Phase Behaviour of Hydrocarbon Systems PDF

PHASE BEHAVIOUR OF HYDROCARBON SYSTEMS – IDEAL AND NON-IDEAL SYSTEM Petroleum reservoirs are broadly classified as oil or gas reservoirs. These broad classifications are further subdivided depending on: The composition of the reservoir hydrocarbon mixture Initial reservoir pressure and tempera...

PHASE BEHAVIOUR OF HYDROCARBON SYSTEMS – IDEAL AND NON-IDEAL SYSTEM Petroleum reservoirs are broadly classified as oil or gas reservoirs. These broad classifications are further subdivided depending on: The composition of the reservoir hydrocarbon mixture Initial reservoir pressure and temperature Pressure and temperature of the surface production The conditions under which these phases exist are a matter of considerable practical importance. These are conveniently expressed in different types of diagrams commonly called phase diagrams. One such diagram is called the pressure-temperature diagram. Pressure-temperature diagram and pressure-volume diagram of a single component are shown in Fig.1&2. 1 A saturated liquid is a state of a substance where it is at its boiling point and any additional heat will cause it to start vaporizing into a gas without increasing its temperature. In other words, it’s the point where the liquid is in equilibrium with its vapor at a given pressure and temperature. For example, in the graph you provided, the line between the 2 liquid and vapor regions represents the conditions under which ethane is a saturated liquid. At these points, ethane is ready to transition to a vapor if more heat is added.  Saturated liquid and vapour densities of single component system, ethane are shown in Fig.3. The graph in the image shows the saturated liquid and vapor densities of ethane. Boundary Line: The line between the liquid and vapor regions shows the phase transition of ethane. This is where ethane changes from liquid to vapor or vice versa at specific temperatures and densities. Phase behaviour of a two component or binary system is shown in Fig.4. Please note that the critical points of various mixtures are shifted. The image shows a pressure-temperature diagram for a binary system of ethane and n-heptane The image shows a pressure-temperature diagram for a binary system of ethane and n-heptane Critical Points: These are marked along the curves and indicate the conditions at which the mixture reaches its critical state, where liquid and vapor phases become indistinguishable. Bubble Point Line: This line indicates the conditions under which the first bubble of vapor forms from the liquid mixture. Dew Point Line: This line shows the conditions under which the first drop of liquid condenses from the vapor mixture. binary system refers to a mixture composed of two different components. These components can be different chemical substance The behavior of such mixtures under various conditions of3 temperature and pressure is often studied to understand their phase transitions, such as from liquid to vapor. In a binary system, the interactions between the two components can significantly affect properties like boiling points, critical points, and phase boundaries  The effect of composition divergence on the critical point refers to how changes in the mixture’s composition affect the critical point of a binary system Composition Divergence: In a binary system (a mixture of two components), the ratio of the two substances can vary. This variation in composition can shift the critical point.his is because the interactions between the molecules of the two different substances affect the overall behavior of the mixture. The critical point for each specific composition will be different, and this is often represented by a curve on a phase diagram. Effect of divergence in the composition on the critical point is shown in Fig.5. 4 MULTI-COMPONENT SYSTEM Phase diagram of a multicomponent system showing nomenclature of retrograde condensation is shown in Fig.6. These multicomponent pressure-temperature diagrams are essentially used to: Classify reservoirs Classify the naturally occurring hydrocarbon systems Describe the phase behaviour of the reservoir fluid To fully understand these diagrams the following terms are defined: Critical point Critical pressure and critical temperature Intensive properties Extensive properties Bubble point curve Dew point curve Phase envelope (two-phase Cricondentherm region) Cricondenbar Quality lines (iso-vol lines) Critical point: The critical point for a multicomponent mixture is referred to as the state of pressure and temperature at which all intensive properties of the gas and liquid phases are equal (point C). Critical pressure and critical temperature: At the critical point, the corresponding pressure and temperature are called the critical pressure pc and critical temperature Tc of the mixture. Intensive properties: Those properties that are independent of the amount of material under consideration (e.g. density, sp. Gravity, etc.). pressure also Extensive properties: Those properties that are directly proportional to the amount of material under consideration (e.g. weight, volume, heat content, etc.). Bubble point curve: The locus of the points of pressure and temperature at which the first bubble of gas is formed in passing from the liquid to the two phase region. 5  Dew point curve: Locus of the points of pressure and temperature at which the first droplet of liquid is formed in passing from the vapour to the two phase region. Phase envelope (two-phase region): The region enclosed by the bubble-point curve and the dew-point curve (line BCA), wherein gas and liquid coexist in equilibrium, is identified as the phase envelope of the hydrocarbon system. Cricondentherm (Tct): The Cricondentherm is defined as the maximum temperature above which liquid cannot be formed regardless of pressure (point E) or liquid and vapour can coexist in equilibrium. Cricondenbar (pcb): The Cricondenbar is the maximum pressure above which no gas can be formed regardless of temperature (point D) or liquid and vapour can co-exist in equilibrium. Quality lines (iso-vol lines): The dashed lines within the phase diagram are called quality lines. They describe the pressure and temperature conditions for equal volumes of liquids. Note that the quality lines converge at the critical point (point C). Classification of fluids in oilfield: 6 7 8 9 10 11 As shown in Fig. 6, at the initial reservoir conditions, corresponding to point L, the reservoir contains two phases, gas and liquid. Thus, in oilfield terminology, the reservoir consists of an oil accumulation with a gas cap. The gas-cap gas is classified as associated free gas and is usually in equilibrium with the contiguous oil accumulation. Thus the gas is at its dew point and the liquid at its bubble point. Phase behaviour of an oil reservoir with a gas-cap: As shown in Figure 12, the phase diagrams for the equilibrium phases are superimposed on the original phase diagram for the total mass of hydrocarbon in the reservoir. The equilibrium gas contains larger percentages of lighter hydrocarbons and lesser percentages of heavier hydrocarbons than does the equilibrium liquid. The gas-cap gas may be dry, wet, or condensate, depending on the composition and phase diagram of the gas. That shown in Figure 12 is a wet gas. 12