Bullheading Dead Oil into Deepwater Wells To Assess Formation Damage Mechanisms

摘要

The Jubilee Field is the first major offshore development in Ghana. The Field was discovered in 2007 and commenced production in November 2010. In mid - 2011 several wells began experiencing productivity declines A formation damage study was commissioned to evaluate and identify the cause (s) of productivity decline. A number of potential causes of productivity impairment were identified and assessed. Amongst those causes were fines migration and plugging of the sandface completion screens and perforation tunnels. A zero cost dead oil bullheading operation was recommended to test the possibility of fines migration and also dislodge any fines or debris that may have accumulated and plugged the Frac-pack screens. Three candidate wells were selected for the bullheading operation and a total of 8 bullhead treatments were carried out. Post-treatment results suggested temporary improvement in well Productivity Index (PI). However, the improvement in PI was short-lived and in most cases declined to pre-treatment levels within 10-14 days. The fact that an improvement in PI was observed for 10-14 days lead to the initial conclusions that the PI decline was the result of near-wellbore damage due to some form of movable debris in the near wellbore region (possibly fines). The ability to perform this operation using the existing subsea and FPSO installation posed several challenges. However, prudent team effort ensured competent job design and execution. Several lessons learnt from previous jobs were reviewed and incorporated in subsequent treatment designs. This paper summarizes the methodology used to assess, identify and eliminate potential causes of productivity decline on the Jubilee Field and how a dead oil bullheading operation conducted on three candidate wells aided in assessing the severity of damage. The paper also discusses the lessons learnt from executing these jobs.