LWD fluid analysis sampling, testing reduces development risk

LWD formation testing and sampling will continue to be important assets to the oil and gas industry for many decades. The vital information provided by these services is used throughout the life cycle of a reservoir, especially during the initial assessment, to determine the commercial potential of a project. This assessment includes estimates for producibility, fluid type and composition, fluid phase behavior, production facility design, and flow assurance.

These estimates are critical to the long-term success of a project because subsequent intervention or redesign could affect the project’s economic viability. The capabilities and efficiencies of LWD formation testing tools are dramatically changing the value of formation sampling and testing programs.

FIGURE 1. The LWD formation sampling and testing tool is positioned within the formation interval by the drillstring. (Images courtesy of Baker Hughes)

With an advanced LWD fluid analysis sampling and testing service it is now possible to obtain pressure measurements and capture fluid samples in environments where wireline operations may be difficult to conduct, such as in extended-reach or horizontal wells.

As real-time pressure-volume-temperature (PVT) simulation and formation sample collection is obtained closer to in situ conditions, a more accurate determination of produced fluid composition is necessary to improve project cycle times and reduce development risks. An optimized single-phase sampling program with real-time PVT predictions and real-time contamination monitoring can improve the data accuracy and quality of fluid samples collected from a reservoir.

New sensor technologies measure real-time physical properties of hydrocarbons such as density and viscosity, which can improve understanding of hydrocarbon PVT properties.

Impact on efficiency, quality

Baker Hughes’ FASTrak LWD fluid characterization service offers wellsite fluid sampling and pressure testing. The portfolio of services was designed to acquire pressure tests and representative single-phase fluid samples.

Flow-assurance designs are heavily dependent on high-quality PVT data. The quality of the data is crucial because an inaccurate gas-oil ratio (GOR) estimate from a contaminated or misrepresentative sample can lead to faulty production facility designs. In turn, this can lead to platform production problems because the facility was inadequately designed to handle the associated gas production.

FIGURE 2. The pump and analyzer section is the heart of the tool with its four sensors: the piezoelectric tuning fork, the acoustic transducer, the temperature sensor, and the refractometer.

It is well known that changes in temperature and pressure regimes affect the overall formation testing and sampling quality. Sample integrity can be monitored with the LWD formation sampling and testing tool from the first time the sample enters the tool until it is transferred to the laboratory facilities for analysis.

Detailed service description

As with any formation testing tool, the fundamentals of sampling and pressure testing remain the same. Operations commence when the LWD formation sampling and testing tool is positioned within the formation interval, as is shown in Figure 1. Annular pressure in the borehole 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 internal test and measurement system of the tool. 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 using a fast-response quartz gauge. As fluid flows into the tool, the pressure increases to a final stabilized value. The pressure buildup profile provides information about the mobility of the formation fluid. A dual-action piston pump draws formation fluid in repeat drawdowns and discharges fluid into the wellbore or into one of the tool’s sample chambers.

A variety of sensors within the fluid analyzer module also is available to measure the properties of the fluid to help distinguish mud-filtrate from formation fluid. The additional fluid compositional properties of density, viscosity, GOR, compressibility, and sound speed can be measured, enhancing understanding of the fluid type, contamination level, and composition. 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.

The tool consists of four modules (Figure 2). The power module is a mud turbine-alternator system that provides the tool string with the power to extend and retract the sealing element, open and close the valves within the modules, and operate the fluid ID sensors. The module also provides power to the drawdown pump during pressure testing and the cleanup operation.

The pump and analyzer section is the heart of the tool. It contains the pad-sealing element, quartz pressure gauge, sample pressure strain gauge, multiple temperature sensors, drawdown pump, core of the fluid analyzer are four sensors: the piezoelectric tuning fork that measures density and viscosity, the acoustic transducer, the temperature sensor, and the refractometer.

The tank module is capable of carrying up to four tanks per module. If required, four tank modules can be connected in a series, providing the capability of retrieving up to 16 single-phase fluid samples in a single run. The termination substructure provides an exit to the wellbore for the contaminated fluids while the system is cleaning and before taking the fluid sample.

Field operation summary

Since the introduction of this technology to the marketplace in 2011, more than 11 projects have been completed. Most of the projects performed have been in highly deviated wells where sampling with wireline was not an option or have involved mitigating the risks of sticking during the sampling and pressure-testing operation.

These field tests demonstrated that the LWD service is capable of taking wireline-quality measurements in extremely challenging borehole environments. In Trinidad the service successfully performed pressure-testing and real-time fluid analysis and recovered three single-phase samples while drilling a 91-m (300-ft) section at 70° deviation.

On a logging run in the Netherlands, the service accurately took 25 formation pressure points and clearly identified three distinct gradients. The customer targeted four zones of interest and acquired 11 samples, including six single-phase samples in the oil zone, two single-phase samples in the water zone, and three single-phase samples in the gas zone. This pressure-testing and sampling operation was executed in a 1,307-m (4,313-ft) tangent section at 73° inclination.

The service also was used to determine the presence of moveable fluids in the reservoir of a horizontal development well for a customer in Norway. The tool was positioned at a sampling station with a mobility of 1.8 mD/cp, and after nearly five hours of pump-out at that station it successfully acquired the first sample. After another four hours of pump-out at the same station, the tool acquired the second sample.