Automatic control system for removal of paraffin deposits in oil well in permafrost region by thermal method
Introduction
Exploitation of many oil wells is complicated by the deposition of asphaltenes, resins and paraffins (hereinafter—the deposits of paraffin) on the walls of the tubing. The main components of sediments are paraffins, whose content varies from 20 to 70% (by mass), and asphaltenes and resins from 20 to 40% (by mass).
In the bottom paraffin is dissolved in oil, because the temperature at the depth of 1000–3500 m is 330–390 K. As the well fluid (hereinafter—the oil) is lifting its temperature decreases and the solubility of paraffin declines sharply, that leads to intense precipitation of paraffin on the inner surface of the tubing (Fig. 1). Because of this the flow of fluid to the bottom reduces and the hydraulic resistance of wells increases, so their productivity decreases (Aiyejina et al., 2011, Martínez-Palou et al., 2011).
Many different methods for the removal of paraffin deposits from the inner surface of tubing were developed. Mechanical method is one of the common methods. The deposits are removed from the tubing by a special scraper—a pig (Wang and Huang, 2014). The need to stop production is a disadvantage of this method. Also a popular method is the application of inhibitors, which prevent wax crystallization (Aiyejina et al., 2011). This method is costly; inhibitors have a bad influence on the tube. To solve the problem, the magnetic field, the microorganisms, the special coatings of pipe are used, but not very often (Davletbaev et al., 2011, Dotto et al., 2006, Xiao et al., 2012). The most common methods are thermal methods of dewaxing. Currently hot oil washing is increasingly being used. But this method is also costly. Today heating cables are used mainly in remote places (Danilovic et al., 2011). However, the thorough calculation of the necessary power of cable is not performed. Cable is turned on by more power, this leads to unnecessary energy costs and to frequent breakage of cables: insulation breakdown. Now we have a situation that the oil price has decreased, therefore it is necessary to reduce production costs. For this purpose a more detailed calculation of the necessary and sufficient power of cable and consideration of changing conditions are required. The paper proposes a calculation program, which allows to automate calculations for a more rapid response to change, and, considering basic properties of the well and production, get the optimal parameters of heating, that will lead to save energy and protect the cable against overheating.
At present, calculations for oilfield are made by using the calculation departments before switching cable. In this situation, timely response to changing conditions is impossible. The program allows operators to enter the necessary changes at the production site and obtain new working parameters of cable.
Section snippets
The physical model
The oil well includes a casing (Fig. 1), well depth is 1000–3500 m. Inside the casing on the tubing the pump with an electric motor is suspended, which is energized by a power cable. Oil from the pump by tubing goes up and goes through the discharge tubing for further processing. The dynamic oil level, located between the casing and tubing, is not constant and depends on the intensity of production, with the increase of oil production level is reduced. Above the dynamic level an air–gas mixture
Mathematical model
The task: to eliminate the paraffin deposits from the inner surface of the tubing by heating oil moved from the bottom to the wellhead to a temperature above the temperature of solidification of paraffin by means of a heating cable located inside the tubing. The problem is the unsteady three-dimensional axisymmetric (Tiyao et al., 2010).
To solve this problem, it is necessary to solve a system of differential equations (energy, motion), closed by boundary and initial conditions (Huang et al.,
Results and discussion
Using block diagrams, the computer program for automatic calculation of heating parameters was developed. Here are some graphs, obtained by using the program.
In Fig. 5 geotherm, the temperature distribution of oil and temperature of cable conductor during operation of the removal of paraffin deposits at the time of working of the heating cable to one of the points in time are presented.
At a depth of 1500 m the heating cable begins. At the depth of 900 m, the dynamic level is, the conditions of
Conclusions
The scheme of calculation of the heating current and the work time of the heating cable to remove deposits of asphaltenes, resins and paraffin on the wall of the tubing in an oil well is developed. Calculation of the heating current is subject to the conditions that the temperature of the cable insulation does not exceed the allowable, and at the same time there is a melting of paraffin. The calculation scheme takes into account the temperature distribution along the well depth, flow rate,
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