MDT Single-Phase Sampling

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Maintaining reservoir fluid conditions for PVT analysis

Accurate compositional and PVT analysis of a formation fluid sample requires that the recovered sample remain in downhole formation conditions. This is accomplished by pairing the Single-Phase Multisample Chamber with the multisample module of the MDT modular formation dynamics tester.

A sample collected at formation pressure in the Single-Phase Multisample Chamber is maintained in single phase by overpressuring it during retrieval. A preset nitrogen charge acting through a piston floating on a synthetic oil buffer adds more than 1,600 psi to the sample pressure, which is sufficient to keep the fluid in single phase during transport to surface. Overpressuring compensates for the temperature-induced pressure drop that would otherwise occur.

Preventing asphaltene precipitation

Not allowing pressure drop in the sealed, pressurized sample chamber in turn does not allow the fluid to pass through the phase-separation pressure to a gas and liquid mixture. For fluids containing asphaltenes, as the pressure approaches the bubblepoint, asphaltene precipitation can occur. Asphaltene precipitation is usually an irreversible process that can go undetected if it occurs in the sample chamber.

Enabling PVT analysis

Monophasic PVT analysis determines critical parameters for optimal completion and production design. Collecting single-phase formation fluid samples with the MDT modular formation dynamics tester makes it possible to reliably conduct laboratory analysis without the additional time, expense, and environmental concerns of drillstem testing.

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Overpressuring for Single-Phase Representative Samples

Phase diagram of a typical crude oil. Liquid hydrocarbon samples taken at reservoir temperature and pressure (A) can change phase at lower temperatures and pressures as they are brought to the surface (D). By overpressurizing the sample downhole (B), the sample will maintain its initial phase as it is brought to the surface (C) at lower temperature.Tool diagram
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