The injection and production wells in a waterflood should be placed to accomplish the following: (1) provide the desired oil productivity and the necessary water injection rate to yield this oil productivity and (2) take advantage of the reservoir characteristics, such as dip, faults, fractures, and permeability trends. In general, two kinds of flooding patterns are used: peripheral flooding and pattern flooding.
Pattern flooding is used in reservoirs having a small dip and a large surface area. Some of the more common patterns are shown in Fig. 11.1. Table 11.1 lists the ratio of producing wells to injection wells in the patterns shown in Fig. 11.1. If the reservoir characteristics yield lower injection rates than those desired, the operator should consider using either a seven- or a nine-spot pattern, where there are more injection wells per pattern than producing wells. A similar argument can be made for using a four-spot pattern in a reservoir with low flow rates in the production wells.
Figure 11.1 Geometry of common pattern floods.
Table 11.1 Ratio of Producing Wells to Injection Wells for Several Pattern Arrangements
The direct-line drive and staggered-line drive patterns are frequently used because they usually involve the lowest investment. Some of the economic factors to consider include the cost of drilling new wells, the cost of switching existing wells to a different type (i.e., a producer to an injector), and the loss of revenue from the production when making a switch from a producer to an injector.
In peripheral flooding, the injectors are grouped together, unlike in pattern floods where the injectors are interspersed with the producers. Figure 11.2 illustrates two cases in which peripheral floods are sometimes used. In Fig. 11.2(a), a schematic of an anticlinal reservoir with an underlying aquifer is shown. The injectors are placed so that the injected water either enters the aquifer or is near the aquifer-reservoir interface. The pattern of wells on the surface, shown in Fig. 11.2(a), is a ring of injectors surrounding the producers. A monoclinal reservoir with an underlying aquifer is shown in Fig. 11.2(b). In this case, the injectors are again placed so that the injected water either enters the aquifer or enters near the aquifer-reservoir interface. When this is done, the well arrangement shown in Fig. 11.2(b), where all the injectors are grouped together, is obtained.
Figure 11.2 Well arrangements for (a) anticlinal and (b) monoclinal reservoirs with underlying aquifers.
Since the 1990s, other water injections schemes have been used. These include horizontal wells and injection pressures above the reservoir fracturing pressure. The use of these schemes has resulted in mixed success. The reader is encouraged to pursue the literature to research these techniques if further interest is warranted.6–9
Leave a Reply