Look ahead pore pressure prediction

Abstract

A method for predicting jumps in pore pressure of a subsurface includes the steps of obtaining a porosity and a resistivity log value while drilling; dividing a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I): R=0.062/ø 1.5 ; averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; and giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø 1.5 . The method may also include giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning down point in its trajectory.

1 . A method for predicting jumps in pore pressure of a subsurface, comprising:
 obtaining a porosity and a resistivity log value while drilling;   dividing, using a processor, a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):
     R= 0.062/ø 1.5   (I)
 
   
       wherein R is the resistivity log value and ø is the porosity log value;
 averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; 
 rejecting intervals where the subsurface is a sand formation that is not water saturated and rejecting porosity values that are larger than a specified threshold; and 
 giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø 1.5 . 
 
     
     
         2 . The method of  claim 1 , further comprising:
 normalizing the first representative values of resistivity and porosity after the first warning is given to obtain a resistivity reference and a porosity reference;   creating a new cross plot of normalized values of resistivity and porosity to form a trajectory;   giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in their normalized trajectory; and   defining a new reference for resistivity and porosity to obtain a new normalized plot.   
     
     
         3 . The method of  claim 1 , further comprising:
 adjusting, using the processor, a drilling operation associated with the drilling location based on the predicted jump in pore pressure.   
     
     
         4 . The method of  claim 2 , further comprising:
 adjusting, using the processor, a drilling operation associated with the drilling location based on the predicted jump in pore pressure.   
     
     
         5 . The method of  claim 3 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         6 . The method of  claim 4 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         7 . The method of  claim 1 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         8 . The method of  claim 2 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         9 . The method of  claim 1 , wherein the first warning is displayed on a graphical user interface. 
     
     
         10 . The method of  claim 2 , wherein the second warning is displayed on a graphical user interface. 
     
     
         11 . A non-transitory computer readable medium comprising instructions to perform a method for predicting jumps in pore pressure of a subsurface, the instructions executable on a processor and comprising functionality for:
 obtaining a porosity and a resistivity log value while drilling;   dividing a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):
     R= 0.062/ø 1.5   (I)
 
   
       wherein R is the resistivity log value and o is the porosity log value;
 averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; 
 rejecting intervals where the subsurface is a sand formation that is not water saturated and rejecting porosity values that are larger than a specified threshold; and 
 giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø 1.5 . 
 
     
     
         12 . The non-transitory computer readable medium of  claim 11 , wherein the instructions further comprise functionality for adjusting a drilling operation associated with the drilling location based on the predicted jump in pore pressure. 
     
     
         13 . The non-transitory computer readable medium of  claim 12 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         14 . The non-transitory computer readable medium of  claim 11 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         15 . The non-transitory computer readable medium of  claim 11 , wherein the first warning is displayed on a graphical user interface. 
     
     
         16 . The non-transitory computer readable medium of  claim 11 , wherein the instructions further comprise functionality for:
 normalizing the first representative values of resistivity and porosity after the first warning is given to obtain a resistivity reference and a porosity reference;   creating a new cross plot of normalized values of resistivity and porosity to form a trajectory;   giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in the trajectory; and   defining a new reference for resistivity and porosity to obtain a new normalized plot.   
     
     
         17 . The non-transitory computer readable medium of  claim 16 , wherein the instructions further comprise functionality for adjusting a drilling operation associated with the drilling location based on the predicted jump in pore pressure. 
     
     
         18 . The non-transitory computer readable medium of  claim 17 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         19 . The non-transitory computer readable medium of  claim 16 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         20 . The non-transitory computer readable medium of  claim 16 , wherein the second warning is displayed on a graphical user interface. 
     
     
         21 . A downhole tool configured to perform a method for predicting jumps in pore pressure 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:   obtain a porosity and a resistivity log value while drilling;   divide a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):
     R= 0 . 062 /ø   1.5   (I)
 
   
       wherein R is the resistivity log value and ø is the porosity log value;
 average the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; 
 reject intervals where the subsurface is a sand formation that is not water saturated and reject porosity values that are larger than a specified threshold; and 
 give a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø 1.5 . 
 
     
     
         22 . The downhole tool of  claim 21 , wherein the memory also enables the downhole tool to adjust a drilling operation associated with the drilling location based on the predicted jump in pore pressure. 
     
     
         23 . The downhole tool of  claim 22 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         24 . The downhole tool of  claim 21 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         25 . The downhole tool of  claim 21 , wherein the first warning is displayed on a graphical user interface. 
     
     
         26 . The downhole tool of  claim 21 , wherein the memory also enables the downhole tool to:
 normalize the first representative values of resistivity and porosity to obtain a resistivity reference and a porosity reference;   create a new cross plot of normalized values of resistivity and porosity to form a trajectory;   give a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in the trajectory; and   defining a new reference for resistivity and porosity to obtain a new normalized plot.   
     
     
         27 . The downhole tool of  claim 26 , wherein the memory also enables the downhole tool to adjust a drilling operation associated with the drilling location based on the predicted jump in pore pressure. 
     
     
         28 . The downhole tool of  claim 27 , wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. 
     
     
         29 . The downhole tool of  claim 26 , wherein the drilling location comprises a location below an operating drill bit in a borehole. 
     
     
         30 . The downhole tool of  claim 26 , wherein the second warning is displayed on a graphical user interface.