Method of calculating pore pressure while drilling
A method for calculating pore pressure of a subsurface includes the steps of obtaining a resistivity value while drilling in a region wherein there is a shallow depth (reference depth) where shale is in a hydrostatic condition, or where shale is not in hydrostatic condition but where the pore pressure at such depth can be estimated and calculating, using a processor, a pore pressure at a drilling location based on the following Formula (I): PP=OvB−(OvB−Hyd)(R/R 0 )ø n wherein PP is pore pressure, OvB is the overburden value associated with the drilling location, Hyd is hydrostatic pore pressure, R is a measured value of resistivity, R 0 is the resistivity at a reference depth or is an imposed value of resistivity, and ø n is a normalized value of porosity.
1. A method of estimating pore pressure (PP) of a subsurface at a drilling location, the method comprising:
measuring a resistivity value (R 0 ) 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 (R 0 ) and the pore pressure (PP [depth of reference] ) at the depth of reference an imposed resistivity value (R 0[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/R 0[depth of reference] )ø n
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, R 0[depth of reference] is the imposed resistivity value at the depth of reference, and ø n 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.
2. The method of claim 1 , wherein adjusting the drilling operation further comprises at least one of adjusting a drilling fluid density and adjusting a drilling trajectory.
3. The method of claim 1 , wherein the drilling location comprises a location below an operating drill bit in a borehole.
4. The method of claim 1 , wherein the estimated pore pressure is displayed on a graphical user interface.
5. 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 (R 0 ) 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 (R 0 ) and the pore pressure (PP [depth of reference] ) at the depth of reference an imposed resistivity value (R 0[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/R 0[depth of reference] )ø n
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, R 0[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.
6. The non-transitory computer readable medium of claim 5 wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density and adjusting a drilling trajectory.
7. The non-transitory computer readable medium of claim 5 , wherein the drilling location comprises a location below an operating drill bit in a borehole.
8. The non-transitory computer readable medium of claim 5 , wherein the estimated pore pressure is displayed on a graphical user interface.
9. 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 (R 0 ) 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 (R 0 ) and the pore pressure (PP [depth of reference] ) at the depth of reference an imposed resistivity value (R 0[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/R 0[depth of reference] )ø n
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, R 0[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.
10. The downhole tool of claim 9 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density and adjusting a drilling trajectory.
11. The downhole tool of claim 9 , wherein the drilling location comprises a location below an operating drill bit in a borehole.
12. The downhole tool of claim 9 , wherein the estimated pore pressure is displayed on a graphical user interface.