Reservoir-Fluid Property Correlations—State of the Art

Abstract

Summary. This paper presents correlations to determine reservoir-fluidproperties from field data. The best available correlations wereselected by comparison with a data base of hundreds of reservoir-fluid studies of samples representing all areas of the free world involvedin active petroleum exploitation from 1980 to 1986. Also, correlationsof formation-water properties are given. Introduction Values of reservoir liquid and gas properties are often needed when laboratory PVT data are not available. This paper shows how touse normally available field data to estimate fluid properties. While at Texas A and M U., I had access to a data base of hundreds of reservoir-fluid studies provided by Core Laboratories Inc. Thegeographical and geological origins of the reservoir samples hadbeen carefully removed from the data but the samples were knownto represent all areas of the free world in which petroleumexploitation was active during the first 6 years of the 1980's. All reservoir-fluid property correlations available in thepetroleum engineering literature were compared with this data base. Thispaper gives the best correlations. Identification of Reservoir-Fluid Type Surprisingly accurate "rules of thumb" are available to identifyreservoir-fluid type from field data. When the initial producing GORis less than 3,300 scf/STB, the fluid is a liquid at reservoir conditions. Possible exceptions occur if the stock-tank liquid is colorless orhas a gravity higher than about 50 degrees API. Reservoir liquids are either black oils or volatile oils; thegeneral material-balance equations work only for black oils. Thebehavior of volatile oils does not fit the assumptions inherent in thederivation of the material-balance equations. Black oils areidentified as having initial producing GOR's below 2,000 scf/STB anddeeply colored stock-tank oil with gravities below 45 degrees API. Reservoir gases are classified as retrograde gases (often called condensate gases or gas condensate), wet gases, and dry gases. Retrograde gases have initial producing GOR's >3,300 scf/STB. The few exceptions of oils that have ratios higher than this areidentified as having deeply colored stock-tank liquids with gravities lessthan 40 degrees API. Retrograde behavior occurs for gases with initialproducing GOR's of 150,000 scf/STB or higher;however, as a practicalmatter, gases with initial producing GOR's > 50,000 scf/STB canbe treated as wet gases. The term wet gas is used for a gas that does not releasecondensate in the reservoir but does form hydrocarbon liquid at thesurface. The term dry gas is used for a gas that does not form any hydrocarbonliquid at the surface. In this context, the terms "wet"and "dry" do not refer to water or water vapor, which is alwayspresent to some extent. Properties of Reservoir Liquids The physical properties discussed next apply only to black oils. Engineering a volatile-oil reservoir requires a special laboratory studynot discussed here. Solution GOR at Bubblepoint, Rsb. The initial producing GORprovides a good estimate of solution GOR for use at pressures equalto and above. bubblepoint pressure. Ms wig not be true if free gasfrom a gas cap or another formation is produced with the oil. Fielddata often exhibit a great deal of scatter; however, a trend ofconstant GOR usually can be discerned before reservoir pressure dropsbelow the bubblepoint. Often the reported values of producing GOR do not includestocktank vent gas. In this case, the use of initial producing GOR forsolution GOR results in values that are low by 10% or more. Thestock-tank GOR can be estimated with log RST =A1+A2 log o +As log gSP+A4 log PSP +a5 log Tsp,............................................ (1) where A1=0.3818, A2=-5.506, A3=2.902, A4=1.327, and A5=-0.7355. Eq. 1 should not be used if the separatortemperature is >140 degrees F. Addition of the estimate of stock-tank GOR from Eq. 1 to theseparator GOR results in an estimate of solution GOR accurate towithin 3 %. Bubblepoint Pressure, Pb. The bubblepoint pressure of the oil atreservoir conditions can be estimated with Pb = 18.2(Cpb − 1.4),...................................... (2) where Cpb =(Rs/ g)0.83 × 10(0.00091T-0.0125 API)..............(3) to an accuracy of 15%. The specific gravity of the separator gascan be used for g; however, Rs should include stock-tank ventgas. The equations are valid to 325 degrees F. A more accurate estimate of bubblepoint pressure can be obtained if reservoir pressure is measured regularly. Plot reservoir pressureand producing GOR vs. cumulative production. For a volumetricsolution-gas-drive reservoir, pressure will decline rapidlyinitially, then flatten when reservoir pressure drops below the oilbubblepoint pressure (the pressure at which the line changes slope). The producing GOR will begin to increase shortly after bubblepointpressure is reached. Solution GOR, Rs. Eqs. 2 and 3 can be used to estimate solution GOR for pressures below the bubblepoint. Enter any pressurebelow bubblepoint in place Of Pb in Eq. 2 and calculate thecorresponding value of solution GOR with Eq. 3. The results shouldbe within 15% of measured values. If a field-derived bubblepoint pressure has been obtained frompressure measurements as described above. the accuracy of theestimates of solution GOR can be improved. Start by creating atable of pressures and solution GOR'S. Subtract the field-derivedbubblepoint pressure from the bubblepoint pressure calculated with Eqs. 2 and 3 to obtain a "delta pressure." Subtract this "deltapressure" from all pressures in the Rs vs. p table. This procedure works very well for pressures near the bubblepoint. It is lessaccurate at low pressures. Oil FVF, Bo. The oil FVF for use at pressures equal to or belowbubblepoint can be estimated with Bob =0.9759+12(10 −5)C Bob 1.2,......................... (4) where C Bob=Rs(g/o)0.5+1.25T..............................(5) The equations can be used for any pressure equal to or below thebubblepoint by inserting the corresponding value of solution GORestimated as discussed above. The resulting FVF value will be within5% of laboratory-measured values if accurate values of solution GOR are used. If solution GOR's are obtained with Eqs. 2 and 3.the accuracy of the resulting FVF values will be some unknowncombination of the 15% accuracy of Eqs. 2 and 3 and the 5%accuracy of Eqs. 4 and 5. Do not use at temperatures above 352degrees F. SPERE P. 266^

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