Abstract

In early 2008, Total E&P-USA sidetracked the Mississippi Canyon 243 #A2 well on its "Matterhorn" TLP, in deepwater Gulf of Mexico. A pre-project geomechanics study identified that the mud weight/fracture pressure window in the depleted and highly unconsolidated 'A' reservoir was very narrow, creating a strong potential for mud losses during drilling and cementing the 7" liner. The risk of losses was a primary concern since the well would be frac-packed, and if a competent cement column did not reach a sufficient height, the ability to fracture the reservoir would have been compromised. To mitigate this risk, the decision was made to drill through the depleted reservoir using a 'flat rheology' synthetic-based fluid, engineered with a high concentration of bridging particles to impart a strengthening effect on the formation.

The 'designer fluid' allowed the reservoir to be drilled through successfully, and the 7" liner to be run and cemented with full returns. Analysis of the frac-pack data showed that the formation breakdown pressure was lower than the wellbore pressures experienced while drilling and cementing the liner, suggesting that the designer fluid improved the fracture resistance of the formation. The results imply that using such a designer fluid can have a strengthening effect on depleted/unconsolidated formations, in which some operators have had limited success applying wellbore strengthening techniques.

The implication for the industry is that this technique can and should be considered on wells with similar challenges and risks as the Matterhorn A2 well. This paper will describe the approach taken in the laboratory for the fluid design, as well as operational practices to apply the treatment on location. A post-mortem analysis will compare formation breakdown pressures taken from the fracturing operations to actual wellbore pressures experienced while drilling and cementing, to demonstrate that a strengthening effect was realized.

Introduction

Total E&P USA conducted sidetrack operations on the A2 well to restore production that was impaired after prolonged shutdowns due to Hurricanes in recent years. The operations included: re-entering, de-completing, side tracking and re-completing the well. The operations were done with a heavy work-over rig installed on the TLP, and used a 'flat rheology' synthetic-based mud (SBM).

A geomechanics study had been conducted prior to the sidetrack, and the analysis identified that a strong potential existed for mud losses in the depleted and highly unconsolidated 'A' reservoir due to the mud weight required (and associated ECD) to control the breakout of the cap rock shales above - a common scenario faced by operators during in-field drilling operations. According to the study, the mud weight/fracture pressure window had essentially disappeared.

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