A new concept that uses piezoelectric material in progressing cavity motors promises a reliable downhole power source.

The continuing increase in the number of deepwater and ultradeep wells has created the requirement for reliable tools that can stay downhole for a much longer time than before. A variety of downhole tools such as measurement-while-drilling (MWD) tools and other instrumented bottomhole assemblies to steer the system in the proper direction are used in these wells. All of these tools have one thing in common - they require power, which is either stored in batteries or conveyed to the tool through umbilical cords.
A reliable supply of power to these tools is necessary so that they can work effectively and stay downhole for long hours along with other tools. Frequent battery power-pack replacement or failure during operations increases the operating cost even as they produce benefits only available from using the tools. For such a system to manage power and reduce or eliminate costs associated with their use, a tool is needed to generate power downhole. This tool or system should be able to produce power by reclaiming the energy lost downhole instead of using the voluble hydraulic energy conveyed downhole.
Such a tool is under development and shows promise in meeting the need for a reliable, downhole electrical power source. It is called the Downhole Energy Harvesting Positive Displacement Motor (DEHPDM).
Converting wasted energy
The positive displacement motor (PDM) is a common and widely used downhole tool. A method has been developed to convert otherwise wasted energy from this tool into a valuable form of useful power. This method is derived from the unique characteristics of the typical, Moineau-type (progressing cavity) motor design, a concept dating to the 1930s that is the basis of most downhole motors. The eccentric rotation of the shaft in this type of motor is a source of vibration. This undesirable, resonant vibration reduces the life of the motor. This method not only serves as a source of power generation but also provides an additional benefit by acting as a vibration damping mechanism, suppressing the resonant mechanical response and acting as a mechanical vibration energy scavenger.
The basic parts of a Moineau-type motor involve a stator (housing) and a shaft enclosed in a casing. The shaft has a wavy-shaped vertical cross section, and each wave corresponds to a lobe. The housing that is contained in a casing accommodates the wavy shaped rotor whose cross section is also wave shaped but the number of lobes is one more than in the shaft.Moineau's pump principle is applied in reverse to rotate the shaft by pumping fluid. This results in a positive displacement motor. There are different designs for positive displacement motors but the basic operating principle is common to all. Motors with the lobe patterns such as 1:2, 3:4, 5:6, 9:10 are now being used. The ratio is known as kinematic ratio, i, of the motor.
The method of energy reclamation converts the strain energy induced by the rotation of the shaft into useful electrical energy using the piezoelectric principle. The primary focus is based on the eccentric motion of the shaft inside the elastomeric housing. This eccentric motion of the shaft compresses the elastomeric housing. Piezoelectric material embedded in the elastomer uses the strain energy to convert mechanical energy to electrical energy.
Figure 1 shows various components of the system in the power section of the positive displacement motor. The figure also shows the piezoelectric material embedded in the elastomer of the housing section. Rotation of the shaft is converted into mechanical stress and further converted into electrical charge through the use of piezoelectric material embedded inside the housing. The pressure applied to the polarized crystals produces a mechanical deformation which, in turn, results in an electrical charge. Then, the electrical charge is rectified and regulated to provide a reliable power supply.
The piezoelectric material is a transition element between the mechanical and electrical domains. When the mud passes through the motor the shaft starts rotating. The rotary motion strains the elastomeric housing material. Because of the nature of the piezoelectric material, the crystals shift and realign developing an electrostatic potential between the opposing faces of the element.
Piezoelectric materials in place also damp the mechanical vibrations caused by the motor rotation. The vibrational mechanical energy is converted and dissipated into electrical energy through piezoelectric material. When the elastomer and the piezoelectric material bonded into it gets compressed due to the rotation of the shaft, electrical charge distribution occurs inside the piezoelectric material, which, in turn, causes flow of electric current. Passive shunt networks are considered to be the easiest and most cost-effective way of suppressing the vibrations.
Figure 2 shows the equivalent circuit between the mechanical (PDM) and the electrical domains (piezoelectric). The force and voltage are the generalized effort variables whereas the speed and current are the generalized flow variables. The transfer of energy from the piezoelectric material to the storage element is beyond the scope of this article.
Contact forces between piezoelectric material and shaft
With the geometrical description and kinematical understanding of the power section, the contact forces are needed to estimate the amount of force the piezoelectric material will be subjected to. The rubbing of rotor with the housing element results in the loss of useful power due to friction and leakage losses. The main causes are contact forces and frictional forces. When the shaft is not rotating, the contacts between the shaft and housing elements are along the seal lines. So, the housing surface is rubbed over by the shaft surface continuously. As the winding ratio of the motor increases the number of seal lines also increases. This further adds to the increase in the intensity of the rub.
The shaft and housing element cause compressive contact stresses when the shaft rolls.Contact stresses are functions of shaft, housing geometry, material properties of housing element, shaft, surface treatment and the forces acting. Dynamic loading is another factor that alters the stress at the contact points.
Power generation
Because of contact forces between the rotor and housing of the motor, the piezoelectric materials embedded undergo strain. The amount of piezoelectric material embedded in the motor will also be a function of the number of lobes in the power section of the motor. It provides a way to position more materials along the seal lines. Since the contact force is also based on the eccentricity of the rotor shaft it can be observed that the power output increase will be less for higher lobes of motors.
Will the piezoelectric materials convert stored electromechanical energy to extractable electrical energy and generate enough power to sustain the requirements of the tools downhole? It has been shown that the dynamic stress produces higher voltage than the quasi-static stress levels, which is the case when using downhole motors. The voltage produced depends on the applied force and the capacitance of the material.
Assume a barium-titanate piezoelectric pickup having dimension 0.2-in. by 0.2-in. by 0.05-in. When subjected to a force of 31 lb per unit length of the seal line, based on the force calculation on unit seal length of a two-lobe motor, the theoretical power production will be approximately 30 watts.
It regard to the cost of power generation, the tool requires additional connectors for using the power from the realized the voltage bias. The piezoelectric wafers will not add additional weight to the DEHPDM. Additional components can be placed in a sub for repeated use. The added advantage of the system is it can be used in both sliding and rotary modes of drilling.

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