Abstract
In the Valhall field in the North Sea, significant resources have been identified in the flanks of the field. In this situation the reservoir targets are the main driver for the wellbore trajectory planning. Even if all of the traditional drilling parameters have been addressed and the well can be drilled on paper, the wellbore can experience wellbore stability problems (lost circulation, stuck pipe, tight hole). These wellbore problems can gradually progress or suddenly occur. The problems may be aggravated by mud properties or mechanical interaction between the rock and the drilling assembly.
Wellbore stability models, using offset drilling information, indicated early in the planning process at Valhall that the drilling of extended reach wells in the field was going to be a challenge. This challenge is the result of a narrow safe operational mud pressure window that narrows progressively with increasing wellbore sail angle. This theoretical window upper bound is given by lost circulation. The lower bound is given by the onset of wellbore collapse. This narrow window leaves hardly any room for the surge and swab pressures associated with pipe movement and pump changes.
The flank locations require sail angles close to 75 degrees in the overpressured unconsolidated claystone of Tertiary age. The record well to date had a build and hold trajectory at 74 degrees. On this record well around 3 MM USD was spent on fighting wellbore stability problems (around 750 hours). Since traditional wellbore stability analyses have already indicated a high risk of instability, and some wells have been successful, while others not, consideration was given to developing new tools to assist in the well planning in the overburden. We wanted to look at potential heterogeneous rock strength changes that could not be picked up using offset well information in the planning. The approach was to develop a more detailed geological model for the overburden with geologic formation top surfaces and 3D seismic coherency data. In this 3D data cube the drilling problems encountered on the offset wells were correlated along the wellbores. The 3D data presentation was done in a standard visualization software (GEOVIZ). The paper will present the geomechanical theory behind wellbore stability problems in faultzones and fractured rock mass, how the geologic model has been developed and how the visualization software is used to select a wellbore trajectory with the lowest risk of wellbore instability problems. Several case histories are included to illustrates the potential this methodology has for reducing unscheduled events due to wellbore instability problems while drilling the wellbore from the seafloor to the reservoir. Potential for further improvement is highlighted
