Waterflooding Candidates

Several factors lend an oil reservoir to a successful waterflood. They can be generalized in two categories: reservoir characteristics and fluid characteristics.

The main reservoir characteristics that affect a waterflood are depth, structure, homogeneity, and petrophysical properties such as porosity, saturation, and average permeability. The depth of the reservoir affects the waterflood in two ways. First, investment and operating costs generally increase as the depth increases, as a result of the increase in drilling and lifting costs. Second, the reservoir must be deep enough for the injection pressure to be less than the fracture pressure of the reservoir. Otherwise, fractures induced by high water injection rates could lead to poor sweep efficiencies if the injected water channels through the reservoir to the producing wells. If the reservoir has a dipped structure, gravity effects can often be used to increase sweep efficiencies. The homogeneity of a reservoir plays an important role in the effectiveness of a waterflood. The presence of faults, permeability trends, and the like affect the location of new injection wells because good communication is required between injection and production wells. However, if serious channeling exists, as in some reservoirs that are significantly heterogeneous, then much of the reservoir oil will be bypassed and the water injection will be rendered useless. If a reservoir has insufficient porosity and oil saturation, then a waterflood may not be economically justified, on the basis that not enough oil will be produced to offset investment and operating costs. The average reservoir permeability should be high enough to allow sufficient fluid injection without parting or fracturing the reservoir.

The principal fluid characteristic is the viscosity of the oil compared to that of the injected water. The important variable to consider is actually the mobility ratio, which was defined earlier in and includes not only the viscosity ratio but also a ratio of the relative permeabilities to each fluid phase:

A good waterflood has a mobility ratio around 1. If the reservoir oil is extremely viscous, then the mobility ratio will likely be much greater than 1, viscous fingering will occur, and the water may bypass much of the oil.