Techniques for optimization of gas extraction from production wells annulus

The article experimentally establishes the regularities of the separated gas pressure growth in the annulus depending on geological and technical characteristics of producing wells; it obtains an experimental dependence for calculating the period of stabilization of the gas pressure in the annulus when the pressure is equal to the pressure in the discharge manifold of the well.


Introduction
One of the ways to increase the efficiency of oil field development is to intensify oil production from wells [1][2][3][4][5][6][7][8]. It is known that to increase the flow rate of wells operated by traditional mechanized methods, various technologies and technical means are used to reduce the gas pressure in the annulus [9]. Ejectors that are installed in the tubing string above the liquid level or directly at the wellhead are used for forced pumping of gas from the annular space of the well into the flow header [10]. There are also technologies for pumping gas with the help of additional plungers installed on the rod string, as well as an overhead compressor driven by the balance beam of the pumping unit [11].
The experience of using the technology of forced pumping of gas from the annulus showed that the flow rate of one well increases on average by more than 2.7 t / day [12].
To calculate the parameters of gas compression from the annulus to the reservoir, it is necessary to have data on the volumes of gas supplied from the pump intake to the annulus of the well per unit of time.
For the experimental determination of these volumes, it is necessary to measure the dynamics of the gas pressure in the annulus after the pressure is reduced to atmospheric value. In this case, the pumping unit must continue to operate continuously in order to avoid violations of the conditions for gas extraction from oil and its separation at the pump intake.
The dynamics of changes in the gas pressure in the annulus ultimately makes it possible to determine the amount of gas separated at the pump intake and supplied to the annulus, which will be described below.

Materials and methods
The essence of the experiment is to pump out gas from the annular space to a pressure corresponding to atmospheric pressure, followed by turning off this compressor and periodically recording the pressure in the annular space for a period sufficient for its stabilization. The simplest and most accessible method for constructing a pressure dynamics curve is its periodic registration after the pump is started up and brought into operation. In the initial period after killing, the downhole pumping unit pumps out fluid from the annulus. Further on, the dynamic fluid level is stabilized. During the period before and after the stabilization of the pumping mode, periodic beating of the dynamic level and recording of the change in gas pressure in the annular space over time are performed. Figure 1 shows the obtained curves of gas pressure dynamics in the annular space of wells No. 740 and No. 716 of the Chutyrsko-Kiengopskoye field. In well No. 740 (curve 1), already after 12 hours, the gas pressure stabilized, which equaled the pressure in the reservoir (about 1.45 MPa). Along with the indicated volume of the well filled with gas, the pressure dynamics are influenced by the gas-oil ratio, the pressure at the pump intake and the conditions for the separation of the free gas phase. Therefore, the gas flow rate entering the annulus above the pump will be proportional to the value:

Results and Discussion
where Gf is the gas factor, m 3 /m 3 ; Z is the gas constant; РО, ТО--pressure and temperature, equal to 0.1 MPa and 273 K, respectively; Тwellfluid temperature at the pump intake, K; Qoilwell oil production rate, m 3 / day; бgas separation coefficient. The Gleft value in (1) is determined from the curve of one-time oil degassing. As an example, Figure 2 shows the curves obtained for fields in Western Siberia. In this figure, at a given pressure value, the amount of gas released from oil is determined, and the residual amount of Gleft gas is determined from it. Considering that during the period of reaching the gas pressure, some changes occur at the pump intake, it is permissible to take its average value as the calculated value of the Gleft coefficient. The Gleft value can also be calculated by the ratio: If we take Vaas the volume of the annular space above the dynamic level, then the change in the ratio Vg/Va in time will characterize the rate of filling this space with gas and increasing pressure in it.
Similarly, if we denote Рa -the current pressure in the annulus, and Рr -pressure in the reservoir, then the change in the Рa / Рr ratio per unit time will show the intensity of the gas pressure growth above the dynamic level.
In this case, the Рa/Рr ratio will depend on the volume of gas Vg entering the annulus. Thus, the parameters Рa/Рr and V/Va are related in a certain way. The amount of gas entering the annulus per day is calculated: where t is time, days.
In order to establish such a relationship in several wells in fields with a large scatter of GOR values, using the measurements described above, the curves of the dynamics of dimensionless pressures were plotted as a function of time ( Figure 3). Figure 3 shows that the nature of change of gas pressure in the annulus is approximately the same. With an increase in the value of Vg/Va, the period after which the pressure in the annulus reaches the pressure in the reservoir (Рa/Рr = 1), decreases.
From a practical point of view, the period of reaching the gas pressure in the annular space of the reservoir pressure is especially interesting. The inverse value of this period(1/tset), characterizes the intensity of gas inflow and growth of its pressure in the annulus. The value of the pressure in the annulus during its stabilization will correspond to the pressure in the discharge manifold of the well due to the presence of a check valve at its wellhead.  2. The study obtains an experimental dependence for calculating the period of stabilization of the gas pressure in the annulus when the pressure is equal to the pressure in the discharge manifold of the well.