Abstract Shell Exploration & Production Company continues to execute redevelopment slim hole sidetracks using Managed Pressure Drilling (MPD) on the Auger TLP in Deepwater Gulf of Mexico. Four sidetracks have been successfully drilled utilizing a Dynamic Annular Pressure Control (DAPC) system to eliminate lost circulation and borehole instability events.
Execution of MPD continues to improve, resulting in operational efficiency gains and allowing access to previously unattainable reservoir targets. Intervals previously considered impossible to drill due to depletion induced frac gradient reduction are being drilled and cased trouble free with MPD.
Recent MPD well designs have incorporated reduced static mud weights below pore pressure to manage the available drilling margin. Bottom hole pressure variation from the defined set point has been reduced and excursions outside of the target pressure window are being eliminated during subsequent MPD well operations.
Auger's field redevelopment history, well designs and Managed Pressure Drilling designs will be reviewed. Execution of MPD operations will be addressed in detail focusing on engineering and operational improvements throughout the four MPD sidetrack campaign.
Introduction The Auger Tension Leg Platform (TLP) is Shell's first deepwater development located in the Gulf of Mexico (GOM). The TLP is moored in 2860 feet of water on Garden Banks block 426 and began production in 1994. Production has surpassed five hundred million barrels equivalent from five main reservoirs, yielding reservoir pressure depletion in excess of 5000 psi from initial conditions.
Common in mature fields, drilling challenges change with the introduction of reservoir pressure depletion. Rock mechanics analysis has concluded that the depletion at Auger results in fracture gradient (FG) reduction due to redistribution of stresses in both the sands and shale overburden. The depletion induced fracture gradient reduction becomes a key component in narrowing the available drilling margin.
Borehole stability must be maintained for the shale overburden and virgin pressure sands. In conventional drilling, borehole stability is achieved solely with the hydrostatic pressure of mud density. The challenge of a reduced fracture gradient is further compounded in redevelopment sidetracks when the existing well geometry yields higher annular friction pressure, or equivalent circulating density (ECD), than during original development drilling. Increased ECD in conjunction with original mud density requires the need to have a larger drilling margin, thus conflicting with the tighter margins created from depletion.
The opportunity of redevelopment drilling is maintaining production rates from normal decline while adding reserves from unique drainage points. Previous redevelopment sidetrack campaigns at Auger conducted between 1999 and 2002 resulted in a success rate less than 30%. Only 2 of 7 sidetrack attempts on three well slots were successfully drilled, cased and completed. The resultant trouble time and cost necessitated an in-depth review and ceasing of future sidetrack drilling.
Figure 1 illustrates the days versus depth curves of the previous redevelopment sidetrack campaign. Post well reviews conclude lost circulation events as the root cause to the failed sidetracks. Common operational events noted on the sidetracks include ballooning, control drilling, tight spots, pack offs and mud weight reduction. These events reaffirm the tight drilling margin challenge in which the combination of mud weight and resulting drilling ECD is larger than the available drilling margin.
Managed Pressure Drilling was identified as the technology to mitigate the challenges of the tight drilling margin environment. The MPD system utilized on Auger provides automated control of surface applied backpressure enabling reduction of surface mud weight to control the ECD magnitude and maintaining a constant bottom hole pressure throughout the drilling process.