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Reservoir vs. Seal Pressure Gradients: Calculations and Pitfalls


There is confusion about calculating pore pressure gradient in permeable beds (reservoirs) versus very low permeable beds (seals), especially in the geopressured section. Geoscientists are inclined to use pressure values in psi and kPa. On the other hand, drilling engineers prefer to calculate subsurface pressure values expressed in pound per gallon mud weight equivalent (ppg mwe). Equating the subsurface geopressured entrapped fluid in the reservoir to the man-made, changeable mud pressure leads to incorrect calibration of pore pressure prediction models. It also creates fictitious pressure regressions on most of the reservoir pressure's P-D plots, displays and gives the impression of a so called centroid effect. Methodical examination, calculation and prediction of subsurface pressure in a widespread data-base from the shallow and deep waters of the Gulf of Mexico demonstrate a distinctive discrepancy between pressure behaviors in seals versus reservoirs. Therefore, the two lithological unit's petrophysical properties should be examined and interpreted separately due to the impact of the geopressure partitions. The four subsurface geopressure zones (A, B, C and D), introduced in this study, explain the fundamentals of pressure measurements and interpretation in reservoirs, in addition to the basics of pressure prediction methods in seals. The top of geopressure (TOG) between the normally pressured and geopressured subsurface sections marks the pivot zone where the pressure gradients in both reservoir and seals divert. A hydrodynamic flow above TOG is usually associated with sediment dewatering. On the other hand, geopressured pressure gradient in a virgin reservoir shows a hydrostatic gradient with cascaded excess pressure envelopes. The gradient's slope in each of the sub-surface's four zones is contingent on the vertical subsurface compartmentalization, stresses, rock formation's permeability and fluid density. Exploration risk, including sealing integrity, hydrocarbon trapping capacity and drilling challenges are greatly impacted by the subsurface geopressure gradients abnormality. Foreseeing the subsurface pressure gradient perturbation is essential for assessing the prospect's risk and the possible drilling hurdles prior to moving the rig on the proposed location.