: The separated light is detected by an array of photodetectors, each sensitive to a specific range of wavelengths. Common detectors include silicon photodiodes or InGaAs (Indium Gallium Arsenide) detectors for NIR spectroscopy.
Data Processing: The detector signals are processed to produce a spectrum, a plot of light intensity versus wavelength. This spectrum can be analyzed to determine the composition and concentration of substances in the sample.
What are the main challenges you need to consider for downhole fluid analysis operations?
Whilst recognising that there are multiple variables which need to be taken into account when design an optimised wireline sampling string (probe inlet type, pump type, sample receivers etc), there are a number of specific challenges which impact the effectiveness of downhole fluid analysis specfically:
1. Contamination from Drilling Fluids:
– Mud filtrate contamination can significantly affect the representativeness of the fluid analysis. Differentiating between formation fluids and drilling mud filtrate can be challenging when the absorption characterics differntial between fluids and filtrate is small (ref, ref);
2. High-Pressure and High-Temperature Conditions:
– Formation environments often have extreme pressures and temperatures that can affect the accuracy and reliability of measurements and the integrity of the sampling tools. Ruggedized equipment is required to withstand these harsh conditions (ref, ref);
3. Phase Changes During Sampling:
– Ensuring that fluid samples remain in a single phase (liquid or gas) throughout the sampling and retrieval process is critical. Phase changes can lead to inaccurate characterization of fluid properties, especially for hydrocarbons prone to gas-liquid phase separation (ref, ref);
4. Real-Time Data Acquisition:
– Traditional methods often require samples to be brought to the surface and analyzed in laboratories, causing delays. Real-time data acquisition is crucial for immediate decision-making but requires advanced downhole fluid analysis tools that can provide accurate and timely information (ref, ref);
5. Representativeness of Samples:
– Obtaining truly representative samples of the in-situ formation fluids is challenging. This involves maintaining the original pressure, temperature, and chemical composition during sampling and retrieval to the surface (ref, ref);
7. Detection and Quantification of Contaminants:
– Accurate detection and quantification of contaminants like OBM filtrate or other non-reservoir fluids require sophisticated analytical techniques. This is essential to determine when the fluid sample is clean enough to be considered representative (ref);
8. Scaling and Precipitation:
– Formation water samples can precipitate solids like scales when there is a change in pressure and temperature during sampling and retrieval. This can affect the flow and quality of the samples and complicate the analysis (ref);
9. Optical and Electronic Sensor Limitations:
– Sensors used for downhole fluid analysis, such as optical and resistivity sensors, can have limitations in terms of resolution, accuracy, and susceptibility to environmental conditions. Improving sensor technology and calibration is an ongoing challenge(ref);
10. Integration of Multiple Measurements:
-Combining data from various sensors (e.g., optical, resistivity, density) to get a comprehensive understanding of the fluid properties can be complex. Ensuring that all measurements are accurately synchronized and interpreted is crucial for reliable fluid analysis(ref);
-Sampling Program Optimisation: The need to optimize sampling programs to obtain the most representative samples with minimal contamination and in a cost-effective manner. Real-time analysis tools help in dynamically adjusting the sampling strategy based on the immediate data, improving the efficiency and quality of the sampling process.
What technologies are out there from suppliers and are they all the same?
Baker Hughes – SampleView
Functionality:
– Integrated within the Reservoir Characterization Instrument (RCI) system.
– Provides real-time downhole fluid characterization and fluid sampling efficiency.
Key Features:
– Real-Time Optical Spectra: Near-infrared (NIR) analysis to monitor fluid contamination.
– 17 Optical Channels: Tracks fluid transition from filtrate to formation fluid.
– Chemo-metric Modeling: Enhanced resolution for methane detection and gas/oil ratio (GOR) estimation.
– Continuous Refractometer and Fluorescence Spectrometer: Differentiates gas, water, and oil; monitors contamination levels.
Measurements:
– Near-Infrared Spectra: Primary measurement for contamination.
– Refractive Index and Fluorescence Spectra: Identify fluid types and hydrocarbon characteristics.
– Methane Channels: Detect methane for GOR estimation.
– Fluid Density and Viscosity, Acoustic Transducer: Measure physical properties and sound speed.
– In-Situ Phase Separation: Real-time bubble-point pressure and compressibility tests.
Versions:
– IB Version: Basic functionalities.
– IC Version: Enhanced with additional sensors for density, viscosity, and sound speed, suited for higher temperature and pressure environments.
Halliburton – ICE Core
Reservoir Xaminer™ Formation Testing Service (halliburton.com)
Functionality:
– Optical sensor integrated into a wireline formation tester for real-time fluid analysis.
– Uses multivariate optical computing (MOC) technology.
Key Features:
– Optical Sensor System: Broadband light source (400-5000 nm) analyzes formation fluids.
– ICE Core Technology: Specific optical elements detect various fluid properties.
– Sensor Carousel: Rotating system for sequential fluid component analysis.
– Real-Time Data Display: Immediate information on fluid composition.
Measurements:
– Methane, Ethane, Saturates, Aromatics Detection: Specific fluid component analysis.
– Validation and Consistency: Cross-references with laboratory results and independent sensors for reliability.
SLB – LFA (Live Fluid Analyzer):
– Part of the Modular Formation Dynamics Tester (MDT).
– Uses visible and near-infrared (Vis-NIR) spectroscopy for fluid analysis.
Key Features:
– Methane Detection: Identifies and measures dissolved methane.
– Contamination Monitoring: Quantitative OBM filtrate contamination monitoring.
– Free Gas Detection: Identifies free gas for accurate sampling.
– Real-Time Data Analysis: Facilitates informed decision-making during sampling.
Measurements:
– Contamination Levels, Fluid Identification, Methane Content: Monitors and differentiates fluid types.
– Phase Changes, Fluid Composition, Free Gas: Ensures accurate downhole conditions.
SLB – InSitu Analyzer:
– Advanced version of LFA with enhanced accuracy and broader measurement range.
Key Features:
– Dual-Spectrometer System: High-resolution near-infrared (1600-1800 nm) and broader wavelength range (400-2100 nm).
– Real-Time Calibration and Algorithms: Ensures precise fluid component quantification.
– Integrated Sensors: Measures density, resistivity, fluorescence, and pH under flowing conditions.
Measurements:
– Hydrocarbon Composition, GOR, Live-Oil Density: Detailed hydrocarbon analysis.
– CO2 Content, pH of Water, Reservoir Fluid Color: Additional fluid properties.
– Free-Gas Detection, Downhole Fluorescence, Flowline Pressure and Temperature, Resistivity of Reservoir Water: Comprehensive fluid profile.
Benefits Over LFA:
– Enhanced Accuracy: Dual-spectrometer system provides higher resolution measurements.
– Comprehensive Analysis: Broader range of measurements including fluorescence and phase behavior analysis.
– Real-Time Data: Provides more detailed insights for informed decision-making.
Technical Advisory
To maximize the effectiveness of downhole fluid analysis operations, operators should consider the following:
Reservoir Conditions
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Pressure and Temperature: The tools must withstand the high-pressure and high-temperature conditions typical of many reservoirs. Ruggedized equipment is necessary for reliable operation under these conditions .
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Fluid Type: Different tools are better suited for different fluid types, such as oil, gas, or water. For example, tools that can handle highly volatile fluids or differentiate between oil and water in OBM environments are essential.
2. Contamination Levels
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Mud Filtrate Contamination: Technologies that can accurately monitor and reduce contamination from drilling mud are crucial. Tools like focused sampling probes and optical spectrometers help in achieving low contamination levels .
3. Real-Time Data Acquisition
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Immediate Analysis: The ability to perform real-time downhole fluid analysis allows for immediate decision-making during drilling and sampling operations. Tools equipped with optical fluid analyzers or other real-time sensors provide this capability .
4. Measurement Capabilities
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Comprehensive Data Collection: Tools that can measure a wide range of properties, such as pressure, temperature, fluid resistivity, density, viscosity, and optical characteristics, are preferred. This allows for a more thorough analysis of the formation fluids and downhole conditions.
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Advanced Optical Sensors: Technologies like VIS/NIR spectrometers and integrated computational elements (ICE) provide detailed compositional analysis and are essential for accurate fluid characterization .
5. Operational Flexibility
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Tool Integration: The ability to integrate multiple tools and sensors into a single logging string enhances the data collection process. Modular tools that allow for easy addition or removal of sensors based on specific requirements are advantageous .
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Sampling Program Optimization: Real-time data allows for dynamic adjustment of the sampling strategy to optimize the collection of representative samples .
6. Environmental and Geographical Factors
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Reservoir Heterogeneity: In reservoirs with significant heterogeneity, tools that can provide detailed fluid property variations and monitor compartmentalization are necessary. This helps in understanding reservoir connectivity and planning optimal production strategies .
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Geological Context: The integration of formation testing data with geological and petrophysical information is crucial for accurate reservoir characterization. Technologies that facilitate this integration are beneficial.
7. Technological Advancements
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Sensor Accuracy and Resolution: High-resolution sensors that provide accurate and reliable measurements are essential. For example, advancements in spectrometer technology have significantly improved the accuracy of downhole fluid analysis
More generally the following has proven to be effective in ensuring DFA operations are efficient and effective:
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Pre-Job Planning and Modeling: Conduct extensive planning and modeling to determine the optimal tool configuration and sampling strategy based on reservoir characteristics. Pump size/speed, probe selection and the sensors available for fluid analysis will all have a direct impact on success;
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Real-Time Monitoring: Utilise vendor modelling tools that provide real-time data on fluid properties and contamination levels to make informed decisions during the sampling process;
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Maintain Reservoir Conditions: Ensure that samples are maintained at reservoir pressure and temperature conditions throughout the sampling and transportation process to preserve their integrity;
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Quality Control: Implement robust quality control measures, including real-time baseline correction and the use of advanced sensors, to ensure the accuracy and reliability of the collected data.
one&zero are uniquely position in the industry having dedicated formation testing domain experience fro within each of the three the main wireline vendors (Halliburton, Baker Hughes and SLB). Hence if you require truly independent planning advice, real-time operations support or post well data intepretation/studies contact us directly – [email protected]
