Case Study: Long Cement Plugs in Ultradeepwater Environment in French Guiana - Challenges and Solutions
Authors: Hatleskog Rune , Gupta Nadish , Fuller George Allen , Bogaerts Martijn , Martell Benjamin Stuart 
Published: June 2014

An abandonment cement plug is placed as part of the permanent barrier system to isolate permeable formations and maintain well integrity. Placing a cement plug in any well is critical. The challenges in placing a successful cement plug in deepwater environments can be accentuated by factors such as the low temperatures associated with the water depth, setting long plugs in a single attempt, inability to use mechanical separators, variety of local regulations and more.

The accepted industry practice is to set a plug of 100 - 250 m in length. Longer plugs increase operational challenges such as stuck pipe. Slurry design in deepwater is also critical due to the temperature profile that the slurry is exposed to. The local regulations in French Guiana require 50 m of cement isolation above and below any permeable zone. Multiple or long permeable zones may require long cement plugs. The long abandonment plugs of more than 250 m in ultra-deepwater require careful attention to design and operational practices. The static time needs to be carefully incorporated in the slurry design to simulate the pipe pull out. After the plug placement, integrity and depth verification may involve either pressure testing or tagging of cement top with weight. The slurry design needs to have early compressive strength while still respecting the long static time and ultra-deepwater temperature profile. Slurry contamination from synthetic based mud (SBM) decreases by using mechanical separators during pumping. If a mechanical separator is not used, spacer fluid volume can be increased to minimize cement contamination. The cement contamination by spacer is affected by different types of surfactants that are used and it is important to understand the effect of each surfactant for successful recipe. Cementing simulation software can help achieve the objectives while planning a cement plug for well abandonment.

Regulations are very stringent in regards to barriers for well integrity. In this particular case a total abandonment of over 2000 m was required. It was achieved by successive long cement plugs. The paper discusses sound engineering and operational practices that increase the chances of good plug placement. These practices may also save millions of dollars worth of rig time.

Requirements for plug and abandonment in French Guiana

The exploration wells in French Guiana were drilled offshore of Cayenne as is shown in Fig. 1. The plug and abandonment process was carried out as per operational planning and agreement with local regulatory standards. Regulations required permanent barriers of 50 m, above and below any permeable zone in any annulus. In addition, 50 m of " good" cement was required above the casing shoe immediately above the open hole. As defined in the well abandonment manual and guidelines (EP 2010–1305), a cement plug is the most acceptable permanent barrier. In addition, any critical permanent barrier by Shell standards must have a back-up permanent barrier that is able to control the maximum anticipated pressure if the first permanent barrier would fail. It must be positioned at a depth where the formation is impermeable and formation's strength exceeds the maximum anticipated pressure. The above requirements were used as guidelines for planning the plug and abandonment process of exploratory wells in French Guiana.