The SPE Foundation through member donations and a contribution from Offshore Europe

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1 Primary funding is provided by The SPE Foundation through member donations and a contribution from Offshore Europe The Society is grateful to those companies that allow their professionals to serve as lecturers Additional support provided by AIME Society of Petroleum Engineers Distinguished Lecturer Program

2 Understanding and Checking the Validity of PVT- reports Klaus Potsch Consultant, EC&C Society of Petroleum Engineers Distinguished Lecturer Program

3 PVT report an old hat? Yes Decades of experiments and reports But with new feathers: Improved equipment Improved reporting Needs new quality control People forgot how to wear it General ignorance about how fluid properties are determined and reported 3

4 Outline Sampling Black Oil systems Gas Condensate systems Flow assurance Summary 4

5 Motivation Formation of a gas cap Condensate dropout Fluid Fluid Interaction -> EOR = Enhanced Oil Recovery Drilling mud invasion Bottom hole sampling Gas coning Water coning Condensate banking To know the fluid properties from the reservoir to the facilities to the refinery 5

6 What are the data good for? Reservoir engineer: simulation (black oil or compositional), well models Facility engineer: designs surface installations (simulation with process SW), design of transportation routes Economics: reserves; commodities (quality = price) stock tank oil, NGL, condensate, gas 6

7 Starting point = reservoir fluid samples Laboratory work starts with good, representative samples. Impossible to get good results from non representative samples (garbage in, garbage out). Goal: information about the composition and quality Essential, whether bottom hole sample (BHS) or surface sample (SS): Reservoir conditions (p,t,depth) experiments Sampling conditions (p,t,v=rates) experiments Sample only single phase streams Sample only a conditioned well = clean and stable flow Sample as early as possible 7

8 concentration mol% Bottom hole sampling Danger: OBM mixed with reservoir fluid decontamination Tracers help! Diesel single carbon number Artificial mud single carbon number OBM clean 8

9 Separator sampling What is the flow rate and the retention time (size of the separator)? Is carry-over or carry-under significant? Are the rates stable? How high is the pressure? How many phases are recorded? gas + carry over inlet oil + carry under water 9

10 absolute pressure Separator sampling, cont d. Where is the reservoir in the phase diagram? Gas phase is at a dew point Liquid phase is at a bubble point p sat liquid p sat vapor res fluid sep gas sep liquid separator condition temperature 10

11 Separator samples Reservoir Surface separator Laboratory reservoir fluid p res,t res,z k separator gas p sep,t sep,y k GLR sep,k k =y k /x k separator liquid p sep,t sep,x k gas 1 p amb,t amb,yy k liquid 1 p amb,=t amb,yx k gas 2 p amb,t amb,xy k liquid 2 p amb,t amb,xx k GLR 1 K yk =yy k /yx k GLR 2 K xk =xy k /xx k BHS SS 11

12 Samples (SS), validity checks Was a valve leaking? Liquid: Determinig p sat at ambient condition and recalculating to sampling condition. Gas: p.v/t should be the same at ambient and sampling conditions 12

13 Samples (SS), analyses Analyses by gas and liquid chromatography earlier times now scn Component scn Component H 2 0 Hydrogen 6 Benzene H 2 S 0 Hydrogen Sulphide 6 Cyclohexane CO 2 0 Carbon Dioxide C 7 7 Heptanes N 2 0 Nitrogen 7 Methyl-Cyclo- Hexane C 1 1 Methane 7 Toluene C 2 2 Ethane C 8 8 Octanes C 3 3 Propane 8 Ethyl Benzene ic 4 4 iso Butane 8 Meta/Para-Xylene nc 4 4 normal Butane 8 Ortho-Xylene neo C 5 5 neo-pentane C 9 9 Nonanes ic 5 5 iso Pentane 9 1,2,4-Tri-Methyl- Benzene nc 5 5 normal Pentane C Decanes C 6 6 Hexanes.. 6 Methyl-Cyclo- Pentane C 36 + Hexatriacontanes Plus 13

14 ln K Quality check of sample compositions via equilibrium- or K-values (Wilson, Hoffman, etc) versus characterization factor, e.g. 8 characterization factor F experimental data Wilson's estimates regression -8 14

15 Reservoir fluid: composition BHS: mathematical recombination SS: mathematical and physical recombination Reservoir fluid: properties Experiments, that mimic the flow process Correlations for Black Oil (BO) EOS 15

16 Experiments, mimicking the flow process Black Oil (BO): Gas gets out of solution, moves upward, forms a gas cap 2phase flow into the well bore, different mobilities, GLR questionable CCE Gas Condensate (GC): Formation of a condensate bank, in case of lean condensate immobile well stream composition reservoir composition Rich condensate: 2-phase flow into the well bore, different mobilities, GLR questionable CCE DLE CVD 16

17 Constant composition expansion (CCE) BO: p 1 > p 2 > p 3 =p b > p 4 > p 5 > p 6 > p 7 GC: Test: Y(p)=(p/p sat -1)/(V t /V sat -1) linear in p p > 1 p 2 > p 3 =p d > p 4 > p 5 > p 6 > p 7 17

18 ln (cell volume) Constant composition expansion (CCE) Determination of saturation pressure BO GC 2ph p b 1ph p.v/z 1ph 2ph p dew 1ph experiment theory absolute pressure dv/dp is discontinuous saturation pressure p b p>p b : compressibility C p =O (10-3 ) [MPa -1 ] absolute pressure d(pv/z 1ph )/dp is discontinuous saturation pressure p d Z 1ph from overall-composition SPE

19 BO: Differential liberation experiment (DLE) Principle: 19

20 Bo BO: Differential liberation experiment (DLE) Gas in solution R s (p) determines the volumetric behavior Test 1: B o (R s (p)) almost linear in R s Rs 20

21 BO: Differential liberation experiment (DLE) Test 2: C t = B o (p atm )-1 = = O (10-3 ) [K -1 ] 21

22 Y BO: Differential liberation experiment (DLE) Test 3: Y-function, use V g,lib (p) for V t, (not in textbooks) and Z g (check with REFPROP from NIST) Y_CCE Y_DLE pr 22

23 ln(µo(p_atm/µo(p)) BO: Differential liberation experiment (DLE) Test 4: ln(µ o (p atm )/µ o (p)) similar shape as B o (p), (not in textbooks) Bo(p) -Bo(p_atm) 23

24 ln concentration BO: Differential liberation experiment (DLE) Test 5: well stream composition, concentation of light components (N 2, C 1, possibly CO 2 and C 2 are convex, the rest is concave) abs.pressure CO2 N2 C1 C2 C3 ic4 nc4 ic5 nc5 C6 24

25 Rsd,Rsf BO: Production process Combination of DLE (reservoir) and CCE (tubing) Flash (CCE) B of,r sf < diffferential (DLE) B od,r sd Rsd Rsfb Rsd-Rsf Rsf Rsf_ lit p R R sf sd p b : const. CCE 0 abs.pressure 25

26 Bod,Bof BO: Production process Bod Bofb Bof Bof_lit Bod-Bof p B R od sd p b : B R of sf const. CCE 1 abs.pressure 26

27 GC: Constant volume depletion (CVD) Principle 27

28 Vl/Vd% GC: Constant volume depletion (CVD) Test 1: V l CCE (p) > V l CVD (p) liquid drop out CCE CVD p abs [bar] 28

29 Yv Constant volume depletion (CVD) Test 2: Y-function, using 1+V well stream (p) slightly curved, close to Y CCE, (not in textbooks) Yv_CVD Yv_CCE p abs [bar] 29

30 Correlations Estimates for Black Oil tables General form: p b = f b (T res,r STO,r STG,R s ) B o = f B (T res,r STO,r STG,R s ) µ o = f µ (T res,r STO,r STG,R s ) µ g calculated from Gonzales, Eakin Z g from REFPROP (developed by NIST) 30

31 Equations of State - EOS input = composition try to match the experiments prerequisite: C 6+ modeling reveals the inconsistencies mind the errors of the field measurments best worst monophasic BHS 0% invalid separator GOR 5 % 25 % human or equipment failure 0 % invalid BHP % 3 % BHT % 5 % 31

32 Flow Assurance Gas hydrates: gas composition necessary Paraffins Saturates, aromates, resins, asphaltenes SARA or PNA analysis Heavy end liquid chromatography Wax appearance temperature (WAT) cold finger test Asphaltenes SARA or PNA or IP44 Flow test with capillary 32

33 Summary Starting point for determination of fluid properties: quality-sampling Tools exist to check the laboratory data Estimates can be gained via correlations or EOS 33

34 Further reading Ahmed, T.; Hydrocarbon Phase Behavior. Gulf Publishing Co, Bon J., Sarma H., Rodrigues T., Bon J.; Reservoir-Fluid Sampling Revisited A Practical Perspective, SPE Danesh, A.; PVT and Phase Behaviour of Petroleum Reservoir Fluids; Elsevier Michelsen, M.L.; Mollerup, J.M.; Thermodynamic Models, Fundamentals and Computational Aspects; Tie-Line Publications, Moffat B.J.; Williams J.M.; Identifying and Meeting the Key Needs for Reservoir Fluid Properties. A Multidisciplinary Approach, SPE

35 Further reading Pedersen, K.S.; Fredenslund, A.; Thomassen, P.; Properties of Oils and Natural Gases. Gulf Publishing Co, Prausnitz, J.M.,Lichtenthaler, R.N.,Gomez de Azevedo, E; Molecular Thermodynamics of Fluid Phase Equilibria. 2 nd edition, Prentice Hall, Riazi, M.R.;Characterization and Properties of Petroleum Fractions, ASTM manual series MNL50; 2005 Whitson, C.H.; Brulé, M. R.; Phase Behavior; SPE Monograph, volume 20,