Many reservoirs are of the volumetric undersaturated type and their production, therefore, is controlled largely by the solution gas-drive mechanism. In many cases, the mechanism is altered to a greater or lesser extent by gravitational segregation of the gas and oil, by small water drives, and by pressure maintenance, all of which improve recovery. The important characteristics of this type of production may be summarized as follows and observed in the graph of Fig. 6.5 for the Gloyd-Mitchell zone of the Rodessa Field. Above the bubble point, the reservoir is produced by liquid expansion, and there is a rapid decline in reservoir pressure that accompanies the recovery of a fraction of 1% to a few percentage points of the initial oil in place. The gas-oil ratios remain low and generally near the value of the initial solution gas-oil ratio. Below the bubble point, a gas phase develops that, in most cases, is immobile until the gas saturation reaches the critical gas saturation in the range of a few percentage points to 20%. During this period, the reservoir produces by gas expansion, which is characterized by a much slower decline in pressure and gas-oil ratios near or in some cases even below the initial solution gas-oil ratio. After the critical gas saturation is reached, free gas begins to flow. This reduces the oil flow rate and depletes the reservoir of its main source of energy. By the time the gas saturation reaches a value usually in the range of 15% to 30%, the flow of oil is small compared with the gas (high gas-oil ratios), and the reservoir gas is rapidly depleted. At abandonment, the recoveries are usually in the range of 10% to 25% by the solution gas-drive mechanism alone, but they may be improved by gravitational segregation and the control of high gas-oil ratio wells.
Figure 6.5 Development, production, and reservoir pressure curves for the Gloyd-Mitchell zone, Rodessa Field, Louisiana.
The production of the Gloyd-Mitchell zone of the Rodessa Field, Louisiana, is a good example of a reservoir that produced during the major portion of its life by the dissolved gas-drive mechanism.16 Reasonably accurate data on this reservoir relating to oil and gas production, reservoir pressure decline, sand thickness, and the number of producing wells provide an excellent example of the theoretical features of the dissolved gas-drive mechanism. The Gloyd-Mitchell zone is practically flat and produced oil of 42.8 °API gravity, which, under the original bottom-hole pressure of 2700 psig, had a solution gas-oil ratio of 627 SCF/STB. There was no free gas originally present, and there is no evidence of an active water drive. The wells were produced at high rates and had a rapid decline in production. The behavior of the gas-oil ratios, reservoir pressures, and oil production had the characteristics expected of a dissolved gas drive, although there is some evidence that there was a modification of the recovery mechanism in the later stages of depletion. The ultimate recovery was estimated at 20% of the initial oil in place.
Many unsuccessful attempts were made to decrease the gas-oil ratios by shutting in the wells, by blanking off upper portions of the formation in producing wells, and by perforating only the lowest sand members. The failure to reduce the gas-oil ratios is typical of the dissolved gas-drive mechanism, because when the critical gas saturation is reached, the gas-oil ratio is a function of the decline in reservoir pressure or depletion and is not materially changed by production rate or completion methods. Evidently there was negligible gravitational segregation by which an artificial gas cap develops and causes abnormally high gas-oil ratios in wells completed high on the structure or in the upper portion of the formation.
Table 6.5 gives the number of producing wells, average daily production, average gas-oil ratio, and average pressure for the Gloyd-Mitchell zone. The daily oil production per well, monthly oil production, cumulative oil production, monthly gas production, cumulative gas production, and cumulative gas-oil ratios have been calculated from these figures. The source of data is of interest. The number of producing wells at the end of any period is obtained either from the operators in the field, from the completion records as filed with the state regulatory body, or from the periodic potential tests. The average daily oil production is available from the monthly production reports filed with the state regulatory commission. Accurate values for the average daily gas-oil ratios can be obtained only when all the produced gas is metered. Alternatively, this information is obtained from the potential tests. To obtain the average daily gas-oil ratio from potential tests during any month, the gas-oil ratio for each well is multiplied by the daily oil allowable or daily production rate for the same well, giving the total daily gas production. The average daily gas-oil ratio for any month is the total daily gas production from all producing wells divided by the total daily oil production from all the wells involved. For example, if the gas-oil ratio of well A is 1000 SCF/STB and the daily rate is 100 bbl/day and the ratio of well B is 4000 SCF/STB and the daily rate is 50 bbl/day, then the average daily gas-oil ratio R of the two wells is
Table 6.5 Average Monthly Production Data, Gloyd-Mitchell Zone of the Rodessa Field
This figure is lower than the arithmetic average ratio of 2500 SCF/STB. The average gas-oil ratio of a large number of wells, then, can be expressed by
where R and qo are the individual gas-oil ratios and stock-tank oil production rates.
Figure 6.5 shows, plotted in block diagram, the number of producing wells, the daily gas-oil ratio, and the daily oil production per well. Also, in a smooth curve, pressure is plotted against time. The initial increase in daily oil production is due to the increase in the number of producing wells and not to the improvement in individual well rates. If all the wells had been completed and put on production at the same time, the daily production rate would have been a plateau, during the time all the wells could make their allowables, followed by an exponential decline, which is shown beginning at 16 months after the start of production. Since the daily oil allowable and daily production of a well are dependent on the bottom-hole pressure and gas-oil ratio, the oil recovery is larger for wells completed early in the life of a field. Because the controlling factor in this type of mechanism is gas flow in the reservoir, the rate of production has no material effect on the ultimate recovery, unless some gravity drainage occurs. Likewise, well spacing has no proven effect on recovery; however, well spacing and production rate directly affect the economic return.
The rapid increase in gas-oil ratios in the Rodessa Field led to the enactment of a gas-conservation order. In this order, oil and gas production were allocated partly on a volumetric basis to restrict production from wells with high gas-oil ratios. The basic ratio for oil wells was set at 2000 SCF/STB. For leases on which the wells produced more than 2000 SCF/STB, the allowable in barrels per day per well, based on acreage and pressure, was multiplied by 2000 and divided by the gas-oil ratio of the well. This cut in production produced a double hump in the daily production curve.
In addition to a graph showing the production history versus time, it is usually desirable to have a graph that shows the production history plotted versus the cumulative produced oil. Figure 6.6 is such a plot for the Gloyd-Mitchell zone data and is also obtained from Table 6.5. This graph shows some features that do not appear in the time graph. For example, a study of the reservoir pressure curve shows the Gloyd-Mitchell zone was producing by liquid expansion until approximately 200,000 bbl were produced. This was followed by a period of production by gas expansion with a limited amount of free gas flow. When approximately 3 million bbl had been produced, the gas began to flow much more rapidly than the oil, resulting in a rapid increase in the gas-oil ratio. In the course of this trend, the gas-oil ratio curve reached a maximum, then declined as the gas was depleted and the reservoir pressure approached zero. The decline in gas-oil ratio beginning after approximately 4.5 million bbl were produced was due mainly to the expansion of the flowing reservoir gas as pressure declined. Thus the same gas-oil ratio in standard cubic feet per day gives approximately twice the reservoir flow rate at 400 psig as at 800 psig; hence, the surface gas-oil ratio may decline and yet the ratio of the rate of flow of gas to the rate of flow of oil under reservoir conditions continues to increase. It may also be reduced by the occurrence of some gravitational segregation and also, from a quite practical point of view, by the failure of operators to measure or report gas production on wells producing fairly low volumes of low-pressure gas.
Figure 6.6 History of the Gloyd-Mitchell zone of the Rodessa Field plotted versus cumulative recovery.
The results of a differential gas-liberation test on a bottom-hole sample from the Gloyd zone show that the solution gas-oil ratio was 624 SCF/STB, which is in excellent agreement with the initial producing gas-oil ratio of 625 SCF/STB.17 In the absence of gas-liberation tests on a bottom-hole sample, the initial gas-oil ratio of a properly completed well in either a dissolved gas drive, gas cap drive, or water-drive reservoir is usually a reliable value to use for the initial solution gas-oil ratio of the reservoir. As can be seen in Fig. 6.6, the extrapolations of the pressure, oil rate, and producing gas-oil ratio curves on the cumulative oil plot all indicate an ultimate recovery of about 7 million bbl. However, no such extrapolation can be made on the time plot (see Fig. 6.5). It is also of interest that, whereas the daily producing rate is exponential on the time plot, it is close to a straight line on the cumulative oil plot.
The average gas-oil ratio during any production interval and the cumulative gas-oil ratio may be indicated by integrals and shaded areas on a typical daily gas-oil ratio versus cumulative stock-tank oil production curve, as shown in Fig. 6.7. If R represents the daily gas-oil ratio at any time, and Np the cumulative stock-tank production at the same time, then the production during a short interval of time is dNp and the total volume of gas produced during that production interval is R dNp. The gas produced over a longer period when the gas-oil ratio is changing is given by
Figure 6.7 Typical daily gas-oil ratio curve for a dissolved gas drive reservoir.
The shaded area between Npl and Np2 is proportional to the gas produced during the interval. The average daily gas-oil ratio during the production interval equals the area under the gas-oil ratio curve between Npl and Np2 in units given by the coordinate scales, divided by the oil produced in the interval (Np2 – Np1), and
The cumulative gas-oil ratio, Rp, is the total net gas produced up to any period divided by the total oil produced up to that period, or
The cumulative produced gas-oil ratio was calculated in this manner in column 11 of Table 6.5. For example, at the end of the third period,
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