Comparison Study of Capillary Pressure Curves Obtained Using Traditional Centrifuge and Magnetic Resonance Imaging Techniques

Abstract

The current study is a comparison trial of a new Magnetic Resonance Imaging (MRI) method for acquiring capillary pressure versus traditional techniques. This study is meant to evaluate the methodology and workflow by performing measurements at the University of New Brunswick, Canada and on-site with CoreLab in Houston, TX, USA. This study focuses on gas-brine primary drainage capillary pressure systems.Traditional centrifuge capillary pressure measurements require the fluid(s) to reach equilibrium at many different speeds. This is very time consuming as each equilibrium step can take a couple of days. In addition, the inlet saturation must be computed using an approximate solution that is known to cause errors. Porous plate capillary pressure measurements are considered the most accurate but acquiring the complete curve can take months.The new method (GIT-CAP) centrifuges the core plugs then directly measures the water saturation distribution inside the core plug using MRI. The measured water saturation together with the known centrifugal force directly leads to a capillary pressure curve.Conventional medical based MRI methods have difficulty in relating the detected signal intensity to water or oil saturation. This is because the MRI image intensity depends on the environment of hydrogen atoms which changes based on saturation level. In this work, we use a new MRI method, one dimensional centric scan Single-point Ramped Imaging with T1 Enhancement (SPRITE), in which the detected signal is directly proportional to the amount of water or oil present.The new technique measures the capillary pressure curve more quickly and accurately. It is also three to five times faster since only two to three centrifuge speeds are required (versus seven to ten). In some rock types, this reduces the measurement duration from many weeks to days. The new technique is also potentially more accurate as it directly measures the water saturation in the rock instead of relying on a calculation using a measurement of the expelled water. The current study focuses on gas-water systems comparing traditional capillary pressure measurements with the new MRI-based method. IntroductionCapillary pressure, Pc, is the difference in pressure across the interface between two immiscible fluids and is dependent on the interfacial tension, pore size, and wetting angle. Capillary pressure is the most fundamental rock-fluid property in multiphase flows, just as porosity and permeability are for single phase flow in oil and gas reservoirs (Lake 1989). Capillary pressure curves directly determine the irreducible water saturation, residual oil saturation, rock wettability, and can be used to determine water oil contact point and approximate oil recovery. Water flood performance is also significantly affected by the capillary pressure of the rock (Masalmeh 2003).Capillary pressure is typically measured in the laboratory by using mercury injection, porous plate, or centrifugation techniques (Dullien 1991). The porous plate method is considered the most direct ...

An Experimental and Numerical Study of Relative Permeability Estimates Using Spatially Resolved $$T_1$$-z NMR

“…A good account on practical aspects and a review of MRI techniques applicable for petrophysical measurements is given in Mitchell et al (2013). Chen and Balcom (2005), Green et al (2007) proposed to apply a T 1 -based SPRITE technique (single-point ramped imaging with T 1 enhancement) to determine fluid saturation along the length of a centrifuged rock core. Composite capillary curves were constructed in assumption of core homogeneity and known capillary pressure as a function of position.…”

“…Here we aim to compare this CM-NMR approach to NMR relaxation-based estimates proposed by AlGhamdi et al (2013). We modified the latter for use with centrifuge (CM-NMR-T 1 ), which enables the acquisition of multiple data points in a single experiment similarly to Green et al (2007). NMR longitudinal relaxation is used to avoid influence of internal gradients effects common in saturated sandstones.…”

Laboratory Measurements on Changes in Carbonate Rock Properties due to CO2-Saturated Water Injection

“…The measurement was T 2 relaxation time which was obtained by CPMG method (Green et al, 2008). The NMR equipment used for these measurements had a frequency of 2.4 MHz, which is almost the same as the frequency used in logging NMR.…”

Capillary Pressures by Fluid Saturation Profile Measurements During Centrifuge Rotation

“…By using MRI to image the distributed water saturation in a core plug after centrifugation, Green et al (2007) recently reported the lack of S w = 1.0 at the outflow face. They speculated that 100% saturation at the outlet was obtained only in a very thin layer, which could not be resolved at the spatial resolutions of the MRI (<1 mm).…”