Patent ID: 7031841
Filing Date: 2006-04-18
Classification: E21B

Abstract:
1. A method for determining the virgin formation pressure at a particular depth region of earth formations surrounding a borehole drilled using drilling mud, and on which a mudcake has formed, comprising the steps of: keeping track of the time since cessation of drilling at said depth region; deriving formation permeability at said depth region; causing wellbore pressure to vary periodically in time and determining, at said depth region, the periodic component and the non-periodic component of pressure measured in the formations adjacent the mudcake; determining, using said time, said periodic component and said permeability, the formation pressure diffusivity and transmissibility and an estimate of the size of the pressure build-up zone around the wellbore at said depth region of the formations; determining, using said time, said formation pressure diffusivity and transmissibility, and said non-periodic component, the leak-off rate of the mudcake at said depth region; determining, using said leak-off rate, the pressure gradient in the formations adjacent the mudcake at said depth region; and extrapolating, using said pressure gradient and said size of the pressure build-up zone, to determine the virgin formation pressure.