Reciprocating Pump Piston Control

Abstract

A sampling tool for conveyance within a well that extends into a subterranean formation. The sampling tool includes a displacement unit having a first cylinder, a second cylinder, a piston, and a magnet. The piston includes a first head located within the first cylinder and a second head located within the second cylinder. A stroke of the piston moves the first and second heads between first positions, in which each of the first and second heads are proximate first end walls of the respective first and second cylinders, and second positions, in which each of the first and second heads are proximate second end walls of the respective first and second cylinders. The magnet is carried with one or more of the first and second heads. The sampling tool further includes a magnetic field sensor operable to measure a magnetic field strength the magnet throughout the stroke.

What is claimed is: 
     
         1 . An apparatus, comprising:
 a sampling tool for conveyance within a well that extends into a subterranean formation, wherein the sampling tool comprises:
 a displacement unit comprising:
 a first cylinder; 
 a second cylinder; 
 a piston comprising a first head disposed in the first cylinder and a second head disposed in the second cylinder, wherein a stroke of the piston moves the first and second heads between first positions each proximate first end walls of the respective first and second cylinders and second positions each proximate second end walls of the respective first and second cylinders; 
 a first magnet carried with the first head; and 
 a second magnet carried with the second head; and 
 
 a single magnetic field sensor operable to:
 measure a first magnetic field strength of the first magnet as the piston moves through a first portion of the stroke that extends at least between a first end of the stroke and a midpoint of the stroke; and 
 measure a second magnetic field strength of the second magnet as the piston moves through a second portion of the stroke that extends at least between a second end of the stroke and the midpoint of the stroke. 
 
   
     
     
         2 . The apparatus of  claim 1  wherein the first and second magnets are relatively oriented with same magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially parallel to a longitudinal axis of the piston. 
     
     
         3 . The apparatus of  claim 1  wherein the first and second magnets are relatively oriented with same magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially perpendicular to a longitudinal axis of the piston. 
     
     
         4 . The apparatus of  claim 1  wherein the first and second magnets are relatively oriented with opposite magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially parallel to a longitudinal axis of the piston. 
     
     
         5 . The apparatus of  claim 1  wherein the first and second magnets are relatively oriented with opposite magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially perpendicular to a longitudinal axis of the piston. 
     
     
         6 . The apparatus of  claim 1  wherein the sampling tool further comprises:
 a hydraulic fluid pump; 
 hydraulic circuitry operatively connecting the hydraulic fluid pump to the first and second cylinders; and 
 system electronics operable to control the hydraulic circuitry based at least on information received from the single magnetic field sensor. 
 
     
     
         7 . An apparatus, comprising:
 a sampling tool for conveyance within a well that extends into a subterranean formation, wherein the sampling tool comprises:
 a displacement unit comprising:
 a first cylinder; 
 a second cylinder; 
 a piston comprising a first head disposed in the first cylinder and a second head disposed in the second cylinder, wherein a stroke of the piston moves the first and second heads between first positions each proximate first end walls of the respective first and second cylinders and second positions each proximate second end walls of the respective first and second cylinders; and 
 at least one magnet carried with at least one of the first and second heads; and 
 
 at least one magnetic field sensor operable to measure a magnetic field strength of the at least one magnet throughout the stroke. 
   
     
     
         8 . The apparatus of  claim 7  wherein:
 the at least one magnet is:
 a first magnet carried with the first head; and 
 a second magnet carried with the second head; and 
 
 the at least one magnetic field sensor is a single magnetic field sensor operable to:
 measure a first magnetic field strength of the first magnet as the piston moves through a first portion of the stroke that extends at least between a first end of the stroke and a midpoint of the stroke; and 
 measure a second magnetic field strength of the second magnet as the piston moves through a second portion of the stroke that extends at least between a second end of the stroke and the midpoint of the stroke. 
 
 
     
     
         9 . The apparatus of  claim 8  wherein the first and second magnets are relatively oriented with the same magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially parallel to a longitudinal axis of the piston. 
     
     
         10 . The apparatus of  claim 8  wherein the first and second magnets are relatively oriented with the same magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially perpendicular to a longitudinal axis of the piston. 
     
     
         11 . The apparatus of  claim 8  wherein the first and second magnets are relatively oriented with opposite magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially parallel to a longitudinal axis of the piston. 
     
     
         12 . The apparatus of  claim 8  wherein the first and second magnets are relatively oriented with opposite magnetic polarities, and wherein the single magnetic field sensor is operable to measure the first and second magnetic field strengths in a measurement direction that is substantially perpendicular to a longitudinal axis of the piston. 
     
     
         13 . The apparatus of  claim 7  wherein:
 the at least one magnet is either but not both of:
 a first magnet carried with the first head; and 
 a second magnet carried with the second head; and 
 
 the at least one magnetic field sensor is:
 a first magnetic field sensor operable to measure a magnetic field strength of the at least one magnet as the piston moves through a first portion of the stroke that extends at least between a first end of the stroke and a midpoint of the stroke; and 
 a second magnetic field sensor operable to measure the magnetic field strength of the at least one magnet as the piston moves through a second portion of the stroke that extends at least between a second end of the stroke and the midpoint of the stroke. 
 
 
     
     
         14 . The apparatus of  claim 7  wherein:
 the at least one magnet is:
 a first magnet carried with the first head; and 
 a second magnet carried with the second head; and 
 
 the at least one magnetic field sensor is:
 a first magnetic field sensor operable to measure a first magnetic field strength of the first magnet as the piston moves through a first portion of the stroke that extends at least between a first end of the stroke and a midpoint of the stroke; and 
 a second magnetic field sensor operable to measure a second magnetic field strength of the second magnet as the piston moves through a second portion of the stroke that extends at least between a second end of the stroke and the midpoint of the stroke. 
 
 
     
     
         15 . The apparatus of  claim 7  wherein the sampling tool further comprises:
 a hydraulic fluid pump; 
 hydraulic circuitry operatively connecting the hydraulic fluid pump to the first and second cylinders; and 
 system electronics operable to control the hydraulic circuitry based at least on information received from the at least one magnetic field sensor. 
 
     
     
         16 . A method, comprising:
 conveying a sampling tool within a well that extends into a subterranean formation, wherein the sampling tool comprises:
 a hydraulically actuated displacement unit comprising a piston and first and second magnets carried with and disposed proximate opposing ends of the piston; and 
 a magnetic field sensor operable to substantially continuously generate:
 first information related to a first magnetic field of the first magnet as the piston moves through a first portion of a stroke that extends at least between a first end of the stroke and a midpoint of the stroke; and 
 second information related to a second magnetic field of the second magnet as the piston moves through a second portion of the stroke that extends at least between a second end of the stroke and the midpoint of the stroke; and 
 
   operating the sampling tool to draw formation fluid from the subterranean formation into the displacement unit, wherein operating the sampling tool causes movement of the piston within the displacement unit to be controlled based on at least the first and second information.   
     
     
         17 . The method of  claim 16  wherein the sampling tool further comprises a hydraulic fluid source and hydraulic circuitry operably connecting the displacement unit to the hydraulic fluid source, and wherein operating the sampling tool causes the hydraulic circuitry to control the displacement unit based on at least the first and second information. 
     
     
         18 . The method of  claim 17  wherein control of the displacement unit by the hydraulic circuitry is based on determination of the piston reaching the first and second ends of the stroke based on the first and second information. 
     
     
         19 . The method of  claim 17  wherein control of the displacement unit by the hydraulic circuitry is based on determination of the piston nearing the first and second ends of the stroke based on the first and second information. 
     
     
         20 . The method of  claim 17  wherein control of the displacement unit by the hydraulic circuitry is based on determination of a position of the piston substantially continuously throughout the stroke based on the first and second information.