4.2 Petroleum fluid properties - Treccani

1 IntroductionReservoir fluids are composed of a large number ofcomponents. At ambient temperatures some of these arepresent in the gas phase, while others (resins andasphaltenes), due to their greater molecular weights, maybe present in solid phases. The ensemble of thesecharacteristics, related to phenomena of migration andheterogeneity of the formation, causes a non-homogeneous partitioning of the individual componentsin the reservoir, which must be taken into account in thedesign of the production process. Under certaintemperature and pressure conditions, solid phases maybe formed that strongly influence the production of fluid .

2 In addition, these fluids are always in contact withwater of a variable degree of salinity, which isproduced together with the crude, and at times in evenlarger quantities; this may cause problems due to saltprecipitation (when there is a mixture of differenttypes of water) and corrosion. On the other hand, the development of ahydrocarbon field requires considerable investment,especially in the case of offshore fields. In order tofinalize the development scheme leading to optimalrecovery (cost, quantity and quality), it is necessary tohave an accurate knowledge of the thermodynamicbehaviour of the fluid .

3 In fact, the surface facilitiesvary according to the type of fluid (oil or gas) and thetemperature and pressure conditions of the there is a risk of precipitation of heavycomponents, it is important to install suitableequipment or to use additives that can avoid suchproblems. Failing to recognize one of these processescan have serious economic and safety implications. To predict the fluid s thermodynamic evolutionunder the temperature and pressure conditionsencountered during production, mathematical modelsdesigned to calculate their behaviour are used. Thedevelopment of models capable of reproducing all ofthe physical phenomena requires knowledge of thefluid s composition.

4 Furthermore, laboratory data arerequired for the calibration of such this reason, fluid samples are taken as soon aspossible from the reservoir in order to performanalyses and thermodynamic experiments aimed atsimulating the temperature and pressure variations towhich the fluid will be subjected, so as to identifypotential problems. To be reliable, these experimentshave to be performed on samples representative of thereservoir fluid . The discussion that follows will be dedicated tothese problems. After a brief account of thethermodynamic behaviour of pure components andbinary mixtures, the various types of reservoir fluidswill be classified.

5 Subsequently, having described thereasons for which the composition of the reservoircomponents is not always homogeneous, the samplingprocedure will be described. A section is thendedicated to laboratory thermodynamic experimentsand finally some empirical correlations and equationsof state used to simulate the phase behaviour of thehydrocarbons will be Phase behaviourThe main components of Petroleum fluids arehydrocarbons. Reservoirs also contain water, howeverits influence on the thermodynamic behaviour of thefluids is secondary, and consequently the oil and gasphases are generally treated separately from the waterphase.

6 The behaviour of hydrocarbon mixtures in thereservoir and during production depends on thecomposition of the fluid as well as on the temperatureand pressure conditions it encounters. Understandingthis behaviour is of crucial importance to the487 VOLUME I / EXPLORATION, PRODUCTION AND fluid propertiesdevelopment of a hydrocarbon field, because it servesas the basis for the design of the production though the behaviour of these fluids is verycomplex, it can be explained on the basis of thebehaviour of simple fluids. As a result, the behaviourof pure components is covered first before going on tothat of binary mixtures, bearing in mind that the realfluids obey the same componentsFig.

7 1illustrates the behaviour of a pure componentby means of a pressure-volume diagram, which can bedescribed in the following way. Starting from point A(the component in the liquid state) and graduallyincreasing the volume (at constant temperature), thefollowing phenomena are observed: a) a rapiddecrease in pressure; b) the appearance of the firstbubbles of gas at point B; c) the increase in volume ofthe gas phase and the decrease in that of the liquidphase at constant pressure (line joining B to R); d) thedisappearance of the last drop of liquid (point R); ande) the much slower decrease in series of phenomena occurs for alltemperatures below the critical temperature (TC).

8 Abovethis temperature, the component remains in a singlephase and is referred to as being in the supercriticalstate. The set of bubble points form the bubble curve,while the dew points give rise to the dew curve. It is also possible to represent the behaviour of apure component on a pressure-temperature diagram(Fig. 2). All of the conditions at which the liquid and gasphases can co-exist are represented by the curve AC,where the bubble and dew curves are merged. In fact,according to the phase rule, at each temperature thereis only one pressure value for which the fluid can havetwo phases: if the number of components of a fluid isgiven by nand the number of phases is given by f,then the variance of the system (V), that is, the numberof intensive properties (temperature, pressure,composition of each phase) that need to be fixed inorder to determine the state of the system, is given by:V n 2 fThis curve is known as the vapour pressure curveand ends at the critical point (point C), beyond whichthe fluid always has a single phase.

9 The line ASrepresents the liquid-solid equilibrium line, whichcorresponds to the line of melting points of the purecomponent. The curve AE is the line of sublimation;on this line the solid is in equilibrium with the intersection of the line AC, AS and AEcorresponds to the triple point representing the onlypair of values of pressure and temperature at which thethree phases can As with a pure component, the behaviour of amixture can be represented on a pressure-volumediagram (Fig. 3). Starting from point I (situated at atemperature below the mixture s critical temperature),and moving towards larger volume, the followingphenomena can be observed: a) a rapid decrease of thepressure in the liquid phase; b) the appearance of the488 ENCYCLOPAEDIA OF HYDROCARBONSOIL FIELD CHARACTERISTICS AND RELEVANT STUDIEST>TCT<TCRAB volumepressureFig.

10 Diagram of a pure component. Two isothermal curves for temperature lower than the critical temperature and one (dashed line) for higher temperatures are shown. Points B and R are the bubble point and the dew point, pointtemperaturecritical pointpressuresolidFig. diagram of a pure +vapourvapourvolumepressureFig. diagram of a bubbles of gas at point B, which represents thebubble point; c) the increase of the volume of the gasphase and the decrease of the volume of the liquidphase (but in this case, instead of remaining constant,the pressure decreases during the phase change); d)the disappearance of the last drop of liquid at point R(the dew point); and e) the slow decrease in pressurebeyond point R, where the entire mixture is in the the behaviour of a mixture is represented on apressure-temperature diagram (Fig.