Abstract

A fracture treatment in offshore Tunisia screened out leaving over 76,000-lbm proppant in the wellbore. The well was significantly under-hydrostatic. The platform was small and had limited deck space and low capacity cranes. The completion incorporated chrome tubulars with a history of causing abrasion failure to coiled tubing strings. The challenge was to efficiently and safely clean out the proppant with coiled tubing (CT).

A prior cleanout campaign had been conducted with concentric CT and jet pumps. An initial design focused on repeating this method. The engineering analysis had to account for fluid and nitrogen pumping being conducted from a supply vessel, limited nitrogen volume, low the solids return rate due to surface handling limitations, and no fluid discharge permitted to sea. A combined engineering, logistical study, laboratory testing and risk assessment was undertaken over the course of three months. Engineering utilized advanced cleanout modelling software to review concentric CT cleaning, forward cleaning (with and without optimizing cleaning Bottom Hole Assembly (BHA) and with various sizes of CT), and reverse circulating. Logistics analyzed the overall operation time, fluid and nitrogen requirements and the number of boat trips to replenish/change well returns and nitrogen. Three additional challenges were present. First, proppant could have packed off creating difficulties for some of the processes under review. Laboratory testing was conducted and confirmed this would not be a concern. Second, the well was sour and considerations for protecting the CT string and handling hydrogen sulfide (H2S) in the return stream were required. Third, CT string optimization was required to reduce potential abrasion failures. Avoiding CT failure was paramount as the string would be boat spooled onto the platform and any failure would severely impact operating time and project finances.

The chosen method was primarily fluid only reverse circulating when cleaning above the formation, changing to forward circulated two phase operation when close to the formation. The downhole pressure gauge in the completion provided early warning of lost returns or of gas kicks. The operation was successfully, efficiently and safely completed in August 2019. The well was handed back to production 8 days ahead of schedule.

The paper will cover the complete concept and detail design, execution and post-job analysis.

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