Residual fluid saturations (Sor, Swir and Srg) are important in order to evaluate the quantity of hydrocarbons that can be produced from a reservoir with a natural occurring underlying aquifer, alternatively when gas cap expansion is expected, or when water flooding is being planned.
Residual saturation is a function of:
- texture of the porous media (pore throat-pore body ratio);
- clay content and clay distribution in the pores.
The complexity of carbonate pore geometry requires the determination of trapped gas or oil saturations. Petrographic services (SEM, thin sections) aid in the understanding of differences in trapped hydrocarbon values for each particular lithology.
|Care must be taken when reading off Sor-Swir values from standard relative permeability curves. Sometimes during coreflow experiments, the actual test has finished without reaching true Sor or Swir. This can be caused by unfavourable mobility ratio, sample heterogeneity or high capillary end-effects. To obtain representative Sor or Swir values, it is advisable to use capillary pressure measurements. With the porous plate technique, one must assure that high enough Pc pressures have been applied to reach the asymptoth of the capillary pressure curve. When using the ultra-centrifuge, care must be taken into the test design: the sample must spin at high enough speed without ‘stripping’ the water from the rock grains.
|Residual gas saturation is usually determined by counter current imbibition techniques or from core flow tests. Counter current imbibition involves submerging the prepared sample into toluene, while simultaneously measuring the sample weight under the liquid. By doing this at various initial saturation levels, one can get information on the relationship of initial saturation versus the residual saturation. This is a cost effective way of estimation Srg values and to obtain relationships of various starting saturations with residual gas saturations. When one is interested in a single representative Srg value, the linear core flow test can be used. The prepared sampe is brought to irreducible oil or water saturation to simulate downhole conditions. Then water or oil (depending of reservoir system) is injected into the core plug to displace the gas. The trapped gas or residual gas is determined volumetrically and by mass balance.