Conclusions of reservoir simulation studies by chu

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Conclusions of reservoir simulation studies by Chu and Trimble 10 included: (1) the optimal constant steam rate is proportional to pattern size rather than sand thickness, (2) economics of a project can be improved over the constant rate case by using higher steam rates in the initial stages and then decreasing the rate with time, (3) decreasing rate in a hyperbolic fashion appeared superior to a linear decrease. Bursell and Pittman 6 reported that rate reductions late in the life of three pilots in the Kern River Field resulted in improved steam/oil ratios. All of these observations are consistent with our current understanding of the steamflood process. Recognition of Heat Management by California Operators As noted in the preceding section, by 1975 there were examples of how to conduct heat balances and evidence supporting the steam over-ride/gravity drainage model, including the principle of reducing injection rates in mature projects. However, rapidly increasing oil prices and a mental analogue of a “displacement” mechanism for the steamflood process led to a common belief that higher injection rates automatically resulted in increased oil production rates. Figure 8 shows the average injection rate per well and average SOR for steamflood operations in four of the largest California thermal fields (Coalinga, Kern River, Midway-Sunset, and South Belridge). It shows that in the late 1960’s, and with current heat management concepts, average injection rates for steamfloods were about 225 BSPD/well. From Figure 7 we see that early life steam chest heat minimum requirements at 60 psig are about 70 MMBTU/day, or approximately 200 BSPD. Thus the rates used in the first steamflood projects were quite reasonable. However, we also see that average rates per well increased to more than 300 BSPD/well during the 1970’s, and did not drop below 300 BSPD/well until 1992. It appears that most operators did not carefully review overall heat and mass balances for their steamflood projects during this period, and they generally appear to have had too high a rate of heat injection. During the 1990’s an increasing amount of effort was applied to heat management 11 , which resulted in a general decrease in the amount of steam injected in each pattern. The significant consolidation which occurred in the thermal industry in the past seven years (from six major operators to two) has also resulted in more rigorous application of heat management. Besides the emphasis placed on heat management by the surviving operators, control of larger contiguous field areas has made coordination and optimization of injection operations easier. Despite the present maturity of many projects and the accompanying production declines, Figure 8 shows that it has been possible to maintain a nearly constant average SOR for the four large fields, despite significant production declines.
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