Obtaining capillary pressure curves in the water–gas system using centrifugation and NMR techniques under atmospheric conditions

About the authors:

Artyom G. Potapov, Head of the Laboratory, Tyumen Petroleum Research Center, Tyumen, Russia; agpotapov@tnnc.rosneft.ru, https://orcid.org/0009-0007-3010-8070

Maxim I. Zagidullin, Chief Specialist, Tyumen Petroleum Research Center, Tyumen, Russia; mizagidullin2@tnnc.rosneft.ru, https://orcid.org/0009-0009-7854-9320

Abstract:

At present, in laboratory practice, three main methods are used to determine capillary characteristics: by centrifuge, semipermeable membrane and mercury indentation, as well as ways to combine the centrifugation method with other methods. This article examines the principles of determination of capillary pressure curves by different methods and briefly discusses their differences, advantages, and disadvantages.

When determining capillary pressure curve by centrifugation and subsequent conversion to the inlet end of the sample, the main source of error is the interpretation process. The shape of the capillary pressure curve also depends on the method of calculating the saturation at the inlet end of the sample. Therefore, a reliable and fast method of controlling the obtained capillary pressure curves is required. By combining NMR and centrifugation, the saturation profile of the sample after centrifugation can be measured and converted to water saturation at the corresponding capillary pressure.

The article thoroughly describes the method of obtaining capillary pressure curve by centrifugation and NMR method, the detailed sequence of operations for measurements is given. The minimum permissible time of water redistribution in the sample, during which no distortion of water saturation profile in core samples will occur, is determined.

The paper compares the capillary pressure curves obtained by direct water saturation measurements with the capillary pressure curves at the inlet end of the sample calculated by 10 different formulas.

The combination of NMR and centrifuge methods allowed us to select 2 most reliable equations for calculating the capillary pressure curves at the inlet face of the core sample — Rajan equation and the second Forbes equation. The maximum deviation was 11.6% and 12.2%, respectively. The maximum time for which the NMR measurement should be carried out so that the saturation profile of the sample would change insignificantly was determined experimentally. It amounted to 40 minutes.

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