11.8. Thermal-gas method of treatment.
The method of thermal-gas treatment (TGT) refers to thermal methods. It is applied on the fields of light oil with high temperatures of above 650 C and high reservoir pressures. As for the in-situ combustion the products of oxidation: nitrogen, carbon dioxide, and the light oils fractions are the displacing gas agent, mingling with oil, and ultimately increasing its mobility. It contributes to the oil recovery factor increase, especially in the development of the fields with difficult-to-recover reserves.
The TGT technology was applied on pilot area of Sredne-Nazymskoye field, reservoir U0, Bazhenovskaya suite. The results of laboratory studies have shown oil reservoir density is 711-767 kg/m3 and saturation pressure is15.4 MPa, reservoir pressure Рпл=33,7 MPa, the bedding depth is 2720-2740 m. The adopted volumetric parameters: porosity is 0.08, oil saturation is 0.85. Permeability coefficients according to the processing results of hydrodynamic researches (0,02 - 7,9) *10-15m2. Thermal-gas treatment on the pilot area is realized by the injection of water-air mixture or by their combined injection or by the alternating injection of air and water.
An average water-air ratio for the well № 219 varies in the range from 0.0001-0.01. Cyclic injection character allows you to combine the properties of thermal and hydrodynamic influence on the reservoir. The volume of air injection is 24000 m3/day, water - 2.4 -240 m3/day. At the well head the pressure of gas injection was 10-35 MPa, water - 15-40 MPa. The total fluid withdrawal of 4 producing wells №№ 401, 3000, 3001, 3002 was 150-400m3/day. Thermal treatment is recommended for the deposits with abnormally high reservoir pressure in bituminous reservoirs.
CHAPTER 12. OTHER METHODS OF ENHANCED OIL RECOVERY
12.1. Formation hydraulic fracturing (fhf)
One of the frequently used methods of enhanced oil recovery is hydraulic fracturing. The technology of formation hydraulic fracturing, the development of the wells after hydraulic fracturing are observed in detail in the course of “Borehole mining”, so these issues will not be discussed here.
The question about the belonging of FHF is rather debatable. It can belong to the methods of enhanced oil recovery or to the production stimulation methods. There are various points of view on this question. On the one hand hydraulic fracturing increases the filtration area and, consequently, increases the well flow rate, so, it refers to EOR methods. On the other hand the length of the fracture reaches tens of meters; it allows you to operate the remote from the bottom-hole oil saturated zones; oil region, the parts of the reservoir that before the fracture formation were not actively involved in the development begin to operate, that is the areal sweep coefficient increases. In this regard, we assume formation hydraulic fracturing as a simple method of enhanced oil recovery, not related neither to the methods of production stimulation (PS), nor to the methods of enhanced oil recovery (EOR).
Let’s сonsider the process of fracture formation. According to the continuum mechanics it is known that in the elastic environment the fracture is formed in the plane of the highest normal stress, that is, in the plane in the direction to the mountain tension (see Chapter 2). Therefore, a fracture is vertical. It spreads toward the minimum normal stress, that is, in the radial from wells direction. Fracture opening occurs in the direction that is perpendicular to the radius of the well, figure 12.1., r=x. Fracture is a violation (gap) of the integrity of the environment, in our case, of the reservoir. The rate of injection of fracturing fluid is chosen in such a way that the dynamic tension that occurs on the bottom-hole, exceeds the tensile strength of rocks. To determine the technological parameters of hydraulic fracturing conduct the mini-hydraulic-fracturing process is carried out.
Fig. 12.1. а) – fracture profile, б) – fracture in the project. h – fracture height at the well bottom-hole., δ – fracture opening at the bottom-hole, L – the length of the semi-fracture, S – semi-fracture side surface area, v – fluid filtration velocity from the reservoir to the fracture.
The choice of well operation drive is determined by the results of hydrodynamic researches of the wells after hydraulic fracturing. The productivity of the schedule-size of the ESP unit must meet the productivity factor of well defined after fracturing.
The fracture, created in the reservoir, is filled with the propant agent, which does not allow it to close. Thus, there is two capacitive system: formation- fracture in the reservoir. The fracture, filled with propant agent, is a fictitious layer, as the diameter of propant particles are the same.
Fracture capacity depends on the packing degree of proppant and volume of liquids with propant agent (liquids of sand carrier), concentration of propant in the fracture. The permeability of the fracture is many times greater than the permeability of the reservoir.
When operating the wells after the hydraulic fracturing the system “reservoir-fracture-well” participates in the filtration process. Through the side surface of the fracture the fluid flows from the reservoir into the fracture. Then along the fracture it moves to the bottom-hole. The pressure in the fracture is distributed unevenly; the lowest pressure is at the bottom of the well. Usually, fracture is modeled by two semi-fractures; that is true only for the homogeneous medium, (fig. 12.1). The volumetric flow of fluid through the lateral surface of the fracture is detected as
(12.1)
Here S is the lateral surface of one semi-fracture side, v – fluid filtration from the reservoir velocity. Unlike the fracture the filtration rate of the reservoir fluid obeys the Darcy law. We will not concern the fluid inflow characteristics to the fracture, let’s consider the linear flow. Then (12.1) takes the form
(12.2)
k
–
reservoir permeability factor, µ
- fluid dynamic viscosity,
- pressure differential, depending on time period and x coordinate.
Thus,
the fluid flow to the fracture essentially depends on reservoir
filtration parameters, fluid viscosity, the total area of the
fracture and the pressure differential. The length of semi-fracture
is considered by the x coordinate, varying in the range of
.
The choice of the optimal operational drive of the well is determined
by the above parameters; the choice of ESP unit drive should match
the flow of fluid in the borehole (12.2). In addition to the
productivity index there is a limit of the well flow rate, and on the
base of these two options there is determined optimal reservoir
depression and pressure on the pump intake. The peculiarities of
wells operation after hydraulic fracturing in complex in structure
reservoirs are observed in Annex 2.
It should be noted that in the common case, not all the side of the fracture surface cracks is the filtration surface. The presence of clay interlayers, not allocated by geophysical researches, can significantly reduce the filtration area, and, consequently, the production rate of the well.