Capillary Pressure Curves for Low Permeability Chalk Obtained by NMR Imaging of Core Saturation Profiles

Abstract

Abstract A new technique for obtaining water-oil capillary pressure curves, based on NMR imaging of the saturation distribution in flooded cores is presented. In this technique, a steady state fluid saturation profile is developed by flooding the core at a constant flow rate, At the steady state situation where the saturation distribution no longer changes, the local pressure difference between the wetting and non-wetting phases represents the capillary pressure. The saturation profile is measured using an NMR technique and for a drainage case, the pressure in the non-wetting phase is calculated numerically. The paper presents the NMR technique and the procedure for calculating the pressure distribution in the sample. In homogeneous samples produce irregular saturation profiles, which may be interpreted in terms of variation in permeability, porosity, and capillary pressure. Capillary pressure curves for North Sea chalk obtained by the new technique show good agreement with capillary pressure curves obtained by traditional techniques. Introduction Accurate petrophysical properties of reservoir rock such as capillary pressure, permeability, and relative permeability functions are essential as input for reliable oil in place estimations and for the prediction of the reservoir performance. Traditional methods for capillary pressure measurements are the mercury injection method, the diaphragm method and the centrifuge method. In the mercury injection method, the non-wetting phase is mercury which displaces a gas. The samples are usually evacuated to a low pressure and Hg is then injected in steps allowing for pressure equilibrium at each step, or alternatively Hg is continuously injected. Corresponding data on injected volume of Hg and the injection pressure are recorded. This technique is widely used for measuring capillary pressure functions for low permeable rocks. This is primarily because it is generally believed that pressure equilibrium in each pressure step is readily obtained, while this is normally a problem for other methods where a liquid is the wetting phase. The disadvantage of this technique is the uncertainty in the scaling of the measured data to reservoir fluid data and conditions. In the diaphragm method or porous plate method, the problem concerning the scaling of the measured data is avoided, since this technique allows for the direct use of reservoir fluids. The water saturated sample is placed on a water wet diaphragm to impose a boundary condition Pc=0 to the wetting phase, i.e. the wetting phase is allowed to drain through the outlet end of the sample, at the same time as the non-wetting phase (oil or gas) is impeded. Pressure is added to the non-wetting phase and through a limited number of pressure steps, the capillary pressure curve is recorded. However, an important requirement is that equilibrium is obtained at each pressure step. This is the major problem when the diaphragm method is used on microporous materials. The drainage time may be considerable during each step e.g. several weeks. In recent studies, thin micropore membranes have been used in an attempt to reduce the experimental time. Such a reduction will be less pronounced for low permeable rocks such as chalk since the flow resistance in the core is relatively more important. P. 807