Wellbore interval densities

Abstract

A method for estimating one or more interval densities in a subterranean wellbore includes acquiring first and second axially spaced pressure measurements in the wellbore. The pressure measurements may then be processed to obtain an interval density of drilling fluid between the measurement locations. A tool string including a large number of axially spaced pressure sensors (e.g., four or more or even six or more) electronically coupled with a surface processor via wired drill pipe may be used to obtain a plurality of interval densities corresponding to various wellbore intervals.

1 . A method for estimating an interval density in a subterranean wellbore, the method comprising:
 (a) deploying a tool string in the wellbore, the tool string including at least first and second subsurface longitudinally spaced pressure sensors deployed at corresponding first and second measured depths in the wellbore;   (b) causing the first and second pressure sensors to acquire first and second drilling fluid pressure measurements at the first and second measured depths; and   (c) causing a processor to process the first and second pressure measurements to compute an interval density between the first and second measured depths in the wellbore.   
     
     
         2 . The method according to  claim 1 , wherein: the first and second pressure measurements are transmitted to the surface and the interval density is computed at the surface. 
     
     
         3 . The method according to  claim 1 , wherein:
 the tool string includes at least first, second, and third subsurface longitudinally spaced pressure sensors deployed at corresponding first, second, and third measured depths;   (b) comprises causing the at least first, second, and third pressure sensors to acquire at least first, second, and third drilling fluid pressure measurements; and   (c) comprises processing the at least first, second, and third pressure measurements to compute at least first and second interval densities, the first interval density between the first and second measured depths and the second interval density between the second and third measured depths.   
     
     
         4 . The method according to  claim 1 , wherein:
 the tool string includes at least n=3 longitudinally spaced pressure sensors;   (b) comprises causing each of the n pressure sensors to acquire a drilling fluid pressure measurement; and   (c) comprises processing the n pressure measurements to compute N interval densities, wherein n−1≦N≦Σ i=1   i=n−1 (n−i).   
     
     
         5 . The method according to  claim 1 , wherein the pressure sensors comprise annular pressure sensors. 
     
     
         6 . The method according to  claim 1 , wherein the pressure sensors comprise internal pressure sensors. 
     
     
         7 . The method according to  claim 1 , wherein the tool string comprises wired drill pipe. 
     
     
         8 . The method according to  claim 1 , wherein drilling fluid is being circulated in the wellbore in (b). 
     
     
         9 . The method according to  claim 8 , wherein a drill bit is rotating on bottom in (b). 
     
     
         10 . The method according to  claim 1 , wherein drilling fluid is substantially static in the wellbore in (b). 
     
     
         11 . A method for drilling a subterranean wellbore, the method comprising:
 (a) deploying a drill string in the wellbore, the drill string including at least first and second subsurface longitudinally spaced annular pressure sensors deployed at corresponding first and second measured depths in the wellbore;   (b) causing the first and second annular pressure sensors to acquire first and second annular pressure measurements at the first and second measured depths;   (c) processing the first and second annular pressure measurements to compute an annular interval density between the first and second measured depths; and   (d) evaluating the annular interval density computed in (c) as an indicator of an adverse drilling condition.   
     
     
         12 . The method according to  claim 11 , wherein the adverse drilling condition is selected from the group consisting of inflow from a subterranean formation to the wellbore, outflow from the wellbore to a subterranean formation, borehole washout, borehole pack-off, poor cuttings transport, and tar intrusion into the wellbore. 
     
     
         13 . The method according to  claim 11 , wherein the drill string comprises wired drill pipe. 
     
     
         14 . The method according to  claim 11 , wherein:
 (b) further comprises causing first and second internal pressure sensors to acquire first and second internal pressure measurements at the first and second measured depths;   (c) further comprises processing the first and second internal pressure measurements to acquire an internal interval density between the first and second measured depths; and   (d) further comprises evaluating the internal and annular interval densities computed in (c) as indicators of an adverse drilling condition.   
     
     
         15 . The method according to  claim 14 , wherein:
 drilling fluid is being circulated in the wellbore in (b); and   (c) further comprises computing (i) a static annular interval density via subtracting a modeled annular friction component from the annular interval density and (ii) a static internal interval density via subtracting a modeled internal friction component from the internal interval density.   
     
     
         16 . A method for drilling a subterranean wellbore, the method comprising:
 (a) deploying a drill string in the wellbore, the drill string including at least first and second subsurface longitudinally spaced annular pressure sensors deployed at corresponding first and second measured depths in the wellbore;   (b) circulating drilling fluid through the drill string;   (c) measuring an annular interval density between the first and second pressure sensors;   (d) comparing the annular interval density measured in (c) with a modeled annular interval density to obtain a difference; and   (e) evaluating the difference obtained in (d) as an indicator of an adverse drilling condition.   
     
     
         17 . The method according to  claim 16 , wherein the adverse drilling condition is selected from the group consisting of inflow from a formation to the wellbore, outflow from the wellbore to a formation, borehole washout, borehole pack-off, poor cuttings transport, and tar intrusion into the wellbore. 
     
     
         18 . The method according to  claim 16 , wherein the modeled annular interval density is obtained via:
 (i) acquiring an internal interval static density measurement;   (ii) processing the internal interval static density measurement to adjust hydraulic model parameters; and   (iii) using the hydraulic model parameters to compute the modeled annular interval density.   
     
     
         19 . The method according to  claim 18 , wherein:
 (i) further comprises acquiring an annular interval static density measurement;   (ii) further comprises processing the annular interval static density measurement to adjust the hydraulic model parameters; and   (iii) using the hydraulic model parameters to compute the modeled annular interval density.   
     
     
         20 . The method according to  claim 16 , wherein
 (c) further comprises computing a static annular interval density via subtracting a modeled annular friction component from the measured annulus interval density;   (d) further comprises comparing the static annular interval density computed in (c) with a modeled static annular interval density to obtain a interval static density difference; and   (e) further comprises evaluating the interval static density difference obtained in (d) as an indicator of an adverse drilling condition.   
     
     
         21 . The method according to  claim 16 , wherein:
 the drill string includes at least first, second, and third subsurface longitudinally spaced annular pressure sensors deployed at corresponding first, second, and third measured depths;   (c) comprises measuring a first annular interval density between the first and second measured depths and a second annular interval density between the second and third measured depths;   (d) comprises comparing the first and second interval densities measured in (c) with first and second modeled annular interval densities to obtain first and second differences; and   (e) comprises evaluating the first and second differences obtained in (d) as indicators of an adverse drilling condition.