Chapter 3 Pressure-Volume-Temperature for Oil

Transcription

1 Chapter 3 Pressure-Volume-Temperature for Oil PVT analysis PVT relationship Gas Oil scf condition underground condition simple complex ( because of bubble point) 1

2 Three main oil PVT parameters The three main parameters required to relate surface to reservoir volumes for an oil reservoir: - s : The solution (or dissolved) gas-oil ratio - o : The oil formation volume factor - g : The gas formation volume factor 2

3 Definition of the basic oil PVT parameters - s : The solution (or dissolved) gas-oil ratio SCF gas at reservoir P&T [] ST oil - o : The oil formation volume factor ( oil + dissolved gas) [ ] ST oil - g : The gas formation volume factor free gas [] SCF gas Note : Standard Condition at reservoir P&T T 60 F p 14.7 psia,, f ( P, T ) s o g For T const.,, f ( P) s o g 3

4 Determination and Conversion of PVT Data Determination of three main oil PVT parameters( S, o, g ) -> PVT f (P only) by routine laboratory analysis Conversion of PVT data, as presented by the laboratory, to the form required in the field, Laboratory - an absolute set of measurements Field - depend up as the manner of surface separation of the gas and oil 4

5 The complexity of Oil PVT For gas, PVT relation -- Simple relation pv nzt p E [ ] scf zt rcf For oil, PVT relation > Complex; PVT parameters must be measured by laboratory analysis of crude oil samples. >elationship between surface and reservoir hydrocarbon volumes. 5

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7 The complexity of Oil PVT The complexity in relating surface volumes of hydrocarbon production to their equivalent volumes in the reservoir can be appreciated by considering the following figures: Undersaturated oil saturated oil gas saturated oil + free gas (or liberated solution gas) They are traveling in reservoir at different velocity 7

8 How to divide the observed surface gas production into liberated and dissolved gas volumes in the reservoir? Control in relating surface volumes of production to underground withdrawal is gained by knowing the three oil PVT parameters which can be measured by laboratory experiments performed on samples of the reservoir oil, plus its originally dissolved gas. 8

9 Oil eservoir & Surface Volume Above ubble Point Undersaturated oil 9

10 Oil eservoir & Surface Volume elow ubble Point Saturated oil gas saturated oil + free gas (or liberated solution gas) They are traveling in reservoir at different velocity 10

11 The instantaneous gas-oil ratio or producing gas-oil ratio ( SCF ) ST the instantaneous gas-oil ratio or producing gas-oil ratio ( Underground withdrawal) ST o + ( s ) g ( ) ST 11

12 o as Function of Pressure 12

13 s as Function of Pressure 13

14 g and E as Function of Pressure 14

15 Producing Gas-oil atio () as Function of Pressure 15

16 Exercise Underground withdrawal Given: q q o g x y ( ST ) D ( SCF ) D measured at t during the producing life Calculate: [ in _( )]? (1) Underground withdrawal rate D expressed in terms of x & y (2) Underground withdrawal rate [ in _( )]? D if p res 2400 psia, q o 2500 ST/D, q g MMscf/D and PVT data in fig. 2.5(a)~(c) (p.51) or table 2.4 (P.65) (3) Pressure gradient of oil? if ρo 52.8 lbm / ft3( at s. c), r g 0.67 ( air 1), p 2400 psia 16

17 (1) y/x q g /q o [] SCF/ST p is known o ; s ; g Underground withdrawal 17 ) ( ) )( ( ) ( ) ( ) ( ) ( SCF ST SCF x y D ST x q D x ST D ST x q g s g o o o ) ( ) ( D x y x g s g s o g o t x y x x q q q + + ) (

18 P 2400 psia from table 2.4 o /ST s 352 SCF/ST g /SCF x qo 2500 ST/D; y qg MMSCF/D q o q g 2500 ( 352) D D q q q t o + g q t D y qo + qg x o + x ( s ) x g 18

19 (3) To find dp dd ρ o ρ o g, applying mass balance, such as Mass of 1 ST of oil Mass of o of oil + + s scf dissolved gas at dissolved gas in the standard condition reservoir condition 19

20 Mass of 1 ST of oil Mass of o of oil + + s scf dissolved gas at dissolved gas in the standard condition reservoir condition [ ρ osc lbm ( SCF ) SCF ( ST) 1 ST ] + ρ gsc lbm ( SCF )[ s SCF ST 1 ( ST)] ρ or lb ( ft m 3 ) 1 ( ST) o ( ST ) ft ρ ρ or osc + s ρ gsc 5.615ρosc + sρ o ρ gsc o or 5.615ρ osc + [ r s g [ ] o ] lb m / ft 3 20

21 21 ft psi ft in lb in ft ft lb s ft ft slug s ft lb slug ft lb g dd dp f f m m or / ) (32.2 ) ( ρ

22 2.3 Collection of fluid samples eservoir fluid samples are usually collected at an early stage in the reservoir s producing life and dispatched to a laboratory for the full PVT analysis. Sampling Direct subsurface sampling, or Surface recombination of the oil and gas phases asic problem : GO in the composite samplego in the reservoir? or Sample at initial condition > 1 ST oil + si SCF 22

23 (a) Subsurface sampling p p wf wf > < a p p b (oil is undersaturated with gas) representativecombined fluid sample b ( saturated oil + free gas) No guarantee oil and gas will be collected in the correct volume proportion in the chamber. 23

24 Subsurface sampling During very early in producing life S g S g critical (no gas flow in reservoir) GO Later in producing life S g > S g critical & free gas from remote GO 24

25 Subsurface sampling Solution to the problems associated with an initially saturated oil reservoir, or an undersaturated reservoir in which p wf < p b If the well has already been flowing, q at stabilized rate for several hrs. pwf (re - dissolve some of well is closed (re - dissolve any remaining free gas) free gas) * * If If p p i i > p p b b, subsurface sampling could be conducted with well still close in, subsurface sampling could be conducted at q very low while p wf > p b 25

26 Subsurface sampling Note : Main drawback in the method small sample (only a few liters) Check!! to take several downhole samples and compare their saturation pressure at ambient temperature on the well sites. Mercury injection pump and pressure gauge gas p p + at sfc temp b gas dissolved in oil at oil Change in fluid compressibility at p b p b 26

27 (b) Surface recombination sampling The well is produced at a steady rate for a period of several hours and the gas-oil rate is measured in scf of separator gas per stock tank barrel of oil. 27

28 Surface recombination sampling Oil samples taken at separator pressure & separator temperature GO () measured at surface conditions (or standard conditions) determined in the lab. as the first stage of PVT analysis. 28

29 Surface recombination sampling The attractive features of the sampling it gives a reliable value of the producing gas-oil ratio measured over a period of hours. it enables the correction of large fluid samples. Notes: --- providing correct GO() only if p wf > p b --- measured is too low if S g < S g critical --- measured is too high if S g >>S g critical 29

30 3.4 Determination of the basic PVT parameters in the laboratory and conversion for field operating conditions The full laboratory analysis ρ, μ & compositions The analysis consists of three parts: to determine p b -- flash expansion of the fluid sample. to determine o, s & g -- differential expansion of the fluid sample. to enable the modification of the lab. derived PVT data to match field data. Separator conditions -- flash expansion of fluid samples through various separator combination. 30

31 Determination of the basic PVT parameters in the laboratory and conversion for field operating conditions 31

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33 Determination of the basic PVT parameters in the laboratory and conversion for field operating conditions The flash expansion experiment. The differential liberation experiment. Difference in (1) gas remain / remove (2) composition unchanged change / change 33

34 Flash Expansion Experiment pressure (in PV cell) >> P Total volume V p p p Gas, P change slightly V t of the cell content is recorded b C t t b change rapidly. 34

35 Differential liberation experiment Differential liberation experiment to obtain the basic PVT parameters to start at p b (for p> p b, the flash and differential experiments are identical). -- after each stage of the differential liberation, the total amount of gas liberated during the latest pressure drop is removed from the PV cell by injecting mercury at constant pressure. for example, p drops from 2700 to 2400 psia volumes of gas are removed. 35

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37 ( 1) ( 2) (3) (2) ( 4) (2) (1) p ( 5) (6) ET 37

38 Which type of experiment will provide the most realistic values of o, s and g, required for relating measured V surface V reservoir at reservoir pressure and temperature? Ans. -- both flash and differential liberator are required for an adequate description of the overall values changes. Differential liberation Provides the better description of how the oil and gas separate in reservoir. Exception : during the brief period after the bubble point has been reached. S g << S g critical Flash expansion Flash expansion Single separator Exception : muti-stage separator differential liberation 38

39 Separator tests To perform a series of separator tests on oil samples as part of the basic PVT analysis and combine the results of these tests with differential liberation data. 39

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41 41 ( ) ( ) CF SCF CF E SCF ST SCF c F ST c ST SCF ST F ST scf ST c V ST tests separator or ansion flash Separator ansion Flash liberation Differential data Experiment g bf si s b bf b s si b b bf o o g s o f f exp exp

42 Thus the laboratory differential data can be transformed to give the required field PVT parameters using the following conversions 42

43 Exercise 2.2 Conversion of differential liberation data (table 2.2) to give the filed PVT parameter o, s and g for the optimum listed in table 2.3. Dodson s PVT analysis technique 43

44 Alternative manner of expressing PVT laboratory analysis results In table 2.2 common way ef. oil V Unit oil V at.p. ef. oil V Volume of residual oil at S.T. condition (14.7psia 60 ) in table2 shrinkage factor C bd residual oil volume (Note :C from separator flash expansion experiments in table 2.3) bf C bd f (no. of pressure steps taken in the differential experiment) The differential liberation results,in this case, do not provide an absolute. Set of data such as that obtained by relating all volumes to the unit volume of oil at.p. 44

45 ST scf C F residual ST scf C F of value Max residual ST scf C F ST C V residual ST C V bf bd bd si b b bf o b b bd o d F at 60 oil of bbl residual relative to the dissoloved gas initial sid oil) residual (SCF / ST - ratio oil Differential Solution gas - sd / / / / formation volume factor Dif. Oil f si s o od 45

46 od sd Corrections are necessary for moderated & high volatility oil o s 46

47 sd sid bd bd sid sd obd obf sd sid sif bf bd bd si bf sif s bd obd bf obf obd obf od bf bd bd o bf o o C F C F Eq C C C F C F C C C C C V C V (2.9) ) ( , 1 ; 47

48 ) / 352( ) ( ) ( / / ST) 510(scf ( / ST) 1/ C From table2.3 ) & / 540( ) & / ( ) & / 378( ) & / ( (from table2.5) 2400psia p For example : sif bf obf ST scf ST psia F at residualoil ST SCF psia F at residualoil ST psia F at residualoil ST SCF psia F at residualoil ST obd obf sd sid sif s obd obf od o sid obd sd od 48