5. 
 
 C.S 
 
 
 STATE OF ILLINOIS 
 ADLAI E. STEVENSON, Govern 
 
 >r 
 
 DEPARTMENT OF REGISTRATION AND 
 
 EDUCATK IN 
 
 
 NOBLE J. PUFFER, Director 
 
 
 
 DIVISION OF THE 
 
 
 STATE 
 
 GEOLOGICAL 
 
 M. M. LEIGHTON, Chief 
 URBANA 
 
 SURVEY 
 
 CIRCULAR NO. 155 
 
 MEASURING 
 
 OIL RESERVES BY 
 
 By 
 FREDERICK SQUIRES 
 
 INJECTED GAS 
 
 Reprinted 
 
 In.m World Oil, pp. 17ii. 17-1, 175, September, 1949 
 
 PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 
 
 URBANA, ILLINOIS 
 1949 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
 FIGURE 3 
 Perspective drawing of oil field showing connections for gas injection. 
 
 3 3051 00004 6056 
 
Measuring Oil Reserves 
 
 By FREDERICK SQUIRES 
 
 Petroleum Engineer, Illinois State Geological Survey 
 
 A HE writer has applied to gas-injec- 
 tion operations on oil sands in Ohio, 
 West Virginia, and Illinois the J. O. 
 Lewis method of raising the over-all 
 pressure on the stratum in order to 
 force gas to traverse its less permeable 
 as well as its more permeable portions. 
 In doing this he has found that pressure 
 may be built up and brought to equilib- 
 rium over an entire field in a short time 
 if the sands are highly permeable and 
 the pool of moderate size. These obser- 
 vations led to the idea that the volume 
 of oil remaining in a partiall} r depleted 
 reservoir might be measured by inject- 
 ing gas. 
 
 All virgin oil and gas reservoirs under 
 pressure are bounded laterally by lens- 
 ing out of the reservoir rock, by imper- 
 meable barriers, or by surrounding wa- 
 ter-bearing rock. Such reservoirs are 
 completely full of fluids, and the liquids 
 will contain gas in solution. Volumetric 
 reservoirs without gas caps produce 
 their oil by expansion of solution gas. 
 This article is concerned with the meas- 
 urement of the amount of oil remaining 
 in such volumetric reservoirs after they 
 are partially depleted. 
 
 Measurement Process 
 
 The process is illustrated in two ways, 
 first by a simple apparatus shown in 
 Figure 1, and second by a simplified 
 diagram of an oil field shown in Figure 
 2. This field is shown in perspective in 
 Figure 3. 
 
 Consider a tank, A (Figure 1), full of 
 oil containing dissolved gas under pres- 
 sure. The size and shape of the tank are 
 unknown, but the pressure and the 
 amount of dissolved gas per unit volume 
 of oil have been determined. Part of the 
 oil is drawn off from A, collected and 
 measured at B (Figure 1); the pressure 
 is allowed to fall to atmospheric, allow- 
 ing all the gas dissolved in all the oil to 
 escape From A. The volume of this gas 
 is measured. (The one atmosphere of 
 
 PARTLY DEPLETED volumetric reser- 
 voirs are evaluated through calcula- 
 tions hinging on the shrinkage of the 
 oil due to loss of dissolved gas as 
 determined earl/ in the life of the 
 producing field. Where no water 
 drive is present, and no gas cap 
 existed early in field history, calcula- 
 tions based on injected gas give 
 conservative estimate of residual oil. 
 
 gas remaining above the oil in the tank 
 is not considered.) 
 
 Subtracting the quantity of gas dis- 
 solved in the known volume of vented 
 oil in B from the total measured volume 
 of gas, the remainder is the volume of 
 gas that was originally dissolved in the 
 oil remaining in A. Dividing this by the 
 amount of gas originally dissolved in 
 each unit volume of oil gives the quan- 
 tity of oil remaining in A. In field prac- 
 tice this means that if you have a record 
 of the original pressure and the amount 
 of dissolved gas in a unit volume of oil 
 and measure the vented oil and gas and 
 water you can determine the amount 
 of oil remaining. 
 
 Assume that the volume of gas dis- 
 solved in each barrel of oil in A is found 
 to be 500 cubic feet, and the shrinkage 
 of the oil from maximum to minimum 
 pressure is 20 percent. 
 
 If the volume of oil in B is four-fifths 
 of a barrel and the total volume of gas 
 liberated from A is 5000 cubic feet, then 
 the oil in B will account for 500 cubic 
 feet, and the difference between 500 
 cubic feet and 5000 cubic feet or 4500 
 cubic feet is the volume of gas which 
 has been liberated from the oil remain- 
 ing in A. Since each barrel of oil in A 
 originally dissolved 500 cubic feet of uas 
 and the shrinkage on the liberation of 
 
 this gas is 20 percent, then there remain 
 nine barrels minus 20 percent or 7.2 
 barrels of oil in A. 
 
 Boyles Law Used 
 
 Next, the same procedure, as described 
 above, is repeated but with no measure- 
 ment of the escaped gas. The problem 
 is the same, namely, to find the volume 
 of oil remaining in A. The space above 
 the oil in A is the sum of the amount of 
 oil removed, shown at'B, and the shrink- 
 age of all the oil due to the loss of all 
 the gas. This space is then measured ac- 
 cording to Boyles law by pumping in a 
 known volume of gas, allowing the pres- 
 sure to equalize and noting the pressure 
 to which it raises the reservoir. As 
 shown in Figure 1, this gas will come 
 to equilibrium pressure immediately. In 
 the field, gas should be introduced 
 through many wells. Figure 2 is a sim- 
 plified diagram showing one injection 
 well. The time required to attain equi- 
 librium will depend on the permeability 
 of the sand and the number of injection 
 wells. The degree to which the method 
 is practical in the field will depend 
 largely on the degree of permeability 
 of the producing strata and the size of 
 the pool. In highly permeable strata, 
 such as in the Johnsonville pool, it 
 would require only a short time to raise 
 the pressure in the reservoir and bring 
 it to equilibrium. 
 
 From the space above the oil at A in 
 Figure 1, deduct the volume of oil B 
 and the shrinkage of this oil due to loss 
 of gas. The remainder is the shrinkage 
 of the oil remaining in A. Since this 
 shrinkage is known to be 20 percent, 
 the volume of this oil is readily figured 
 as four times the amount of this 
 shrinkage. 
 
 The method for measuring shrinkage 
 of the oil by injecting a Known volume 
 of gas and observing the resulting equi- 
 
PRESSURE GAUGE 
 
 METER 
 COMPRESSOR i I I 
 
 METER 
 t4= 
 
 COMPRESSOR 
 
 PLAN 
 
 METER 
 
 I I i GAS 
 
 METER 
 COMPRESSOR^ 
 
 SECTION 
 
 METER GAUGE 
 COMPe4== i | 
 
 OIL SAND OUTLINE 
 
 PLAN 
 
 OIL RESERVES MEASURED BY GAS INJECTION 
 
 FIGURE 1. 
 
 FIGURE 2. 
 
 librium pressure may be used to deter- 
 mine the amount of oil remaining in a 
 partially depleted oil reservoir under the 
 following conditions: 
 
 1. When the reservoir is volumetric. 
 Such pools are common in the Cow 
 Run sand in West Virginia and in 
 the McClosky lime in Illinois. 
 
 2. When pressure, cubic feet of dis- 
 solved gas per barrel of oil at the 
 beginning of production, and shrink- 
 age factor have been determined. 
 
 3. When total oil and water produced 
 have been recorded. 
 
 4. When the sands are highly perme- 
 able. 
 
 5. When the pool is of moderate size. 
 
 Unknown Volume 
 
 In the past it has been the common 
 practice in many areas to produce solu- 
 tion gas without metering it so that its 
 volume is unknown. Such a condition is 
 assumed in the following description of 
 a procedure to determine the quantity of 
 oil remaining in a partially depleted res- 
 ervoir. 
 
 Let x — original oil in field in bbls. total 
 (gas-free basis). 
 a = oil removed from held in bbls. 
 (gas-free basis). 
 
 S = the shrinkage of the oil due to 
 loss of dissolved gas (fraction 
 of original volume). 
 
 V= original oil in field in bbls. (be- 
 fore gas loss). 
 sv = shrinkage in bbls. (for both oil 
 removed and residual oil). 
 
 b = volume of void in field calcu- 
 lated from volume of injected 
 gas required to build up a stated 
 pressure. 
 
 Then we can see that 
 b = a + sv 
 
 or 
 
 _b — a 
 
 v 
 
 s 
 X = V — sv 
 x — a = residual oil 
 
 This calculation requires, primarily, a 
 good measured value for s. It assumes 
 that when gas is injected into the field 
 to determine b, a negligible amount of 
 this gas dissolves in the residual oil. 
 Since the measuring pressure is low and 
 the time of contact of gas and oil is very 
 short, especially in highly permeable 
 sands, this assumption is justified. If 
 conditions are such that a considerable 
 amount of gas is dissolved during this 
 process it may be tested and a proper 
 allowance made for it in the calcula- 
 tions. 
 
 Interstitial water is present in almost 
 every reservoir but since the solubility 
 of gas in water is a small fraction of its 
 solubility in oil, it is believed that the 
 effect of interstitial water is essentially 
 only to reduce the void space so that for 
 the purpose of this procedure water may 
 be considered part of the rock. If there 
 is substantial encroachment of water 
 this procedure is not applicable. 
 
 If there is no record of original pres- 
 sure, it is fairly safe to assume that in 
 most areas it approximates hydrostatic 
 pressure for the depth of the reservoir. 
 If there is no record of the amount of 
 gas originally dissolved per barrel of oil, 
 laboratory tests may be made to deter- 
 mine the solubility of similar gas in that 
 particular oil at the original reservoir 
 pressure and temperature. If the oil in 
 the reservoir was not completely satu- 
 rated, then the error in calculating the 
 amount of remaining oil is on the con- 
 servative side. 
 
 ACKNOWLEDGMENT 
 
 Thanks to J. S. Machin, who set up the 
 mathematical statement; A. Tl. Bell, « l"> 
 clarified the article; and C. A. Bays, F. H. 
 Reed, Paul Torrey, William L. Horner, R. C. 
 Earlougher, Parke A. Dickey, Kurt H. Andre- 
 sen. U E. Elkins. R. V. Hughes. J. A. Culbert- 
 son and E. Charles Patton, Jr., who read the 
 paper and made valuable criticisms and sug- 
 gestions.