Mud Motor: Design and Technological Advantages for Complex Drilling Conditions
A mud motor refers to the type of positive-displacement rotary hydraulic power units and is widely used to create wells of various depths and directions. The first developments were made as far back in the 60s of the last century, however, mud motors were put into active commercial operation much later, towards the end of the millennium. Initially, such motors were considered as an addition to turbodrills in particularly difficult areas of work, but gradually, with the development of technologies and materials, mud motors took leading positions — they were used by oil & gas and service companies around the world.
The new drilling era, which began with the start of development of fields that are characterized by complex mining and geological conditions and an increase in the share of controlled directional and horizontal drilling, brought the mud motor equipment to a new level of relevance. Today, in the world practice of drilling there are no alternatives to the mud motors (even at the level of design developments). Mud motors gained such unique positions due to a combination of design, technological and economic advantages. Power units of this type consist of several key elements: a power section, a transmission assembly, and a skew angle regulator.
Power Section
The working bodies of the motor are actually a rotor and a stator, and together they constitute a power section. It is rotated by a flow of fluid (drilling mud). The number of stages of the stator and rotor differs by one, which allows the mud to be flowed through the chambers and turn the rotor inside the stator. The rotor is made of high-strength alloy steel and generally has an additional protective coating, although it is not always applied — much depends on the economic component.
The power section of mud motor may have a standard or profiled stator design.
The latter is used to increase the mud motor power parameters when the length of working bodies is reduced. It is the power section that determines the main characteristics of the mud motor: power, torque, rotor speed index, and efficiency.
Transmission Section
The torque from the power assembly to the bit is transmitted by a transmission section. It consists of a body, a shaft and supports (axial and radial). In case of controlled directional or horizontal drilling, the transmission part is subjected to high axial loads. In this regard, the quality of body construction materials and the reliability of articulated joints and radial bearings are of great importance. The transmission section body and the power assembly rotor are made of high-strength alloy steel.
The transmission assembly in a standard version has an open design, when the working bodies are in contact with the drilling mud that lubricates and cools them. Mud motors with a closed transmission section when its elements are in a sealed oil bath under pressure (10–20 atm) and are not in contact with hydraulic liquids are used in particularly difficult drilling areas characterized by a large percentage of cuttings and/or the presence of impurities and solid particles in the drilling mud. However, this design is difficult-to-operate, and the forces resulting from drilling often damage seals very quickly.
Angle Regulator
The skew angle regulator is one of key mud motor assemblies used for controlled directional and horizontal drilling. It allows you to change a skew-axis angle of the structure relative to the BHA for a specified range of angles. The regulator design includes the top and bottom subs, the core and the gear coupling. This is the most complex process component of the mud motor, in respect to which constant scientific and engineering surveys aimed at achieving the simplicity of structure, the high accuracy of setting angular values, the minimization of mismatched element axes and curvature planes to prevent deviations from the specified angle of drilling, are carried out. The technical solution to the problem of maintaining a zero angle value and a given path when drilling straight areas in a horizontal directional well is of particular importance. In this case, it is necessary to provide the sufficient lifetime and reliability of the motor when it runs at maximum values of the skew angle.
Additional Equipment
Mud motors may include other necessary additional components. Their list depends on the drilling conditions at a particular area. Mud motors can be equipped with near-bit stabilizers and above-bit calibrators, slurry filters, check and overflow valves. Some manufactures also equip their mud motors with casing centralizers by non-replacing of drive sub body additionally. Overflow valves are intended for the BHA and provide the connection between its internal cavity and the annulus. During run-in-hole/put-out-of-hole operations, the overflow valve prevents the motor from idling and reduces the hydrodynamic load on the working area. Stabilizers and calibrators are used to calibrate the diameter of the wellbore. The development of the latest generation is variable-diameter calibrators, allowing to change the diameter of the wellbore walls without mud motor put-out-of-hole to the surface.
Telemetry
The complex profile wells cannot be drilled without the use of modern telemetry systems. They include a downhole instrument package as close as possible to the bottom of a well, a power source, a system for retrieving, transmitting and receiving information from the well bottom to the surface, a computerized data processing system for solving problems to monitor and control the well drilling process. The WS software and hardware suites are usually located directly at the facility and have a stationary design. However, today, oilfield service companies go further and create their own drilling design and support centers, which carries out an additional remote monitoring over wellbore passage. For example, a center somewhere in Samara can remotely monitor 8–15 facilities located in different regions of Russia at the same time. Process drilling parameters are collected and transmitted using a survey and wellfield data remote monitoring system. Data from the measure-while-drilling and from the sensors located at the drilling site sends to the center in real time and is visualized on the video wall. Based on the information received, the well path is promptly corrected, and process issues are resolved. This allows you to monitor all the parameters of the equipment and the drilling process in the aggregate and make accurate predictions regarding the work deadlines and plans. Accumulated knowledge is also used to design new wells. Moreover, this approach allows preventing major accidents and minimizing the number of minor emergencies at the facilities.
Bits
There are various mud motor modifications by overall dimensions and energy characteristics: rotation speed, torque, and power. The standard mud motor size for a certain drilling interval is selected on the basis of a combination of geological, technical and process factors, considering drill through tools to be used. It is of paramount importance that an effective joint operation of the bit and the mud motor is ensured. Polycrystalline Diamond Cutters together with high-torque mud motors are currently used to drill most of controlled directional and horizontal wells, including in difficult mining and geological conditions. The rational choice of a bit/mud motor pair allows to increase the mechanical penetration rate, reduce the number of trips and improve the technical and economic indicators of drilling The manufacture of PDC cutters is characterized by high manufacturability and quality of the materials used: metal, diamond cutters, carbide tips and inserts, hard-facing and soldering materials, since these components directly affect the performance and durability of this equipment. As general, the PDC cutters are made in small batches considering the specifics of a particular facility and customer requirements. In Russia, the most modern plants that manufacture PDC cutters, are located in Samara, Ufa, and Kurgan.
Mud Motors vs. Turbodrills
Mud motors have a number design advantages over classic turbodrills and allow carrying out operations to install controlled directional and horizontal wells with maximum efficiency indicators. This also applies to carry out operations on particularly difficult areas, for example, when drilling additional wellbores through the hole in a production casing string. The comparable models of mud motors have a smaller diameter in comparison with the turbodrills, realizing the high power characteristics of motor torque at the same time. In addition, it is also worth noting the simplicity of design of the mud motor power section, the relatively low metal consumption and low wear parameters due to the low rotational speeds.
Mud Motor Modifications
The greatest demand comes from motors with a flow-through transmission section equipped with a skew angle regulator and a standard power section. In terms of the special orders, these are modifications with a profiled power section, a hollow rotor, and a oil-protected drive sub. Mud motors with a flow-through drive sub have universal application and are selected for the conditions of a certain well according to the parameters of diameter, rotational speed, power, and lobes. Mud motors with a profiled power section are more often used for drilling of wells with a complex profile, since they have a lower length, a high power, and an optimal indicator of curvature. However, such models are more sensitive to cuttings than the mud motor in a standard version and require installation of additional cleaning systems. The hollow-rotor motor modifications are used for drilling with increased mud flow rate, which is due to the high requirements for bottom-hole treatment. A mud portion is flowed through the rotor in them, allowing the working area to be flushed.
Power Section Resistance
The mud motors are effectively operated in contact with drilling muds of various densities, but the various components of muds can adversely affect the power section elements. The stator elastomer insert is the most vulnerable motor component in terms of increased wear when using oil-base muds (OBM). It is made of nitrile butadiene rubber (NBR) or hard rubber (HR). The elastomer swells when interacted with an OBM, and its physical and mechanical properties change. As a result, the preload increases in a rotor-stator engagement, and the elastomer is destroyed due to increased load.
The chromium coating of the rotor is damaged to the greatest extent, when muds with a high content of highly active components (salts, chlorides, acids) are used. Due to the formation of underlayer corrosion, chromium is peeled, and cavities are formed on the rotor surface, which in turn destroys the stator elastomer.
Today, the rotor surface coatings and impregnations are developed, which allow minimizing the detrimental effect of the highly active components of drilling muds. As far as the elastomer is concerned, manufacturers do not have a rubber mixture of the desired level of resistance, there is a need for new research and development.
With regard to the use of mud motors in general, it is recommended to use new developments of drilling muds, which not only have better performance, but also prevent damage to mud motors. In particular, water-based micro-emulsion muds can reduce the frictional force that occurs when the drilling and casing strings come in contact with the wellbore wall, which significantly reduces the risks of differential stickings and promotes the reduction in wear of friction surfaces of mud motor elements.
Market Requirements
The customers of mud motor equipment impose two main requirements on it: ensuring high drilling rates and a low accident rate. The power sections of mud motor that meet the parameters of speed and stability of work are selected as part of the planning phase of work in a particular area. However, there are objective limitations: when a high-speed power section is used, the mud motor power inevitably decreases, and a lower speed and more powerful equipment shall be applied with increasing a bottom-hole depth. In such conditions, the rotation of the entire BHA is additionally created by a rotor to increase the rate of penetration. In this case, there are difficulties associated with limiting the speed of operations at large skew angles. If the design of mud motor drive sub does not have an adequate margin of safety, the probability of equipment failure in the well increases significantly.
Engineering Solutions
To eliminate these limitations, several engineering solutions have been developed. In particular, the reduction of the lower mud motor arm to the point of curvature, the implementation of unloading elements into the most loaded areas of motor, the use of flexible subs in the BHA that reduce the probability of occurrence of accidents that lead to emergencies in the well. Let’s consider an example of the implementation of these solutions using a PetroEngineering ISC DMR178 model of the mud motor.
Changing in the design of transmission section made it possible to minimize a distance between the skew point of drive sub axis and the bit to 1796 mm in a standard version and to 1662 mm in a special version. Due to the modernization of an adjustable skew angle of drive sub axis, the tolerances for possible rotor speed are significantly increased, depending on the set angle. The clearance grooves that reduce a load on this part at high rotor speeds are provided in the top sub of the diverter. Improvements in the mud motor design allowed not only to expand the rotation range of motor as part of the BHA, but also improved the wellbore passage indicators. As a result, a more reliable modification, which allows the BHA to be rotated at a significantly higher speed, was obtained. If the rotation of mud motor power section at a set skew angle value of 1°29’ was limited to 20 rpm (with the power section length of more than 5 m), now it is possible to rotate it with a speed of 50–80 rpm at an angle of 1°50’. This gives a whole range of advantages in ensuring the speed and quality of drilling operations.
Mud Motor Manufacturing
With regard to mud motor manufacturers, actively working in the industry, they can be divided into two large groups. The first group includes companies that invest substantial funds in the R&D and offer new relevant developments to the market. The second group consists of companies that copy technologies demanded by customers and save on costs. This approach allows them to offer lower prices and win major tenders. As a result, unfair conditions of competition are created when the leading players who invest in R & D and feed the industry with new technologies turn out to be less successful commercially than their brothers in trade copying their achievements. Not only and not so much specialized government support programs, as the creation of a responsible customer approach to the formation of bidding procedures, mud motor requirements, as well as ensuring the transparency and openness of data on the conditions of actual equipment operation can contribute to a radical recovery of the situation. It is in such conditions that the industry will receive new incentives for development and will increase in quantitative indicators through the successful activities of professional and bona fide players.
Alexander Gerasimenko,
General Director, ISC PetroEngineering