3.3. The well patterns - development systems of production facilities on natural recovery modes.
At the first stage of development or during the development of production facilities on natural recovery modes; when the activity of edge stratum water is weak, there are applied evenly distributed well systems by a three - or four-point grid:
а
)
б)
Fig.3.2. а) Three-point and б) four-point well patterns. 1 – oil pool outline; 2 – producing wells.
When the development of oil part of the reservoir of oil-and-gas deposit is advanced (under-gas-cap zone) the producing wells are situated between the outer gas-pool outline and inner oil-pool outline, i.e. in pure oil zone (POZ).
Fig. 3.3. 1- outer oil pool outline; 2 – inner oil pool outline; 3- producing wells; 4 and 5- inner and outer gas-pool outlines.
3.4. Enhanced recovery systems
3.4.1. Oil-displacement and sweep efficiency
Enhanced recovery systems with water injection are called primary enhanced oil recovery methods [6]. They are used at the end of the first stage of field development. At this time according to the project and approved development system the injection wells are being put into operation. The injected water performs two functions. The first – during the reservoir development on natural drives there is the falling of reservoir pressure, as a result, production rate is decreased. The injected water fills the loss of reservoir pressure, restores, supports reservoir pressure (RPM), increases reservoir energy that is necessary for oil displacement and, therefore, for enhanced oil recovery. The second – because of the water injection to the formation there is the process of oil displacement. With continuous injection process water washes out the remaining oil particles, separates them from the surface of the primarily washed pore channels.
There are suggested the oil-displacement в and sweep о efficiencies by A.P. Krylov during the process of oil displacement by water.
The oil-displacement efficiency в is the ratio of the oil volume, displaced from the reservoir area ( accumulated) Q(t), busy in sweep agent (water, gas) to the initial content of oil in this area G1:
The reservoir sweep efficiency is defined as о - the ratio of net pay rock volume, covered by displacement, to the total oil volume in the reservoir:
The oil recovery factor will be equal to the product of oil-displacement and sweep efficiencies.
= во
The displacement efficiency в depends on the oil viscosity to the viscosity of displacing agent, on reservoir homogeneity, pore channels diameters, i.e. on permeability, wettability of the rock.
в =0,8÷0,9 for high-permeable reservoirs with low oil viscosity, в =0,25÷0,4 for low- permeable reservoirs. When we have mixed oil displacement by water and gas, then в =0,9÷0,98 (0,7÷0,8), [3,5].
Sweep efficiency characterizes oil loss on the thickness and the reservoir area in the zones of the central rows of producing wells, rows of dividing injection wells and unswept zones. It depends on the reservoir heterogeneity in general and selected development system.
о = 0,7 ÷ 0,9 for water,
о = 0,2 ÷ 0,3 for gas.
The theory of oil fields enhanced recovery system has received its continuation in the works of Soviet and Russian scientists. The oil recovery factor is assumed to be equal to the product of three, four, five factors [7].