Reservoir liquid density calculation

The results of the reservoir liquid density calculations can be used to calculate oil formation volume factors. 

Calculation of Reservoir Liquid Density at Saturation Pressure Using Ideal-Solution Principles... 

Calculation of Liquid Density Using Ideal-Solution Principles — Calculation of Reservoir Liquid Density at Saturation Pressure Using Ideal-Solution Principles — Calculation of Reservoir Liquid Density at Pressures Above the Bubble Point... 

The volume of stock-tank liquid which condenses during production of 1 lb mole of reservoir gas can be computed with values which result from the recombination calculation. The molecular weight divided by the stock-tank liquid density (both calculated in the first step of Example 7-1) is the volume of one lb mole of stock-tank liquid. The sum of column 7 of the third step of Example 7-1 gives the pound moles of reservoir gas per pound mole of stock-tank liquid. 

This chapter begins with bubble-point pressure and solution gas-oil ratio, and then explains methods of estimating the density of reservoir liquids. The results of the density calculations are used to estimate oil formation volume factors. A technique for adjusting the results of the correlations to fit field derived bubble-point pressure is presented. 

The petroleum engineer needs to be able to estimate the density of the reservoir liquid at reservoir conditions. Then the shrinkage in volume that a reservoir liquid undergoes while progressing from the reservoir to the stock tank can be estimated. There are several methods of calculating the volume occupied by a given mass of liquid at elevated pressures and temperatures. We will consider only one method the method most applicable to the liquids encountered in the oil fields. This. method is based on ideal-solution principles. 

We will study three different applications of ideal solution theory to the calculation of density of a liquid. Each application will depend on the amount of information available. The first applies when the composition of the reservoir liquid is known. The second applies when solution gasoil ratio, gas composition, and stock-tank oil gravity are known. The third is used when solution gas-oil ratio, gas specific gravity, and stock-tank oil gravity are known. 

Now we will examine the methods of estimating the density of a reservoir liquid at reservoir conditions. First, we will consider liquids at their bubble points or liquids in contact with gas in either case, we will call these saturated liquids. The first step in the calculation procedure is to determine the density of the liquid at standard condition. The next step is to adjust this density to reser >oir conditions. 

The solution to the second problem is to create, by calculation, a pseudoliquid at standard conditions. The pseudoliquid has the same composition as the reservoir liquid even though a mixture of that composition is partially gas at standard conditions. The density of this pseudoliquid, called pseudoliquid density, is calculated with ideal solution procedures. 

EXAMPLE 11-4 Use the data given in Figure 11—2 to calculate the density of a reservoir liquid at its bubble point of 2635 psia at a reservoir temperature of 220°F. The composition of the well stream is as follows. 

This pseudoliquid density then is adjusted to density at reservoir temperature and pressure using Figures 11-3 and 11-4. Densities calculated by this method are as accurate as densities obtained experimentally for both black oils and volatile oils. If the liquid mixture contains hydrogen sulfide, an additional adjustment (given in Figure 11-7) is necessary.4... 

EXAMPLE 11-8 The producing gas-oil ratio of the well given in Example 11-7 is 768 scf/STB, and the gravities of the gas and stock-tank oil are 0.786 and 40.7°API, respectively. Reservoir conditions are 5000 psig and 220aF, but the bubble point of the reservoir liquid is 2635 psia at 220°F. Calculate the density of this liquid at reservoir conditions given that the coefficient of isothermal compressibility of the reservoir liquid is 15.4 X 10 6 psia-1. 

First, calculate the density of the reservoir liquid at bubble-point pressure of 2635 psia at 220°F. 

The oil formation factor can be calculated using the results of ideal-solution calculations of the liquid density at reservoir conditions. 

Calculate the density of the reservoir liquid of Exercise 11-13 at the initial reservoir pressure of 4800 psia given that the coefficient of isothermal compressibility between bubble-point pressure and 4800 psia is 14.5 x 106 psi-1. 

Step 5 Calculate the density of the reservoir liquid at reservoir conditions using the feed stream composition from Step 1, reservoir temperature, and bubble-point pressure. The procedure is described in Chapter 11. 

Step 5 Calculate the density and molecular weight of the reservoir liquid at reservoir conditions. 

Depending on the density In the vlsclnlty of the reservoir walls some slip might be observed. Therefore, the actual shear rate that the liquid slab experiences might be lower than the Imposed one. This actual shear rate 7 Is determined empirically from the simulation by calculating the average velocity of the liquid slab particles which are located next to the reservoir walls. The actual shear rate 7 rather than the Imposed shear rate imp s used In Equation 49 for the calculation of the effective viscosity r/eff. 

Samples of gas and liquid are taken from a first stage separator operating at 500 psia and 75°F. The separator gas-oil ratio is constant at 2347 scf/SP bbl. The compositions of the samples are given in the table below. The density of the separator liquid at separator conditions, calculated with procedures given in Chapter 11, is 47.7 Ib/cu ft. Calculate the composition of the reservoir fluid. What type of reservoir fluid is this ... 

If the composition of the crude oil and the equilibrium constants of the components were known p could be calculated using the principles outlined in the previous chapter. The composition and amount of liquid and vapor could be calculated at any temperature and pressure. Using these results, the volume of the liquid could be calculated from the known densities of the various components in the liquid. Similarly, the volume of the residue liquid at stock tank conditions could be computed, p would be the ratio of these two volumes. Again, for most crude oils, calculations of this type are impossible since the composition of the original crude oil is seldom known. Consequently, it is usually necessary to make experimental determinations of p versus P in the laboratoiy using a sample of the reservoir crude oil. If experimental data are not available it is possible to estimate a value of p using one of the following methods for this purpose. 

Some laboratories use a gas pycnometer to determine the density with Ar or He to intrude into the sample and, not surprisingly, results do differ from methods that use a liquid. The gas pycnometer uses the ideal gas law to determine the volume of a sample and given a known volume of the sample, test chamber and the gas reservoir, together with the change in pressure, the absolute density can be calculated from the volume of the sample and its weight. The method does require a larger sample (0.5-10 g), but the test is non-destructive. Normally 10 iterations are taken to ensure accuracy. 

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