Method of calculating pore pressure while drilling

A non-transitory computer readable medium comprising instructions to perform a method for pore pressure (PP) estimation at a drilling location of a subsurface, the instructions executable on a processor and comprising functionality for:

• measuring a resistivity value (R0) while drilling in a region having a pore pressure (PP[depth of reference]) that is known or can be approximated at a depth of reference;
• determining from the resistivity value (R0) and the pore pressure (PP[depth of reference]) at the depth of reference an imposed resistivity value (R0[depth of reference]) at the depth of reference;
• measuring a resistivity value (R) in an interval of the subsurface from the depth of reference to a depth of the drilling location;
• calculating, using a processor, the pore pressure (PP) at the drilling location based on the following Formula (I):
• PP=OvB−(OvB−Hyd)(R/R0[depth of reference])øn
• wherein PP is the pore pressure, OvB is an overburden pressure value associated with the drilling location, Hyd is hydrostatic pore pressure, R is the measured resistivity value for the interval from the depth of reference to the depth of the drilling location, R0[depth of reference] is the imposed resistivity value at the depth of reference, and øn is a normalized value of porosity;
• adjusting, using the processors, a drilling operation associated with a new drilling location based on the estimated pore pressure, wherein said adjusting comprises optimizing a number of casing strings in the borehole;
• and
• repeating the steps of the method using the depth of the drilling location as a new depth of reference to estimate pore pressure at the new drilling location of the subsurface wherein a pore pressure at the new depth of reference is the estimated pore pressure (PP) at the drilling location.

A downhole tool configured to perform a method for pore pressure (PP) estimation at a drilling location of a subsurface, the downhole tool comprising:

• a processor;
• a memory comprising software instructions for enabling the downhole tool under control of the processor to:
• measure a resistivity value (R0) while drilling in a region having a pore pressure (PP[depth of reference]) that is known or can be approximated at a depth of reference;
• determine from the resistivity value (R0) and the pore pressure (PP[depth of reference]) at the depth of reference an imposed resistivity value (R0[depth of reference]) at the depth of reference;
• measure a resistivity value (R) in an interval of the subsurface from the depth of reference to a depth of the drilling location;
• calculate, using a processor, the pore pressure at the drilling location based on the following Formula (I):
• PP=OvB−(OvB−Hyd)(R/R0[depth of reference])øn
• wherein PP is the pore pressure, OvB is an overburden pressure value associated with the drilling location, Hyd is hydrostatic pore pressure, R is the measured resistivity value for the interval from the depth of reference to the depth of the drilling location, R0[depth of reference] is the imposed resistivity value at the depth of reference, and on is a normalized value of porosity;
• adjust, using the processors, a drilling operation associated with a new drilling location based on the estimated pore pressure, wherein said adjusting comprises optimizing a number of casing strings in the borehole;
• and
• repeat the steps of the method using the depth of the drilling location as a new depth of reference to estimate pore pressure at the new drilling location of the subsurface wherein a pore pressure at the new depth of reference is the estimated pore pressure (PP) at the drilling location.