Saturation Measurement
The Retort Distillation Method
The retort distillation method uses the same apparatus ( Figure 1 , Schematic of summation-of-fluids retort) as is used in the summation-of-fluids estimation of porosity. In this procedure the sample is weighed and its bulk volume measured or calculated; it is then placed in a cylindrical metal holder with a screw cap on the top and a hollow stem projecting from the bottom. The top is sealed and the sample holder is placed within a retort oven. A temperature controller raises the temperature of the core to a selected level, at which point the water within the core is vaporized and recovered. The temperature is then increased to 1200ºF (650 ºC) to vaporize and distill oil from the sample.
The apparatus is referred to as a downdraft retort because vaporized fluids build up pressure in the sample holder and then move vertically downward through the hollow stem. They are subsequently condensed and measured in a calibrated receiving tube. The retort oil correction curve is used to correct the volume measured in the receiving tube to the actual volume that was present in the core sample.
This laboratory procedure is normally restricted to small samples that are destroyed in the process; it is not generally used for full diameter cores. When a full diameter core is analyzed in a full diameter retort, a vacuum is pulled on the system and a temperature not greater than 450 ºF (232 ºC) is normally used. The lower temperature is selected to avoid damaging the rock sample, which may subsequently be used to measure permeability. If the saturating oil has an API gravity of 30 º or less, some residual oil is left in the full diameter core after retorting. This volume can be estimated by using additional test information from high temperature retorting of adjacent rock samples, or by monitoring the volume of oil recovered during retorting and applying a correction factor.
The correction factor for low API gravity oils can be substantial. If helium is subsequently used to estimate porosity of the sample and compensation is not made for the volume occupied by residual oil, the Boyle's law porosity value will be too low. This full diameter retort procedure for estimating residual saturations is used in limited situations and under those conditions where speed of analysis is paramount. It is not the preferred technique for full diameter analysis.
During the retort distillation procedure, water and oil content are measured concurrently. When the volume of the collected water is measured during the temperature rise to 1200º, a distinction must be made between pore water held by capillary forces (the value we are seeking) and that which is the result of mineral decomposition. One approach to distinguishing between the two is to plot the water recovered against the elapsed time of retorting.
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.
Accuracy of Measurement
Both the retort distillation and the Dean-Stark analysis are capable of yielding residual saturation values within ±5%
of the true value. In a study made by Rathmell (1967), where high temperature retort oil saturation measurements were compared with gravimetric measurements, average residual oil saturation agreed to within ±1.4% pore volume. The average residual oil saturation in these cores was approximately 45 % pore space, and the scatter of the data was ± 6.1 % pore volume. API RP 40 reports the accuracy of the retort distillation technique to be ±5%
of the true value.
Wyman (1977) points out what many investigators have recognized: it is difficult to relate oil saturation values measured from conventional cores with true in situ values. He shows that routine saturation measurements from conventional cores are not adequate to describe in situ saturations, not even residual oil saturations after waterflooding, unless a pressure core barrel is used and a carefully designed mud and core handling program is followed.
Rathmell, Brown, and Perkins (1972) qualify this statement by suggesting that routine core analysis oil saturations adjusted for bleeding and shrinkage can give reliable values for waterflooding residual oil saturation in many sandstone reservoirs.
Exercise 1.
Given the following data, calculate the porosity, oil, and water saturations:
Fresh sample (oil + gas + water) weight = 28.519 g
Clean and dry sample weight = 26.490 g
Water recovered in Dean-Stark apparatus = 1.59 cc
Pore volume from helium injection = 2.89 cc
Bulk volume from Archimedes' law = 12.85 cc
Oil gravity = 36.1°API
Solution 1:
1. Fresh sample (oil + gas + water) weight = 28.519 g
2. Clean and dry sample weight = 26.490 g
3. Water recovered in Dean Stark = 1.59 cm3
4. Weight water (cm3 recovered 1.0 g/cm3) = 1.59 g
5. Oil + water weight = (1) - (2) = 2.029 g
6. Oil weight = (1) - (2) - (4) = 0.439 g
7. Oil volume = (6) ÷ 0 = 0.520 cm3
8. Pore volume from helium injection = 2.89 cm3
9. Bulk volume from Archimedes = 12.85 cm3
10. Porosity = (PV ÷ BV)(100) = ((8) + (9))(100) = 22.5%
11. Oil saturation = [(7) ÷ (8)] [100] = 18.0% PV
12. Water saturation = [(3) ÷ (8)] [100] = 55.0% PV
°API = 36.1
SG = 141.5/(131.5 + API°) = 141.5/(131.5 + 36.1)
= 0.844 g/cm3 = r0
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