Pore Pressure Prediction Before and During Drilling Applying the Four Zones Method

Abstract

This method integrates the geological building blocks of sedimentation, dehydration to lithification and the formation water hydrodynamic forces associated with each of these phases. The current known conventional velocity – pore pressure transformation models are lacking this relationship. In the current widely used conventional methods, it is controversial to consider the shallow section as normally pressured and at the same time extract a compaction trend from its petrophysical properties. Moreover, there is confusion about which part of the subsurface section the Effective Stress theorem should be applied to. The novelty of dividing the subsurface into four zones using this new approach reduces the risk of predicting the pore pressure before drilling and the uncertainty of its correct calibration during drilling. This new pore pressure calculation is done separately in each zone based on the predominant formation-water dynamic. Normal hydrostatic pressure is only assigned to the loosely compacted very shallow section (A). Hydrodynamic zone B with upward formation water flow is associated with compaction and reduction of porosity due to sediments load. Petrophysical trends such as velocity, density, resistivity follows a compaction trend in this zone. As result of depositing shale seal in zone C, due to high stand sea level, fluid is prevented from permeating upward. This low permeable top seal is referred to as top of geopressure (TOG). The geopressured section of zone D below the pressure ramp in zone C follows a cascade outline where the pressure in permeable beds show linear trends and shale exhibits an exponential trend. The petrophysical properties of the deeper shale beds below the top seal represent several passive compaction trends. Pore pressure prediction in the deep geopressured section D is derived from calculating at the same depth the disparity between the extrapolated velocity compaction trend (CT) values and the measured ones. A unique mathematical calculation is introduced here to establish the compaction trend (CT) instead of the manual graphically extrapolated so called NCT. Before drilling seismic velocity, semblance is a key for defining the four zones. Velocity – pore-pressure transformation modeling is an important aspect of the drilling cost for a proposed location. Moreover, LWD’s during drilling and conventional logs post drilling are the fine-tuning tools of calibrating the pre-drilling seismic-pressure model. The calibrated model is the backbone of any predicted pore pressure in future drilling locations in the same basin. The pore pressure prediction applying this method facilitates assigning the casing setting and mud programs at the appropriate depths before drilling. Furthermore, it reduces the non-productive time (NPT) and challenges by assessing the subsurface formation pressure including the shallow water flow (SWF), risk of kicks and loss of circulation along the proposed bore-hole trajectory before moving rig on location.