Lost Circulation and Fractures in Wamsutter, WY

Nandi, Papia
BP America, Inc, Houston, TX

     Circulation losses are the biggest source of non-production drilling delays in Wamsutter, resulting in 426 days of downtime over 2001 to 2005 or roughly ten million dollars in wells costs. Analysis of these wells suggests that fractures play a large role. Wells experiencing circulation losses align spatially with field-wide directions of natural stress and are concentrated along sparsely distributed trends, suggesting that they could be related to regional faults/fractures. These wells take less pressure to induce fracturing during completions, indicating that they are inherently weaker. They also have lower rates of initial production, perhaps from depletion via fracture conduits. In comparison to wells which experienced no losses, the difference in pressure required to induce fractures is directly proportional to rock quality. Geologically, loss wells are restricted to only part of the Wamsutter field, underlying the northeast corner of the Washakie Basin, shown from drill stem tests to have high values of hydraulic head at reservoir depths. In total, these factors suggest that loss wells occur an area with significant overpressures and containing fractures and faults, causing lost circulation problems while drilling. Normally pressured weaker rock at shallow depths is exposed to higher mud weights to combat overpressures and then fractures, providing a pathway for drilling mud to flow. Fortunately, the spatial distribution of fracture and fault trends allows the prediction of which wells might experience lost circulation. Analysis from wells drilled in 2007 show this method to be quite successful.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah