Shale Instability

Situation:

Offset well had shown severe well instability issue with the large volume of caving. The drill string got stuck and it took 12 days more to finish the section. The next well was in the same field in close vicinity of the troubled well and hence we were expecting the same hole instability issue.

Actions Taken:

·        Analyzed the shale sample from offset well in the lab.

·        The results identified it as montmorillonite clay, which is prone to severe hydration from water base mud.

·        Analysis of the stratigraphic map of the area and review of formation logs indicated the presence of a fault nearby. That raised the possibility that the shale might be fractured as well with loose bonding.

·        Prepared a detailed roadmap that included the min/max operating parameters to achieve the most efficient drilling performance. Held a series of meetings with the team of rig and service personnel to ensure that everyone is fully aware of the challenges and the plan of action

·        Planned the well with minimum overbalance and fluid loss to minimize fluid invasion.

·        Used inhibited mud system with high salt concentration. Ensured strict adherence to the planned mud rheology.

·        Drilled with low RPM and avoided back-reaming. This reduced the possibility of hitting and whirling the pipe around the wellbore walls.

·        Kept a close watch on the shale shaker and ensured hole cleaning.

·        Controlled tripping speed to avoid sudden acceleration and stopping to minimize vibrations.

·        With these steps, we were able to avoid hole collapse and were able to keep the caving to the minimum level.

·        Made casing and cementation preparations well in advance.

·        Since the trip out did not indicate any tight hole, ran casing without the reaming trip. It minimized exposure time ensuring casing and cementing of the shale section quickly.