Method to predict overpressure uncertainty from normal compaction trendline uncertainty

Abstract

A method for predicting a pressure window for drilling a borehole in a formation includes: obtaining a pore pressure related data value of the formation using a data acquisition tool; predicting pore pressure uncertainty from the pore pressure related data value of the formation using a processor; estimating uncertainty of a pressure window for drilling fluid using the predicted pore pressure uncertainty using a processor; and applying the estimated uncertainty to the pressure window to provide a modified pressure window using a processor.

What is claimed is: 
     
         1 . A method for predicting a pressure window for drilling a borehole in a formation, the method comprising:
 obtaining a pore pressure related data value of the formation using a data acquisition tool;   predicting pore pressure uncertainty from the pore pressure related data value of the formation using a processor;   estimating uncertainty of a pressure window for drilling fluid using the predicted pore pressure uncertainty using a processor; and   applying the estimated uncertainty to the pressure window to provide a modified pressure window using a processor.   
     
     
         2 . The method according to  claim 1 , further comprising defining an operating margin and applying the operating margin to the modified pressure window to provide an operating pressure window using a processor. 
     
     
         3 . The method according to  claim 2 , further comprising monitoring at least one equivalent of drilling fluid pressure and determining if the monitored drilling fluid pressure equivalent is within equivalents of an upper bound and a lower bound of the operating pressure window. 
     
     
         4 . The method according to  claim 2 , further comprising:
 defining a drilling parameter for drilling a borehole in the formation within the operating pressure window using a processor; and   drilling into the formation using a drilling tool and the operating pressure window for the drilling fluid.   
     
     
         5 . The method according to  claim 4 , wherein the drilling parameter comprises at least one of a drilling fluid density, a drilling fluid flow rate, an equivalent circulating drilling fluid density, an equivalent static drilling fluid density, and a standpipe pressure. 
     
     
         6 . The method according to  claim 1 , further comprising determining at least one pore pressure related trendline using the pore pressure related data value and extrapolating the at least one pore pressure related trendline. 
     
     
         7 . The method according to  claim 6 , wherein the pore pressure related value is obtained from a pore pressure related log acquired by the data acquisition tool. 
     
     
         8 . The method according to  claim 6 , wherein the formation comprises a normal compaction zone and an overpressure zone below the normal compaction zone and method further comprises determining the at least one pore pressure related trendline from data from the normal compaction zone and extrapolating the at least one pore pressure related trendline into the overpressure zone. 
     
     
         9 . The method according to  claim 6 , wherein the pore pressure uncertainty accounts for at least one selection from a group consisting of instrument error, equipment calibration error, statistical error of measurement apparatus or method, regression error of trendlines when the trendline comprises a plurality of trendlines, and variation of trendlines when the trendline comprises a plurality of trendlines. 
     
     
         10 . The method according to  claim 9 , further comprising identifying a correlation between pore pressure uncertainty and the uncertainty of the pore pressure related data value using data from at least two previously drilled boreholes and wherein calculating the pore pressure uncertainty further comprises using the uncertainty of the pore pressure related data value and the correlation. 
     
     
         11 . The method according to  claim 6 , further comprising deriving a representative pore pressure related trendline from the at least one pore pressure related trendline. 
     
     
         12 . The method according to  claim 6 , wherein the at least one pore pressure related trendline comprises a plurality of pore pressure related trendlines and the method further comprising determining an upper bound line having an upper bound line slope and a lower bound line having a lower bound line slope, wherein the upper bound line slope is less than a slope of the plurality of pore pressure related trendlines and the slope of the plurality of pore pressure related trendlines is less than the lower bound line slope, the upper bound line indicating positive uncertainty with respect to the pore pressure related trendline and the lower bound line indicating negative uncertainty with respect to the pore pressure related trendline. 
     
     
         13 . The method according to  claim 12 , wherein the upper bound line is a function of an uncertainty of the plurality of pore pressure trendlines and the lower bound line is a function of an uncertainty of the plurality of pore pressure trendlines. 
     
     
         14 . The method according to  claim 6 , wherein the at least one pore pressure related trendline comprises a plurality of pore pressure related trendlines and the method further comprising determining an upper bound line having an upper bound line slope and a lower bound line having a lower bound line slope, wherein the upper bound line is a pore pressure related trendline in the plurality of pore pressure related trendlines having a minimum slope and the lower bound line is a pore pressure line in the plurality of pore pressure related trendlines having a maximum slope. 
     
     
         15 . The method according to  claim 1 , wherein calculating pore pressure uncertainty in the overpressure zone comprises calculating a Q-factor by solving: 
       
         
           
             
               
                 Q 
                 = 
                 
                   
                      
                     
                        
                       z 
                     
                   
                    
                   
                     ( 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         R 
                         N 
                         * 
                       
                     
                     ) 
                   
                 
               
               , 
             
           
         
       
       where d/dz is the derivative of ΔR* N  with depth z, and
   ΔR* N =log 10    R   N   u −log 10    R   N   l  
 
 
       represents the difference between the upper (R N   u ) and lower (R N   l ) bounds at depth z that envelope an estimate of a pore pressure related value. 
     
     
         16 . The method according to  claim 15 , wherein Q=constant value q. 
     
     
         17 . The method according to  claim 1 , wherein the pressure window is defined at least in part by a fracture gradient, a pore pressure gradient, and a collapse gradient and the pore pressure uncertainty affects at least partly one of the fracture gradient and the collapse gradient. 
     
     
         18 . An apparatus for predicting a pore pressure window for drilling a borehole in a formation, the apparatus comprising:
 a data acquisition tool configured to perform formation measurements related to pore pressure of the formation at a plurality of depths in the borehole; and   a processor in communication with the downhole tool and configured to implement a method comprising at least one of the steps:
 obtaining a pore pressure related data value of the formation from the data acquisition tool; 
 predicting pore pressure uncertainty from the pore pressure related data value of the formation; 
 estimating uncertainty of a pressure window for drilling fluid using the predicted pore pressure uncertainty; and 
 applying the estimated uncertainty to the pressure window to provide a modified pressure window. 
   
     
     
         19 . The apparatus according to  claim 18 , wherein the processor is further configured to:
 define an operating margin and apply the operating margin to the modified pressure window to provide an operating pressure window; and   define a drilling parameter for drilling a borehole in the formation within the operating pressure window.   
     
     
         20 . The apparatus according to  claim 19 , further comprising a drilling tool configured to drill the borehole within the operating pressure window. 
     
     
         21 . The apparatus according to  claim 19 , further comprising a controller configured to control a drilling fluid pump or a drilling fluid control valve to maintain drilling fluid pressure equivalent within the operating pressure window. 
     
     
         22 . The apparatus according to  claim 19 , further comprising a controller configured to control a drilling fluid flow control valve to maintain drilling fluid pressure within the operating pressure window. 
     
     
         23 . The apparatus according to  claim 19 , further comprising a drilling fluid sensor configured to sense a drilling fluid parameter and to provide input to a controller configured to provide an output to maintain drilling fluid pressure within the operating pressure window. 
     
     
         24 . The apparatus according to  claim 18 , wherein the data acquisition tool comprises a downhole tool comprising at least one of a gamma ray tool, a resistivity tool, a dielectric permittivity tool, a density tool, a neutron porosity tool, a pulsed neutron tool, a nuclear magnetic resonance tool, and an acoustic tool. 
     
     
         25 . The apparatus according to  claim 18 , wherein the data acquisition tool is configured to acquire formation data at the surface of the formation.