Analysis of a porosity-based pore pressure model derived from the effective vertical stress

Abstract

Without accurately predicting the pore pressure, serious wellbore challenges like kicks, blowout, lost circulation, and wellbore instability threaten. Terzaghi's total stress furnishes a vital approach for predicting pore pressure, but the challenge is how to find an accurate value for effective stress. The conventional approach assumes that compaction disequilibrium is the main mechanism in action; but at greater depths, more than one overpressure mechanism can be active at once. Besides, at subsurface depths greater than 6560 feet, viscoelastic compaction becomes significant. Nevertheless, irrespective of the active overpressure mechanism, the formation deforms as a poro-elastic composite material (comprising a solid matrix and pore-grain frame) under the action of geomechanical and geologic stresses. The bulk volume deformation of this composite was used to derive a porosity-based equation for the effective vertical stress and then applied to Terzaghi's total stress in a way that helped to improve the ratio method of pore pressure prediction. The boundary conditions applied were the initial (or mudline) porosity and the normal porosity; thus, making the model an effective stress method that depends on the normal compaction trend. The results show the following: the derived effective vertical stress confirmed an exponential-inverse trend with porosity; the poro-elastic composite technique is as significant as the compaction disequilibrium technique; the pore pressure at a given depth increases with porosity in a monotonic logarithmic manner, and rock's pore pressures (or porosity) varies with the stiffness. The developed model requires the use of quality log data and proper calibration by offset data.