The Dean-Stark Apparatus
With the Dean-Stark method of measuring residual fluid saturations we can obtain residual saturations, porosity, and permeability of a cylindrical sample from the same piece of rock. This assures compatability of data; that, for example, a plot comparing permeability and porosity is valid. The measuring device (Figure 1 , Dean-Stark apparatus for measuring residual fluids) furnishes a direct determination of the water content of the sample. The oil content is calculated from weight difference and therefore it is important that no sand grains be lost from the core during the analysis, as this would result in an erroneously high calculated residual oil saturation. Rock grain loss is easily controlled by maintaining the sample within a tare ( Figure 1 ) throughout the Dean-Stark analysis — and good quality work utilizes this technique.
The principle of operation is straightforward. When the core to be analyzed is weighed, the resulting measurement will consist of the weight of the sand grains, as well as the oil and water present in the pore space. The sample is then placed within a tare in the apparatus, and this unit is suspended above a flask containing a solvent such as toluene.
Whatever the solvent, it must have a boiling point higher than water and be both immiscible with and lighter than water.
Next, heat is applied to the solvent, causing it to boil (toluene boils at approximately 240ºF (115ºC)). The hot solvent vapor rises, surrounds the sample, and moves up into the condensing tube, where it is cooled and condenses. The condensate falls to the bottom of the offset calibrated tube. The tube slowly fills until the liquid reaches the spill point, whereupon solvent condensate runs down the connecting side arm and drips onto the sample, which contains residual fluids. The dripping solvent mixes with oil from the sample, and both the solvent and oil are returned to the solvent flask.
The process continues until the sample is raised to the boiling point of water. When it does, the water vaporizes, rises in the condensing tube until it is condensed, and falls back into the calibrated tube. Because it is heavier than the solvent, it collects at the bottom of the tube, where its volume can be measured. When successive readings indicate no additional water recovery has occurred, we know all water has been removed from the sample, and the water volume is recorded for further calculations. The rock sample may be retained in the Dean-Stark apparatus until all oil is removed or it may be moved to another apparatus for subsequent cleaning and drying.
After all water and oil have been removed from the sample, it is dried and again weighed. The difference between the original and final weight equals the weight of oil and water originally in the sample. Because the water collected in the calibrated tube is distilled water with a density of 1.0 g/cm3 and the volume of water recovered is known, the weight of oil in the sample can be calculated. Knowing the density of the oil allows its volume to be calculated. When this information is subsequently combined with the estimated porosity of the clean, dry sample, the volumes of residual oil and water can be converted to percent pore space.
Salts, originally dissolved in the residual water, remain in the core sample when water is vaporized from the core. The water volume collected in the calibrated tube contains no dissolved ions; consequently, the volume occupied by the residual water in the pore space is greater than that read. The difference is small and is typically ignored in conventional core analysis, but is measured, when appropriate, in the special core analysis tests.