Redundancy enhanced removal of pressure-effect offset for drill bit strain gauge measurements

Abstract

As a wellbore is extended into a formation, hydrostatic and hydrodynamic pressures change due to variations in drilling mud weight, fluid density, etc. Strain gauges downhole measure forces experienced by drilling equipment also experience strain due to hydrostatic and hydrodynamic pressures. In order to measure strain due to drilling, hydrostatic and hydrodynamic pressure contributions are removed from strain measurement by removal of a pressure-effect offset, which zeroes or tares the strain measurements. Values of the pressure-effect offset can also be monitored to check that strain measurements are accurately zeroed or to monitor wellbore operations and conditions.

1 . A method comprising:
 obtaining strain measurements from a strain gauge at a first location associated with a drillstring, based on detection of a pause in drilling;   determining an offset value corresponding to a fluid pressure based on the strain measurements, wherein the offset value indicates contribution of fluid pressure to the strain measurements and the fluid pressure comprises at least one of hydrostatic pressure and hydrodynamic pressure; and   indicating the offset value for zeroing of the strain measurements.   
     
     
         2 . The method of  claim 1 , wherein obtaining strain measurements from a strain gauge further comprises:
 determining if the drillstring is off bottom; and   selecting a subset of the strain measurements corresponding to the drillstring being off bottom, based on a determination that the drill string is off bottom,   wherein determining the offset value comprises determining the offset value based on the selected subset of the strain measurements.   
     
     
         3 . The method of  claim 2 , wherein determining that the drillstring is off bottom comprises:
 obtaining weight on bit measurements and torque on bit measurements for a drill bit associated with the drillstring;   determining if a local minimum in the weight on bit measurements occurs based on the weight on bit measurements;   determining if the torque on bit measurements are substantially equal to zero; and   determining that the drillstring is off bottom, based on the determination that there is a local minimum in the weight on bit measurements and that the torque on bit measurements are substantially equal to zero.   
     
     
         4 . The method of  claim 1 , wherein obtaining strain measurements further comprises:
 determining that drilling fluid is not flowing; and   selecting a subset of the strain measurements that correspond to the drilling fluid not flowing, based on a determination that the drilling fluid is not flowing,   wherein determining the offset value comprises determining a hydrostatic offset value based on the selected subset of strain measurements.   
     
     
         5 . The method of  claim 4 , wherein determining that the drilling fluid is not flowing comprises:
 obtaining at least one of rotational velocity measurements for a drill bit associated with the drillstring and motor speed measurements for a drilling fluid pump associated with the drillstring;   determining if the at least one of the rotational velocity measurements for the drill bit and the motor speed measurements for the drilling fluid pump is substantially equal to zero; and   determining that the drilling fluid is not flowing, based on a determination that the at least one of the rotational velocity measurements for the drill bit and the motor speed measurements for the drilling fluid pump is substantially equal to zero.   
     
     
         6 . The method of  claim 1 , wherein obtaining strain measurements from a strain gauge further comprises:
 determining that drilling fluid is flowing; and   selecting a subset of the strain measurements corresponding to the drilling fluid flowing, based on a determination that the drilling fluid is flowing,   wherein determining the offset value comprises determining a hydrodynamic offset value based on the selected subset of the strain measurements.   
     
     
         7 . The method of  claim 6 , wherein determining that the drilling fluid is flowing comprises:
 obtaining at least one of rotational velocity measurements for a drill bit associated with the drillstring and motor speed measurements for a drilling fluid pump associated with the drillstring;   determining if the at least one of the rotational velocity measurements for the drill bit and the motor speed measurements for the drilling fluid pump is substantially not equal to zero; and   determining that the drilling fluid is flowing, based on a determination that the at least one of the rotational velocity measurements for the drill bit and the motor speed measurements for the drilling fluid pump is substantially not equal to zero.   
     
     
         8 . The method of  claim 1 , wherein detection of the pause in drilling comprises at least one of detection of an off-bottom event, detection of a stand addition, and detection of an addition of a drillstring component. 
     
     
         9 . The method of  claim 1 , wherein the strain measurements comprise at least one of axial strain measurements and transverse strain measurements. 
     
     
         10 . The method of  claim 1 , wherein determining an offset value comprises:
 determining a hydrostatic offset value and a total offset value on the strain measurements; and   determining a hydrodynamic offset value based, at least in part, on a difference between the total offset value and the hydrostatic offset value.   
     
     
         11 . The method of  claim 1 , wherein indicating the offset value further comprises:
 determining if the offset value is erroneous;   indicating the offset value for zeroing of the strain measurements, based on a determination that the offset value is not erroneous; and   indicating at least one previously determined offset value for zeroing of the strain measurements, based on a determination that the offset value is erroneous.   
     
     
         12 . The method of  claim 11 , wherein determining if the offset value is erroneous comprises:
 determining a difference between the offset value and the at least one previously determined offset value;   determining if the difference corresponds to an expected value of the difference between the offset value and the at least one previously determined offset value;   determining that the offset value is not erroneous, based on a determination that the difference does correspond to an expected value; and   determining that the offset value is erroneous, based on a determination that the difference exceed the expected value.   
     
     
         13 . The method of  claim 12 , wherein determining if the difference corresponds to an expected value of the difference further comprises determining the expected value of the difference between the offset value and the at least one previously determined offset value. 
     
     
         14 . A non-transitory, machine-readable medium having instructions stored thereon that are executable by a computing device to perform operations comprising instruction to:
 obtain strain measurements from a strain gauge at a first location associated with a drillstring, based on detection of a pause in drilling;   determine an offset value corresponding to a fluid pressure based on the strain measurements, wherein the offset value indicates contribution of fluid pressure to the strain measurements and the fluid pressure comprises at least one of hydrostatic pressure and hydrodynamic pressure; and   indicate the offset value for zeroing of the strain measurements.   
     
     
         15 . The machine-readable medium of  claim 14 , wherein instructions to obtain strain measurements from a strain gauge comprise instructions to:
 determine if the drillstring is off bottom;   determine that drilling fluid is not flowing; and   select a subset of the strain measurements corresponding to the drillstring being off bottom and drilling fluid not flowing, based on a determination that the drill string is off bottom and that drilling fluid is not flowing,   wherein instructions to determine the offset value comprises instructions to determine a hydrostatic offset value based on the selected subset of the strain measurements.   
     
     
         16 . The machine-readable medium of  claim 14 , wherein instructions to obtain strain measurements from a strain gauge comprise instruction to:
 determining if the drillstring is off bottom;   determine that drilling fluid is flowing; and   selecting a subset of the strain measurements that correspond to the drillstring being off bottom and drilling fluid flowing, based on a determination that the drill string is off bottom and that drilling fluid is flowing,   wherein instruction to determining the offset comprises instruction to determine a hydrodynamic offset value based on the selected subset of the strain measurements.   
     
     
         17 . The machine-readable medium of  claim 14 , further comprising instruction to:
 determine a difference between the offset value and at least one previously determined offset value;   determine if the difference corresponds to an expected value of the difference between the offset value and the at least one previously determined offset value;   indicate the offset value for zeroing of the strain measurements, based on a determination that the difference does correspond to an expected value; and   indicate at least one of the at least one previously determined offset value for zeroing of the strain gauge, based on a determination that the difference exceed the expected value.   
     
     
         18 . An apparatus comprising:
 at least one strain gauge at a first location associated with a drillstring;   a processor; and   a computer-readable medium having instructions stored thereon that are executable by the processor to cause the apparatus to,
 obtain strain measurements from the at least one strain gauge, based on detection of a pause in drilling; 
 determine a offset value corresponding to a fluid pressure based on the strain measurements, wherein the offset value indicates contribution of fluid pressure to the strain measurements and the fluid pressure comprises at least one of hydrostatic pressure and hydrodynamic pressure; and 
 indicate the offset value for zeroing of the strain measurements. 
   
     
     
         19 . The apparatus of  claim 18 , wherein the first location is associated with a drill bit of the drillstring. 
     
     
         20 . The apparatus of  claim 18 , further comprising instruction to:
 determine a difference between the offset value and at least one previously determined offset value;   determine if the difference corresponds to an expected value of the difference between the offset value and the at least one previously determined offset value;   indicate the offset value for zeroing of the strain measurements, based on a determination that the difference does correspond to an expected value; and   indicate at least one of the at least one previously determined offset value for zeroing of the strain gauge, based on a determination that the difference exceed the expected value.