«Oilfield engineering»

Unrevealed properties of inertial forces of a pump rod column in dynamograms of downhole rod pumping units

UDC: 622.276.53
DOI: 10.33285/0207-2351-2023-2(650)-29-34

Authors:

¹ Ufa State Petroleum Technological University, Ufa, Russia
² Bashkir State University, Ufa, Russia
³ Bashneft-Dobycha, Ufa, Russia

Keywords: vertical line in the dynamometer chart, initial state of the rod string, forced and natural oscillations of the sucker rod string, elongation of the plunger stroke due to the action of inertial forces

Annotation:

On all modern dynamometer charts of downhole rod pumping units (DRPU), when changing the direction of the polished rod stroke, there are vertical lines, most likely related to inertial forces. When changing the direction of rods column stroke, two types of inertial forces arise, which are different in nature – from forced and natural oscillations. Forced vibrations are generated by the action of the pumping unit and natural vibrations are generated by the elasticity of the rod string in response to the removal of the compressive load after the discharge valve opening during the downward stroke. A similar process occurs during the upward movement of the rod string, when, after the opening of the suction valve, the tensile load weakens.

Involvement in the appearance of vertical loads in the dynamometer chart caused by the action of forces of forced and natural vibrations should not cause discrepancies. However, the problem of "vertical lines" is not discussed in scientific papers or is managed by general reasoning, which is more related to forced oscillations. The concept of clarifying the situation with a vertical line is posed by the authors more broadly: from clarifying the properties of oscillations to their successful combination with the main processes. These tasks are solved step by step.

The first task was to find out the reasons for the vertical line appearance. If it appears at the moment of stopping the polished rod when approaching the bottom point, this is the action of inertial forces of slowing down the rod string movement; if at the moment of the beginning of the polished rod movement upwards – this is the action of inertial acceleration forces; if at the moment of "downtime" of the pumping unit (PU) – this is the action of natural oscillations forces. The verification of this postulate is implemented by removing the dynamometer chart with the suspension of the PU operation in its lower position.

The second task lies in differentiation of the forces of forced and natural oscillations in the dynamometer chart, the third one – in the mathematical support of the research methodology. Based on the results of the research, the properties of the inertial forces of the rod column in the dynamometer charts of the downhole rod pumping units have been revealed so far.

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