Multi-Component Systems Determination of Reservoir - Lab 3

Lab 3: Multi-Component Systems: Determination of Reservoir Fluid Parameters Cover Letter Dr. Ebrahim Fathi, The objective of this experiment was to determine three main parameters used to relate surface to reservoir volumes. These parameters were the gas formation volume factor (Bg), the oil formation volume factor (Bo), and the gas solubility/solution gas-oil ratio (Rs). The parameters were determined through a series of flash liberation and differential vaporization tests. These parameters can help predict the amount of liquid and gas that will be produced at surface conditions for a unit volume of reservoir fluid at reservoir conditions. The flash vaporization test was performed at a value of pressure much higher than the bubble point obtained. This test helped find the first drop of gas removed from the solution. The differential vaporization testing was conducted using a pressure much lower than the bubble point obtained; this measured specific amounts of gas that were extracted from the solution. In order to conduct this experiment, the PVT simulator was required. Specific initial conditions needed to be met in order to properly perform the experiment. The visual cell at initial conditions contained mole fractions of propane, hexane and methane of 0.2, 0.4 and 0.4, respectively. The pressure in the cell was 2000 psia and the temperature inside the cell was 100 F. After the conditions were met and verified, the bubble point was obtained through proper flash liberation and differential vaporization testing. The flash vaporization test was performed by setting the back pressure regulator and evacuating the gas vessel. These actions allowed the liquid to expand from the visual cell onto the surface. The servo pump was then connected and used to pump fluid out of the visual cell. The liquid and gas present then expanded at surface conditions. Liquid at a pressure larger than the back pressure regulator pressure was drained to the surface, which left the liquid and gas in the visual cell equal to the back pressure regulator pressure. Connecting the helium supply in the simulator to the chromatograph would charge the contents of the cell. The liquid and gas left could then be analyzed and used to calculate the initial solution gas-oil ratio of the contents and the shrinkage factor relative to it. The differential liberation testing was used in a similar fashion as the flash vaporization test, but only 10 cc of liquid was contained initially in the visual cell and the pressures used were below the bubble point found. Extracting different values of mercury from the cell for each separate test was necessary in finding the parameters needed for calculations. The amounts of mercury extracted (which can be found in table 2) produced different pressures for each test. Data collected from these tests was used for calculations of the parameters. Differential parameters (Vo, F, E) were obtained at separate pressures, and the values were used in formulas to calculate the field parameters (Bo, Bg, Rs) mentioned at the beginning of this letter. The shrinkage factor (Cbf) was 0.398 and the initial solution gas-oil ratio (Rsif) was 446.6. The Bo values for each liberation were 2.395, 2.2824, 2.1697, 2.0669, 1.9738, 1.8881, 1.8097, and 1.7338, respectively. The Bg values were 0.0099, 0.0118, 0.0141, 0.0175, 0.0228, 0.0323, 0.0481, and 0.0933, respectively. The Rs values for each liberation were 404.96, 365.38, 325.43, 287.97, 253.33, 222.19, 194.09 and 168.49, respectively. All values were recorded using accuracies of (+/- 0.001) for pressure and (+/- 0.0001) for liquid and gas compositions. Fluids undergo a drastic change as they arrive to the surface of the earth, because the pressures below the surface are larger than the pressures at the surface. The composition of the fluid changes as the pressure of the fluid decreases, and gas is liberated from the solution. “The gas formation volume factor measures the volume of gas at reservoir conditions divided by the volume of gas at standard conditions. The oil formation factor measures the amount of oil at reservoir conditions divided by the oil at standard conditions. The gas solubility measures the volume of gas at standard conditions relative to the unit volume of the stock tank oil” (McCain) The main objective of this experiment was to calculate this values, and understanding what each value represents is necessary to conduct this experiment properly. Theory, Concepts, and Objective of Experiment The main objective of this experiment was to determine the gas formation volume factor, the oil formation volume factor, and the gas solubility of a mixture of propane, hexane, and methane with mole fractions of 0.2, 0.4 and 0.4, respectively. With the use of these parameters, the amount of liquid and gas produced at surface conditions could be predicted (per unit volume of reservoir fluid). The data needed in order to calculate the parameters was provided by performance of the flash liberation and differential vaporization testing. These parameters were able to be calculated due of the difference in pressure between the reservoir and the surface. Reservoir pressure, in every case, is much larger than the pressure at the surface. Due to this, gas is liberated from the oil in the reservoir; This means there is more gas at the surface than in the reservoir, and there is more oil in the reservoir than at the surface. With this being said the oil formation volume factor (Bo), will always be greater than one, and the gas formation volume factor (Bg), will always be less than one. During the flash liberation test, the gas that is created from the oil did not change the composition of the contents, because it remained in contact with the oil at all times. During the differential vaporization test, the gas created from the oil was separated from the system, which changed the composition of the contents significantly. Several different hydrocarbons accounted for most of the composition. These two processes are illustrated below with figures from the lab manual. (Figures 1 & 2). The flash liberation test allowed to find the shrinkage factor and the initial solution gas-oil ratio. “The shrinkage factor is the amount of oil collected at the surface, divided by the amount of oil left over in the cell. The initial solution gas-oil ratio is the amount of gas produced at surface conditions divided by the amount of oil produced”. (McCain) These parameters combined with the values of the cumulative relative volume at standard temperature, the gas expansion factor, and the data acquired from the differential vaporization tests will allow for the calculation of the field parameters (Bo, Bg, and Rs). Figure 1 – Flash Liberation Sequence Figure 2 – Differential Liberation Sequence After obtaining the results for the three field parameters, a graph of each parameter versus the reservoir pressure should look similar to these graphs from the textbook… Figure 3 – Gas formation volume factor vs. Reservoir Pressure Figure 4 – Oil formation volume factor vs. Reservoir Pressure Figure 5 – Solution oil-gas ratio vs. Reservoir Pressure Symbols and Equations Gas Vaporization = Gas actually expanded (cc) = gas cell content before expansion – gas cell constant after expansion Mass of Gas = m = gas density gr/cc * Gas actually expanded (cc) R – Universal Gas Constant T – Temperature Standard P – Pressure Standard C1 – Methane mole fraction in gas chromatography C3 – Propane mole fraction in gas chromatography C6 – Hexane mole fraction in gas chromatography Ma – apparent molecular weight n = m/Ma V – Gas volume F1 – relative gas volume at standard condition – V/Vb F – cumulative relative gas volume at standard condition E – gas expansion factor – Gas volume (V) / Gas Actually Expanded. X(i) – Liquid composition Y(i) – Gas composition Bo – Oil Formation Volume Factor Bo = (Vo/Cbf) Bg – Gas Formation Volume Factor Bg = (1/E) Rs – Gas Solubility Rs = (Rsif – (F/Cbf)) Peng Robinson Equation of State = ((RT/Vm-b)-(a/Vm^2+2Vmb-b^2)) a – 0.457236 R^2*a*Tc^2 b – 0.0777961 R*Tc/Pc Experimental Procedure The procedure for this experiment required the use of the PVT simulator. The flash liberation and differential vaporization tests were required to obtain the values for the oil formation volume factor, gas formation volume factor, and the gas solubility. Before the tests could be performed, the initial conditions of the cell needed to be verified. The cell initially contained mole fractions of propane, hexane, and methane of 0.2, 0.4, and 0.4, respectively. The first step in the process was to record these initial conditions by displaying the gas chromatograph. After obtaining the values, the bubble point pressure was obtained by opening values 8 and 9, which allowed the visual cell and the hand pump to communicate. Operating the pump and removing mercury from the cell helped find the bubble point pressure. After finding bubble-point pressure, the contents in the cell needed to be mixed or “shook” ensure constant equilibrium for the contents inside of the cell. Failure to mix the contents within the cell would affect the accuracy of the data. Figure 6 – PVT Simulator After recording the data for the step above, the flash liberation test was conducted. This test provided a sample of the fluid to the surface conditions. After bringing the contents to equilibrium, the back pressure regulator was set to the next highest integer pressure above the cell pressure shown on the simulator (Note: the BPR did not accept decimal values, so the pressure was rounded up to the neared whole number). The gas collection vessel was evacuated, and the pressure in the cell dropped from 14.7 to 0 as a result. Following this, valve 9 was closed. After closing this cell, the liquid expanded from the cell to the surface along the path connected by valves 14, 16, 18 and 19. Any liquid that got past the BPR encountered 0 psia pressure and flashed to both the liquid and gas states. Due to this, the liquid collected in the centrifuge tube, and the gas collected in the gas vessel. The mass of the liquid, and the pressure of the gas were displayed in the upper right hand corner of the simulator. Opening valves 14 and 16 allowed for the density of the fluid in the cell to be shown, and this value was recorded. Opening valves 18, 19 and 7 allowed for the connection of the servo pump to the cell. The main difference between the servo pump and the hand pump is that the servo pump can operate at a controlled rate. Data needed to be entered to run the servo pump properly, and a value of 10 was entered for the rate of the flow and the value of 6 was entered to cap the maximum value of amount fluid that allowed the gas to expand. Running the servo pump and allowing it to stop will leave liquid in the centrifuge tube and gas in the gas vessel, the gas pressure (in psia) and the mass of the liquid (in grams) were recorded as a result. Closing valves 18 and 19, and running a chromatographic analysis of the contents was required next. These values were not the same as the original analysis measured at bubble-point conditions. To complete the liquid expansion, the contents needed to be charged into the chromatograph. This was done by opening valves 20 and 21, connecting the helium supply to the chromatograph. An analysis of both the gas and the liquid were obtained and recorded. After completing the flash liberation test, the differential vaporization was performed. The initial conditions in the visual cell were identical to the first test, beside the fact that there was only 10 cc of liquid in the cell (opposed to 60cc). The smaller volume of liquid in the cell gave the gas more space to expand throughout the cell. The bubble point needed to be obtained through a similar method as the