CONTENTS
1 DEFINITIONS 2 PHASE BEHAVIOUR OF PURE SUBSTANCES 2.1 The Phase Diagram 3 TWO COMPONENT SYSTEMS 3.1 Pressure - Temperature Diagrams 3.2 Pressure Volume Diagram 4 MULTI-COMPONENT HYDROCARBON 4.1 Pressure Volume Diagram 4.2 Pressure Temperature Diagram 4.3 Critical Point 4.4 Retrograde Condensation 5 MULTI-COMPONENT HYDROCARBON 5.1 Oil Systems (Black Oils and Volatile Oils) 5.2 Retrograde Condensate Gas 5.3 Wet Gas 5.4 Dry Gas 6 COMPARISON OF THE PHASE DIAGRAMS OF RESERVOIR FLUIDS 7 RESERVOIRS WITH A GAS CAP 8 CRITICAL POINT DRYING
LEARNING OBJECTIVES
Having worked through this chapter the Student will be able to:
General • Define; system, components, phases, equilibrium, intensive and extensive properties. Pure Components • Sketch a pressure-temperature (PT) diagram for a pure component and illustrate on it; the vapour-pressure line, critical point, triple point, sublimation-pressure line, the melting point line, the liquid, gas and solid phase zones. • Define the critical pressure and critical temperature for a pure component. • Describe briefly with the aid of a PT diagram the behavior of a pure component system below( left|) and above ( right) of the critical point. • Sketch the pressure- volume (PV) diagram for a pure component illustrating the behavior above the bubble point, between the bubble and dewpoint and below the dewpoint. • Sketch a series of PV lines for a pure component with a temperature below, at and above the critical temperature. • Sketch the three dimensional phase diagram for pure component systems. Two Components • Plot a PV diagram for a 2 component system and identify key parameters. • Plot a PV diagram for a 2 component system and identify key parameters and the relationship to the vapour pressure lines for the two pure components. • Sketch the critical point loci for a series of binary mixtures including methane and indicate how a mixture a mixture of methane and another component can exist as 2 phases at pressures much greater than the 2 phase limit for the two contributing components. • Draw a PT diagram for a two component system, to illustrate the cricondentherm, cricondenbar and the region of retrograde condensation. • Define the terms cricondentherm and cricindenbar. • Explain briefly what retrograde condensation is. Multicomponent Systems • Sketch a PT and PV diagrams to illustrate the behaviour at constant temperature for a fluid in a PVT cell. Identify key features. • Draw a PT diagram for a heavy oil, volatile oil, retrograde condensate gas, wet gas and dry gas. Illustrate and explain the behaviour of depletion from the undersaturated condition to the condition within the phase diagram. • Describe briefly with the aid of a sketch, the reasons for and the process of gas cycling, for retrograde gas condensate reservoirs. • Plot a PT diagram for a reservoir with a gas cap to illustrate the gas at dew point and oil at bubble point. Miscellaneous • With the aid of sketch explain the process of critical point drying.
Oil and gas reservoir fluids are mixtures of a large number of components which when subjected to different pressure and temperatures environments may exist in different forms, which we call phases. Phase behaviour is a key aspect in understanding the nature and behaviour of these fluids both in relation to their state in the reservoir and the changes which they experience during various aspects of the production process. In this chapter we will review the qualitative aspects of the behaviour of reservoir fluids when subjected to changes in pressure and temperature.
1 DEFINITIONS
Before we consider the effect of temperature and pressure on hydrocarbon systems we will define some terms: • System - amount of substances within given boundaries under specific conditions composed of a number of components. Everything within these boundaries are part of the system and that existing outside of the boundaries are not part of the system. If anything moves across these boundaries then the system will have changed.• Components - those pure substances which produce the system under all conditions. For example, in the context of reservoir engineering, methane, ethane, carbon dioxide and water are examples of pure components. • Phases - This term describes separate, physically homogenous parts which are separated by definite boundaries.1 Examples in the context of water are the three phases, ice, liquid water and water vapour. • Equilibrium - When a system is in equilibrium then no changes take place with respect to time in the measurable physical properties of the separate phases.
• Intensive and extensive properties - physical properties are termed either
intensive or extensive. Intensive properties are independent of the quantity of material present. For example density, specific volume and compressibility factor are intensive properties whereas properties such as volume and mass are termed extensive properties; their values being determined by the total quantity of matter present. The physical behaviour of hydrocarbons when pressure and temperature changes can be explained in relation to the behaviour of the individual molecules making up the system. Temperature, pressure and intermolecular forces are important aspects of physical behaviour. The temperature is an indication of the kinetic energy of the molecules. It is a physical measure of the average kinetic energy of the molecules. The kinetic energy increases as heat is added. This increase in kinetic energy causes an increase in the motion of the molecules which also results in the molecules moving further apart.
The pressure reflects the frequency of the collision of the molecules on the walls of its container. As more molecules are forced closer together the pressure increases. Intramolecular forces are the attractive and repulsive forces between molecules. They are affected by the distance between the molecules. The attractive forces increases as the distance between the molecules decreases until however the electronic field of the molecules overlap and then further decrease in distance causes a repulsive force, which increases as the molecules are forced closer together. The molecules in gases are widely spaced and attractive forces exist between the molecules whereas for liquids where the molecules are closer together there is a repelling force which causes the liquid to resist further compression. The hydrocarbon fluids of interest in reservoir systems are composed of many compo-nents however in understanding the phase behaviour of these systems it is convenient to reflect on the behaviour of single and two component systems.
2 PHASE BEHAVIOUR OF PURE SUBSTANCES
2.1 The Phase Diagram
It is beneficial to study the behaviour of a pure hydrocarbon under varying pressure and temperature to gain an insight into the behaviour of more complex hydrocarbon systems. Phase diagrams are useful ways of presenting the behaviour of systems. They are generally plots of pressure versus temperature and show the phases that exist under these varying conditions. Figure 1 gives a pressure - temperature phase diagram for a single-component system on a pressure temperature diagram and the following points are to be noted.
Pressure Temperature Melting Point Sublimatio n Vapour Pressu re Triple Point Critical Point C 1 2 3 Vapour Liquid Solid Gas Figure 1 Pressure temperature diagram for a single component system • Define the black oil model description of the composition of a reservoir fluid. • Explain briefly what PNA analysis is and its application.
Vapour Pressure Line
The vapour pressure line divides regions where the substance is a liquid, 2, from regions where it is a gas, 3. Above the line indicates conditions for which a substance is a liquid, whereas below the line represent conditions under which it is a gas. Con-ditions on the line indicate where both liquid and gas phases coexist. Critical Point The critical point C. is the limit of the vapour pressure line and defines the critical
temperature, Tc and critical pressure, Pc of the pure substance. For a pure substance
the critical temperature and critical pressure represents the limiting state for liquid and gas to coexist. A more general definition of the critical point which is both applicable to multi component as well as single component systems is; the critical point is the point at which all the intensive properties of the gas and liquid are equal. Triple Point The triple point represents the pressure and temperature at which solid, liquid and vapour co-exist under equilibrium conditions. Petroleum engineers seldom deal with hydrocarbons in the solid state, however, more recently solid state issues are a concern with respect to wax, asphaltenes and hydrates. Sublimitation-Pressure Line The extension of the vapour-pressure line below the triple point represents the con-ditions which divides the area where solid exists from the area where vapour exists and is also called the sublimation - pressure line.
Melting Point Line
The melting line divides solid from liquid. For pure hydrocarbons the melting point generally increases with pressure so the slope of the line is positive. (Water is ex-ceptional in that its melting point decreases with pressure).
3 USE OF PHASE DIAGRAMS
3.1 Pressure -Temperature Diagrams (PT)
Consider the behaviour of a cell charged with a pure substance and the volume varied by the frictionless displacement of a piston as shown in figure 2, below. P1 Pb P Pd P2 Liquid Gas Figure 2 Phase Changes With Pressure at Constant Temperature For example, following the path 1 - 2 in figure 3 on the pressure-temperature diagram, ie holding temperature constant and varying pressure by expansion of the cylinder.
c Pc Tc Pressure Temperature Solid Liquid
Melting - Point Line
Vapour - pressure line T Gas E A B G F 1 2 3 4 Figure 3 Pressure-Temperature Diagram for a Single-Component System As the pressure is reduced, the pressure falls rapidly until a pressure is reached lying on the vapour pressure line. A gas phase will begin to form and molecules leave the liquid. At further attempts to reduce the pressure the volume of gas phase increases, while liquid phase volume decreases but the pressure remains constant. Once the liquid phase disappears further attempts to reduce pressure will be successful as the gas expands. Above the critical temperature, following the path 3 - 4, a decrease in pressure will cause a steady change in the physical properties, for example a decrease in density but there will not be an abrupt density change as the vapour pressure line is not crossed. No phase change takes place. Consider the behaviour of the system around the critical point. If we go from point A to point B, by increasing the temperature, we go though a distinctive phase change on the vapour pressure line where two phases, liquid and gas co-exist. If we now go a different route to B, starting with the liquid state at ‘A’ increase the pressure iso-thermally (constant temperature ) to a value greater than Pc at E. Then keeping the pressure constant increase the temperature to a value greater than Tc at point F. Now decrease the pressure to its original value at G. Finally, decrease the temperature keeping the pressure constant until B is reached. The system is now in the vapour state and this state has been achieved without an abrupt phase change. The vapour states are only meaningful in the two phase regions. In areas far removed from the two phase region particularly where pressure and temperature are above the critical values, definition of the liquid or gaseous state is impossible and the system is best described as in the fluid state. The pressure-temperature diagram for ethane is given in Figure 4.
400 500 600 700 800 40 60 80 100 120 Liquid Vapor c Temperature - º F Pressure - PSIA Figure 4 Pressure-Temperature diagram of Ethane
3.2 Pressure Volume Diagram (PV)
The process just described in 3.1 can also be represented on a pressure-volume dia-gram at constant temperature (Figure 5). As the pressure is reduced from 1, a large change in pressure occurs with small change in volume due to the relatively low compressibility of the liquid. When the vapour pressure is reached gas begins to form. This point is called the bubble point, ie the point at which the first few mol-ecules leave the liquid and form small bubbles of gas. As the system expands more liquid is vaporised at constant pressure. The point at which only a minute drop of liquid remains is called the dew point. Sharp breaks in the line denote the bubble point and dew point.
4
PVT CELL PV DIAGRAM
All Liquid
All Gas First Gas Bubble
Last Drop of Liquid
1
2
Liquid state-rapid change of pressure with small volume change
Pressure remains constant while both gas and liquid are present
Dew Point Gas Bubble Point
Volume
Pressure
TWO PHASE REGION SINGLE PHASE T > Tc T < Tc T2 > Tc Figure 5 Pressure-Volume diagram for a Single-Component System For a pure substance vapour pressures at bubble point and dew point are equal to the vapour pressure of the substance at that temperature. Above the critical point, ie 3 - 4 , the PV behaviour line shows no abrupt change and simply shows an expansion of the substance and no phase change. This fluid is called a super critical fluid. A series of expansions can be performed at various constant temperatures and a pressure volume diagram built up and the locus of the bubble point and dew point values gives the bubble point and dew point lines which meet at the critical point. Conditions under the bubble point and dew point lines represent the conditions where two phases coexist whereas those above these curves represent the conditions where only one phase exists. At the critical temperature the P,T curve goes through the critical point. Figure 6
10 Bubb le P oint Cur ve Dew Point C urve 4 3
Liquid state rapid change of temperature with small volume change
Critical Point 1
2
Volume
Pressure
TWO PHASE REGION
SINGLE PHASE
T = Tc
T < Tc
T > Tc
Pressure remains constant while both gas and liquid are present
Figure 6 Series of PV lines for a pure component The pressure volume curve for pure component ethane is given in figure 7 The locus of the bubble points and dew points form a three-dimensional diagram when projected in to a P-T diagram give the vapour pressure line (Figure 8). 400 500 600 700 800 900 0 0.05 0.10 0.15 0.20 0.25 Liquid Vapor C D B A
Specific Volume - Cu. Ft. per lb.
Pressure - PSIA Two Phase Region
110 º F 90 º F
60 º F
Volume Temperature Temperature Liquid Gas Liquid
Gas and Liquid
Gas
Critical Point
Critical Point
Vapor Pressure Curve Dew Point Line
Bubble Point Line
Pressure
Pressure
Figure 8 Three Dimensional Phase Diagram for a Pure Component System
4 TWO COMPONENT SYSTEMS
Reservoir fluids contain many components but we will first consider a system con-taining two components, such a system is called a binary.
4.1 Pressure Volume Diagram
The behaviour of a mixture of two components is not as simple as for a pure sub-stance. Figure 9 shows the P-V diagram of a two-component mixture for a constant temperature system. Pressure Volume Liquid Gas
Liquid and Gas Bubble Point
Dew Point
1 The isotherm is very similar to the pure component but the pressure increases as the system passes from the dew point to the bubble point. This is because the composi-tion of the liquid and vapour changes as it passes through the two-phase region. At the bubble point the composition of the liquid is essentially equal to the composi-tion of the mixture but the infinitesimal amount of gas is richer in the more volatile component. At the dew point the composition of vapour is essentially the mixture composition whereas the infinitesimal amount of liquid is richer in the less volatile component. Breaks in the line are not as sharp as for pure substances. The pressure-volume diagram for a specific n-pentane and n-heptane mixture is given in Figure 10. Clearly a different composition of the two components would result in a different shape of the diagram. 100 200 300 400 500 600 0 0.1 0.2 0.3 0.4 0.5 Critical point
Specific Volume - Cu. Ft. per lb.
Pressure - PSIA 454 º F 450 º 425 º 400 º 350 º 300 º
Dew Point Line
Bubble Point Line
Figure 10 Pressure-Volume Diagram for N-Pentane and N-Heptane (52.4 mole % Heptane) ref. 4
4.2 Pressure Temperature Diagram
Compared to the single line representing the vapour pressure curve for pure substances there is a broad region in which the two phases co-exist. The two-phase region of the diagram is bounded by the bubble point line and the dew point line, and the two lines meet at the critical point. Points within a loop represent two-phase systems (Figure 11). Consider the constant temperature expansion of a particular mixture composition. At 1 the substance is liquid and as pressure is reduced liquid expands until the bubble point is reached. The pressure at which the first bubbles of gas appear is termed the bubble point pressure. As pressure is decreased liquid and gas co-exist until a minute amount of liquid remains at the dew point pressure. Further reduction of pressure causes expansion of the gas.
By carrying out a series of constant temperature expansions the phase envelope is defined and within the envelope contours of liquid to gas ratios obtained. These are called quality lines and describe the pressure and temperature conditions for equal volumes of liquid. The quality lines converge at the critical point.
4.3 Critical Point
In the same way as pure components, when more than one component is present liquid and gases cannot coexist, at pressures and temperatures higher than the criti-cal point. The critical point for a more than one component mixture is defined as a point at which the bubble point line and dew point line join, ie. it is also the point at which all the intensive properties of the liquid are identical. This aspect is a very severe test for physical property prediction methods. If the vapour pressure lines for the pure components are drawn on the P-T diagram then the two-phase region for the mixture lies between the vapour pressure lines. In the figure 11 the critical temperature of the mixture TcAB lies between TcA and TcB whereas the critical pressure PcAB lies above PcA and PcB. It is important to note thatthe PcAB and TcAB of the mixture does not necessarily lie between the Pc & Tc of the
two pure components. CA CB PCAB TCA Pressure Temperature Liquid Gas 1 2 TCAB TCB PCA PCB Bubble - Poin t Line Dew Point % Liq. 100 75 50 25 0 Critical Point Figure 11 Pressure-Temperature Diagram for a Two Component System A specific mixture composition will give a specific phase envelope lying between the vapour pressure lines. A mixture with different proportions of the same components will give a different phase diagram. The locus of the critical point of different mix-ture compositions is shown in Figure 12 for the ethane and n-heptane system, and in Figure 13 for a series of binary hydrocarbon mixtures. Figure 13 demonstrates that for binary mixture e.g. Methane and n-decane two phases can coexist at conditions of pressure considerably greater than the two phase limit, critical conditions for the
1 0 100 200 300 400 1400 1200 1000 800 600 400 200 0 500 600 Temperature º F C2 C1 A1 A2 A3 B1 B2 B3 B A C C3 C7 Dew Po intlin e N-Hep tane Etha ne Bubble Poi nt Line Composition No Wt % Ethane C 100.00 C1 90.22 C2 50.25 C3 9.78 C7 N-Heptane Pressure, lbs./Sq. In. ABS Figure 12 Pressure-Temperature Diagram for the Ethane-Heptane System 2
0 1000 2000 3000 4000 5000 6000 0 -100 0 100 200 300 400 500 600 700 Temperature º F Pressure Lbs. (psia) Meth ane
Ethane PropaneN-ButaneN- Pentan e
N-Hexane N-HeptaneN-Decane
Two Phases Single Phase Figure 13 Critical Point Loci for a Series of Binary Hydrocarbon Mixtures 2
4.4 Retrograde Condensation
Within the two phase region our two component system there can be temperatures and pressures higher than the critical temperature where two phases exist and similarly pressures. These limiting temperatures and pressures are the cricondentherm and cricondenbar . The cricondentherm can be defined as the temperature above which liquid cannot be formed regardless of pressure, or expressed differently, as the maxi-mum temperature at which two phases can exist in equilibrium. The cricondenbar can be defined as the pressure above which no gas can be formed regardless of tem-perature or as the maximum pressure at which two phases can exist in equilibrium. (Figure 14). These limits are of particular significance in relation to the shape of the diagram in figure 14. Consider a single isotherm on Figure 14. For a pure substance a decrease in pressure causes a change of phase from liquid to gas. For a two-component system below Tc a decrease in pressure causes a change from liquid to gas. We now consider the constant temperature decrease in pressure, 1-2-3 , in figure 14 at a temperature between the critical temperature and the cricondentherm. As pressure is decreased from 1 the dew point is reached and liquid forms, i.e., at 2 the system is such that 5% liquid and 95% vapour exists, i.e. a decrease in pressure has caused achange from gas to liquid, opposite to the behaviour one would expect. The phenom-1 and vaporisation occurs and the dew point is again reached where the system is gas. Retrograde condensation occurs at temperatures between the critical temperature and cricondentherm. The retrograde region is shown shaded in the figure. Bubb le Po int Li ne
Dew Point Line
% Liq. 100 75 50 25 5 10 0 Pressure Temperature Liquid Gas 1 2 3 Cricondenbar Cricondentherm
Region of retrograde condensation
Figure 14 Phase Diagram Showing Conditions for Retrograde Considerations
5. MULTI-COMPONENT HYDROCARBON
Using two component systems we have examined various aspects of phase behaviour. Reservoir fluids contain hundreds of components and therefore are multicomponent systems. The phase behaviour of multicomponent hydrocarbon systems in the liq-uid-vapour region however is very similar to that of binary systems however the mathematical and experimental analysis of the phase behaviour is more complex. Figure 15 gives a schematic PT & PV diagram for a reservoir fluid system. Systems which include crude oils also contain appreciable amounts of relatively non-volatile constituents such that dew points are practically unattainable.
PVT CELL PHASE DIAGRAM
All Liquid
Gas / 40% Liquid
All Gas First Gas Bubble
Last Drop of Liquid
"a" Critical Point Dew Point Bubble Point Bubble Point Temperature Pressure Pressure Volume Liquid Bubb le Po int Li ne Dew Po int Lin e 80% L iquid 60% 40% 20% Dew Point Figure 15 Phase Diagrams for Multicomponent Systems We will consider the behaviour of several examples of typical crude oils and natural gases: Low-shrinkage oil (heavy oil - black oil) High-shrinkage oil (volatile oil) Retrograde condensate gas Wet gas Dry Gas Figure 16 is a useful diagram to illustrate the behaviour of the respective fluid types above. However it should be emphasised that for each fluid type there will be different scales. The vertical lines help to distinguish the different reservoir fluid types. Isothermal behaviour below the critical point designates the behaviour of oil systems and the fluid is liquid in the reservoir, whereas behaviour to the right of the critical point illustrates the behaviour of systems which are gas in the reservoir.
1 X5 Pressure Temperature % Liquid Gas (Gas) Black
Oil Volatile Oil Condensate Gas Gas
TM 2 75 100 50 25 20 15 10 5
0 Single Phase Region
Single Phase Region (Liquid)
Single Phase Region
Two Phase Region
CP
Where:
Pb = Bubble point pressureat indicated temperature Pm = Maximum pressure at whichtwo phases can coexist Tm = Maximum temperature at which two phases can coexist C = Critical conditions X5 = Cricondentherm Bubbl e Poin t Line Dew Point L ine Pm Pb Figure 16 Phase diagram for reservoir fluids
5.1 Oil Systems ( Black Oils and Volatile Oils)
Figures 17&18 illustrate the PT phase diagrams for black and volatile oils. The two-phase region covers a wide range of pressure and temperature. Tc is higher than the reservoir temperature. In figure 17 the line 1-2-3 represents the constant reservoir temperature pressure reduction that occurs in the reservoir as crude oil is produced for a black oil. These oils are a common oil type. The dotted line shows the conditions encountered as the fluid leaves the reservoir and flows through the tubing to the separator. If the initial reservoir pressure and temperature are at 2, the oil is at its reservoir bubble point and is said to be saturated, that is, the oil contains as much dissolved gas as it can and a further reduction in pressure will cause formation of gas. If the initial reservoir pressure and temperature are at 1, the oil is said to be undersaturated, i.e. The pressure in the reservoir can be reduced to Pb before gas is released into the
Sep. Pressure Temperature Liquid Gas 1 Undersaturated 2 Saturated 3 100 75 50 25 0 Critical Point Dew Point line Bubb le Po int Li ne Mole % Liq. Pb Figure 17 Phase Diagram for a Black Oil As the pressure is dropped from the initial condition as a result of production of flu-ids, the fluids remain in single phase in the reservoir until the bubble point pressure corresponding to the reservoir temperature is reached. At this point the first bubbles of gas are released and their composition will be different from the oil being more concentrated in the lighter ( more volatile) components. When the fluids are brought to the surface they come into the separator and as shown on the diagram, the separa-tor conditions lie well within the two phase region and therefore the fluid presents itself as both liquid and gas. The pressure and temperature conditions existing in the separator indicate that around 85% liquid is produced, that is a high percentage and as a result the volume of liquid at the surface has not reduced a great amount compared to its volume at reservoir conditions. Hence the term low-shrinkage oil. As the pressure is further reduced as oil is removed from the reservoir, point 3 will be reached and 75% liquid and 25% gas will be existing in the reservoir. Strictly speaking once the reservoir pressure has dropped to the bubble point, beyond that the phase diagram does not truly represent the behaviour of the reservoir fluid. As we will see in the chapter on drive mechanisms, below the bubble point gas produced flows more readily than the associated oil and therefore the composition of the reservoir fluid does not remain constant. The system is continually changing in the reservoir and therefore the related phase diagram changes. The summary characteristics for a black oil sometimes termed a heavy oil or low shrinkage oil are as follows. Broad-phase envelope High percentage of liquid High proportion of heavier hydrocarbons GOR < 500 SCF/STB
0 Oil gravity 30˚ API or heavier Liquid - black or deep colour Volatile oil contains a much higher proportion of lighter and intermediate hydocar-bons than heavier black oil and therefore they liberate relatively large volumes of gas leaving smaller amounts of liquid compared to black oils. For this reason they used to be called high shrinkage oils. The diagram in figure 18 shows similar behaviour to the black oil except that the lines of constant liquid to gas are more closely spaced. Points 1 and 2 have the same meaning as for the black oil. As the pressure is reduced below 2 a large amount of gas is produced such that at 3 the reservoir contains 40% liquid and 60% gas. At separator conditions 65% of the fluid is liquid, i.e. less than previous mixture. The summary characteristics for a volatile sometimes termed a heavy oil or high shrinkage oil when compared to black oils are as follows. Not so broad phase envelope as black oil Fewer heavier hydrocarbons Deep coloured API < 50˚ GOR < 8000 SCF/STB Pressure Temperature Liquid Gas 1 2 3 100 75 50 25 0 Critical Point Mole % Liq. Sep.
Bubble point line
Dew point lin e 40 Figure 18 Phase Diagram for a Volatile Oil Clearly, for these fluids, it is the composition of the fluid that determines the nature of the phase behaviour and the relative position of the saturation lines, (bubble point and dew point lines), the lines of constant proportion of gas/liquid and the critical point.
For both of these fluids types one can prevent the reservoir fluid going two phase by maintaining the reservoir pressure above its saturation pressure by injecting flu-ids into the reservoir. The most common practise is the use of water as a pressure maintenance fluid.
5.2 Retrograde Condensate Gas
If the reservoir temperature lies between the critical point and the cricondentherm a
retrograde gas condensate field exists and Figure 19 gives the PT diagram for such
a fluid. Above the phase envelope a single phase fluid exists. As the pressure de-clines to 2 a dew point occurs and liquid begins to form in the reservoir. The liquid is richer in heavier components than the associated gas. As the pressure reduces to 3 the amount of liquid increases. Further pressure reduction causes the reduction of liquid in the reservoir by re-vaporisation. It is important to recognise that the phase diagram below for a retrograde condensate fluid represents the diagram for a constant composition system. Before production the fluid in the reservoir exists as a single phase and is generally called a gas. It is probably more accurate to call it a dense phase fluid. If the reservoir drops below the saturation pressure the dew point, then retrograde condensation oc-curs within the formation. The nature of this condensing fluid is only in recent years being understood. It was previously considered that the condensing fluid would be immobile since its maximum proportion was below the value for it to have mobil-ity. It was considered therefore that such valuable condensed fluids would be lost to production and the viability of the project would be that from the ‘wet’ gas. Bubb le Point Line Dew Po int Line Pressure Temperature Liquid Gas 1 2 3 100 75 50 25 10 5 0 Critical Point Mole % Liq. Sep. Figure 19 Phase Diagram for a Retrograde Condensate Gas One of the development options for such a field therefore is to set in place a pressure maintenance procedure whereby the reservoir pressure does not fall below the saturation pressure. Water could be used as for oils but gas might be trapped behind the water as the water advances through the reservoir. Gas injection, called gas
cycling ( Figure 20 ), is the preferred yet very expensive option. In this process the produced fluids are separated at the surface and the liquid condensates, high value product relative to heavy oil, are sent for export, in an offshore situation probably by tanker. The ‘dry’ gas is then compressed and reinjected into the reservoir to maintain the pressure above the dew point. Clearly with this process the pressure will still decline because the volume occupied by the gas volume of the exported liquid is not being replaced. Full pressure maintenance is obtained by importing dry gas equivalent to this exported volume from a nearby source. Eventually the injected dry gas displaces the ‘wet’ gas and then the field can be blown down as a conventional dry gas reservoir, if a suitable export route for the gas is then in place. The process described is very costly and carries with it a number of risks not least the possibility of early dry gas breakthrough. Imported Gas Gas Surface Separation
Gas Water Contact
Dry Gas Reinjection
Injection Well Production Well Condensate Sales Figure 20 Gas cycling process Recent research has shown that the nature of oil forming in porous media by this ret-rograde process may not be as first considered. The isolation of condensing liquids in porous rock is dependant on the relative strength of the interfacial tension and viscous forces working in the rock. If the relative magnitude of these is high then the fluid will be trapped however if they are low as a result of low interfacial tension, which is the case nearer the critical point, then the condensing liquids may be mobile and move as a result of viscous and gravity forces. Condensate liquids have been able to flow at saturations well below the previously considered irreducible saturation proportion. Established relative permeability thinking is having to be reconsidered in the context of gas condensates. The phenomena just described may give explanation to the observation sometimes made of an oil rim below a gas condensate field. Looking at the PT phase diagram one might consider that "blowing the reservoir down"
quickly might be an option and as a result vaporise the condensed liquids in the for-mation. This is not a serious option since once the reservoir pressure falls below the dew point the impact of the increasing liquid proportion remaining in the reservoir causes the phase diagram to move to the right relative to reservoir conditions, and any vaporising will be of the lightest components which are likely to be in good supply and therefore not of significant value. The summary characteristics for a retrograde gas condensate fluid are as follows. Contains more lighter HC’s and fewer heavier HC’s than high-shrinkage oil API up to 60˚ API GOR up to 70,000 SCF/STB Stock tank oil is water-white or slightly coloured
5.3 Wet Gas
The phase diagram for a mixture containing smaller hydrocarbon molecules lies well below the reservoir temperature. Figure 21. The reservoir conditions always remain outside the two-phase envelope going from 1 to 2 and therefore the fluid ex-ists as a gas throughout the reduction in reservoir pressure. For a wet gas system, the separator conditions lie within the two-phase region, therefore at surface heavy components present in the reservoir fluid condense under separator conditions and this liquid is normally called condensate. These liquid condensates have a high propor-tion of light ends and sell at a premium. The proportion of condensates depend on the compositional mix of the reservoir fluid as represented by the iso-volume lines on the PT diagram. Pressure Temperature Liquid Gas 1 2 100 75 50 25 5 0 Critical Point Mole % Liq. Sep. Figure 21 Phase Diagram for a Wet Gas The reference wet gas, clearly does not refer to the system being wet due to the pres-ence of water but due to the production condensate liquids.In some locations where there are natural petroleum leakages at the surface, when condensates are produced they are sometimes called white oil. The summary characteristics for wet gas are as follows. GOR < 100,000 SCF/STB Condensate liquid > 50˚ API
5.5 Dry Gas
The phase envelope of the dry gas, which contains a smaller fraction of the C2-C6 components, is similar to the wet gas system but with the distinction that the separator also lies outside the envelope in the gas region (Figure 22). The term dry indicates therefore that the fluid does not contain enough heavier HC’s to form a liquid at surface conditions. The summary characteristics for a dry gas are as follows. GOR > 100,000 SCF/STB Pressure Temperature Liquid Gas 1 2 75 50 25 Critical Point Sep. Figure 22 Phase Diagram for a Dry Gas6 COMPARISON OF THE PHASE DIAGRAMS OF RESERVOIR
FLU-IDS
Figure 16 gave a rather simplistic representation of the various types of fluids with respect to the relative position of reservoir temperature with respect to the phase diagram. In reality it is the phase diagram which changes according to composition and the relative position of the reservoir temperature and separator conditions, and these determine the character of the fluid behaviour. Figure 23 gives a better indica-tion of the various reservoir types with respect to a specific pressure and temperaturescales. As the proportion of heavier components in the respective fluids increases the phase envelope moves to the right.
Dry Gas Wet Gas Gas Condensate Separator
Critical Point
Volatile
Oil BlackOil
Temperature (ºC)
Pressure
Figure 23 Relative positions of phases envelopes
7 RESERVOIRS WITH A GAS CAP
Figure 24 illustrates a simplification of the phase diagrams associated with an oil reservoir with a gas cap. The phase diagram for the gas cap fluid, the oil reservoir fluid and for a fluid representing the combination fluid of a mixture of gas and liquid in the same proportions as they exist in the reservoir are presented.
Pressure Temperature CG CL Reservoir Liquid Total Reservoir Fluid Reservoir Temperature Reservoir Gas C Separator Initial Reservoir Pressure Pd=Pb Figure 24 Phase Diagram for an Oil Reservoir with a Gas Cap The diagram illustrates that at the gas-oil contact the gas is at its dew pressure, the oil is at its bubble point pressure and the combination fluid lies on the constant propor-tion quality line representing the ratio of the gas and oil as they exist in the reservoir system. The gas cap may be dry, wet or condensate depending on the composition and phase diagram of the gas.
8 CRITICAL POINT DRYING
Although not part of the topic of phase behaviour in the context of reservoir fluids it is useful to illustrate the application in a very practical application in the context of the evaluation of rock properties. Critical point drying has been used by a number of sciences to prepare specimens of delicate materials for subsequent micro visual analysis where conventional preparation techniques will destroy delicate fabric. Critical point drying takes advantage of the behaviour of fluids around the critical point where one can go from one phase type, like liquid to gas without a visually observed phase change. In the 1980’s it was observed in a UK offshore field that the interpreted permeability for a well sand in the zone where water injection was proposed was different from well injectivity tests when compared to the core analysis value where the value was many times more. The extent of this difference was such that permeabilities from the well test gave values which would prevent injection to take place whereas those from the core tests would result in practical injectivities. Clearly the difference was important.
The company concerned embarked on a more sophisticated core recovery and analy-sis process suspicious that perhaps the fabric of the rock was being affected by core preparation methods. They resorted to critical point drying.
The core recovered from the water zone of the reservoir from a subsequent new well was immersed and transferred to the test laboratory submerged in ‘formation water’. At the laboratory a core plug sample was extracted, cut to size and loaded into a core holder still submerged in the water. The core was then mounted in a flow rig (figure 25) and an alcohol which is miscible with water displaced the water in the core. Carbon dioxide at a pressure and temperature where it is in the liquid state was then introduced which miscible displaced the alcohol. The temperature and pressure was then adjusted taking them around the critical point rather than across the vapour pressure line of the PT phase diagram (figure 26) ending up with a temperature and pressure below the vapour pressure line with the fluid now in a gaseous state. After this process the permeability was measured to be of the same order as that interpreted from the well injectivity test. The reason for this difference was subsequently demonstrated to be a very fragile clay which during conventional core recovery and cleaning was damaged to an extent that its pore blocking structure was destroyed.
P
T
Core In Holder Figure 25 Critical point drying system Temperature Pressure Vapour Pressure Line GAS LIQUID Critical Point Critical Point Drying RouteFigure 26 Critical point drying
REFERENCES
1. Fig 1 Daniels, F Farrington: “Outlines of Physical Chemistry,” John Wiley & Sons,Inc New York, 1948
2. Fig 2 Brown,GG et al. “ Natural Gasoline and Volatile Hydrocarbons,” Natural Gasoline Association of America, Tulsa, Okl., 1948.
Fig 10 Sage, S.G.,Lacy,W.N. Volumetric and Phase Behaviour of Hydrocarbons, Gulf Publishing Co.Houston 1949
