4.4. Reciprocating and non-reciprocating oil displacement.

In paragraphs 4.1,4.2 the models, reflecting features of the geological reservoir structure, pool - reservoir properties and the degree of their studying are under the view. From the physical point of view in each of these models there are realized two options. The first – reciprocating oil displacement by water or other agent, the second – nonreciprocating displacement.

4.4.1. Reciprocating displacement.

Let’s consider reciprocating oil displacement by water on a simple example - linear displacement or straight - parallel filtering, fig.4.7.

Fig. 4.7. Reciprocating oil displacement by water, x_ф(t)-oil displacement front by water, _св - bound water coefficient, _{но –} bound oil coefficient, h – formation thickness, _но=1-^*, ^{* -} maximum water saturation coefficient, when the oil filtration stops.

The displacement front moves from injection wells to producing wells, displacing oil. After displacement only water moves, oil does not move, its amount is determined by the coefficient _он=1-^*, fig. 4.4. Before displacement front only oil moves, an amount of bound water is characterized by the parameter _св. The water cut on the displacement front and behind it is constant and equals to ^*.

When the displacement front reaches the gallery of producing wells, the output is completely watered out. In reality, k^*_н(^*)=0, and according to (4.5) f(^*)==1. Thus, the water cut is equal to 1 and the saturation is less than 1. Reciprocating displacement can happen only in homogeneous high-permeable reservoirs or in high-permeable interlayers of the formations. It is used for the approximate calculation of development indicators.

4.4.2. Non-reciprocating displacement.

As in the previous paragraph, let’s observe the linear displacement of oil by water. During non-reciprocating displacement outside the displacement front there is organized the zone of two-phase filtration oil - water, shown in figure 4.8.

Fig. 4.8. The scheme of non-reciprocating oil displacement, _{н –} oil saturation coefficient in the zone of combined filtration.

In contrast to the reciprocating displacement there is a combined filtration two phases - oil and water occurs behind the front of displacement. Moreover, due to the heterogeneity of the reservoir and chaotic spatial distribution of pore channels of different radii, the Jamen effect appears [1]. Bars, particles of oil are displaced with water, and the water columns are displaced by oil. On the border of the particles phase interaction - the menisci and pore channels walls there is additional resistance - capillary pressure that must be overcome by the external pressure of injected water into the reservoir.

When the displacement front reaches the gallery of the producing wells the output begins to be watered out gradually and unlike reciprocating displacement the well operation continues as the oil saturation during nonreciprocating displacement on the displacement front is less than the maximum *.

The size of the zones of two-phase filtration can be considerable, and the joint production time of oil and water can reach tens of years. The share of water in crude production rate increases, if the methods of its restriction are not used, for example, the changes of profiles intake capacity of injection wells, isolation, disabling of high-permeable interlayers on the bottom of producing wells (Kazemi or Serra models).