Capillary Pressure Measurement on Cores by MRI
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 19234, "Capillary Pressure Measurement in Petroleum-Reservoir Cores With MRI," by Prisciliano F. de J. Cano Barrita, Bruce J. Balcom, and Michael J. McAloon, University of New Brunswick, and Derrick P. Green, SPE, and Josh Dick, Green Imaging Technologies, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.
The quantitative nature of the double half-K-space single-point ramped imaging with longitudinal-relaxation-time (T1) enhancement (2HK-SPRITE) magnetic-resonance-imaging (MRI) method enables measurement of fluid distribution in core plugs, reducing the number of speeds necessary to obtain a full capillary pressure curve compared with the standard centrifuge method. This centrifuge/MRI method requires the use of only a moderate-rotational-speed centrifuge and a low-field permanent magnet. Capillary pressure curves were obtained for primary drainage, imbibitions, and secondary drainage in water/oil systems.

Introduction
The SPRITE-MRI method has proved suitable to establish the capillary pressure curve of core plugs. To determine the capillary pressure with this technique, the distribution of the fluids along the length of the core is obtained without assumptions required in traditional centrifuge testing. This technique is directly quantitative compared to traditional, less-quantitative, spin-echo-based MRI. The procedure to establish capillary pressure curves for core plugs by use of double half-K-space MRI, combined with a moderate-speed centrifuge, is presented.

Capillary Pressure
Two immiscible fluids in contact with each other in the pore network of a porous medium produce a discontinuity in pressure across the interface. The pressure difference across the interface is called the capillary pressure. Capillary pressure is a result of the curvature of the fluid interfaces and of the interfacial tension.

Traditional Centrifuge Methods
Capillary pressure curves for rocks are used to predict potential hydrocarbon recovery from a reservoir. Traditional methods to establish these curves include mercury intrusion, porous plate, and centrifuge. Mercury intrusion is considered a rapid method, but it has limitations in providing information on reservoir wettability. Porous plate is a direct and accurate technique, but it is time-consuming because the equilibration time can be weeks or months for each pressure point. The centrifuge uses centrifugal force by rotating a core plug at different speeds and allowing time for saturation equilibrium in the core at each speed. The amount of fluid expelled from the core at each speed is measured, and an average saturation in the core is determined. An approximate solution is used to obtain the inlet saturation corresponding to the inlet capillary pressure. This solution makes the assumptions that the outlet face of the core remains 100% saturated with the wetting phase at all centrifuge speeds, and that the core length is negligible with respect to the radius of rotation.