Analysing Down Hole Scale Deposition in Offshore ESP Wells: A Case Study on Formulating an Effective Mitigation and Inhibition Strategy

Abstract

Analysis of recovered scale samples deposited within Electrical Submersible Pumps in offshore wells was carried out. The primary objective includes the identification of root cause of deposition and suggesting remedial measures for removal and prevention. Many wells in an offshore field in ONGC are facing an acute problem of decrease in pump efficiency and ultimate failure of Electrical Submersible Pumps due to deposition of inorganic scale. The methodology for carrying out the study consists of a detailed Compositional Analysis of a sample of scale retrieved from inside the ESP of one of the affected wells. Additionally, X-Ray Diffraction analysis of the debris and compositional analysis of produced water sample from the same well were also carried out. The results of the experiments were corroborated by software simulation in Scale Prediction Software. Experiments were also carried out to identify a suitable inhibitor which could operate at high bottom hole temperatures experienced in the affected wells for preventing any future deposition. Based on the compositional analysis of the scale and simulation runs using the produced water analysis, it was established that deposition of hard, adherent sulphate scale was the primary reason behind the choking and subsequent failure of the ESPs. In-situ conditions leading to deposition of the scale including variations in the operating environments and the effect of pump sizing have also been discussed. The paper analyses the problem of inorganic scale deposition in offshore ESPs of ONGC utilising actual scale samples retrieved while dismantling the pump. An attempt has been made to provide an insight into the driving mechanism behind deposition inside the pump and various factors leading to it. The paper also details extensive lab experiments carried out to identify suitable inhibitors for high temperature application.