New LWD Service Integrates Formation Fluid Analysis and Sampling
Determining the fluid properties of a reservoir by using pressure/volume/temperature (PVT) analysis is essential to petroleum reservoir studies, production equipment design, and reservoir recovery efficiency estimation. The properties of the formation fluid are used to determine reserves and to predict reservoir performance and economics. PVT properties such as bubblepoint pressure, gas/oil ratio, viscosity, oil formation volume factor, and detailed composition are important to well performance analysis, material balance calculations, reservoir simulation, and production engineering calculations.
Once the reservoir information is available, the well team makes the critical field development decisions. Conventional post-well wireline formation testing operations can delay decision making for days, sometimes months, depending upon the logistics involved in transporting a sample from the wellsite to a PVT laboratory. Additionally, wireline deployment is expensive in horizontal and highly deviated wells because of the extra time and equipment required to convey the tools to the test intervals.
The latest logging-while-drilling (LWD) technology integrates downhole fluid analysis and sampling with formation pressure while drilling (FPWD), thereby providing environmental, economic, time-saving, and data quality benefits over traditional methods of reservoir characterization. The integration enables three distinct services: real-time formation pressure tests, real-time in-situ measurements of fluid properties, and downhole capture and retrieval to the surface of fluid samples.
The real-time formation pressure testing provides important information on fluid dynamics within the reservoir, mobility measurements, and zone productivity predictions. It is also important for accurate gradient analysis. Measuring multiple formation pressures at different depths delivers a formation fluid gradient that makes it possible to find contact points between different formation fluids such as water, gas, and oil. Pressure testing in an LWD environment also provides important information for safety and drilling optimization, including data that are valuable for controlling hydraulic overbalance and equivalent circulating density.
Representative fluid samples provide information on the production potential of the reservoir. LWD fluid analysis and sampling enables fluids samples to be collected closer to in-situ conditions for a more accurate determination of fluid composition. Additionally, sample integrity can be monitored continually from the first time the sample enters the LWD tool until it is transferred to the laboratory for detailed analysis. Greater accuracy improves project cycle times and reduces development risks.
Although the vital information provided by the integrated LWD service is useful throughout the life cycle of a reservoir, it is particularly valuable during the initial assessment to determine the commercial potential of a project. The assessment includes estimates for producibility, fluid type and composition, fluid phase behavior, production facility design, and flow assurance.
Baker Hughes’ FASTrak LWD fluid analysis and sampling service can collect an unlimited number of pressure and fluid analysis tests, and capture and recover up to 16 fluid samples under PVT conditions. The service is based on the reservoir characterization instrument and offers automatic sequencing during pressure testing, pump-through operations, and sampling.
The tool consists of four modules: the power module, the pump and analyzer section, the tank module, and the termination sub. The power module is a dedicated mud turbine-alternator system that provides the power for extending and retracting the sealing element; opening and closing the valves within the modules; and operating the fluid identification sensors. It also provides power to the drawdown pump during pressure testing and cleanup operations.
The pump and analyzer section is the heart of the tool. It contains the pad-sealing element, the quartz pressure gauge, the sample pressure strain gauge, multiple temperature sensors, the drawdown pump, and the fluid analyzer. The pad and pump design is similar to that of the field-proven TesTrak FPWD service. Filters within the flowline and pad prevent plugging. The pad-sealing element is exchangeable with different probe diameters for varying formation mobilities. A high-temperature pad for temperatures above 275˚F (135°C) is also available. The quartz pressure gauge is the same as in FPWD tools. Strain gauges monitor the tank-filling pressure during each sampling process. Multiple temperature sensors in the flowline and on the pressure sensor electronics monitor tool and formation fluid temperatures. The drawdown pump is designed for, and operated up to, 8,700 psi (600 bar) differential pressure at rates of up to 25 cm3/s for maximum sampling efficiency.
The fluid analyzer contains four sensors: the piezoelectric tuning fork, the acoustic transducer, the temperature sensor, and the refractometer. They measure fluid properties to help distinguish mud filtrate from formation fluid and enhance the understanding of fluid type, contamination level, and composition. Density and viscosity are measured using the tuning fork. The high-frequency acoustic transducer makes sound speed measurements by measuring the response of the acoustic travel time through the fluid. The high-resolution response makes it possible to detect even very small changes in contamination. The refractometer, which is highly sensitive to changes in water salinity, is used for optical analysis of the downhole fluid.
Each tank module is capable of carrying up to four tanks. The tool can carry four tank modules, providing the capability for retrieving up to 16 single-phase fluid samples per run. The termination sub provides an exit to the wellbore for the contaminated fluids while the system is cleaning, before taking the fluid sample.
The FASTrak service operates like other formation testing services. Operations begin by positioning the tool within the formation of interest. Annular borehole pressure is measured before the pad or packer extends from the tool and seals itself against the borehole wall. The packer isolates the hydrostatic pressure in the borehole from the tool’s internal test and measurement system. Initially, a small quantity of fluid is drawn into the tool to confirm that a packer-to-formation seal has been achieved. The formation pressure is then recorded. As fluid flows into the tool, the pressure increases to a final stabilized value. The pressure buildup profile provides information about the local reservoir mobility. A dual-action piston pump draws formation fluid in repeat drawdowns and discharges it either to the wellbore or into one of the sample chambers. When the mud-filtrate contamination is at a minimum level, the sample can then be directed into one of the sample tanks for future analysis.
Operational efficiency and test accuracy are enhanced by two continuous control closed loop systems. The SmarTest system enables fluid pressure and mobility testing to be performed automatically and the test parameters to be monitored by the engineer at the surface. The system then selects the optimum pretest and prepares to take a sample on the command of the engineer. The SmartPad system automatically maintains seal integrity throughout the operation.
Although still undergoing field testing, the new LWD fluid and pressure testing service is building a track record worldwide.
The service provided accurate pressure testing and gradient analysis in a highly deviated, unstable formation in the Netherlands. The project set an area record by acquiring more samples in a single run than other comparable services. Twenty-five pressure tests were performed and three gradients were identified, targeting four zones of interest for sample acquisition. Twelve single-phase samples were acquired—six oil, three water, and three gas—with 100% sealing efficiency. This test illustrated how the system mitigated risk while improving drilling safety and efficiency.
In Australia, the service acquired 25 formation fluid samples and a range of pressure tests in challenging, near horizontal wellbores. The high number of samples acquired and excellent operational efficiency enabled the operator to make critical field development decisions while saving valuable rig days.
In an oil-bearing reservoir in the deepwater Gulf of Mexico, the LWD service obtained three single-phase samples with less than 5% contamination and identified a disconnect that, when verified, confirmed a fault that was sealing within the reservoir.
Numerous jobs are scheduled for Europe, Latin America, and the Asia Pacific region.
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