Pvt analysis 1

4/9/2018 PVT Analysis Nguyen Le Anh Tu 9th April 2018 4/9/2018 Why PVT? We need to know Composition of the production well stream and its temporal variation Separator specifications including LPG Design of the completions by identifying the spatial fluid distribution in the vicinity of the wellbore Gas injection/re-injection • Identifying gas composition • The interaction of injected gas and reservoir fluid 4/9/2018 Why PVT? 3 Ultimate recoveries of components, under different drives, mixing/no mixing, single depletion, etc Amounts and composition of liquids left in the reservoir not recovered (especially in gas condensate reservoirs) and its properties: density, Surface Tension, viscosity Detect spatial variation of PVT properties Identify and adjust data inconsistencies 4/9/2018 Why PVT? Why PVT? Calibrate Equations of State (EOS) 4/9/2018 True Objective of gathering and analyzing PVT data Reason: • We cannot measure all characteristic of hydrocarbon fluids EOS provides one consistent source of PVT data • Experiment problems (cost, reliability, accuracy and precision) Reservoir Fluid Composition 𝑪𝒏 𝑯𝟐𝒏+𝒉 𝑺𝒂 𝑵𝒃 𝑶𝒄 4/9/2018 • Hydrocarbon components can be expressed by the general formula: dependent of hydrocarbon classes Reservoir Fluid Composition • Alkanes or Paraffins (h=2) have completely saturated hydrocarbon chains that are chemically very stable 4/9/2018 The major classes of hydrocarbon are: • Alkenes or Olefins (h=0) have unstable straight chains (unstable nature) and rarely found in reservoir but an important part in downstream business • Naphthenes or Cycloparaffins (h=0) saturated ring or cyclic compounds rarely found in crude oil • Aromatics or Benzene series (h=6) are unsaturated cyclic compound • Asphaltenes and Resins (increasingly negative h) are aromatics and polycyclic hydrocarbon with fuse rings contain N,S,O and metals such as Nickel and Vanadium Alkanes or Paraffins Long chain paraffins (carbon no >15) forming solids at surface but will remain in solution at reservoir condition 4/9/2018 • What are Waxes? 4/9/2018 Naphthenes or Cycloparaffins • At low carbon numbers (below 5) Cycloparaffins are less stable than normal Paraffins, hence rarely found in the reservoir Aromatics 4/9/2018 • Aromatics contain one or more Benzene rings 10 Volatile Oil (cont.) • GOR increases when the reservoir pressure falls below the bubble point during the production life 4/9/2018 • Initial GOR’s typically range between 1750-3200 scf/stb • Liquid is colored with an API higher than 40 • Gas produced below the bubble point as quite rich and behave as retrograde gases so the amount of liquid recovered from the gas constitutes a significant portion f the total oil recovery 41 • During production below the bubble point, API increases particularly at high producing GOR A significant liquid production is due to condensation of the rich associated gases 4/9/2018 Volatile Oil (cont.) • Saturation pressure of volatile oils are high • Compositional material balance methods should be applied generally to study volatile oil reservoirs • Accurate measurement of temperature is of particular importance when dealing with near critical fluid 42 4/9/2018 Volatile Oil (cont.) 43 Volatile Oil Properties Formation volume factors and solution gas oil ratios normally are not measured for volatiles oils These quantities are primary used in material balance calculations which don’t apply to volatiles oil 4/9/2018 All the properties are the same as black oil If FVF and Rs are measured for volatiles oils, a sharp decline is observed in both measurements just the below bubble point due to the evolution of large quantities of gas in the reservoir This is indicated by the close spacing of the iso-volume lines just below the bubble point The coefficient of isothermal compressibility is important Viscosity of volatile oils are much lower than oils Value of 0.1cp are common at the bubble point and value above 0.2cp are rare 44 4/9/2018 Equation of States 45 QC Lab Data PVT Analysis depending on type of fluid 46 QC Lab Data Composition & Flash Data Simple summation of all compositions to ensure no typewritten & typographical errors Apply material Balance tests: • Evaluate compositional consistency between feed composition and separator vapor & liquid compositions • Useful when feed compositions is measured separately from separator vapor & Liquid Composition • Deviation from straight line indicative of errors or uncertainty in the measurements 𝑦 𝐹 𝐿 𝑥 • 𝑖= − ∗ 𝑖 𝑧𝑖 𝑉 𝑉 𝑧𝑖 Where: F = Total moles of feed L = Total moles of separator liquid V = Total moles of separator vapor 𝑧𝑖 = mole fraction of component i in the feed 𝑥𝑖 = mole fraction of component i in the liquid 𝑦𝑖 = mole fraction of component i in the vapor 47 QC Lab Data Composition & Flash Data (cont.) Apply Hoffman Plots: • • • • • Evaluate consistency of K-value (y/x) Note: Hoffman plot is not a pass or fail check, just a checking of consistency log(K) vs F should be straight line with possible curvature at heavier components Extreme curvature, however, indicates potential data issue 1 1 𝐹 = (log 𝑃𝑐 − log 14.7 )( − )/( − ) 𝑇𝑏 𝑇 𝑇𝑏 𝑇𝑐 Where: K = K-value (y/x) P = separator pressure (psia) T = separator temperature (R) Tb = boiling temperature (R) Tc = critical temperature (R) Pc = critical pressure (psia) 48 QC Lab Data Consistency Test for Gas Data (CCE & CVD) Much have been focused on evaluating compositional data using Whitson 1983 (Material Balance & Composite Hoffman plot) Focusing on bulk properties of both CCE & CVD: Liquid dropout data • • • • • Check dew point (both should be identical) CVD liquid dropout should be greater than CCE (on total volume basis) CVD maximum liquid dropout should occur at lower pressure than CCE This only a consistency between CCE and CVD, not the accuracy of the tests To bring CCE value to same level as CVD: Liq (% of V) = liq (% of Vsat)/(V/Vsat) 49 QC Lab Data Consistency Test for Gas Data (CCE & CVD) Cont Z-factor or/and vapor phase density • A vapor phase density from both CCE & CVD should show agreements • In case of no vapor phase density ideal gas law is in used with available z-factor • 𝜌 = 𝑃/𝑧𝑅𝑇 Where: 𝜌 = Molar density P = Pressure Z = z-factor R = Gas constant T = Temperature (absolute) 50 QC Lab Data Consistency Test for Gas Data (CCE & CVD) Apply material balance & Hoffman plot • A backward calculation applying material balance to determine consistency in each stage of CVD • A Hoffman plot is apply in which a plot should show parallel trend without deviation or bumps (Note: Hoffman plot is not a pass or fail check, just a checking of consistency) 51 QC Lab Data Consistency Test for Oil Data (CCE & DL) A visual check: • Plotting report properties as a function of pressure  Compressibility decrease with increasing pressure  ρ & Vis increase with increasing pressure  Note: A subtle change might mean a lot Understanding the issue helps determining the accuracy of tuned EOS SCN at later stage • Comparison of the residual API gravity at sock tank cond between DL and Flash/SEP for consistency checking  API in DL < API in flash/SEP because of denser liquid, due to higher temperature condition of the DL test 52 QC Lab Data Consistency Test for Oil Data (CCE & DL) Y-functions commonly used for smoothing CCE data and predicting bubble point pressure: • Calculate Y-function for all pressure below the saturation pressure • Plot the Y-function vs pressure on a Cartesian 𝑃 −𝑃 𝑌 = 𝑠𝑎𝑡 𝑃(𝑉𝑟𝑒𝑙 −1) Where: 𝑃𝑠𝑎𝑡 = saturation pressure (psia) P = pressure (psia) 𝑉𝑟𝑒𝑙 = relative volume at pressure P • Determine the coefficients of the best straight fit of the data 𝑌 = 𝑎 + 𝑏𝑃 Where a & b are intercept and slope of the line, respectively • Recalculate the relative volume at all pressure below Psat 𝑃 −𝑃 𝑉𝑟𝑒𝑙 = + 𝑠𝑎𝑡 𝑃(𝑎+𝑏𝑃) 53 QC Lab Data Consistency Test for Oil Data (CCE & DL) Conversion of solution GOR to cumulative GOR in DL to eliminate the errors in measurements while performing EOS tuning (matching) • The conversion of the propagation of these errors are as follow: 𝑟𝑐𝑏𝑝−𝑅𝑠  𝑅𝑐𝑏𝑝 =  𝑉 𝑉𝑠𝑎𝑡 = 𝐵𝑜𝑏𝑝 𝐵𝑜 𝐵𝑜𝑏𝑝 Where: Rcbp = cumulative GOR referenced to bub-point volume Rsbp = Solution GOR from DL at bub-point Rs = solution GOR from DL V/Vsat = liquid shrinkage Bo = FVF from DL Bobp = FVF from DL at bub-point 54 QC Lab Data Contamination Level Determination The approach assume linear declination of component C10 to Cn+ using log wt% vs MW The decontaminated composition can be calculated as follow: • Decontaminated component wt% = (X-Y*(Z/100)))/((100-Z)/100) Where: X = measured component wt% composition of contaminated reservoir fluid Y = measured component wt% composition of OBM Z = Level of contaminate expressed as a percentage of the weight of the whole fluid • It is also useful to plot mud composition on the same plot for reference • Note: initially Z = Calculated 'pure' fluid composition Mud B 100.00 Wt% (Log scale) 10.00 1.00 0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00 450.00 500.00 0.10 0.01 0.00 Component MW (g mole-1) 55