Small bubbles formation and contribution to the overall gas holdup in large diameter columns of very high viscosity oil

•Small bubbles creation in high viscosity oils is presented for the first time.•Small bubbles are created from the rising large bubbles eruption and coalescence.•Small bubbles diameter varies between millimetres to centimetres.•Small bubble characteristics are mainly controlled by bubbles generation location.•Small trapped bubbles contribute by 6.6- 30% to the total gas holdup in the column.