Extracting Pore Throat Size and Relative Permeability from Mri Based Capillary Pressure Curves

Capillary pressure (Pc) curves acquired using the Magnetic Resonance Imaging (MRI) technique, call GIT-CAP, have 70-100 data points. This extra data, when compared with traditional porous plate or centrifuge techniques, allows for the interpretation of more information from the Pc curve. In particular, the pore throat size and relative permeability can be extracted. The pore throat size can be extracted from the capillary pressure curve by computing the change in saturation at each pressure and converting the pressure into a radius (similar to the mercury intrusion method). This information is valuable on its own but it can also be correlated with the T2 NMR pore size distribution to extract more information, such as the NMR relaxivity parameter. The relaxivity parameter is required to convert the T2 distribution into a quantitative pore size distribution. The T2 NMR pore size distribution can easily be measured as part of the same test protocol that measures the MRI-based Pc. The relative permeability of the rock can be modeled from the capillary pressure data using a Burdine model (or similar). The advantage of the MRI-based capillary pressure measurement is that with the additional measured data points, a Pc model (like BrooksCorey) does not need to be applied to use the relative permeability model. This work describes the techniques used in the acquisition and analysis of the data and a series of results on a variety of rocks from sandstones to carbonates. INTRODUCTION The pore throat size and its distribution is important in many fluid transport processes in porous media (like reservoir rock)[1]. The pore throat size affects the fluid saturation distribution, porosity, permeability and, to some extent, wettability. Pore size and pore throat size are often related. NMR is a common way of determining the pore size distribution in a rock. Capillary pressure curves acquired using an MRI-based technique called GIT-CAP can be used to determine pore throat size distributions. Typical methods of acquiring capillary pressure, such as centrifuge and porous plate, do not have the resolution required [2]. Permeability is a measure of a porous media’s ability to transmit fluid. Absolute permeability is a measurement of the permeability when only a single fluid is present. Effective permeability is the permeability of one fluid with a second fluid present. Relative permeability is the ratio of effective to absolute permeability.