9.3. Cyclic water flooding.
The main criteria of cyclic water flooding implementation:
1) the presence of stratified-heterogeneous by permeability or fractured-porous hydrophilic reservoirs;
2) high residual oil-saturation of low-permeability interlayers;
3) technical-technological ability to create pressure fluctuations of high amplitude that can reach 0,5-0,7 from the average pressure differential between the injection and producing wells.
Cyclic flooding is used for the formations, corresponding to Kazemi model, Chapter 4, paragraph 4.2.3. Stratified reservoir consists of at least two layers: high permeability (HP) and low-permeability (LP). Between the HP and LP interlayers there is hydrodynamic connection. The cycle is divided in two semi-cycles. In the first semi-cycle during the injection of the displacing liquid there is partial water cross flow from the HP into the LP. Another part of the water is filtered by the HP in the direction of the producing well, simultaneously, oil displacement by water from the PH happens.
Figure 9.2. shows the steps of the first semi-cycle process. In the second semi-cycle when the pressure of the injection well reduces or injection is terminated then the pressure in high-permeable interlayer falls below the pressure in the LP. As the oil has higher compressibility than water, and due to the hydrophilicity of the reservoir, the water is retained in the LP by capillary forces and the oil flows from the LP to the HP. At the first semi-cycle of the second cycle the injection well starts to operate again, the reservoir pressure increases, oil is displaced from the HP, come from the to the LP to the bottomholes of the producing wells.
V
Fig. 9.2. The scheme of oil displacement by water. The first semi-cycle.
R – the distance between the injection and producing wells; h1 – HP thickness; h1 ,h2 – the thicknesses of HP and LP, V- the cross-flow rate from HP to LP, r –coordinate, qв – water expenses, q – liquid production rate.
Let Р0 – reservoir pressure in low-permeability interlayer; Рн – bottom-hole injection well pressure; Р1(r,t) – current pressure of HP; r – coordinate; t – time.
During the first semi-cycle there is satisfied the ratio: [21].
(9.1)
Cross-flow rate from HP to LP is determined by
for
t<=t2
(9.2)
for
t>t2,
(9.3)
Where
- achievement time of reservoir upper boundary;
- the HP piezoconductivity coefficient.
For each fixed r the cross-flow rate V is modified firstly by (9.2), then when it reaches the upper boundary of the formation h2 the ratio (9.3) is fulfilled and the cross -flow rate decays. The parameters λ1 , λ2 are the characteristics of the LP.
(9.4)
Where
-
hydraulic conductivity of LP; к2
– permeability of LP;
- water dynamic viscosity. Here is not taken into consideration the
porosity changes depending on pressure and cross-flow rate on
saturations.
During the first semi-cycle in the low-permeability part of the reservoir the water saturation will increase and oil saturation will decrease. In the second semi-cycle the pressure in the high-permeability part of the reservoir drops, oil from LP will be displaced by water in the HP. The volume of the displacing oil will be less than the volume of water, broken into LP. In subsequent cycles, the volume of oil, transported from the LP to the HP will be reduced. At cyclic flooding each of the producing and injection wells operates in the periodic pressure changing drive (withdrawal, consumption). To ensure more regular load of the equipment, the deposit should be divided into separate blocks with the implementation of shifting of semi-periods of injection and withdrawal.
If the reservoir is represented by several layers, interlayers of different permeability according to GIS interpretation, then it is possible to use stratified reservoir model or Serra model to describe physical processes, paragraph 4.2.3.