11.1. Physical processes, happening during oil displacement by heat-transfer agents.
The initial value of reservoir temperature and its distribution in the deposit is defined by geothermal conditions of the field. Usually reservoir temperature corresponds to the geothermal gradient. In the process of development of the field reservoir temperature can vary, thus, the injected into the reservoir water has another temperature. In the formation there are the processes associated with the heat release or absorption. The temperature change will occur due to the hydraulic resistance of the filtering fluids, due to the Joule-Thomson effect.
The distribution of reservoir temperature and its change is called temperature mode. The change of the temperature mode occurs primarily due to the thermal conductivity and convection (warm fluids’ density is less, that’s why they are lighter) [7,22].
The peculiarity of the application of thermal methods is that along with hydrodynamic oil displacement the temperature of the deposit increases. There is formed an additional heat oil displacement front by hot water. Hydrodynamic displacement front passes ahead the heat displacement front, because the heat transfer from the heat-transfer agent that is used to heat viscous oil occurs not at once, later, figure 11.1.
Fig.11.1. The scheme of oil displacement by hot water. 1 – the displacement zone of cold oil by water, 2 – the displacement zone of warm oil by hot water, ρв(t) - the radius of hydrodynamic displacement front, ρt(t) –the radius of thermal displacement front.
The increase of oil, water and rock temperature leads to the decrease of oil viscosity, change in the ratio of oil and water mobility to the change of relative permeabilities, residual oil saturation, to the evaporation of light fractions; there is occurred the thermal expansion of the reservoir (varies porosity, the fluids volume that fills it, i.e. saturation).
11.2. Oil displacement by hot water and steam.
Hot water and steam, in other words – heat-transfer agents, are produced in steam generators (boilers) of high pressure and are injected into the formation through the injection wells of special construction and with special equipment designed for operation in the conditions of high temperatures. The disadvantage of using of surface steam generators provides high level of heat losses (temperature) in the surface communications and in the well bore. During the movement of the heat-transfer agent in the reservoir there are happened the heat losses through the top and the bottom of the reservoir. To reduce heat loss there should be chosen the formations with the thickness of more than 6 meters, applied rectangular well pattern with the distance of up to 100-200m between the injection and producing wells. The perforation interval is chosen in the middle part of the reservoir; the pipes are isolated; steam generator is situated to the wells as close as possible.
The injected to the reservoir steam can turn into hot water, depending on thermodynamic conditions. That’s why, in the design and implementation of hot water and steam injection to the reservoir, you need to know the thermodynamic state of water: liquid, steam, or a mixture of water and steam [7], that is determined by using the P-T diagram (fig. 11.2.).
The
critical point – condensation point C - matches the state of the
water where the physical properties of the liquid and gas phases are
the same. Water Ркр=22,12MPa,
Ткр=
647,30
К
(374,120
С),
density
.
If pressure and temperature corresponds to the point of being on the
saturation line OC, then water is simultaneously in liquid and
vaporous state; the steam is called saturated. Water, situated above
the line of saturation OC is liquid, below the line of OC is in the
form of superheated steam.
Fig.11.2 . P-T diagram –water pressure- temperature давление, С - condensation point
In atmospheric conditions oil and water are insoluble. In 1960 E.B. Chekaliuk found with the help of laboratory researches that the solubility of oil in water is achieved at the temperature of 320-3400 C and pressure 16-22MPa. It means that solubility occurs at thermobaric conditions that are close to critical. When the temperature of oil-water solution decreases up to the temperature of 18-200C oil is completely extracted from water. If the density of the water in normal conditions is1000 -1020 kg/m3, then with the increasing of temperature the density is falling and at pressure close to the critical the complete mixing of water and oil happens, the phase boundary is blurred.
Saturated steam as thermal dissolvent of oil operates in the temperature range of 100-3700 C and pressures that are equal to atmospheric to 22MPa. The reservoir sweep coefficient by hot water is higher than for steam. Steam as low-viscosity operating agent usually moves near the top of formation, the thickness sweep coefficient doesn’t exceed 0,4; by the area is 0.5 -0.9. In the result, the ORF is 0,3-0,35.
According to Y.P. Zheltov data, when oil is displaced by hot water to recover 4000m3 additional oil it is necessary to burn out 1770m3 of oil from the additional quantity. Under conditional burning of oil is understood the consumption of an equivalent amount of energy for water heating [7].