【正文】 how production of a dry gas from reservoir conditions (A) to separator conditions (A) to separator conditions (B) still does not cross the dewpoint and allow the formation of condensate.Figure (b) shows the phase diagram for a retrograde condensate reservoir. Here, the behavior of the mixture is such that a reduction to reservoir pressure at reservoir temperature initially causes liquids to condense in the reservoir. Further pressure reduction, however, causes some of these liquids to revaporize. The production of the lighter gaseous hydrocarbons from the reservoir mixture will cause the mixture to bee heavier and the entire phase envelope to shift to the right. This means that significant amounts of condensate could be left behind in the reservoir.Figure (c) shows a typical phase diagram for an oil mixture. If the reservoir conditions of pressure and temperature are at point A, there is only one phase, liquid oil, in the reservoir. If we follow the isothermal pressure reduction line, we can see that after crossing the bubble point tine, we enter the region of pressure/temperature conditions where two phases can exist. Here is a volume of liquid oil containing an amount of dissolved gas mensurate with the pressure and a volume of liberated, or free, gas. The volume of liberated gas increases and that of dissolved gas decreases as we continue to decrease the pressure. As more and more light hydrocarbons are liberated from the oil, the density and viscosity of the heavier oil phase remaining in the reservoir increases. In a parallel manner, as more and more heavier ponents are vaporized, the gas phase viscosity and density increases also. Because the gas is produced more easily than the oil, the increase in gasoil ratio as the reservoir pressure is reduced can have severe consequences on recovery, as shown in section , SolutionGas Drive.There is a formation volume factor for oil just as there is for gas. Oil will shrink in volume between reservoir and surface conditions primarily as a result of the solution gas evolved. Figure shows schematically how this occurs for oil and. water, and how the gas volume expands. Figure shows how the formation volume factors for oil and gas change with decreasing reservoir pressure. For example, to reservoir barrels per stock tank barrel (bbl/STB) is a mon range for oil formation volume factors. As oil is produced from a reservoir above the bubble point, the formation volume factor will increase slightly as the oil expands. At this point, no gas is being liberated and the pressed oil is simply expanding as volumes are removed. However, when the bubble point is reached, gas begins to e out of solution, and a volume of the remaining reservoir oil will have less of a difference between it and its surface volume. Consequently the formation volume factor will decrease as the pressure is decreased and more gas is liberated from the oil. In addition, the gas, which is liberated from the oil, will begin to form a free gas phase in the reservoir. This gas volume will react to the change in pressure with production. When the reservoir pressure is reduced from 5000 psia to 2000 psia (13,790 kPa), the same reservoir volume of produced gas that expanded to 1500 cu ft ( m3) will now only expand to 702 cu ft ( m3) at standard conditions. Consequently, there are less standard cubic feet of gas to each reservoir barrel of gas produced as the pressure depletes with production. The phase diagram and PVT properties are different for every mixture of hydrocarbons, within general ranges. For a given reservoir fluid, the phase behavior is usually determined by catching a sample at reservoir conditions, subjecting it to pressure and temperature variations in the laboratory, and measuring the resulting changes in liquid and gas volumes and their individual phase properties. Where samples are not present, the reservoir engineer often uses published correlations. Natural Drive MechanismsThe production of oil and gas is possible only because of potential energy stored in the pressed fluids and rock of the reservoir or because of energy added to the reservoir. Reservoir energy is released when a pressure difference is imposed between the wellbore and the reservoir, and a well is produced. While this pressure differential is maintained, fluids will flow from high to lowpressure. If the pressure at the wellbore is sufficient to lift the column of fluid, the well will flow。 if not, it must be artificially lifted. Energy sources in a reservoir vary and this largely determines the efficiency of oil and gas recovery. The sources of energy for oil production are:(1) Gas dissolved in oil(2) Free gas under pressuregas reservoiroil reservoir with free gas cap(3) Fluid pressurehydrostatichydrodynamicpressed water, gas, oil(4) Elastically pressed rock(5) Gravity(6) Combinations of the aboveThe dominant type of energy determines the type of drive mechanism attributed to a given reservoir. These mechanisms are:Solutiongas drive: gas dissolved in oil Gascap drive: free gas cap under pressure Water drive: hydrostatic/hydrodynamicGravity drainage: pressure and pressed water differences of fluids. SolutionGas DriveNatural gas dissolved in oil will e out of solution and form bubbles, which expand as the fluid pressure is reduced. This action, similar to that occurring in the uncorking of a champagne bottle, provides the driving force in a solutiongas drive reservoir (also called dissolved gas drive, internal gas drive, and depletion drive). When production is first initiated, pressed oil expands in response to the pressure reduction at the wellbore. This continues until the bubblepoint pressure is reached. At the bubblepoint, gas bubbles begin to evolve from solution. With further pressure reduction, the expanding bubbles continue to support production. This occurs until they reach a critical saturation—the saturation