Abstract

Objectives/Scope: When drilling a micro fractured formation, the fluid differential pressure for wellbore support can be lost and the unsupported formation may fail, causing losses, pack-off and instability. This may be minimized or resolved if the zones of pre-existing fractures are known and the trajectory is optimized to avoid them.

In the Malay Basin, one structure of interest has undergone several periods of stress and faulting, at times, strike-slip in nature. Wrench faults in the area are causing compression and extension. This strain causes micro fractures and voids to be generated. The formation has a secondary porosity but seismic resolution does not allow the pre-existing fractures to be seen. By using a technique known as Geomechanical Forward Modelling (GFM), it is possible to determine natural fracture distribution and avoid these zones, thereby decreasing the risk of encountering instability caused by secondary porosity.

The problems of borehole instability encountered in the area are demonstrated in wells, A1, A2 and A3. The most problematic well was A3. It had many incidents of pack off causing side tracks. In each case, the hole condition was better on side-tracks made higher up the well. It was considered that there is a 3 dimensional extent to the presence of micro fractures. The fracture intensity may have a vertical as well as areal extent. Sometimes wells crossing above or below a zone are not as difficult as through it.

On wells that experienced pack off, cavings at the shaker had an abundance of linear breaks oblique to shale bedding surfaces and suggests the probability of pre-existing fractures. Pre-existing fractures were also seen on image logs.

During planning for an appraisal well, zones with lower plastic shear strain were identified to the North and West of the planned well trajectory by applying GFM. The plastic shear strain profile along the planned trajectory suggested that the target depth will intersect a potential fractured zone. By moving the surface location by only 500 meters, the chances of encountering pre-existing fractures was significantly diminished.

By applying Geomechanical Forward Modelling, (GFM), it is possible to determine distribution and possible locations of secondary fractures, too small to be determined from seismic. This allows for the selection of well trajectories to avoid these naturally fractured zones, thereby decreasing the risk of encountering instability from secondary porosity (pre-existing shear fractures).

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