A pore pressure analysis was conducted for an offshore California field which indicated that abnormal pressure in a shale section was the reason for hole instability problems in this area. Previously, the common belief was that the chemical reactivity of the shale caused the instability.
The wells were encountering a thick shale section which caused considerable drilling problems due to sloughing shale and eventual stuck pipe. The shale section was sandwiched between two permeable intervals that were normally pressured. The question presented was: Can a thick shale section be abnormally pressured even though normally pressured formation lie just above and below?
This paper illustrates a technique of creating a pore pressure overlay for the general geographic area to determine if the shale section could be abnormally pressured. Since conclusive abnormal pressure data from the subject area did not exist, known abnormal pressure data from a nearby field was used for the pore pressure overlay development. The similarity in the overburden gradients and depositional environment between the two fields is what made this analysis feasible.
The pore pressure analysis clearly defined the thick shale section as being abnormally pressured. Based on the predicted pore pressure schedule, mud weights in the subsequent well were increased and the well was drilled to TD without problems. The shale instability problems, which had plagued this field In the past, were not evident.