Method for determining pressure of earth formations

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:

• causing wellbore pressure to vary periodically in time;
• 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 periodic component, an estimate of the size of the pressure build-up zone around the wellbore at said depth region of the formations;
• determining, using said non-periodic component, the leak-off rate of the mudcake at said depth region;
• and
• determining, using said leak-off rate, and said size of the pressure build-up zone, the virgin formation pressure.

A method for determining the virgin reservoir 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;
• deriving formation permeability at said depth region;
• causing wellbore pressure to vary periodically in time, and measuring, at said depth region, the time varying pressure in the borehole and the time varying pressure in the formations adjacent the mudcake;
• determining, at said depth region, an estimate of the flow resistance of the mudcake from said derived permeability and components of said measured pressure in the borehole and said measured pressure in the formations adjacent the mudcake;
• determining, at said depth region, the leak-off rate of the mudcake from said estimated flow resistance and said measured pressure in the borehole and said measured pressure in the formations adjacent the mudcake;
• determining, at said depth region, the pressure excess in the formations adjacent the mudcake from said derived permeability, said leak-off rate, and said time since cessation of drilling;
• and
• determining, at said depth region, the virgin reservoir pressure from said measured pressure in the formations adjacent the mudcake and said pressure excess in the formations.

A method for determining the leak-off rate of a mudcake formed, at a particular depth region, on a borehole drilled in formations using drilling mud, comprising the steps of:

• deriving formation permeability at said depth region;
• causing wellbore pressure to vary periodically in time, and measuring, at said depth region, the time varying pressure in the borehole and the time varying pressure in the formations adjacent the mudcake;
• determining, at said depth region, an estimate of the flow resistance of the mudcake from said derived permeability and components of said measured pressure in the borehole and said measured pressure in the formations adjacent the mudcake;
• and
• determining, at said depth region, the leak-off rate of the mudcake from said estimated flow resistance and said measured pressure in the borehole and said measured pressure in the formations adjacent the mudcake.