Determining a drilling path in a subterranean formation

Abstract

A downhole tool includes a milling assembly configured to couple to a tubular string and run into a vertical wellbore formed from a terranean surface to one or more subterranean formations. The milling assembly includes a tubular mandrel; a first mill coupled to the tubular mandrel at a downhole end of the tubular mandrel; and a second mill coupled to the tubular mandrel uphole of the first mill. The downhole tool includes an inclination sensor mounted in or to the tubular mandrel between the first mill and the second mill. The inclination sensor is configured to determine an angle between a lateral wellbore formed by the milling assembly from a window in the vertical wellbore and the vertical wellbore.

1 . A downhole tool, comprising:
 a milling assembly configured to couple to a tubular string and run into a vertical wellbore formed from a terranean surface to one or more subterranean formations, the milling assembly comprising:
 a tubular mandrel; 
 a first mill coupled to the tubular mandrel at a downhole end of the tubular mandrel and configured to mill through a wellbore casing and form a lateral wellbore; and 
 a second mill coupled to the tubular mandrel uphole of the first mill and configured to mill through the wellbore casing and form the lateral wellbore; and 
   an inclination sensor mounted in or to a portion of the tubular mandrel separate from and between the first mill and the second mill, the inclination sensor configured to determine an angle between the lateral wellbore formed by the milling assembly from a window in the wellbore casing of the vertical wellbore and the vertical wellbore.   
     
     
         2 . The downhole tool of  claim 1 , wherein the inclination sensor is mounted within a pocket formed in a wall of the tubular mandrel. 
     
     
         3 . The downhole tool of  claim 2 , comprising a cover plate coupled to the tubular mandrel over the pocket to enclose the inclination sensor. 
     
     
         4 . The downhole tool of  claim 3 , comprising one or more fasteners configured to couple at least one of the inclination sensor or the cover plate to the tubular mandrel. 
     
     
         5 . The downhole tool of  claim 1 , wherein the inclination sensor is configured to determine:
 a first angle between the lateral wellbore formed by the milling assembly from the window in the casing of the vertical wellbore and the vertical wellbore; and   a second angle between the lateral wellbore formed by the milling assembly from the window in the casing of the vertical wellbore and the vertical wellbore.   
     
     
         6 . The downhole tool of  claim 1 , wherein the inclination sensor comprises a printed circuit board that comprises:
 at least one accelerometer; and   at least one memory module communicably coupled to the at least one accelerometer.   
     
     
         7 . The downhole tool of  claim 1 , comprising a third mill coupled to the tubular mandrel uphole of the second mill and configured to mill through the wellbore casing and form the lateral wellbore. 
     
     
         8 . A drilling system, comprising:
 a whipstock configured couple to a casing positioned in a vertical wellbore formed from a terranean surface to one or more subterranean formations; and   a bottom hole assembly comprising a milling assembly configured to couple to a tubular string and run into the vertical wellbore through the casing, the milling assembly comprising:
 a tubular mandrel; 
 a plurality of mills, each mill coupled to the tubular mandrel and configured to mill through the casing and form a window in the casing to a lateral wellbore that extends from the vertical wellbore; and 
 an inclination sensor mounted in or to a portion of the tubular mandrel separate from and between two of the plurality of mills, the inclination sensor configured to determine an angle between the lateral wellbore formed by the milling assembly from the window in the casing positioned in the vertical wellbore and the vertical wellbore. 
   
     
     
         9 . The drilling system of  claim 8 , wherein the inclination sensor is mounted within a pocket formed in a wall of the tubular mandrel. 
     
     
         10 . The drilling system of  claim 9 , comprising a cover plate coupled to the tubular mandrel over the pocket to enclose the inclination sensor. 
     
     
         11 . The drilling system of  claim 10 , comprising one or more fasteners configured to couple at least one of the inclination sensor or the cover plate to the tubular mandrel. 
     
     
         12 . The drilling system of  claim 8 , wherein the inclination sensor is configured to determine:
 a first angle between the lateral wellbore formed by the milling assembly from the window in casing positioned in the vertical wellbore and the vertical wellbore; and   a second angle between the lateral wellbore formed by the milling assembly from the window in the casing positioned in the vertical wellbore and the vertical wellbore.   
     
     
         13 . The drilling system of  claim 8 , wherein the inclination sensor comprises a printed circuit board that comprises:
 at least one accelerometer; and   at least one memory module communicably coupled to the at least one accelerometer.   
     
     
         14 . The drilling system of  claim 8 , wherein the plurality of mills comprise a first mill positioned at a distal end of the tubular mandrel and a second mill positioned uphole of the first mill, and the inclination sensor is mounted in or to the tubular mandrel between the first and second mills. 
     
     
         15 . A drilling method, comprising:
 running a downhole tool into a vertical wellbore formed from a terranean surface to one or more subterranean formations, the downhole tool comprising a milling assembly that comprises:
 a tubular mandrel; 
 a first mill coupled to the tubular mandrel at a downhole end of the tubular mandrel; 
 a second mill coupled to the tubular mandrel uphole of the first mill; and 
 an inclination sensor mounted in or to a portion of the tubular mandrel separate from and between the first mill and the second mill; 
   forming a window in a casing of the vertical wellbore with the first and second mills;   urging the milling assembly through the window in the casing of the vertical wellbore with a whipstock positioned in the vertical wellbore;   milling at least a portion of a lateral wellbore with the first and second mills from the window formed in the casing; and   during the milling, determining an angle between the lateral wellbore and the vertical wellbore with the inclination sensor.   
     
     
         16 . (canceled) 
     
     
         17 . The drilling method of  claim 15 , comprising installing the inclination sensor within a pocket formed in a wall of the tubular mandrel. 
     
     
         18 . The drilling method of  claim 17 , comprising securing a cover on or to the tubular mandrel with one or more tension bolts to enclose the inclination sensor in a pocket. 
     
     
         19 . The drilling method of  claim 15 , wherein determining the angle between the lateral wellbore and the vertical wellbore with the inclination sensor comprises:
 determining a first angle between the lateral wellbore and the vertical wellbore; and   determining a second angle between the lateral wellbore and the vertical wellbore.   
     
     
         20 . The drilling method of  claim 15 , wherein determining the angle between the lateral wellbore and the vertical wellbore with the inclination sensor comprises determining the angle with at least one accelerometer. 
     
     
         21 . The drilling method of  claim 20 , comprising:
 storing the determined angle in at least one memory module communicably coupled to the at least one accelerometer; and   subsequent to running the milling assembly out of the vertical wellbore to the terranean surface, accessing the stored determined angle from the at least one memory module.   
     
     
         22 . The drilling method of  claim 15 , wherein the milling assembly comprises a third mill coupled to the tubular mandrel uphole of the second mill, and the method comprises:
 forming the window in the casing of the vertical wellbore with the first, second, and third mills; and   milling at least the portion of the lateral wellbore with the first, second, and third mills from the window formed in the casing.