INFLUENCE OF SANDSTONE RESERVOIR DEFORMATIONS DURING BOTTOMHOLE AND RESERVOIR PRESSURE DECREASING ON THE PERMEABILITY AND WELL PRODUCTIVITY CHANGES

Abstract

The relevance of the research is caused by the need to take into account the effects of sandstone reservoir rocks deformations and changes in reservoir permeability for predicting oil wells productivity under conditions of varying values of reservoir and bottomhole pressures. The main aim of the research is based on the results of laboratory experiments, to identify relationships of changes in the sandstone reservoir core samples permeability with changes in reservoir and bottomhole pressures and to assess the degree of these effects influence on the oil well productivity. Objects: sandstone reservoir rocks of the oil fields located in the south of the Perm Region. Methods: laboratory research of the core samples permeability using the laboratory equipment at various values of pore pressure and sample compression pressure; numerical simulation of the near-wellbore zone by the finite element method using the basic relationships of the Biot theory of poroelasticity to determine the magnitude of effective stresses and their effect on the reservoir rocks permeability and oil well productivity. Results. The authors have obtained the analytical relationships that describe the change in permeability with a decrease and recover of reservoir and bottomhole pressure, which showed a decrease in the permeability of core samples by 20,6 % with a limiting decrease in pore pressure from 14,5 to 5 MPa. When the pressure is recovered to the initial value, the permeability is restored only by 84,5 %, which indicates a significant role of rocks irreversible plastic deformations. The results of numerical modeling of the effective stresses distribution in the near-wellbore zone showed that in case of creating the maximum bottomhole pressure drawdown of 9,5 MPa the productivity factor decreases by 10 %. If the drawdown is reduced and the bottomhole pressure is recovered, then the productivity factor recovers just to 92 % of the initial value, which again indicates the negative role of plastic deformations of the reservoir rock. If the reservoir pressure drops to 8,5 MPa, then the reducing in the productivity factor is 13 % compared to the maximum value.

Keywords

• sandstone reservoir
• permeability
• core sample
• reservoir pressure
• bottomhole pressure
• effective stress
• well
• productivity ratio