The current scenario of many Pre-salt wells considers high rate stimulation process after a long-term shut-in due to temporary abandonment. Thus, the wellbore is at geothermal temperature in the beginning of an acidizing operation. In an ultra-deepwater environment, like the Pre-salt cluster, the temperature reaching the wellhead during acidification is near 4 °C. Numerical simulations have shown that the wellbore cooling down during short transient injection operation can lead to well integrity failure collapsing the intermediate casing.
The approach commonly employed in the industry considers just steady state injection operations which are not enough to describe what is happening in the field and are also not sufficient to support a suitable casing design. The industry must put efforts to develop and consolidate a methodology based on transient thermal analysis to deal with thermal shock events in the well, as proposed in this paper.
This paper aims to show the main findings of Petrobras wellbore design team to deal with reverse APB during well cooling down operation. In order to mitigate the decrease in pressure, unusual fluids were assessed as packer fluids. In addition, a comprehensive analysis was developed and proposed to deal with transient thermal phenomena in commercial wellbore thermal-structural simulators once some of the important identified loads are currently neglected by O&G industry standards.
Results have shown that several alternative packer fluids might reduce or prevent casing failure due to fluid thermal isolation and/or shrinkage. Several case studies that led to loads under the collapse safety factor before, are now meeting appropriate values. Also, it was demonstrated the key parameters that affect wellbore transient analysis. Overall, predicting reverse APB, especially during short transient wellbore cooling down must be faced carefully.