Power Section Knowledge Centre

With more than 25 years of power section experience, PV is a leading expert in high performance downhole equipment. Here are the answers to some of the most common questions we receive about power sections and their performance.

  • What is Aniline Point?

    One key measurement that can be used to “predict” elastomer swell is the “aniline point” of the hydrocarbon based fluid.  The main rule of thumb is:

    • Fluids with an aniline point higher than the bottom hole circulating temperature will minimize the amount of swelling in the stator rubber.
    • Fluids with an aniline point lower than the bottom hole circulating temperature will result in greater amounts of swell.

    The reason for this is the relationship between the hydrocarbon and the nitrile rubber.  By definition, aniline point is defined as:

    “Temperature at which equal volumes of a hydrocarbon fluid becomes miscible with aniline”

    The aniline point is really a direct measurement of the polarity (polar’ refers to the positive and negative charge separation in a chemical bond) of the aniline fluid and the hydrocarbon which can then be extrapolated to correlate with swell of the rubber.  In the “aniline point” test, the more polar the hydrocarbon, the lower the temperature will be in which it becomes miscible in the aniline. Because “aniline” and hydrocarbons are “polar” compounds, when they are mixed and heated, they want to “react” when a certain temperature is met.

    In relating this to how the stator rubber reacts with the drilling fluid it is necessary to understand the polar relationship of the compounds. Nitrile and HSN are both polar compounds so when they are immersed into a hydrocarbon based fluid, which is also a polar compound, and heated, the aniline point of the fluid then becomes critical.  If the bottom hole drilling and circulating temperature is greater than the aniline point of the fluid, the elastomer and fluid will begin to react and swelling occurs.  The greater the difference between the aniline point and the bottom hole temperature, the greater degree of reaction will take place.

  • 6:7 vs 7:8 Lobe Configuration

    If you had a choice of running a 6:7 or a 7:8 lobe power section and assuming that both power sections are the same length, running the same flow rate, which one would you choose?

    From pure design principles, it makes more sense to choose the 6:7 over the 7:8.  Why?  For a number of reasons:

    1. The less number of lobes in the power section, the more efficient the design.
    2. Off-bottom pressure (losses due to friction and other mechanical / hydraulic losses) is lower in power sections with less lobes.
    3. If both motors had a maximum differential pressure of 1000 psi, then the 6:7 would have more available torque to drill with as the off-bottom losses are less than the 7:8.
    4. Less nutation, reducing the heat buildup and allowing for a greater torque output, potentially even allowing more differential pressure per stage.

     

    The following chart illustrates the differences in performance between the 6:7 5.0 stage and the 7:8 5.0 stage power section:

  • De-pressurization of Stators to Avoid Explosive Gas Decompression

    Gas under high pressure tends to permeate and dissolve in to an elastomer. The rate of permeation depends upon several factors. While all stator elastomers are susceptible to gas permeation to some degree, the type of elastomer and its ingredients can affect control the rate of permeability.

    The type of gas and the mud have an effect on the permeation. Carbon dioxide and Hydrogen sulfide (H2S) dissolve more readily into an elastomer when immersed in water based muds. Methane and Nitrogen tend to permeate more in oil based muds.

    Temperature of immersion plays a significant role also. Higher pressures and lower temperatures lead to more permeation.

    Too rapid a decompression rate means that the dissolved gas cannot escape from the elastomer quickly enough and therefore forms gas bubbles within the elastomer (explosive decompression).  The decompression rate becomes critical below 1000 psi. A gradual depressurization is preferable to a rapid depressurization and a wait time.

    For every 100 psi of pressure, it is recommended that the elastomer is depressurized for 15 minutes to ensure a slow decompression and reduce the chance for explosive decompression.

    If these recommendations are not adhered to, the risk of stator damage is high and therefore the stator should be relined prior to the next run to prevent a premature failure.

  • How to Measure Fit

    One very important concept when discussing power sections is fit. The stator fit relates to the level of interference between the rotor and stator. The fit of a given power section is typically measured in thousands of an inch and can be calculated by using the formulas below.

    As the power section drills deeper into the formation, the bottom hole temperature generally increases. This causes the elastomer to swell and tighten the fit. In order to compensate for this phenomenon, PV offers multiple fits, designed for different temperature ranges. These different fits are commonly called “Group Numbers” and typically range from 0 (tightest) all the way to 7 (loosest).

  • Understanding Compression Fit and Optimum Downhole Compression

    Compression fit percentage (CFP) is a metric commonly used to benchmark the relative rates of compression between various power section manufacturers. When calculating the compression fit percentage (CFP) we need to take into account the thickness of the stator lobes. The CFP can be calculated using the following formula:

    Where Fit, Stator Minor and Nominal Tube ID are all measured in inches.

    There are limitations to Compression Fit %. Compression fit percentage only takes into account one aspect of the stator profile (stator minor). It also cannot accurately account for the highly application specific effects of the elastomer reacting with the drilling fluid. Therefore, the use of compression fit percentage should be used with caution. Power section fit is application specific and compression fit percentage is an oversimplification of a complex relationship which is driven by multiple variables including geometry, temperature, mud chemistry, operating parameters and time.

    PV designers, with extensive knowledge of the elastomer behavior, have created fit charts with the recommended operating temperatures for each model to achieve optimal compression and performance.

  • Max Working Pressure vs Effective Pressure

    Differential pressure refers to the difference in pressure between the inlet and outlet of a hydraulic system. The pressures present in a power section can be described using the following equation:

    The maximum recommended differential pressure on a spec sheet is the Working Pressure and it represents the total pressure loss across the power section during operation. The Off Bottom pressure (OFB) is the pressure loss that results from frictional losses of the fluid moving through the power section. The Effective Pressure is the remaining pressure (after OFB taken into account) that is left to generate torque.

    It’s important to understand the relationship between these pressures. The OFB is a function of the flow rate and fluid properties. It can be significantly increased by increasing the flow rate or by increasing the rheology of the drilling fluid. As the OFB increases, there is less Effective Pressure available to generate torque. This is shown graphically in the figure below.

  • What is Nutation?

    Nutation is the motion of the center of the power section rotor about the center of the stator. As drilling fluid is pumped through the power section, pressurized pockets cause the rotor to rotate in a counterclockwise motion, opposite the direction of the bit.

    Nutation is very important since it determines the frequency with which the lobes of the power section are loaded. Each time the rotor lobe contacts with a stator lobe, the lobe is flexed and absorbs a stress. This flexion also generates heat (hysteresis). Both the repetitive stresses put on the stator lobes, combined by the resulting heat build-up, can ultimately lead to the failure of the elastomer.

    Below is the formula for nutation.

    The nutation rate is a function of the rotor RPM and the rotor geometry. Therefore, higher lobe configuration power sections tend to have higher nutation rates than lower configurations.

  • Understanding Rotor Corrosion (Effect of chlorides on rotor surfaces)

    One component of the mud system that will affect and shorten the life of a rotor is the “chloride” content in the mud system. The rotor itself is 17-4 stainless steel, coated with chrome.  The chrome is applied via an electrolysis process and is dispositioned in such a way that there is more chrome on the major diameter than the minor diameter.  By the nature of electrolysis, the chrome adheres itself to the base metal in “layers” and can be seen as micro cracks under a microscope.

    When the chromed rotor is exposed to a corrosive environment, the chrome coating is at risk of what is known as corrosion pitting.  The corrosive elements of the drilling fluid permeate into the chrome layers via the micro cracks and upon reaching the base metal, break the bond between the chrome and the steel.  This causes the chrome to peel off and can be seen as corrosion or simply pitting in the chrome.  The following pictures are examples of this corrosion.

    The chloride content in the drilling mud is a factor to consider in determining rotor life downhole.  In water based mud (WBM) applications, chrome coated rotors are generally acceptable up to approximately 30,000 ppm (mg/L).  Once beyond that limit, the chrome coating will start to deteriorate.  With chloride contents in the >30 < 100K ppm, the rotor life will be shortened and damage can be expected.  Depending on other drilling mud characteristics and other drilling parameters, it may be shortened by anywhere from 10 to 50% of its normal life.  Chloride content above 100K ppm will reduce the rotor life significantly.  Cases have been reported to have rotors fail in a matter of hours at higher chloride concentrations. Generally, in these applications we recommend the use of a carbide coated rotor, which has a much denser porosity than chrome and has greater resistance to chlorides and other corrosive agents.  A secondary effect of the chrome pitting is that generally the stators also fail due to the chrome coming off the rotor and slicing into the stator elastomer.

  • How to Read Power Section Curves

    How to properly read power section curves is an important concept that often leads to confusion. Power sections are designed to operate over a range of flow rate and differential pressures, with established upper limits for each. The figure below shows a typical power section curve.

    The power section curve is made up of a series of RPM curves and a torque slope line. The red RPM curve represents the expected RPM drop for the maximum flow rate, as a function of differential pressure. The blue curve represents the RPM drop for a lower flow rate, as a function of differential pressure. If the power section is being operated at a flow rate that is between these two curves, first calculate the no-load RPM by multiplying the rev/gal from the spec sheet by the flow rate (gpm). Find this RPM along the Y-axis where the differential pressure is equal to zero. Then draw a new RPM drop curve starting at this point on the Y-axis that then runs parallel to the other two RPM curves and terminates at the dashed max differential line on the right.

    As the differential pressure is increased, slippage will begin to occur and result an RPM drop. To calculate the RPM at a given differential pressure, simply find the differential pressure on the x-axis and draw a line until you intersect the RPM curve. Then draw a line from the point of intersection to the Y-axis on the left side of the chart. Power section torque, is directly proportional to the differential pressure. This is true even when the power section is in a stall with zero RPM. To determine the torque first find the current differential pressure on the X-axis and draw a line vertically until the green torque slope curve is intersected. Now draw a horizontal line from the point of intersection until the right-handed Y-axis is intersected (as shown in above figure).

  • Temperature De-rating of Power Sections

    PV’s fit philosophy has always been about the right amount of compression at a given downhole temperature which moderates the stress on the rubber, and maximizes the horsepower output of the motor. As the fit becomes tighter, the compression between the rotor and stator increases as does the amount of heat buildup (hysteresis) in the stator. This increased heat buildup can result in a reduction in stator life. To compensate for this, PV has created temperature de-rating charts (see below).

    These charts provide a suggested safety factor for the application. First you must find the bottom hole temperature for the application. Then draw a vertical line upwards until the blue curve is intersected. Now trace a horizontal line to the Y-axis on the left and record the “dP Reduction Factor” percentage. Now multiply this percentage by the recommended max differential pressure on the spec sheet.

    It’s important to note that this is simply a guideline and whether or not the power section is de-rated depends on the objectives of the run. If the run is intended to be relatively short and high ROP is the key driver, then the power section may not need to be de-rated since power section longevity is not the key issue. If you have any questions about a specific application, please feel free to get in touch with a member of our Application Engineering Team for assistance.

  • What is Slip?

    Like most mechanical systems, power sections are not 100% efficient. Power sections convert hydraulic energy in the form of flow and differential pressure into mechanical energy in the form of torque and RPM. Some of this input energy is lost as a portion of the flow bypasses the seal lines of the cavities in the power section. This is known as slip.

    The degree of slip and corresponding reduction in power section efficiency is directly related to the differential pressure acting on the power section. As differential pressure is increased, the cavities in the power section are forced to hold more fluid pressure and they begin to leak (slip). Having a very tight fit can reduce slip but this also increases contact stress between the rotor and stator which may compromise power section longevity. Having a loose fit will typically result in increased slippage as the cavities are less capable of holding pressure.

    It should be noted that slippage only affects RPM but not torque. As can be seen form the power curves below, the RPM for a given flow rate will tend to decrease with increased differential pressure. However, even in a stall condition, torque is always directly related to differential pressure.

     

  • What Materials is an Elastomer Made From?

    An elastomer (rubber) is an engineered system comprised of

    • A base polymer which defines characteristics – examples are NBR and HNBR
    • A cure system which vulcanizes the rubber to cross-link the sulfur with the polymer chemical chains
    • Fillers and reinforcing agents of many varieties and structures, which help the vulcanization process, and help define the mechanical properties (ie. tensile, tear, modulus)
    • Plasticizers and softeners which aid in the blending of the polymer and the other additives, and in processing characteristics
    • other aids, agents, accelerators

    The chemical structure of an Acrylonitrile-Butadiene Rubber (NBR) is shown below.

    The chemical structure of a Hydrogenated Acrylonitrile-Butadiene Rubber (HNBR) is shown below.

  • Stator Shelf Life

    Elastomers age when exposed to heat, light, ozone, oxygen and radiation. Aging causes the elastomer to harden and crack and changes the mechanical properties. The recommended shelf life for a NEW stator stored with both ends covered is:

    It is recommended that stators stored outside are painted a light color and are located out of direct sunlight. It is also recommended that stators are relined after each run, particularly when used with invert or oil base mud and in high temperature applications.

    If used stators are to be re-run, local knowledge and experience should be used to determine shelf life. Running the power section and exposing it to increased temperatures and drilling fluids will accelerate the aging process of the elastomer. Additionally, whenever the stator is exposed to drilling fluids and wellbore gases, the rubber can absorb chemicals and/or potentially lose components of its composition due to extraction. These events may decrease the mechanical strength of the compound and/or effect the integrity of the rubber-metal bond.

    Used stators should be flushed out with clean water before being laid down. The rotors should not be stored inside the stator.