ABSTRACT

Pore pressure prediction provides an important risk assessment in the oil and gas industry. Most predrill pore-pressure prediction methods from seismic and/or well-log sonic velocities are based on the effective stress principle, which relates velocity variation to the combined effect of overburden stress and pore pressure. In the current practice of pore pressure prediction, the effective stress coefficient n is often assumed as unity, which is not always the case, especially when sediments are deeply buried and consolidated. To understand the variation of n with depth, I analyzed density and velocity trends from more than 100 Gulf of Mexico wells near the Louisiana continental shelf edge. In the study area, overpressure zones are present in most wells and compaction disequilibrium is the dominant overpressure mechanism. Normal compaction trends for velocity and density were built. The overburden pressure model was refined by taking into account that the density gradient approaches zero at the onset depth of overpressure. Based on the effective pressure principle, values for n in the overpressure intervals were estimated in the study area. The average n values varied from 0.6 to 0.9 inclusive of errors associated with assuming the gradient of mud weight and pore pressure is the same.

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