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Revised GOR indicates oil type

Revised GOR indicates oil type - MANAGEMENT...

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Unformatted text preview: MANAGEMENT ~5931'“'|=‘-3"-3T,’I:='R\n:'iIR .. Revised Gas—Oil Ratio Criteria Key Indicators Of Reservoir Fluid Type Part 5: The previous four articles in this series revealed the differences and similarities among the five reservoir fluids in detail.“4 This concluding article discusses guidelines for using field data to determine the fluid type, the laboratory evidence that verifies fluid type and the production behavior of the five fluids. able 1 gives the guidelines for determining fluid type » . from field data. Three prop- ' erties are readily available: the initial producing gas-oil ratio (GOR), the gravity of the stock—tank liquid and the color of the stock-tank liquid.l Initial producing GOR is by far the most important of the indica- tors and should be considered first, with the other two indicators used to confirm fluid type. Stock-tank liquid gravity and color are both indicators of the quantity of heavy components present in the initial reservoir fluid. Darker colors are associated with the largest, heaviest molecules in the petroleum mixture. - If any one of these three proper- ties fails to meet the criteria of Table 1, the test fails and a repre- sentative sample of the TABLE 1. FIELD INDENTIF'IDATIDN aF RESERVOIR FLUIDS reservoir fluid must be examined in a laboratory to establish fluid type. The initial producing GOR guidelines given in Table 1 are somewhat differ- ent than rules presented by other authors. The ration- ales for selection of the val- ues in Table 1 are given in the previous articles in this series}4 These articles are the first to present empirical evidence to sup— port the selection of GOR criteria for identifying fluid type. Table 2 shows the expected labo- ratory analysis results of the five flu- ids. The oils will exhibit bubble points, the retrograde gases will dis- play dew points, and the other gases will demonstrate no phase change throughout the pressure range expected in the reservoir. The hep- tanes plus composition cutoff between black oils and volatile oils (20 mole %) is not exact. Values from 19 to 22 mole % might be observed.z However, the cutoff of 12.5 mole % - between volatile oils and retrograde ~ gases is fairly sharp.3 The composi- tions of 4 mole % and 0.7 mole % for .‘ :Eor EngineE g purp the other gases are based on engi- neering applications. Some retro- grade liquid will likely occur in the reservoir in either case.4 Oil formation volume factor has been defined for use in oil material balance calculations. Since these cal— culation procedures are not applica- ble to volatile oils, formation volume factor usually is not measured for volatile oils. But one laboratory result that indicates the presence of a volatile oil is an oil formation vol- ume factor at bubble-point pressure of 2.0 res be/STB or greater. Production Characteristics Production trends for-the five fluids are shown in Table 3. Producing GOR is constant for oils as long as reser- by William D. McCain Jr., S.A. Holditch and Associates, College Station, Texas Petroleum Engineer International \u ,_.w RESERVOIR MANAGEMENT _——_——_————_—-——_—_—_——____ 2 pomt »' voir pressure is above bubble—point pressure. Both oils exhibit increas- ing producing GORs when two phases exist in the reservoir. This increase is due to the existence of reservoir gas which has much lower viscosity and, therefore, moves more easily than the oil to the well bore. Of course, as reservoir pressure declines further, the amount of gas in the reservoir increases. This caus- es an increase in the effective per- meability to gas and a decrease in the effective permeability to oil. As a result, the ratio of gas to oil in the reservoir flow stream increases. Gases Dry gases associated with black oils leave the flow stream in the first stage of separation. The retrograde gases associated with volatile oils release some condensate in the first stage of separation. Therefore, black oils typically have higher surface GORs than volatile oils during most of the producing time.2 Notice the decrease in producing GORs for both oils late at the end of the pro- duction period in Table 3. This tum- down is primarily due to the severe increase in gas formation volume factor at low reservoir pressures. Retrograde gases also demon-' strate constant producing GORs early when the pressure is above the dew-point pressure of the gas. And retrograde gases have increasing producing GORs at pressures below the dew point. However, the reason for this increase is different than for the oils. Very little of the liquid released from retrograde gases in the reservoir will flow. This is liquid which would be a part of the con- densate at the surface were it not lost in the reservoir. Thus the con- ) .-—' 2 TABLE 2. LABORATORY ANALYSIS " ' I. I B'fikei'fiivelé1962a“ 'efib'blég. Bubble ' ' '. point “Retrograde gas _3 :EWet gas} , Dev. gent ' densate yield at the surface decreas- es and the GOR increases as reser- voir pressure declines during pro- duction. The producing GOR of a true wet gas remains constant throughout the life of the reservoir as shown in Table 3. Remember, though, that guidelines for identifying a wet gas for engineering purposes cut fairly deep into the range of fluids that exhibits some retrograde behavior. Therefore, an increase in GOR later in the production period of a wet gas might be expected. Liquids The changes in API gravity of the stock-tank liquids during produc- tion, as shown in Table 3, are inter- esting. These changes are often help- ful in differentiating between black oils and volatile oils. Stock-tank gravity remains constant when the reservoir pressure is above the bub- ble-point pressure of the oil. How- ever, as pressure falls below the bub- ble point, the trends are different for ”y Dry gas -_ iNoifphase r-‘No phase ',_change:.‘- .v'chang’e ><0‘.7'*f 1.3. | black oils and volatile oils. The increasing propor- tions of dry gas produced with black oils as reservoir pressure declines strip some of the lighter components from the oil. Therefore, the gravity of the stock-tank oil gradually decreases throughout most of the life of the reservoir. This decrease is not significant (usually about 2" API from start to end). Late in the life of a black oil reser- voir, the gravity of the stock-tank oil will increase. At low reservoir pres- sures, the gas which comes out of solution from the oil in the reservoir is rich enough (wet gas) to release condensate when it is produced. This dilutes the stock-tank liquid with condensate causing the gravity to increase. On the other hand, the incr'easing proportions of retrograde gas pro- duced with volatile oils release increasing quantities of condensate at the surface. This condensate mixes with the decreasing propor- tions of produced oil, causing the gravity of the stock-tank liquid to increase. This change in gravity can be significant, on the order of 10 or more API units. Therefore, the trend of stock-tank oil gravities is another indicator of fluid type between black oils and volatile oils. The gravities of the stock—tank liquids produced 'with retrograde 7‘ TABLE 3..PRDDUGTIDN TRENDS Petroleum Engineer International RESERVOIR MANAGEMENT -. _ Bubble point g Fig. 1. The effect of composition on initial producing GOR is indicated b}, the composition cutoffs of the five reservoir fluids. International (November 1993) 24-27. McCain, W. D., Jr. and Piper, L.D.: "Volatile Oils And Retrograde Gases—What's The Difference?" Petroleum Engineer International (January 1994) 35-36. McCain, W. D., Jr.: ”Reservoir Gases Exhibit Subtle Differences," Petro- leum Engineer International (March 1994) 45-46. gases also remain constant when reservoir pressure is above the dew— 3. point pressure of the gas and increase as reservoir pressure declines below the dew point. The trend below the dew point is a result of the heavier components of 4. the gas being lost to the retrograde liquid in the reservoir and, there— fore, not reaching the stock-tank. Part 1 of this series presented a set of data showing the effect of composition (represented by the mole percent of heptanes plus in the fluid) on initial producing GOR.l Using the same data, Fig. 1 indicates the composition and initial produc- ing cut-offs for the five fluids. The rationalizations for these cutoffs have been explained throughout this series of articles. 0 ' ABOUT THE 'AUTHORS' I William D. McCain Jr. is senior executive consultant with SA. Holditch 8: Associates in College Station, Texas, and a part time visiting professor at Texas A&M University. He has BS, MS and PhD degrees in chemical engineering. Previously, he served as head of the petroleum engineering department at Mississippi State University, gained experience as a reservoir References 1. McCain, W. 0., Jr.: "Chemical Composition Determines Behavior of Reservoir Fluids," Petroleum En- gineer International, (October 1993) 18-25. McCain, W. D., Jr. and Bridges, 8.: "Black Oils And Volatile Oils—What’s The Difference?" Petroleum Engineer Petroleum Engineer international engineer and wrote two editions of the Properties of Petroleum Fluids textbook. ...
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