Compensation for rotor tilt in hybrid pdm stators

Abstract

Minor diameter enlargements are engineered into a stator in a positive displacement motor (PDM) power section to relieve stator stress concentrations at the lower (downhole) end of the power section in the presence of rotor tilt. The stator may be a hybrid construction of heterogenous materials.

We claim: 
     
         1 . A positive displacement motor (PDM) power section, comprising:
 a rotor and a stator, the rotor configured to be received within the stator, the rotor having a rotor length such that a first portion of the rotor length has a substantially uniform transverse cross-sectional profile;   the stator having a longitudinal centerline;   the stator further including an internal liner such that the liner provides the stator with a stator internal surface, wherein the stator internal surface has lobes formed therein, wherein the lobes define helical pathways in the stator internal surface, wherein the helical pathways define a contour length between a contour inlet end and a contour outlet end, wherein the contour length includes an outlet region starting at the contour outlet end and extending up to 25% of the contour length towards the contour inlet end;   wherein the liner includes an elastomer portion and a non-elastomer portion, wherein the non-elastomer portion extends from the contour outlet end towards the contour inlet end, wherein the non-elastomer portion is either (a) plastic or (b) metal;   wherein transverse cross-sections of the liner taken orthogonal to the longitudinal centerline provide alternating lobes and valleys around a circumference of the stator such that 2× a distance between the longitudinal centerline and a zenith point of the lobes defines a minor diameter and 2× a distance between the longitudinal centerline and a nadir point of the valleys defines a major diameter;   wherein the outlet region further includes a first relief length disposed thereon, wherein the first relief length extends between a first relief length inlet towards the contour inlet end and a first relief length outlet towards the contour outlet end;   wherein the first relief length inlet has a first relief length inlet minor diameter and the first relief length outlet has a first relief length outlet minor diameter;   wherein the first relief length outlet minor diameter is larger than the first relief length inlet minor diameter by more than 0.0002 inches per inch of first relief length; and   wherein all of the first relief length opposes the first portion of the rotor length when the rotor is received within the stator.   
     
     
         2 . The PDM power section of  claim 1 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a taper profile. 
     
     
         3 . The PDM power section of  claim 2 , in which the taper profile is non-linear. 
     
     
         4 . The PDM power section of  claim 1 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a step function. 
     
     
         5 . The PDM power section of  claim 1 , in which the elastomer portion of the liner has at least one tensile stress value selected from the group consisting of:
 (a) a tensile stress in a range between about 250 psi and about 1000 psi at 25% elongation;   (b) a tensile stress in a range between about 400 psi and about 1200 psi at 50% elongation; and   (c) a tensile stress in a range between about 500 psi and about 1600 psi at 100% elongation.   
     
     
         6 . The PDM power section of  claim 1 , in which:
 (i) both the first portion of the rotor length and a second portion of the rotor length have a common and substantially uniform transverse cross-sectional profile; and   (ii) the contour length further includes a second relief length disposed thereon;   wherein the first relief length is located nearer the contour outlet end than the second relief length;   wherein the second relief length extends between a second relief length inlet towards the contour inlet end and a second relief length outlet towards the contour outlet end;   wherein the second relief length inlet has a second relief length inlet minor diameter and the second relief length outlet has a second relief length outlet minor diameter;   wherein the first relief length inlet minor diameter increases towards the first relief length outlet minor diameter according to a first constant taper profile;   wherein the second relief length inlet minor diameter increases towards the second relief length outlet minor diameter according to a second constant taper profile;   wherein the first constant taper profile is more aggressive than the second constant taper profile; and   wherein all of the second relief length opposes the second portion of the rotor length when the rotor is received within the stator.   
     
     
         7 . The PDM power section of  claim 6 , in which the first and second relief lengths are contiguous. 
     
     
         8 . A positive displacement motor (PDM) power section, comprising:
 a rotor and a stator, the rotor configured to be received within the stator, the rotor having a rotor length such that a first portion of the rotor length has a substantially uniform transverse cross-sectional profile;   the stator having a longitudinal centerline;   the stator further including an internal liner such that the liner provides the stator with a stator internal surface, wherein the stator internal surface has lobes formed therein, wherein the lobes define helical pathways in the stator internal surface, wherein the helical pathways define a contour length between a contour inlet end and a contour outlet end, wherein the contour length includes an outlet region starting at the contour outlet end and extending up to 25% of the contour length towards the contour inlet end;   wherein the liner includes an elastomer portion and a non-elastomer portion, wherein the non-elastomer portion is either (a) plastic or (b) metal; wherein the non-elastomer portion has a higher modulus than the elastomer portion, wherein the non-elastomer portion extends from the contour outlet end towards the contour inlet end;   wherein transverse cross-sections of the liner taken orthogonal to the longitudinal centerline provide alternating lobes and valleys around a circumference of the stator such that 2× a distance between the longitudinal centerline and a zenith point of the lobes defines a minor diameter and 2× a distance between the longitudinal centerline and a nadir point of the valleys defines a major diameter;   wherein the outlet region further includes a first relief length disposed thereon, wherein the first relief length extends between a first relief length inlet towards the contour inlet end and a first relief length outlet towards the contour outlet end;   wherein the first relief length inlet has a first relief length inlet minor diameter and the first relief length outlet has a first relief length outlet minor diameter;   wherein the first relief length outlet minor diameter is larger than the first relief length inlet minor diameter by more than 0.0002 inches per inch of first relief length; and   wherein all of the first relief length opposes the first portion of the rotor length when the rotor is received within the stator.   
     
     
         9 . The PDM power section of  claim 8 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a taper profile. 
     
     
         10 . The PDM power section of  claim 9 , in which the taper profile is non-linear. 
     
     
         11 . The PDM power section of  claim 8 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a step function. 
     
     
         12 . The PDM power section of  claim 8 , in which the elastomer portion of the liner has at least one tensile stress value selected from the group consisting of:
 (a) a tensile stress in a range between about 250 psi and about 1000 psi at 25% elongation;   (b) a tensile stress in a range between about 400 psi and about 1200 psi at 50% elongation; and   (c) a tensile stress in a range between about 500 psi and about 1600 psi at 100% elongation.   
     
     
         13 . The PDM power section of  claim 8 , in which:
 (i) both the first portion of the rotor length and a second portion of the rotor length have a common and substantially uniform transverse cross-sectional profile; and   (ii) the contour length further includes a second relief length disposed thereon;   wherein the first relief length is located nearer the contour outlet end than the second relief length;   wherein the second relief length extends between a second relief length inlet towards the contour inlet end and a second relief length outlet towards the contour outlet end;   wherein the second relief length inlet has a second relief length inlet minor diameter and the second relief length outlet has a second relief length outlet minor diameter;   wherein the first relief length inlet minor diameter increases towards the first relief length outlet minor diameter according to a first constant taper profile;   wherein the second relief length inlet minor diameter increases towards the second relief length outlet minor diameter according to a second constant taper profile;   wherein the first constant taper profile is more aggressive than the second constant taper profile; and   wherein all of the second relief length opposes the second portion of the rotor length when the rotor is received within the stator.   
     
     
         14 . The PDM power section of  claim 13 , in which the first and second relief lengths are contiguous. 
     
     
         15 . A positive displacement motor (PDM) power section, comprising:
 a rotor and a stator, the rotor configured to be received within the stator, the rotor having a rotor length such that a first portion of the rotor length has a substantially uniform transverse cross-sectional profile;   the stator having a longitudinal centerline;   the stator further including an internal liner such that the liner provides the stator with a stator internal surface, wherein the stator internal surface has lobes formed therein, wherein the lobes define helical pathways in the stator internal surface, wherein the helical pathways define a contour length between a contour inlet end and a contour outlet end, wherein the contour length includes an outlet region starting at the contour outlet end and extending up to 25% of the contour length towards the contour inlet end;   wherein the liner includes an elastomer portion and a non-elastomer portion, wherein the non-elastomer portion is either (a) plastic or (b) metal; wherein the non-elastomer portion has a higher modulus than the elastomer portion, wherein the non-elastomer portion extends from the contour outlet end towards the contour inlet end; wherein the elastomer portion and the non-elastomer portion are contiguous;   wherein transverse cross-sections of the liner taken orthogonal to the longitudinal centerline provide alternating lobes and valleys around a circumference of the stator such that 2× a distance between the longitudinal centerline and a zenith point of the lobes defines a minor diameter and 2× a distance between the longitudinal centerline and a nadir point of the valleys defines a major diameter;   wherein the outlet region further includes a first relief length disposed thereon, wherein the first relief length extends between a first relief length inlet towards the contour inlet end and a first relief length outlet towards the contour outlet end;   wherein the first relief length inlet has a first relief length inlet minor diameter and the first relief length outlet has a first relief length outlet minor diameter;   wherein the first relief length outlet minor diameter is larger than the first relief length inlet minor diameter by more than 0.0002 inches per inch of first relief length; and   wherein all of the first relief length opposes the first portion of the rotor length when the rotor is received within the stator.   
     
     
         16 . The PDM power section of  claim 15 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a taper profile. 
     
     
         17 . The PDM power section of  claim 16 , in which the taper profile is non-linear. 
     
     
         18 . The PDM power section of  claim 15 , in which the first relief length inlet minor diameter increases at least in part towards the first relief length outlet minor diameter according to a step function. 
     
     
         19 . The PDM power section of  claim 15 , in which the elastomer portion of the liner has at least one tensile stress value selected from the group consisting of:
 (a) a tensile stress in a range between about 250 psi and about 1000 psi at 25% elongation;   (b) a tensile stress in a range between about 400 psi and about 1200 psi at 50% elongation; and   (c) a tensile stress in a range between about 500 psi and about 1600 psi at 100% elongation.   
     
     
         20 . The PDM power section of  claim 15 , in which:
 (i) both the first portion of the rotor length and a second portion of the rotor length have a common and substantially uniform transverse cross-sectional profile; and   (ii) the contour length further includes a second relief length disposed thereon;   wherein the first relief length is located nearer the contour outlet end than the second relief length;   wherein the second relief length extends between a second relief length inlet towards the contour inlet end and a second relief length outlet towards the contour outlet end;   wherein the second relief length inlet has a second relief length inlet minor diameter and the second relief length outlet has a second relief length outlet minor diameter;   wherein the first relief length inlet minor diameter increases towards the first relief length outlet minor diameter according to a first constant taper profile;   wherein the second relief length inlet minor diameter increases towards the second relief length outlet minor diameter according to a second constant taper profile;   wherein the first constant taper profile is more aggressive than the second constant taper profile; and   wherein all of the second relief length opposes the second portion of the rotor length when the rotor is received within the stator.   
     
     
         21 . The PDM power section of  claim 20 , in which the first and second relief lengths are contiguous.