Capillary Pressure Measurement in Petroleum Reservoir Cores with MRI
Authors: Cano-Barrita Prisciliano F. de J. , Balcom Bruce John , Green Derrick , McAloon Michael , Dick Josh 
Published: October 2008

The centrifuge method to determine capillary pressure curves involves increasing the centrifugation speed in steps and measuring the amount of water expelled from a core plug at equilibrium. This approach requires the use of several assumptions related to the unknown fluid distribution in the core and gravity effects. It is also time consuming since only one point in the curve is obtained at each speed. The quantitative nature of the Double Half K-Space Single Point Ramped Imaging with T1 Enhancement Magnetic Resonance Imaging method makes it ideally suited for the measurement of fluid distribution in core plugs, thus reducing the number of speeds necessary to obtain a full capillary pressure curve. The Centrifuge-Magnetic Resonance Imaging method requires only the use of a moderate rotational speed centrifuge and an inexpensive low field permanent magnet. Capillary pressure curves were obtained for primary drainage, imbibition, and secondary drainage in water/oil systems. By establishing a free water level at half height of the core, negative capillary pressure data was obtained for imbibition and secondary drainage. Attention is necessary to the fluid redistribution when high permeability cores are tested.

INTRODUCTION

Capillary pressure curves for rocks are used to predict the potential hydrocarbon recovery from a reservoir. Traditional methods to determine 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 since the equilibration time can be weeks or months for each pressure point. The centrifuge method (Hassler and Brunner, 1945) involves rotating a core plug at different speeds and allowing time for saturation equilibrium in the core at each speed. The amount of fluid expelled is measured at each speed and an average saturation in the core is determined. By using an approximate solution, Hassler and Brunner (1945) proposed a method 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 centrifugal speeds, and that the core length is negligible with respect to the radius of rotation.

The SPRITE Magnetic Resonance Imaging method has proved to be a suitable technique to determine the capillary pressure curve of core plugs (Chen and Balcom, 2005, Chen et al., 2006, Green et al., 2007). To determine the capillary pressure with this technique, the distribution of the fluids along the length of the core is obtained without any of the assumptions required in traditional centrifuge testing. Better still this technique is directly quantitative compared to traditional, less quantitative, spin echo based MRI (Baldwin and Spinler, 1998). In this paper, we present the procedure to determine the capillary pressure curves in core plugs using Double Half K-Space Single Point Ramped Imaging with T1 Enhancement MRI, combined with a moderate speed centrifuge.