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Daniel Moos1, Thomas Finkbeiner1
(1) GeoMechanics International, Palo Alto, CA

Abstract: Predicting reservoir pressure and compaction drive in clastic rocks using velocity and porosity

Many pore pressure prediction methods that rely on measurement of a single material property (for example, velocity, resistivity, or porosity) implicitly assume that the material lies on a compaction trend, and that the mechanisms that maintain elevated pore pressure have resulted in under compaction relative to normally pressured materials at the same depth. If the material is over consolidated (that is, at one point in its history it was under a higher effective stress than at present, as can occur if overpressure results from recharge), most standard methods will underestimate the actual pore pressure. Through measurement of more than one property, for example, porosity and velocity, materials in which the elevated pore fluid pressure results from recharge can be differentiated from materials that still lie on the compaction trend. In addition, materials which have undergone diagenesis can also be identified using velocity-porosity relationships, because diagenesis generally results in a larger reduction in material compliance with a smaller accompanying pore volume reduction, compared to the changes that result from compaction.

Knowledge of stress and pore pressure history is also important to evaluate the effects of depletion. If the reservoir lies on a velocity-porosity compaction trend, depletion will be accompanied by additional compaction providing energy to maintain the pore pressure through compaction drive. If on the other hand the material is over consolidated, its response to production will be elastic and compaction drive will not occur until a finite amount of depletion has taken place. As in the case of under compacted materials, diagenetic effects will prevent compaction from occurring until a finite amount of depletion has occurred. We illustrate these concepts theoretically and through use of field examples from the Gulf of Mexico.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana