Abstract:
A method for measuring load performance of a positive displacement motor (PDM) test coupon. The test coupon comprises a partial length of a PDM stage and is received inside a sealable test chamber. In some embodiments, the test coupon may be cut from a failed PDM stage. The test chamber is filled with test fluid. In some embodiments, the test fluid may be drilling fluid sampled from a live well. Rotation of the rotor on the test coupon actuates rotation of the stator. A braking torque is applied to the stator rotation, enabling evaluation of, for example, fatigue load performance of test coupon. Additional embodiments comprise the rotor axis and the stator axis being offset in order to simulate rotor/stator eccentricity in a full size PDM stage.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of, and priority to, commonly-invented and commonly-assigned U.S. Provisional Patent Application Ser. No. 62/311,278 filed Mar. 21, 2016. The entire disclosure of 62/311,278 is further incorporated herein by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    This disclosure is directed generally to devices that test the performance of a subterranean positive displacement motor (“PDM”), and more particularly to a device that miniaturizes the performance testing of full-scale PDMs while still maintaining accurate simulation of downhole conditions during such miniaturized performance testing. 
       BACKGROUND OF THE DISCLOSED TECHNOLOGY 
       [0003]    Currently, PDMs are tested using standardized ASTM protocols in order to simulate performance of parameters such as elastomer material performance and elastomer bonding performance under projected drilling loads. Conventional testing may be performed on failed PDM parts (typically stators) that have been retrieved from downhole service as part of an investigation into the cause of the failure. Alternatively, conventional testing may be performed as part of well planning activity, prior to drilling, in order to optimize selection of PDM components such as stator elastomer in view of the expected downhole environmental conditions and anticipated loading. 
         [0004]    Conventional testing is done according to current applicable ASTM protocols. The ASTM tests are not particularly representative of specific expected or encountered downhole environments. First, the ASTM tests are not able to replicate the cyclic loading stresses encountered by PDMs in service. Second, the ASTM tests only peripherally simulate the elevated temperatures and pressures encountered by PDMs in service. 
         [0005]    Generally, the ASTM tests are basic immersion tests in which an elastomer ample from a stator is first exposed to drilling fluid for a prescribed period at elevated temperatures, followed by performance evaluation by pull or tear testing. The performance evaluation is normally done at ambient temperatures and pressures. The purpose of the ASTM testing is to evaluate loss of physical properties in the rubber material and/or bond integrity. The ASTM testing may be done with a small volume of drilling fluid used in the specific drilling application. The drilling fluid sample may be taken from the actual drilling site. 
         [0006]    Examples of specific conventional ASTM protocols include the following:
       ASTM D2240—Durometer Hardness (Shore A)   ASTM D412—Tensile properties (Die C)   ASTM D5289—Vulcanization using Rotorless Cure Meter (MDR tests)   ASTM D624—Tear Strength (Die C)   ASTM D3182—Practice for preparing standard vulcanized sheets (for tensile and tear bars   ASTM D429—Adhesion   ASTM D6370—Compositional Analysis by Thermogravimetry (TGA)   ASTM D471—Rubber Property—Effect of Liquids (Swell or immersion test)       
 
         [0015]    In addition to the traditional ASTM testing, two other conventional test methods are known to evaluate the performance of PDM power sections. In contrast to the traditional ASTM protocols described above, however, these two test methods require large volumes of test drilling fluid to perform each test. 
         [0016]    Flow loop testing attempts to simulate downhole PDM service by placing a full downhole motor in an enclosed dynamometer (“dyno”). Drilling fluid is introduced at full operating pressure to drive the PDM power section. It often takes hundreds or even thousands of gallons of drilling fluid to fill and operate the flow loop. Each well drilled may have different drilling fluid types, making a full scale dyno test cost prohibitive to perform on a frequent basis. Further, the specialized equipment required to handle drilling fluids under pressure and high temperature make this approach logistically challenging and often not economically viable. 
         [0017]    Recirculating pump vessels have also been used in the past to evaluate the performance of PDMs. While pump vessels do not require the volume of drilling fluid needed by flow loops, pump vessels still require a minimum of 25 gallons of drilling fluid to operate. Further, pump vessels have limited load ranges that correspondingly limit the range of downhole environments that can be simulated. 
         [0018]    The volumes of drilling fluid needed by flow loop testing and recirculating pump vessel testing present a further drawback that is inherent to these conventional tests. Often the need for testing and evaluation arises after there has been a specific downhole PDM failure. Investigation of the failure advantageously includes evaluation and testing of the power section in an environment that closely simulates the downhole conditions in which the failure occurred. Such simulation is enhanced greatly when the evaluation and testing can be done using a sample of the actual drilling fluid flowing through the power section at the time the failure occurred. Such samples can be taken, and are available, but they come in small volumes, typically 0.5 to 2 gallons—not nearly enough to do flow loop or recirculating pump vessel testing. 
         [0019]    There is therefore a need in the art for a test apparatus that can closely simulate, evaluate and measure PDM performance under (1) the elevated temperatures encountered downhole, and (2) the cyclic loads encountered downhole. The test apparatus should nonetheless provide embodiments in which only small quantities of drilling fluid are required, ideally less than about 10 gallons of drilling fluid, preferably less than about 5 gallons and most preferably, less than about 2 gallons. Embodiments of the test apparatus should also ideally have a small enough footprint to be suitable to be housed in a conventional laboratory or at a wellsite location. 
       SUMMARY AND TECHNICAL ADVANTAGES 
       [0020]    These and other drawbacks in the prior art are addressed by a test apparatus using a PDM test coupon that is a partial length of one stage of power section, mounted within a test chamber. In some embodiments, the longitudinal length of the test coupon is up to about 12 inches. The test chamber also holds a small volume of test fluid, sufficient to immerse the test coupon and occupy all of its internal progressing gaps. In some embodiments, the test fluid is drilling fluid maintained at predetermined and monitored elevated temperatures so as to simulate temperature performance in desired downhole conditions. The volume of test fluid/drilling fluid required to fill the test apparatus is further advantageously small enough to allow, if desired, use of actual drilling fluid samples drawn from full operational power sections that may have failed downhole. It will be appreciated that the test coupon cannot be sealed because it is only a partial length of one stage of a PDM power section, and thus presents less than one complete progressing cavity. Therefore, the test apparatus does not pump drilling fluid through the test coupon per the conventional operation of a PDM. Instead, an external motor rotates the rotor section in the test coupon, actuating corresponding rotation of the stator section by contact between rotor section and the stator section. The rotation of the rotor section, with corresponding actuated rotation of the stator section, is all in the presence of test fluid in the progressing gap(s) between rotor section and stator section in the test coupon. In this way, traditional PDM interaction between rotor and stator via drilling fluid flow can be simulated without pumping drilling fluid. The test fluid in the test coupon is present to interact with the test coupon materials to simulate degradation that might be seen in actual downhole conditions. Note that the relative differential rotation of the rotor section and the stator section in the test coupon may cause the test fluid to flow under low pressure through progressing gaps between rotor and stator sections. In some embodiments, a flow loop of test fluid may form through two or more progressing gaps. In other embodiments where openings are provided in the stator section, a flow loop of test fluid may form through the progressing gaps, out of the stator section openings, into the annular space between the test coupon and the test chamber, and back into the progressing gaps again. 
         [0021]    Once the rotor section and stator section are rotating, an external brake mechanism intentionally applies a braking torque to the rotation of the stator section against the externally-driven rotation of the rotor, causing the test coupon to come under controlled load. The braking force may be controlled, for example, by measuring the output torque of the stator section and adjusting the braking force to obtain a desired torque. This ensures the interface stress between the rotor and stator sections remains constant, resulting in a highly controlled fatigue loading being placed on the test coupon. Controlling the applied torque in this way enables multiple modes of evaluating performance of the test coupon. The performance of the test coupon in the test fluid environment can be monitored continuously over a wide range of loads placed cyclically on the coupon over time. In this way, a stress field can be generated between the rotor and stator sections in the test coupon that simulates very closely the same loading placed on a corresponding full scale power section in downhole service. Variables such as materials selection, performance optimization and useful downhole life, for example, can all be evaluated against the fatigue caused by the loading placed on the test coupon. Performance of the test coupon may be evaluated via measuring metrics including, but not limited to: (1) recording cycles to test coupon failure, via, for example, counting rotor section and/or stator section cycles through to failure, (2) examining the rotor section and/or the stator section for wear, (3) monitoring temperature change over time in the test chamber, and (4) monitoring temperature change over time in an elastomer layer provided by the stator section at its contact surface with the rotor section. 
         [0022]    In other embodiments, an external motor may drive the stator section in the test coupon, and an external brake mechanism may intentionally slow the rotor section in order to place load on the test coupon. 
         [0023]    In other embodiments, the separate external motors may drive the rotor section and the stator section in the test coupon. Controlled differential rotation of separate motors driving the rotor and stator may be used to place the test coupon under desired loads. 
         [0024]    In other embodiments, the disclosed test apparatus provides a method of evaluating PDM power section materials at elevated drilling temperatures and under similar stress profiles as seen in downhole service by full scale PDM power sections. According to currently preferred embodiments, the test apparatus design uses a small section of the full scale PDM power section as a test coupon and preferably less than about two gallons of drilling fluid within the test chamber. Many of the advantages of conventional flow loop or recirculating pump vessel testing can thus be obtained using the disclosed test apparatus, while at the same time obviating the need for large footprint or high volumes of drilling fluid. 
         [0025]    According to a first aspect, therefore, this disclosure describes an embodiment of a method for measuring load performance of a positive displacement motor (PDM) test coupon, the method comprising the steps of: (a) providing a PDM test coupon, the test coupon comprising a partial length of a PDM stage, the test coupon including a rotor section received inside a stator section, wherein the rotor section and the stator section are independently rotatable, wherein further at least one progressing gap on a helical pathway is formed between the rotor section and the stator section when the rotor section and the stator section are differentially rotated; (b) receiving the test coupon inside a sealable test chamber; (c) filling the test chamber with test fluid; (d) sealing the test chamber; (e) rotating the rotor section, thereby actuating corresponding rotation of the stator section in the presence of test fluid in the progressing gap; (f) applying a braking torque to said stator section rotation actuated in step (e); and (g) responsive to step (f), evaluating performance of the test coupon, wherein said evaluating step includes at least one substep selected from the group consisting of: (g1) controlling torque across the rotor section and the stator section; (g2) counting, through to failure of the test coupon, at least one of (1) rotor section rotation cycles and (2) stator section rotation cycles; (g3) examining at least one of (1) the rotor section and (2) the stator section for wear; and (g4) monitoring temperature change over time in the test chamber. 
         [0026]    According to a second aspect, this disclosure describes an embodiment of a method for measuring load performance of a positive displacement motor (PDM) test coupon, the method comprising the steps of: (a) providing a PDM test coupon, the test coupon comprising a partial length of a PDM stage, the test coupon including a rotor section received inside a stator section, wherein the rotor section and the stator section are independently rotatable about a longitudinal rotor axis and a longitudinal stator axis respectively, wherein further the rotor axis and the stator axis are substantially parallel and offset by a preselected axis offset distance, wherein further at least one progressing gap on a helical pathway is formed between the rotor section and the stator section when the rotor section and the stator section are differentially rotated; (b) receiving the test coupon inside a sealable test chamber; (c) filling the test chamber with test fluid; (d) sealing the test chamber; (e) rotating the rotor section, thereby actuating corresponding rotation of the stator section in the presence of test fluid in the progressing gap; (f) applying a braking torque to said stator section rotation actuated in step (e); and (g) responsive to step (f), evaluating performance of the test coupon, wherein said evaluating step includes at least one substep selected from the group consisting of: (g1) controlling torque across the rotor section and the stator section; (g2) counting, through to failure of the test coupon, at least one of (1) rotor section rotation cycles and (2) stator section rotation cycles; (g3) examining at least one of (1) the rotor section and (2) the stator section for wear; and (g4) monitoring temperature change over time in the test chamber. According further to the second aspect, embodiments of the test coupon may be selected to be representative of a full size PDM stage having a known rotor/stator eccentricity, in which case the preselected axis offset distance is selected to be substantially the same as the known rotor/stator eccentricity. 
         [0027]    Embodiments according to the first or second aspects may also include the stator section providing an elastomer interface at a contact surface with the rotor section inside the test coupon, and in which the evaluating step (g) includes at least one substep selected from the group consisting of: (g1) controlling torque across the rotor section and the stator section; (g2) counting, through to failure of the test coupon, at least one of (1) rotor section rotation cycles and (2) stator section rotation cycles; (g3) examining at least one of (1) the rotor section and (2) the stator section for wear; (g4) monitoring temperature change over time in the test chamber; and (g5) monitoring temperature change over time in the elastomer interface. 
         [0028]    In other embodiments according to the first or second aspects, the at least one progressing gap comprises a plurality of progressing gaps, and in which a first flow loop of test fluid is formed through the plurality of progressing gaps when the test chamber is sealed and the rotor section is rotated. 
         [0029]    In other embodiments according to the first or second aspects, an annular cavity is formed between an external periphery of the stator section and an internal periphery of the test chamber, wherein the at least one progressing gap, a plurality of openings in the stator section, and the annular cavity form a second flow loop of test fluid when the test chamber is sealed and the rotor section is rotated. 
         [0030]    According to third and other aspects and embodiments, step (c) of the method according to the first or second aspects requires no more than about 25 gallons of test fluid, and most preferably, no more than about 2 gallons of test fluid. Embodiments of the test coupon may have an overall longitudinal length of no more than about 12 inches. Embodiments of the method according the first or second aspects may further comprise further steps of maintaining a preselected temperature and/or pressure inside the test chamber during at least steps (e) and (f). Embodiments of the test coupon may further be cut from a full size PDM stage through which drilling fluid was previously caused to flow. Step (f) of the method according to the first or second aspects may be accomplished using a continuous-slip brake. 
         [0031]    It is therefore a technical advantage of the disclosed test apparatus to miniaturize the performance testing of PDM power sections in downhole conditions by accurately simulating such conditions (for example, temperature and stress loading) in a partial section of one stage of a PDM power section. The footprint of the disclosed test apparatus is suitable to house the apparatus in a conventional laboratory or even at the wellsite. 
         [0032]    A further technical advantage of the disclosed test apparatus is to provide a PDM test apparatus that enables performance testing using a small volume of drilling fluid. In this way, if desired, the disclosed test apparatus can accurately simulate downhole service conditions using samples of actual drilling fluid extracted from a PDM previously in downhole service (such samples known to be available only in small quantities). 
         [0033]    A further technical advantage of the disclosed test apparatus is to avoid eccentric rotation vibration in the PDM power section test coupon during testing. By avoiding such vibration, the disclosed test apparatus enables performance evaluation of a PDM power section test coupon that is isolated from the effects of vibration caused by the eccentric rotation of the rotor within the stator. Conventionally, the construction of PDM power sections provides a hard (e.g. metal) rotor surface contacting a resilient stator (usually providing an elastomer or rubber through part or all of its cross-section). In normal PDM power section operations, the eccentric rotation of the rotor within the stator imparts cyclic loads (including, without limitation, compressive, shear and tensile loads) on the resilient stator material, particularly on the stator lobes. These loads imparted by cyclic contact are all included in the suite of performance metrics sought to be simulated and evaluated by the disclosed test apparatus. 
         [0034]    However, the eccentric rotation of the rotor during normal PDM power section operations also creates many modes of vibration throughout the PDM assembly and elsewhere on the drill string. The rotor on a full-size downhole PDM distributes torque and filling reaction forces along the full length of the elastomer component of the stator. The large length to diameter ratio of the power section allows these forces to be distributed along the length of multiple power section stages. The PDM, over its entire multi-stage length, can therefore absorb the vigorous dynamic forces created as the rotor orbits eccentrically in the stator. In contrast, the disclosed test apparatus evaluates performance on only a partial section of one PDM power section stage. It is therefore not possible in the disclosed test apparatus to stabilize the rotor section within the test coupon against the vibrational effect of eccentric rotation and tilting forces as might be seen in “live” downhole operations, since the length of the rotor section in the test coupon is too short to have sufficient longitudinal flexure to compensate for eccentric rotation vibration. 
         [0035]    Embodiments of the disclosed test apparatus therefore substantially eliminate eccentric rotation vibration from the test coupon by setting and holding the rotor rotation axis on a predetermined, fixed offset from the stator rotation axis. The offset is selected to be approximately equal to the design eccentricity of the full-size, operational power section whose performance the test coupon seeks to replicate and evaluate. The disclosed test apparatus thus generates test results that reflect performance isolated from substantially all eccentric rotation vibration, in a test coupon that can be very short in length and requiring only a small volume of drilling fluid. In this way, the results generated by the disclosed test apparatus will more accurately predict corresponding or vibration-compensated or vibration-isolated performance expected in full-size PDM power sections operating downhole. 
         [0036]    The disclosed test apparatus provides yet a further advantage in embodiments in which eccentric rotation vibration is substantially eliminated. In embodiments where an offset is provided between axes of rotor rotation and stator rotation (in order to elimination eccentric rotation vibration), the external drive train rotating the rotor is greatly reduced in complexity which still giving a very accurate stress field within the test coupon. In embodiments where an offset is not provided between axes of rotor rotation and stator rotation, a transmission is required in the external drive train to counteract or eliminate eccentric rotation vibration. The rotor must also be longer in such embodiments in order to generate flexure. Such an additional transmission and rotor length adds cost, complexity, and inevitably (1) increases potential footprint and (2) requires a higher volume of drilling fluid. 
         [0037]    The foregoing has rather broadly outlined some features and technical advantages of the disclosed test apparatus, in order that the following detailed description may be better understood. Additional features and advantages of the disclosed technology may be described. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same inventive purposes of the disclosed technology, and that these equivalent constructions do not depart from the spirit and scope of the technology as described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    For a more complete understanding of embodiments described in detail below, and the advantages thereof, reference is now made to the following drawings, in which: 
           [0039]      FIGS. 1 -A through  1 -J depict the operation of a conventional power section  100  in a series of freeze-frame cutaway section views of the conventional power section in operation; 
           [0040]      FIGS. 2 -A through  2 -J depict movement of corresponding components to  FIGS. 1 -A through  1 -J in the disclosed new test apparatus  200 , wherein such movement is also depicted in a series of freeze-frame cutaway section views; 
           [0041]      FIG. 3  shows an exemplary layout for test bed  300  on which components may be secured for enabling the disclosed new test apparatus  200 ; 
           [0042]      FIG. 4  illustrates test chamber  325  in cutaway view with internals and surrounding components; 
           [0043]      FIGS. 5 and 5A  illustrate two different embodiments of test chamber  325  in cutaway view, in which test fluid  323  may follow different flow loops FL 1  and FL 2 ; and 
           [0044]      FIG. 6  illustrates exemplary embodiments of drive train  310  as more generally illustrated on  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    As discussed summarily above in this disclosure, this application is directed to a PDM testing apparatus using a rotor/stator test coupon that is a partial length of one stage of a power section. A stage of a power section is defined by the minimum length to seal a single helical progressing cavity. The relationship describing the helical length of a single progressing cavity is based on the rotor to stator lobe count ratio and can be expressed as: 
         [0000]      Stator Pitch Length=( N+ 1)/ N ×Rotor Pitch Length
 
         [0000]    Stator pitch length is a PDM design parameter chosen to give the desired volumetric fluid flow ratio for the selected rotor/stator configuration. 
         [0046]      FIGS. 1 -A through  1 -J depict the mechanics of a conventional progressing cavity power section, as is well known in the prior art. Such progressing cavity power sections are also well known as “Moineau” devices.  FIGS. 1 -A through  1 -J depict the operation of such a conventional power section  100  in a series of freeze-frame cutaway section views of the power section in operation. The series of freeze-frame views depicted in  FIGS. 1 -A through  1 -J are in sequence. Parts and other features of conventional power section  100  are identified by reference number or letter, as described in detail further below. Where the same reference number or letter is used in  FIGS. 1 -B through  1 -J, the same part or feature of power section  100  is being identified on that Figure as depicted on  FIG. 1 -A. In this way, reference numbers and letters on  FIGS. 1 -B through  1 -J can be omitted for clarity on some views while still allowing the reader to understand the subject matter depicted on  FIGS. 1 -B through  1 -J. 
         [0047]    As noted, power section  100  as depicted in  FIGS. 1 -A through  1 -J is conventional. Such power sections are designed using intermeshing continuous helical pathways that provide, in cross section, cooperating lobed gear profiles that intermesh with one another in an “n” and “n+1” combination. Typically the inner rotor has n lobes and the outer stator gear has n+1 lobes. Referring to  FIG. 1A , rotor  120  is depicted with 6 lobes, and stator  110  has 7 lobes, although these values are purely by way of example. As will be seen generally from  FIGS. 1A through 1 -J viewed in sequence, the precise cross-sectional profiles of these these-gear-within-a-gear devices are normally described by hypocycloid geometry created by rolling circle techniques. The resulting gear devices are helically swept over the axial lengths of the rotor and stator. The intermeshing along the axial length creates progressing cavities on a helical pathway between the rotor and stator, identified in cross-section on  FIG. 1A  as PC, through which a flow of fluid drives rotor  120  around the inner periphery of stator  110 . 
         [0048]    The hypocycloid geometry of the intermeshed lobes on rotor  120  and stator  110  thus compels that in conventional PDM power sections such as power section  100  on  FIG. 1 -A, rotor  120  orbits within stator  110  at a known eccentricity from a central longitudinal axis. This eccentricity is a derived relationship and is related to the curvature and lobe heights generated from the mathematical expressions defining hypocycloid and true rolling motion used in the rotor and stator geometries. Review of  FIGS. 1 -A through  1 -J in sequence illustrate this conventional eccentricity in more detail. 
         [0049]    Referring first to  FIG. 1 -A, stator  110  includes rotational marker  112  and point marker  113 . Rotational marker  112  sets a fixed point on the outer periphery of stator  110 , and point marker  113  sets a fixed point on the inner periphery of stator  110  (adjacent reference point “S 1 ” on stator  110 ). It will be seen that the location of markers  112  and  113  on subsequent views in  FIGS. 1 -B through  1 -J does not change, indicating that stator  110  is stationary throughout the operation of conventional power section  100  depicted by  FIGS. 1 -A through  1 -J. 
         [0050]    With further reference to  FIG. 1 -A, rotor  120  includes fixed reference point “R 1 ” on the outer periphery of one of its lobes.  FIG. 1 -A also shows progressing cavity (PC) position marker  130 , on which PC marker line  132  indicates the position of the approximate most open point of progressing cavity PC. PC position marker  130  rotates in synch with progressing cavity PC around a central longitudinal axis through stator  110 . PC position marker  130  thus also serves to illustrate and highlight the eccentric rotation of rotor  120  within stator  110 . It will be seen on  FIG. 1 -A that that PC position marker  130  is offset from a central longitudinal axis through rotor  120 . 
         [0051]    Referring now to  FIG. 1 -B, a flow of drilling fluid (not illustrated) has displaced rotor  120  within stator  110  such that progressing cavity PC has rotated approximately 90 degrees clockwise within stator  110  from the corresponding progressing cavity position depicted on  FIG. 1 -A. This rotation of progressing cavity PC is illustrated by the new position of PC marker line  132  on  FIG. 1 -B, as well as a new general position on  FIG. 1 -B of progressing cavity PC.  FIG. 1 -B also shows that in response to clockwise movement of PC, rotor  120  has rotated counterclockwise as shown by the new position of rotor reference point R 1 .  FIG. 1 -B illustrates the clockwise movement of progressing cavity PC by arrow R PC , and the responsive counterclockwise movement of rotor  120  by arrow RR. 
         [0052]      FIG. 1 -C illustrates that the flow of drilling fluid has displaced rotor  120  within stator  110  such that progressing cavity PC has rotated approximately a further 90 degrees clockwise within stator  110  from the corresponding progressing cavity position depicted on  FIG. 1 -B. The relative movement of components within stator  110  can be seen from the new positions of progressing cavity PC, PC marker line  132 , and rotor reference point R 1  on  FIG. 1 -C, as compared to their corresponding positions on  FIGS. 1 -A and  1 -B. 
         [0053]      FIGS. 1 -D and  1 -E each show a further rotation of approximately 90 degrees clockwise of progressing cavity PC over the previously illustrated positions. Again, the relative movement of components within stator  110  can be seen from the new positions of progressing cavity PC, PC marker line  132 , and rotor reference point R 1  on each advancing view on  FIGS. 1 -A through  1 -E. It will be appreciated that on  FIG. 1 -E, progressing cavity PC has made one complete revolution of stator  110  over the starting position on  FIG. 1 -A. 
         [0054]      FIGS. 1 -F through  1 -J illustrate the counterclockwise rotation of rotor  120  within stator  110  responsive to subsequent full revolutions clockwise of progressing cavity PC. In each of  FIGS. 1 -A, and then  1 -F through  1 -J, progressing cavity PC has made one further full clockwise revolution over the previously illustrated view. When  FIGS. 1 -A, and then  1 -F through  1 -J are viewed in sequence, the new position of rotor reference point R 1  can be seen in response to one additional revolution of progressing cavity PC. It will be appreciated that in  FIG. 1 -J, progressing cavity PC has made 6 clockwise revolutions of stator  110  over the view depicted in  FIG. 1 -A, during which time rotor reference point R 1  has made one counterclockwise revolution, indicating that the eccentricity of rotor  120  within stator  110  is in the same state in  FIG. 1 -J as it was initially in  FIG. 1 -A. 
         [0055]    As noted throughout the disclosure immediately above,  FIGS. 1 -A through  1 -J illustrate movement of components within a conventional power section  100 .  FIGS. 2 -A through  2 -J depict movement of corresponding components within the disclosed new test apparatus  200 . As with  FIGS. 1 -A through  1 -J,  FIGS. 2 -A through  2 -J depict such movement within test apparatus  200  in a series of freeze-frame cutaway section views of the test apparatus in operation. The series of freeze-frame views depicted in  FIGS. 2 -A through  2 -J are in sequence. Similar to conventional power section  100  depicted in  FIGS. 1 -A through  1 -J, test apparatus  200  in  FIGS. 2 -A through  2 -J provides a rotor  220  with 6 lobes operating inside a stator  210  with 7 lobes. Where the same reference number or letter is used in  FIGS. 2 -A through  2 -J, the same part or feature of test apparatus  200  is being identified. In this way, reference numbers and letters on  FIGS. 2 -A through  2 -J can be omitted for clarity on some views while still allowing the reader to understand the subject matter depicted on  FIGS. 2 -A through  2 -J. 
         [0056]    The “Summary” section describes above how the disclosed test apparatus  200  tests less than one full length of a full downhole PDM power section stage, and thus necessarily cannot provide a series of full progressing cavities (one full stage of a power section being defined by the minimum length to seal a single helical progressing cavity, see above). Test apparatus  200  thus cannot be sealed to operate conventionally with moving drilling fluid driving a rotor in a stationary stator (as illustrated and described above with reference to  FIGS. 1 -A through  1 -J). Test apparatus  200  thus does not provide progressing cavities as illustrated as PC on  FIGS. 1 -A through  1 -J. In contrast, as illustrated on  FIGS. 2 -A though  2 -J, test apparatus  200  provides progressing gaps PG on helical pathways formed between rotor  220  and stator  210  when rotor  220  and stator  210  are differentially rotated. Progressing gaps PG on  FIGS. 2 -A though  2 -J are necessarily only a partial section of corresponding progressing cavities PC found on a full PDM power stage, as illustrated on  FIGS. 1 -A through  1 -J. 
         [0057]    In operation, currently preferred embodiments of test apparatus  200  on  FIGS. 2 -A through  2 -J provide an external motor (not illustrated on  FIGS. 2 -A through  2 -J) to rotate rotor  220  while submersed in test fluid/drilling fluid inside stator  210 . As rotor  220  rotates, it actuates corresponding rotation of stator  210  via contact between rotor  220  and stator  210 . An external brake mechanism (again not illustrated on  FIGS. 2 -A through  2 -J) is then activated to apply braking torque to intentionally slow the rotation of stator  210  against the powered rotation of rotor  220 . Applied braking torque can be finely controlled. In this way, a controlled stress field can be intentionally introduced on the components inside test apparatus  200  that approximate closely in kind, location, and strength the operational stresses experienced inside conventional power section  100  on  FIGS. 1 -A through  1 -J when drilling fluid is pumped through progressing cavity PC to rotate rotor  120  within stationary stator  110 . 
         [0058]    Looking at  FIGS. 2 -A through  2 -J in more detail,  FIG. 2 -A illustrates test apparatus  200  with the following parts and features, many of which are counterparts to the corresponding parts and features described above with respect to  FIGS. 1 -A through  1 -J:
       Stator  210     Stator rotational marker  212     Stator point marker  213     Stator reference point S 1     Rotor  220     Rotor reference point R 1         
 
         [0065]    Progressing gap PG along the axial length of rotor  220  and stator  210 , on a helical pathway formed between rotor  220  and stator  210  when rotor  220  and stator  210  are differentially rotated. 
         [0066]    Additionally,  FIG. 2 -A illustrates rotor rotation marker line  222 , which, along with rotor reference point R 1 , indicates rotation of rotor  220  relative to other components in test apparatus  200  as views advance through  FIGS. 2 -A through  2 -J. 
         [0067]    Referring now to  FIG. 2 -B, external rotor motor (not illustrated) has rotated rotor  220  approximately 90 degrees counterclockwise within stator  210  from the corresponding position of rotor  220  depicted on  FIG. 2 -A. As a result, rotational motion forces from rotor  220  have exerted themselves on stator  210 , actuating rotation of stator  210  in a counterclockwise direction. This displacement of stator  210  can be seen by comparing the relative positions of stator rotational marker  212 , stator point marker  213  and stator reference point S 1  on  FIG. 2 -B as compared to  FIG. 2 -A. Additionally the displacement of rotor  220  by approximately 90 degrees counterclockwise in  FIG. 2 -B has caused progressing gap PG to move in a clockwise direction away from its corresponding position in  FIG. 2 -A. This clockwise movement of progressing gap PG is best seen by comparing its displaced position relative to stator rotational marker  212  in  FIG. 2 -A and then  FIG. 2 -B.  FIG. 2 -B illustrates the counterclockwise movement of rotor  220  by arrow RR, the responsive counterclockwise movement of stator  210  by arrow Rs, and the resulting clockwise movement of progressing gap PG by arrow R PG . 
         [0068]      FIG. 2 -C illustrates that external rotor motor (not illustrated) has rotated rotor  220  approximately 90 degrees further counterclockwise within stator  210  from the corresponding position of rotor  220  depicted on  FIG. 2 -B. As a result, rotational motion forces from rotor  220  have acted on stator  210 , causing stator  210  to rotate further in a counterclockwise direction. This displacement of stator  210  can be seen by comparing the relative positions of stator rotational marker  212 , stator point marker  213  and stator reference point S 1  on  FIG. 2 -C as compared to  FIG. 2 -B. Additionally the displacement of rotor  220  by approximately 90 degrees further counterclockwise in  FIG. 2 -C has caused progressing gap PG to move in a clockwise direction away from its corresponding position in  FIG. 2 -B, as seen by comparing the position of progressing gap PG in  FIG. 2 -C relative to stator rotational marker  212  in  FIG. 2 -B. 
         [0069]      FIGS. 2 -D and  2 -E each show a further rotation of approximately 90 degrees counterclockwise of rotor  220  over the previously illustrated positions. Again, the relative movement of components within stator  210  can be seen from the new positions of rotor rotation marker line  222 , rotor reference point R 1 , stator rotational marker  212 , stator point marker  213 , stator reference point S 1 , and progressing gap PG on each advancing view on  FIGS. 2 -A through  2 -E. It will be appreciated that on  FIG. 2 -E, external rotor motor (not illustrated) has caused rotor  220  to make one complete revolution of stator  210  over the starting position on  FIG. 2 -A. 
         [0070]      FIGS. 2 -F through  2 -J illustrate the counterclockwise rotation of stator  210  responsive to subsequent full revolutions counterclockwise of rotor  220 . In each of  FIGS. 2 -A, and then  2 -F through  2 -J, rotor  220  has made one further full counterclockwise revolution over the previously illustrated view. When  FIGS. 2 -A, and then  2 -F through  2 -J are viewed in sequence, the new relative position of stator rotational marker  212 , stator point marker  213 , stator reference point S 1 , and progressing gap PG can be seen in response to one additional revolution of rotor  220 . It will be appreciated that in  FIG. 2 -J, rotor reference point R 1  has made 6 counterclockwise revolutions as compared to the view depicted in  FIG. 2 -A, during which time stator reference point S 1  has made almost one counterclockwise revolution in the orbital distance separating rotor reference point R 1  and stator reference point R 1  during rotation. It will be appreciated that with one further complete counterclockwise revolution of rotor reference point R 1  (a seventh overall rotation), stator reference point S 1  will have made one complete counterclockwise revolution in the orbital distance separating rotor reference point R 1  and stator reference point S 1 , and will further have returned to the same position relative to rotor reference point R 1  as depicted in  FIG. 2 -A. 
         [0071]    Returning now to view  FIGS. 1 -A through  1 -E in sequence, it will be appreciated rotor  120  is in an eccentric orbit within stator  110  during in the operation of conventional power section  100 . Conventionally, the construction of PDM power sections provides a hard (e.g. metal) rotor surface contacting a resilient stator (usually providing an elastomer or rubber through part or all of its cross-section). As described above in the “Summary” section, in normal PDM power section operations, the eccentric rotation of the rotor within the stator imparts cyclic loads (including, without limitation, compressive, shear and tensile loads) on the resilient stator material, particularly on the stator lobes. The eccentric rotation of the rotor during normal PDM power section operations also creates many modes of vibration throughout the PDM assembly and elsewhere on the drill string. It will be appreciated that these vibrations, as experienced downhole, can be addressed over the length of a full power section stage by fixing the ends of the rotor and allowing the natural flexure of the rotor to compensate. However, in short lengths of conventional power section configured according to  FIGS. 1 -A through  1 -E, such eccentric rotation vibrations cannot easily be compensated for, since the rotor is too short to have the required flexure. 
         [0072]    Turning now to view  FIGS. 2 -A through  2 -E in sequence, it will be appreciated that embodiments of the disclosed test apparatus  200  are reconfigured in a way to optimize, and in some cases to substantially eliminate, any eccentric rotation vibration that might potentially arise in the test coupon, so that performance evaluation of the components of the test coupon can be conducted free of the effects of such vibration.  FIGS. 2 -A through  2 -E show that stator  210  and rotor  220  each rotate independently about their own fixed, substantially parallel longitudinal axes, and that the rotor rotation axis is set at a fixed offset distance from the stator rotation axis. The fixed offset distance is a predetermined design choice, selected according to the size and lobe count of the rotor and stator, and further according to the amount of operational contact desired by the rotor on the stator as the rotor is externally rotated. Comparison should now be made with the eccentric orbit of rotor  120  within stator  110  on  FIGS. 1 -A through  1 -E in sequence, where rotor  120  makes operational contact on stator  110  as rotor  120  moves around the inner periphery of stator  110 . Returning to  FIGS. 2 -A through  2 -E, it will be appreciated that by selecting, setting and holding the rotor rotation axis of rotor  220  at a fixed offset distance from the rotation axis of stator  210 , the same operational contact between rotor  220  and stator  210  can be replicated in test apparatus  200  as is experienced in conventional PDM power section  100  on  FIGS. 1 -A through  1 -E. In test apparatus  200  on  FIGS. 2 -A through  2 -E, however, there is no eccentric orbital rotation of rotor  220  within stator  210 , thereby substantially eliminating vibration that might otherwise be caused by such eccentric orbital rotation. 
         [0073]    Previous disclosure described how in most test environments, test apparatus  200  will be configured such that the offset between of rotor  220  and stator  210  is substantially the same as the eccentricity of the rotor&#39;s orbit in a full-size PDM whose performance the test coupon is designed to evaluate. For purposes of this paragraph, such offset between longitudinal rotational axes of rotor  210  and stator  220  will be referred to as the “ideal eccentricity”. Additional embodiments of test apparatus  200  may be configured with variations in offset (greater or smaller) away from the ideal eccentricity. Such additional embodiments will simulate (and enable corresponding performance evaluation under) extreme loading conditions experienced by full-length PDM power sections in environments where the ambient dynamic loading conditions are deflecting rotor&#39;s eccentric orbit beyond design. 
         [0074]    Physical embodiments of the disclosed test apparatus are now described with reference to  FIGS. 3 through 6 . As before, where the same reference number or letter is used in  FIGS. 3 through 6 , the same part or feature is being identified on more than one Figure. 
         [0075]      FIG. 3  shows an exemplary layout for test bed  300  on which components may be secured for enabling the disclosed test apparatus. External motor  305  provides rotational power to rotor section  315  via drive train  310 . External motor  305  is illustrated on  FIG. 3  as an electric motor. The scope of the disclosed test apparatus is indifferent, however, to the type of external motor selected. Drive train  310  is illustrated in  FIG. 3  as a belt-and-pulley drive train. Embodiments of test bed  300  that provide drive train  310  as a belt-and-pulley train will gain further advantages as described below with reference to  FIG. 6 . However, this disclosure is not limited to embodiments whose drive train  300  is a belt-and pulley train. 
         [0076]    Rotor section  315  on  FIG. 3  is set rotationally in place on rotor bearings  318 . It will be understood from momentary reference to  FIG. 5  that a distal end of rotor section  315  terminates inside test chamber  325 , and functions as the rotor portion of the test coupon under analysis in test chamber  325 . Test chamber  325  is shown sealed on  FIG. 3 . Embodiments of the internals of test chamber  325  are discussed further below with reference to  FIGS. 4, 5 and 5A . Returning to  FIG. 3 , stator shaft  320  exits test chamber  325  and will be understood to be rotationally connected to stator section  321  inside test chamber  325  (again, refer momentarily to  FIG. 4 ). Stator bearings  328  set stator shaft  320  rotationally in place. Stator shaft  320  is rotationally connected to torque sensor  330 , which measures the torque generated by stator shaft  320  as stator shaft  320  rotates in response to external motor  305  driving rotor  315 . Torque sensor  330  is further subject to intentional slowing of rotation via a braking torque supplied by continuous-slip brake  335 , whereby fine control of the braking torque induces a controlled stress field (not illustrated) between rotor  315  and stator  321  inside test chamber test chamber  325 . While the embodiments described with reference to  FIG. 3  refer to a continuous-slip brake  335 , it will be understood that the scope of the test apparatus is not limited in this regard, and that other types of conventional brakes may be substituted for continuous-slip brake  335 . 
         [0077]    Test chamber  325  will now be discussed in more detail with reference to  FIGS. 4, 5 and 5A .  FIGS. 4, 5 and 5A  are section views shown generally on  FIG. 3 . Note  FIG. 3  omits specific reference to  FIG. 5A  mainly for clarity, it being understood from disclosure further below that  FIGS. 5 and 5A  are alternative embodiments and therefore the section line for  FIG. 5  shown on  FIG. 3  is representative for both  FIGS. 5 and 5A . 
         [0078]    Referring first to  FIG. 4 , test chamber  325  is seen in cutaway view. Rotor section  315  is omitted for clarity. Stator section  321  is seen rotationally and coaxially fixed to stator shaft  320 . Sealable test chamber  325  also provides annular cavity  322  formed between an external periphery of stator section  321  and an internal periphery of test chamber  325 . Per the embodiment of  FIG. 5A , described further below,  FIG. 4  depicts openings  324  provided stator section  321 . (It will be appreciated from disclosure further below that openings  324  are not provided in the embodiment of  FIG. 5 ). In operation, test chamber  325  is filled with test fluid  323  (advantageously drilling fluid) prior to being sealed so that stator section  321  is completely immersed. 
         [0079]      FIGS. 5 and 5A  illustrate two exemplary alternative embodiments of test chamber  325  in more detail with surrounding components, again in cutaway view.  FIGS. 5 and 5A  each depict progressing gaps PG formed on helical pathways between stator section  321  and rotor section  315 . Test chamber  325  is sealed and filled with test fluid  323 . It was noted in earlier disclosure that test fluid  323  is present in test chamber  325  to interact with the materials from which rotor section  315  and stator section  321  are made, in order to simulate degradation that might be seen in actual downhole conditions. It was also noted in earlier disclosure, however, that once test chamber  325  was sealed, the relative differential rotation of rotor section  315  and stator section  321  may cause the test fluid  323  to flow under low pressure through progressing gaps PG. Referring to the embodiment of  FIG. 5 , and as illustrated by the darker arrows on  FIG. 5 , first flow loop FL 1  for test fluid  323  is formed through progressing gaps PG.  FIG. 5  depicts first flow loop FL 1  in one exemplary flow direction, although it will be understood that flow loop FL 1  may arise in either direction according to user selection of the direction in which to rotate rotor section  315 . 
         [0080]    Referring now to the embodiment of  FIG. 5A , openings  324  are provided in stator section  321 . The darker arrows on  FIG. 5A  illustrate that second flow loop FL 2  for test fluid  323  may form in one exemplary flow direction, through progressing gaps PG in a direction away from the end near rotor bearing  318 , then through openings  324  in stator section  321 , then through annular cavity  322  back to the rotor bearing end of progressing gaps PG. Again, although not specifically illustrated on  FIG. 5A , it will be understood that, according to user selection of the direction in which to rotate rotor section  315 , second flow loop FL 2  may flow in either direction. In general, first and second flow loops FL 1  and FL 2  on  FIGS. 5 and 5A  are indifferent to the direction in which test fluid  323  may be caused to flow. 
         [0081]      FIGS. 5 and 5A  further show rotor section  315  held rotationally in place by one of rotor bearings  318 . Stator shaft  320  is also shown held rotationally in place by one of stator bearings  328 .  FIG. 5  further illustrates a cross-section cut for  FIGS. 2 -A through  2 -J. It will be understood that freeze-frame views seen on  FIGS. 2 -A through  2 -J represent movement of the disclosed test apparatus within test chamber  325  along an exemplary cross-section cut line as shown on  FIG. 5 . 
         [0082]      FIGS. 5 and 5A  also illustrate the preselected offset  317  of rotor rotation axis  316  and stator rotation axis  326 . As discussed extensively above with reference to  FIGS. 2 -A through  2 -E, for example, offset  317  is introduced to eliminate eccentric rotation vibration during operation of test chamber  325 . It will be understood that relative adjustment of rotor bearings  318  and stator bearings  328  allows test chamber  325  to accommodate a range of offsets  317  to be selected, set and held according to user requirements. 
         [0083]      FIG. 6  illustrates exemplary embodiments of drive train  310  as more generally illustrated on  FIG. 3 . On  FIG. 6 , three alternatives are illustrated in which varying drive speeds and torques may be delivered to rotor section  315  by external motor  305  via drive train  310 . It will be understood, however, that the disclosed test apparatus is not limited to the three alternatives illustrated on  FIG. 6 , and that the scope of the disclosed test apparatus contemplates many alternative drive speeds and torques delivered to rotor section  315 . It will be further appreciated that by providing different drive speeds and torques to rotor section  315 , the disclosed test apparatus can simulate the mutation speed of a PDM power section, i.e. the “step down” effect of the “gear within the gear”. 
         [0084]    As suggested in earlier disclosure, external motor  305 , drive train  310 , rotor and stator bearings  318  and  328 , torque sensor  330 , continuous-slip brake  335 , and other seals not called out by part number are all off-the-shelf components whose performance characteristics are selected to suit a particular design of the disclosed test apparatus. In presently preferred embodiments, a suitable external motor  305  is a Brook Crompton 75 HP AC 3-phase 230/460 V motor delivering up to 1800 rpm; a suitable continuous-slip brake  335  is a Wichita Clutch model KKB  208 ; suitable seals for test chamber  325  are UTEX models MP; suitable rotor/stator bearings  318 / 328  are available from Dodge; and a suitable torque transmitter  330  is a Hirnmelstein MRCT 39000X. The scope of the disclosed test apparatus is nonetheless not limited to any particular selection or combination of such off-the-shelf components. Likewise, the control of the disclosed test apparatus is advantageously via conventional PLC and PM control, and the scope of the disclosed test apparatus is not limited in this regard. 
         [0085]    Referring now to commonly-invented and commonly-assigned U.S. Provisional Patent Application Ser. No. 62/311,278 (the “Provisional Application”), to which this disclosure claims priority and whose provisional disclosure is incorporated herein by reference, FIG. 7 of the Provisional Application is an exemplary finite element analysis (FEA) image of static displacement (strain) based upon known torque stresses placed on a rotor to be used in conjunction with the disclosed test apparatus. The image is color coded to show increasing strain. FEA images of the type shown on FIG. 7 of the Provisional Application are useful, for example, for sizing the external motor driving the rotor and deriving specifications for the interconnecting drive train. 
         [0086]    There now follows description of an exemplary operation of the disclosed test apparatus. It will be understood that the following disclosure is for illustration only, and that the disclosed test apparatus is not limited thereby. 
         [0087]    The objectives of an exemplary test protocol may include to evaluate new elastomer compounds in an environment that accurately simulates expected downhole service in an operational PDM power section. With this in mind, a test stator section is prepared with the elastomer, molded into the actual stator profile to be expected downhole. The test stator section is placed into the test chamber. In accordance with the disclosed test apparatus, the stator section is then (1) exposed the actual drilling fluids expected downhole; (2) exposed to the actual elevated temperatures expected downhole; (3) loaded with comparative (or higher) forces and cycle frequencies expected downhole. The disclosed test apparatus may then, for example, measure the number of cycles to failure under defined loads and conditions. In some embodiments, the cycles to failure may be determined by counting, through to failure, the rotor section cycles and/or the stator section cycles. In other embodiments, the rotor section and/or stator section may be examined for wear. In other embodiments, temperature change over time inside the test chamber may be monitored. In other embodiments, temperature change over time may be monitored in an elastomer layer provided by the stator section at its contact surface with the rotor section. The test data yielded by the disclosed test apparatus will be expected to correlate closely to comparative test data that might have been extracted from a hypothetical power section in downhole service, and may be used to develop elastomer compounds with improved performance characteristics according to the service. Alternatively, without limitation, the disclosed test apparatus may be used to test the performance of actual test coupons and surrounding drilling fluids taken from PDMs in service in wells with their own chemistry. 
         [0088]    Embodiments of the disclosed test apparatus may be expected to achieve the following exemplary target performance parameters (again, the following list is not exhaustive, and the scope of the disclosed test apparatus is not limited in any of the following regards):
       Test chamber temperatures up to 350-400 degrees F.;   Ability to use stator sections or rotor sections cut from actual downhole tools; and   Ability to load stator elastomers up to approximately 40% strain.       
 
         [0092]    It will be appreciated that the scope of the disclosed test apparatus is not limited to the construction of stator sections that may be put in the test coupon, and includes, without limitation, all-elastomer construction, hybrid metal/elastomer constructions (“evenwall”) or other types of construction. Likewise, the disclosed test apparatus is not limited to the size, type or construction of rotor that may be put in the test coupon. 
         [0093]    Alternative embodiments of the disclosed test apparatus could further include, without limitation, the following features and aspects:
       (a) Substituting the disclosed offset shaft mounting of rotor and stator and convert the rotor to an eccentric transmission by which to receive rotational torque;   (b) Adapting the disclosed test apparatus to evaluate miniaturized 1.0 to 2.0 stage motors, advantageously with small diameters; and   (c) Adapting the disclosed test apparatus to evaluate test coupons with straight pathways for drilling fluid rather than helical pathways.   (d) Adapting the input motor to deliver impulse loads to the rotor or stator shaft to simulate downhole torsional impacts and stall events.   (e) Varying the number and locations of rotor/stator bearings for support of the rotor/stator.   (f) Varying the number and locations of the test chamber fluid seals.       
 
         [0100]    Although the inventive material in this disclosure has been described in detail along with some of its technical advantages, it will be understood that various changes, substitutions and alternations may be made to the detailed embodiments without departing from the broader spirit and scope of such inventive material as set forth in the following claims.