Case History: Successful Wellbore Strengthening Approach in a Depleted and Highly Unconsolidated Sand in Deepwater Gulf of Mexico

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.

Finite Element Studies of Wellbore Strengthening

“…Barrett, Manescu et al 2010 also show that sized granular LCM at a suitable concentration markedly strengthened the wellbore in both permeable and impermeable intervals of the Cashiriari field. Other work shows that fluids designed for a sidetracked well can significantly improve fracture resistance pressure of depleted/unconsolidated formations (Fett, Martin et al 2010). …”

Wellbore Fracture Mode and Fracture Pressure Drilled in Depleted Reservoir

“…Therefore, the fracture mode and fracture pressure are both changed when wellbore is drilled through highly depleted reservoirs to make new horizontal wells as producers or injectors, or develop new untapped reservoirs in deeper horizons. Field evidences have indicated that the fracture pressure was decreased with reservoir depletion and more drilling fluid losses occurred which have serious impact on drilling safety and costs [3][4][5][6][7][8][9][10].…”

Finite-Element Studies of Hoop-Stress Enhancement for Wellbore Strengthening

“…In high-fluid-loss and high-strength lost-circulation conditions, a fiber-based treatment pill has proved to provide more-effective sealing than traditional treatments (Friedheim et al 2012). After the use of the designed treatment pill for wellbore strengthening, an additional 0.8 lbm/gal of fracture resistance may be obtained (Song and Rojas 2006).…”

“…Wellbore strengthening by use of LCM has been experimentally and numerically studied by many researchers (Aston et al 2004;Dupriest 2005;Song and Rojas 2006;Aston et al 2007;Fuh et al 2007;Wang et al 2007aWang et al , b, 2008Wang et al , 2009Zhang and Standifird 2007;Growcock et al 2009;Barrett et al 2010;Fett et al 2010;Kumar et al 2010;Otutu 2011;Bassey et al 2012;Friedheim et al 2012;Grandis et al 2012;Morita and Fuh 2012;Savari et al 2012;Nwaoji et al 2013). Although various techniques have been used, the several factors affecting wellbore strengthening are not thoroughly understood.…”