8/25/2023 0 Comments Viscosity units cst to cp![]() During this time period, the volume of gas in solution in the crude oil remains constant at the initial value of R so i. If this reservoir were to undergo pressure depletion from oil production, then the average reservoir pressure would decline over time, and the pressure would eventually reach the bubble-point pressure. The initial reservoir pressure, pi, is greater than the bubble-point pressure. In this figure, we can see that the reservoir is an undersaturated oil reservoir. When measured data are not available, Standing’s correlation can be used to estimate the bubble-point pressure: For all measurements made up to and including the bubble-point pressure, the constant composition expansion test and the differential liberation test give identical results. The constant composition expansion test is similar to the differential liberation test, however, the evolved gas is not expelled from the PVT cell during the test. Other PVT tests, such as the Constant Composition Expansion Test, can be used to determine the bubble-point pressure of the crude oil. The laboratory method for calculating the bubble-point pressure, p b, of a crude oil was discussed earlier in the context of the differential liberation test. MW o is the molecular weight of the crude oil, lb/lb-moleīubble-Point Pressure of the Crude Oil, p bĪs already discussed, the bubble-point pressure is the pressure that first bubble of gas evolves from an undersaturated crude oil during pressure reduction.It is the properties of the separated phases that we are most interested in, as these are more representative of the processes occurring in the reservoir. In addition, due to the density differences between oil and gas phases, gravity will also act to separate the two phases. Consequently, as gas evolves from the oil, this difference in the viscosity allows the gas to move faster than the oil and to separate from the source oil from which it evolved. ![]() What is the differential liberation test trying to model? In the reservoir, as gas comes out of solution, it typically has a much lower viscosity than the oil phase. The pressure, liquid volume, and gas volume are then used in the calculation of the appropriate properties for black oils. This process is then repeated until the desired final pressure is reached (Step 8). The gas is then expelled from the piston under isobaric (constant pressure) conditions by reducing the piston volume and allowing the gas to escape through a valve in the system (Step 5). At this point, the pressure and the oil and gas volumes in the cell are measured. Up until the bubble-point pressure is reached, all measurements have been single-phase (liquid hydrocarbon) measurements.Īfter the bubble-point pressure has been reached, the volume is increased further until a significant volume of free gas has developed (Step 4). This pressure is the bubble-point pressure of the crude oil. This process is continued for several pressure steps until the first bubble of gas (red) is observed through a window in the cell (Step 3). At Step 2, the pressure in the cell will be less that the original pressure due to the expansion of the crude oil. The volume of the cell is then increased by extending the piston outward (Step 2), and the pressure and volume are recorded. Results should be reported in Zahn-Seconds at a specified temperature for a particular cup.In a differential liberation test, a crude oil sample (green) is introduced into the cell at the initial reservoir pressure and temperature (Step 1 in Figure 3.07). Īscott Zahn Cups are based on measuring the Flow of liquid in seconds. Gardco signature Zahn cups and Gardco EZ Zahn cups use different conversions. Note the above equations are for a Brookfield series of Zahn cups. One can convert efflux time to kinematic viscosity by using an equation for each cup specification number, where t is the efflux time and ν is the kinematic viscosity in centistokes. Zahn Cup Technical Specification Conversion On paint standard specifications, one denotes viscosity in this manner: efflux time, Zahn cup number. After lifting the cup out of the substance the user measures the time until the liquid streaming out of it breaks up, this is the corresponding "efflux time". To determine the viscosity of a liquid, the cup is dipped and completely filled with the substance. They are manufactured in accordance to ASTM D 4212, ASTM D1084 and ASTM D816 Large number cup sizes are used when viscosity is high, while low number cup sizes are used when viscosity is low. There are five cup specifications, labelled Zahn cup #x, where x is the number from one through five (see table below). There is also a long handle attached to the sides. It is commonly a stainless steel cup with a tiny hole drilled in the centre of the bottom of the cup. ![]() A Zahn cup is a viscosity measurement device widely used in the paint industry.
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