Economic and Technical Problems of Raising the Oil Recovery Factor of Seams in the USSR

Summary

Over 90 per cent of oil in the USSR is obtained from oilfields where the reservoir is treated in various ways to increase the oil recovery factor. The most widely used systems are based on displacement of the oil by water. This method is now used in almost 300 oilfields and extends to more than 75 Per cent of reserves currently in exploitation.

Water flooding is likely to be the main method of increasing the oil recovery factor until the end of this century. Given favourable geological conditions (high porosity and permeability of the reservoir, low viscosity of the oil) this method makes it possible to recover 60–65 per cent of reserves from the deposit. Such deposits are exploited by protracted gushing with a relatively sparse network of operational wells.

In oilfields with less favourable geological conditions, not more than 40–45 per cent of the reserves can be recovered by the water flooding method. The main factor reducing the efficiency of water flooding in this case is the large difference between the viscosity of the oil and that of water, which lowers the water-oil displacement coefficient and thus the over-all efficiency of flooding. In a number of oilfields in the USSR chemical reagents (water thickeners) are used ot balance or reduce the ratio between the viscosity of the oil and that of the injection agent. Practical experience shows that the oil recovery factor can thereby be increased by 7–10 per cent.

Around 20 per cent of oil reserves being prepared for exploitation or already being exploited are not suitable for the efficient use of the water flooding method because they contain highly viscous oils (over 60–70 cp). In oilfields with such oils, thermal drive methods have been widely developed in the USSR over the last 10 yearsf they include area and local injection of live steam, the creation of an advancing fire front in the reservoir combined with injection of a water/air mixture, and steam formation in the fire zone. These methods are used on an industrial scale in the oilfields of Sakhalin Island, the Ukraine, Azerbaidzh and other regions. As calculations show, thermal treatment can facilitate recovery of the oil up to 45–60 per cent, as against 20–25 per cent with the depletion method. The use of various methods of increasing the oil recovery factor involves not only-technical but also economic problems.

In many cases where ordinary systems of water flooding are used, the oil recovery factor can be raised by increasing the density of the well network; oil is then recovered from hydrodynamically insulated sectors of the reservoir which would not be drained if the network is sparse.

Consequently, for most oilfields attainment of the maximum possible oil recovery factor for the method of exploitation adopted is a purely economic problem. Material and manpower resources being limited, bringing into exploitation new reserves in another oilfield is always a possible alternative to achieving maximum recovery by increasing the density of the well pattern. This problem is solved by calculating the ratio between the additional oil produced. For example, drilling 6,000 wells to increase the oil recovery factor in the Tatar ASSR will yield around 10 million t of extra oil in 1980. The production cost of this additional oil is approximately 15 per cent below the average for the industry.

The economic advisability of applying any new method of exploitation to increase the oil recovery factor is ultimately determined by the value of the additional reserves recovered as a result as well as by the value of the additional oil obtained.

Calculations show that up to the year 2000 the most economically efficient method of increasing the oil recovery factor will be the use of water thickening agents (polyacrylamides), alkalis (NaOH) and mi cellar solutions. The production cost of the additional oil recovered with these methods will be 40–60 per cent below the sectoral average.

When the oil is displaced by steam and when surface—active substances are used, the production cost of the additional oil will, until 1990f be approximately equal to, and after 1990 20–25 per cent lower than, the sectoral average. The use of carbonic acid is likely to be limited until the year 2000 on account of the relatively high production cost of the additional oil, which at some stages is twice the sectoral average. All calculations were, of course, made on the basis of the notional cost of chemical reagents and the efficiency of the technology employed. It is possible that these estimates will change with the development of new technologies using more effective and cheaper reagents and with general economic factors increasing the profitability of new methods of oilfield exploitation.