In this study, by using a large cylindrical sandpack model with a length of 30.48cm and a diameter of 15.24cm, six sets of experiments with different pressure decline rates (12.5kPa/min, 8kPa/min, 5kPa/min, 3kPa/min, 1kPa/min and a varying pressure decline rate) were conducted. The effects of pressure decline rates on production performance and different driving mechanisms during CSI process were analyzed. The results suggested that CSI production performance presented two distinct phases in terms of oil production rate due to different driving mechanisms. Phase 1 is dominated by foamy oil flow and Phase 2 is dominated by gas-liquid two-phase flow. In Phase 1, 5kPa/min pressure decline is the optimum pressure decline rate. In Phase 2, a larger pressure decline rate leads to a higher production rate because a larger pressure decline rate provided a larger pressure gradient in the physical model and a shorter production time, which carried more oil out and prevented the crude oil from regaining its viscosity. One test was conducted by using a varying pressure decline rate, in which 5kPa/min pressure decline rate was applied in early timeperoid and then a larger pressure decline rate (12.5kPa/min) was applied. The results indicated that with varying pressure decline rate, both a relative large recovery factor and a large average production rate can be obtained.Another important finding in this study is the minimum production pressure, which is defined as the pressure less than which the instantaneous oil production decreased to almost zero. The production well could shut down when the pressure declined to the minimum production pressure to recover the oil economically. Due to the different driving mechanisms involved in the CSI process, the minimum production pressure will vary with different pressure decline rates. Final residual oil saturation was also measured to map the residual oil distribution and solvent chamber shape which were also significantly affected by the pressure decline rate.
Journal of Petroleum Science and Engineering – Elsevier
Published: Sep 1, 2016
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