Downhole Motors PDF

Downhole motors INTRODUCTION

The idea of using a downhole motor to directly turn the bit is not a new one. Indeed, the first commercial motor used was turbine driven. The first patent for a turbodrill existed in 1873. The USSR focused efforts in developing downhole motors as far back as the 1920s and has continued to use motors extensively in their drilling activity. After 1945, the West focused efforts more on rotary drilling but the field of application for downhole motors increased spectacularly from about 1980 onwards.

The turbine consists of a multistage vane type rotor and stator section, a bearing section, a drive shaft and a bit rotating sub. Each stage consists of a rotor and stator of identical profile. The stators are stationary, locked to the turbine body, and deflect the flow of drilling fluid onto the rotors which are locked to the drive shaft. The rotors are forced to turn; the drive shaft is thus forced to turn, causing the bit sub and the bit to rotate.

A positive displacement motor is a hydraulically driven downhole motor that uses the Moineau principle to drive the drilling bit, independent of drill string rotation.

Figure1 : Differences between the turbine motor (left) and positive displacement motor (right) designs

Prepared By: Eng. Fayez Amin Shehata 1

TURBINES The turbine is made up of several sections:

• The drive stages or motor section. • The axial thrust bearing assembly and radial bearings. • The bit drive sub.

The drive stages, or motor section, consists of a series of stators and rotors of a bladed design. One stator and one rotor together form a stage. Turbines will be referred to as 90 stage, 250 stage, etc. The number of stages will determine the torque generated. Each stage, theoretically, applies an equal amount of torque to the control shaft and it is the sum of those torques which will be output to the bit.

Figure 2 : Turbine details

The drive sub is simply the bit connection and bearing shaft. The radial bearings protect the shaft from lateral loading. The thrust bearings support the downwards hydraulic thrust from drilling fluid being pumped through the tool and the upward thrust of weight being applied to the bit. Theoretically, weight on bit should be applied to equalize the hydraulic thrust and therefore unload the bearings and prolong their life.


DRIVE SECTION This consists of a series of bladed stators, fixed to the outer tool housing and bladed rotors fixed to the central rotating shaft. Drilling fluid flow is deflected at a pre-determined angle off the stator blades to hit the rotor blades and cause the shaft to rotate. The angle of the blades will affect the torque and speed output of the turbine. (Figure 2)

BEARING SECTION Usually, thrust bearings are made up of rubber discs (Figure 2) which are non-rotating, being fixed to the outer housing of the tool, and rotating steel discs attached to the central rotating shaft. Long bearing sections known as cartridges are used for long life in tangent or straight hole drilling sections. These are changeable on the rig. If the bearings wear past the maximum point, considerable damage can be inflicted as the steel rotors will crash into the stators below.

DIRECTIONAL TURBINE This is a short tool which has a set number of stages and its bearing section entirely within one housing. That is, it is not a sectional tool and will be typically less than 30 ft (9 m) long. It is designed for short runs to kick off or correct a directional well, using a bent sub as the deflection device. Steerable turbodrills do exist and will be discussed later.

CHARACTERISTICS

• Torque and RPM are inversely proportional (i.e., as RPM increases, torque decreases and vice versa).

• RPM is directly proportional to flow rate (at a constant torque). • Torque is a function of flow rate, drilling fluid density, blade angle

and the number of stages, and is affected by varying weight on bit. • Optimum power output takes place when thrust bearings are

balanced. • Changing the flow rate causes the characteristic curve to shift. • Off bottom, the turbine RPM will reach "runaway speed" and torque

is zero. • Optimum performance is at half the stall torque and at half the

runaway speed, the turbine then achieves maximum horsepower. • A stabilized turbine used in tangent sections will normally cause

the hole to "walk" to the left.


OBSERVATIONS • There is minimal surface indication of a turbine stalling. • Turbines do not readily allow the pumping of LCM. • Sand content of the drilling fluid should be kept to a minimum. • Due to minimal rubber components, the turbine is able to operate in

high temperature wells. • Pressure drop through the tool is typically high and can be

anything from 500 psi to over 2,000 psi. • Turbines do not require a by-pass valve. • Usually, the maximum allowable bearing wear is of the order of 4

mm

POSITIVE DISPLACEMENT MOTORS The PDM is made up of several sections:

• The by-pass valve or dump sub. • The motor section. • The universal joint or connecting rod section. • The bearing section with drive sub.

BY-PASS VALVE A by-pass valve allows fluid to fill the drill string while tripping in the hole and drain while tripping out. While drilling fluid is being pumped, the valve closes to cause fluid to move through the tool. Most valves are of a spring piston type which closes under pressure to seal off ports to the annulus. When there is no downward pressure, the spring forces the piston up so fluid can channel through the ports to the annulus. (Figure 3).

Figure 3 : Bypass Valve


MOTOR SECTION This is a reverse application of Rene Moineau's pump principle. The motor section consists of a rubber stator and steel rotor. The simple type is a helical rotor which is continuous and round. This is said to be a single lobe type. The stator is moulded inside the outer steel housing and is of an elastomer compound. The stator will always have one more lobe than the rotor. Hence motors will be described as 1/2, 3/4, 5/6 or 9/10 motors. Both the rotor and stator have a certain pitch length and the ratio of the pitch lengths is equal to the ratio of the number of lobes on the rotor to the number of lobes on the stator.

As drilling fluid is pumped through the motor, it fills the cavities between the dissimilar shapes of the rotor and stator. The rotor is forced to give way by turning or, in other words, is displaced; hence the name positive displacement motor. It is the rotation of the rotor shaft which is eventually transmitted to the bit.

Figure 4. Motor details



Geo-Pilot Rotary Steerable System Sperry, in collaboration with JNOC (Japan National Oil Corporation) and Security DBS, has designed a new approach to rotary steerable drilling. The Geo-Pilot® system uses point-the-bit technology, where bit direction is controlled by deflection of the bit drive shaft rather than relying on bit side forces to steer the well while rotating the drillstring. Because the tool does not try to push the bit sideways to deviate, as other systems do, the Geo-Pilot system can utilize patented Security DBS FullDrift extended-gauge bits. These bits have been proved to yield truly high-quality wells with lower friction factors than have ever before been measured.

Push-the-bit concepts, in contrast, deliberately use bits that have strong side-cutting capability in order to deviate the well. These bits tend to drill a hole slightly larger than the bit diameter and are the root cause of bit spiraling, whirl, and high vibration, all of which lead to premature bit and MWD/LWD tool failures, excessive time for hole cleaning and backreaming, and difficulty running casing and logging tools. The Geo-Pilot system also uses the Geo-Span two-way communication downlink system, the most advanced downlink system on the market. Furthermore, the Geo-Pilot system is built using the same Sperry engineering and manufacturing expertise that has created the industry's most reliable MWD/LWD systems.

Although the push-the-bit concept dominated early rotary steerable tool commercial designs, its limitations and detrimental effects on wellbore quality were soon noticed. As such, Sperry aggressively proceeded with designing and manufacturing the industry's most robust point-the-bit system to date. The 7600 Series (6-3/4-inch tool) is used for hole sizes 8-3/8 to 10-5/8 inches, and the 9600 Series (9-5/8-inch tool) is used for hole sizes 12-1/4 to 17-1/2 inches.

The Geo-Pilot system minimizes the vibration caused by aggressive sidecutting bits and can be used for extended-reach and designer well applications in which excessive torque and drag could inhibit drilling operations. It helps to eliminate hole spiraling, minimizes wellbore tortuosity, and improves directional control, drilling efficiency, and hole cleaning while reducing drill time to TD. It also allows more efficient casing running and wireline operations. Tests comparing MWD reliability of conventional drilling equipment with that of the Geo-Pilot system show clear evidence that MTBF (mean time between failure) is almost tripled with the Geo-Pilot system.

Innovative features of the Geo-Pilot® rotary steerable system include:


� Compact bias unit—A pair of eccentric rings, one nested inside the other, deviate a drive shaft mounted on bearings at each end and enclosed in a non-rotating outer sleeve. � Automatic self-correcting guidance system—Using near-bit inclination sensors, the self-correcting electronics automatically maintain directional tool face and inclination. � "On-the-fly" two-way control communications between the tool and surface software systems allow directional drillers to precisely control the wellbore trajectory while on bottom with zero rig time impact.

An anti-rotation device prevents slippage of the outer housing while the tool is in the deflected, steering mode, allowing for a stable toolface orientation. Shaft deflection is achieved by a pair of eccentric rings that can be rotated to any desired toolface setting and to varying degrees of offset from center. The tool's precision electronics work with downhole software to automatically avoid excessive deviations from desired toolface orientation.

There are two methods for steering the Geo-Pilot® system. In manual steering mode, the tool is pointed and controlled by the directional driller. In automated mode, the tool is self-guiding along a pre-programmed trajectory using the Geo-Span™ downlink system, which allows the tool to be controlled on the fly. With this downlink, changes to the azimuth and inclination can be made while on bottom. Quick acknowledgement of the commands is then sent back to the surface. Average time to send a command and receive confirmation is 90 seconds. Should this system ever fail, it is always possible to control the system by starting pumps and rotation in a prescribed pattern that is recognized by the tool.

Because the Geo-Pilot system has the ability to place the wellbore through multiple targets, it can greatly improve recovery from a single well. By using the Geo-Pilot system's unique ability to perform "flat-turn" open hole sidetracks, six laterals were drilled from two main bores for an operator in the North Sea. The open hole sidetracks in the second quad (four-branch) lateral were performed while keeping the wellbore within +/-1 ft of the planned TVD horizon.


Figure 5

Figure 6

For maximum operating life and reliability, the Geo-Pilot system's bearings, seals, and other internal moving components are all immersed in lubricating oil. And, because the tool operates independently of wellbore fluids, there are virtually no mud compatibility problems.

In addition to having its own internal electronic controls and sensors, the Geo-Pilot system interfaces with our LWD system. While you drill with Geo- Pilot technology, our LWD system pulses detailed steering information about at-bit inclination, toolface, percent, deflection, and system status information back to the surface, along with standard


formation evaluation data from our standard sensors located above the Geo-Pilot system.

A specially designed pressure compensator maintains a slight positive internal pressure across the rotary seals. It features an integral displacement transducer to provide an indication of low compensation volumes to the surface system. Other advanced sensors—all powered by a long-lasting internal battery— monitor bit position, string RPM, and internal tool parameters.

Plus, the Geo-Pilot system is integrated into our INSITE rig information system, making the tool even easier to control. The INSITE system features an enhanced driller's display unit that enables you to monitor the current status of the toolface and precisely what the Geo-Pilot® system is doing downhole. This capability maximizes your wellbore steering quality and provides the best steering control possible for your well.

The Geo-Pilot system thinks for itself: "smart" software monitors toolface setting and maintenance and continuously modifies adjustment procedures in the light of lessons learned. With operational costs on the rise and dollars at a premium, you need to make the most of your valuable assets and extend their reach.

Sperry Drill motors are primarily classified by their output speed and output power characteristics, which relate to the rotor/stator lobe configuration and power unit length. Power units are designated as standard or performance, the performance power units being longer and capable of producing greater output power than standard power units of similar lobe configuration. Some performance power units have lobe configurations that are not available in standard power unit form.

For specific applications, tandem motors can be configured, in which two power units are connected. Specialty motors are available for air/foam and short-/intermediate- and medium-radius drilling.

Low-Speed Motors Low-speed motors are characterized by rotor-to-stator lobe configurations of 5:6, 6:7, 7:8, 8:9, and 9:10. The use of adjustable bent housings with high torque output and relatively low bit speed makes the low-speed motors ideal for use in steerable drilling applications, including horizontal wells and wells in troublesome formations. The output characteristics of the low-speed motors can be utilized in various specialist applications, such as short- and intermediate-radius drilling.


Medium-Speed Motors Medium-speed motors are characterized by rotor-to-stator lobe configurations of 3:4 and 4:5. The operational characteristics of medium-speed motors permit fine-tuning of operational parameters and rates of penetration while maximizing bit life and on-bottom time, therefore reducing costs. Applications for medium-speed motors include steerable straight hole drilling, extended-reach drilling, and performance drilling.

High-Speed Motors High-speed motors are characterized by rotor-to-stator lobe configurations of 1:2 and 2:3. The high-speed motor operational characteristics make them suitable for use in correction and sidetrack applications where precise directional control permits efficient establishment of the req nd direction. uired well inclination a

Figure 6


Figure 7

Adjustable Gauge Motor The AGM is the first tool to incorporate an adjustable stabilizer directly

The AGM system has been used in the Gulf of Mexico, Middle East, and

It is available in three tool sizes—4 3/4, 6 3/4, and 8 inches—and allows inclination control between 1 and 3 degrees/100 feet build and drop in the

into a PDM. Using an adjustable stabilizer above a motor allows for small changes in inclination (+/- 1degree/30m) to help maintain a smooth well profile. But adjustable gauge stabilizers are not effective when larger inclination changes are desired or when extended length power sections push the stabilizer so far back that it becomes ineffective. Experiments using an adjustable gauge stabilizer below a PDM (between the motor and the bit) were successful in producing larger inclination changes, but motor reliability suffered as a result and the number of BHA connections was increased.

North Sea, where it has proved to be particularly effective in long horizontal sections. Using the AGM in extended-reach and horizontal drilling applications allows inclination to be fine tuned while continuously rotating. The result is a higher quality borehole with tighter TVD control, better hole cleaning, and higher rates of penetration.


extended/flush positions. The exact variation is dependent on the power section length, motor top stabilizer position, and the formation being drilled.

Adjustable Gauge Stabilizers s—the AGS

(adjustable gauge stabilizer) tool and the TRACS™ telemetry

or run independently in rotary assemblies. Both can be used with the full range of Sperry Drill®

mple, rugged, purely mechanical device that works off of pump pressure and has only two positions. The

ticated and is typically programmed with six different positions. If the two tools are used

of pump cycles. Two cycles tell the tool to get ready to receive data. The third time

Sperry offers two different adjustable gauge stabilizer

regulated angle control system. Adjustable gauge stabilizers allow changes in the build or drop tendencies of directional drilling assemblies in rotary mode without tripping to reposition stabilizers or change the gauge.

These tools can be combined

mud motors. Both offer the ability to adjust the BHA's rotary tendency from build to drop, thus giving control over inclination while rotating. Inclination tends to be more important than azimuth on most wells. They are, in effect, two-dimensional rotary steerable systems and improve efficiency by allowing you to spend more time rotating the pipe. This improves hole cleaning and weight transfer to the bit, resulting in faster drilling and straighter boreholes.

The AGS™ tool is a si

mandrel has several inclined surfaces or ramps machined into the outer surface. Each ramp has an associated piston that is locked into the ramp face via a T-slot. By toggling between two different positions, the inclination tendency of the BHA can be manipulated while in rotary mode. More time rotating means more efficient weight transfer, better hole cleaning, and higher quality wellbores with greater reach capacity—all with fewer trips for BHA changes. This makes Sperry's AGS™ tool perfectly suited for extended-reach and horizontal well applications.

The TRACS™ tool is more sophis

together, one tool can be used to make radical BHA behavior changes and the other can be used for fine-tuning. The BHA ration determines which tool is used for which purpose.

The TRACS™ tool is controlled from surface by a sequence

configu

the pumps come up, the duration of the pumps-up cycle determines the new blade position. The internal electronics control the movement of the downhole piston or mandrel and ramp. The stroke of the mandrel determines the distance the blade moves outward.


On the 8-1/2-inch tool, the maximum stroke is 1 1/4 inches. On the 12-1/4-inch tool, the maximum stroke is 1 3/4 inches. When using the TRACS™ tool, the MWD signal is simply set to delay transmission until after the

n provides a powerful and efficient drilling assembly that can achieve the drilling

window required for telemetry, which is typically 3 minutes.

Both the AGS™ and the TRACS™ tools can be used in conjunction with the SlickBore® matched drilling system. The combinatio

efficiencies of rotary steerables at a much lower cost. This makes the SlickBore Plus™ stabilizer system an alternative for areas that do not justify the cost of a rotary steerable system.

ABI™ Sensor The ABI™ sensor consisinstalled next to the bit

ts of a triaxial accelerometer package and is box of specially modified Sperry Drill® mud

motors. An acoustic telemetry link transmits the inclination data from the ABI™ sensor to the standard MWD/LWD tool located above the motor. The ABI™ data are then transmitted to the surface via mud pulse telemetry. Real-time inclination from the ABI™ sensor can be compared with the inclination from the standard MWD/LWD directional sensor above the motor to determine the angle-building or -holding tendencies of the BHA. This information is particularly useful when building angle in a medium-radius horizontal well or when maintaining angle in a tangent or lateral section. The ABI™ service can be a cost-effective alternative to more expensive and complex geosteering tools in many applications.

Figure 7


he ABI™ sensor can be used in the following: ne

whether the BHA is building or dropping by comparing the inclination at

erator to know whether e required build rates are being achieved at the bottom of the hole. The

e drilling out of the shoe, corrective teering can be performed by sliding high side instead of rotating out of

The ABI™ sensor enables inclination at the bit be determined within ±0.2 degrees, which is accomplished by taking

T• Horizontal sections—With the ABI™ sensor, it is possible to determi

the bit and inclination at the MWD tool when taking surveys. It aids in reducing wellbore tortuosity, which in turn reduces the effects of drag, enabling longer horizontal sections to be drilled. � Build sections—The ABI™ sensor allows the opthoccurrence of plug backs or unnecessary tripping is reduced because the BHA build performance can be examined immediately rather than waiting for measurements from the MWD tool, which in many cases is located more than 60 feet behind the bit. � Detecting casing sag— beforsthe shoe. Often, the final inclination at a casing point can be reduced by the washout that occurs while conditioning the well prior to running casing. This casing sag can be detected immediately by the ABI™ sensor once tripped to bottom. � High-accuracy mode—tomultiple rotational check shots so that systematic inclination errors associated with the influence of the motor bend are removed.


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Downhole Motors PDF