Drilling Fluid Functions

The following is the function of Drilling Fluid: 

1. Cool the drill bit and lubricates its teeth: one of the prime functions of the drilling fluid or mud is to cool the drill bit and lubricate its teeth. The drilling action requires a considerable amount of mechanical energy in the form of weight on bit, rotation, and hydraulic energy. A large proportion of this energy is dissipated as heat, which must be remove to allow the drill bit to function properly, the drilling mud also helps the removing of the rock cuttings from the space between the bit teeth, thereby preventing bit balling which is one of the common problems in drilling process. 
2. Lubricates and cool the drillstring: a rotary drillstring generates a considerable amount of heat which must be dissipated outside the hole. The drilling mud helps to cool the drillstring by absorbing the heat and releasing it, by convection and radiation, to air surrounding the surface mud tanks (pits). The mud also, provides lubrication by reducing friction between drillstring and borehole walls. Lubrication is usually achieved by the addition of bentonite, oil, graphite, etc.
3. Control formation pressure: for safe drilling, high formation pressure must be contained within the hole to prevent damage to equipment and injury to personnel. The drilling mud achieves this by providing a hydrostatic pressure just greater than the formation pressure. For effective drilling, the difference between the hydrostatic pressure and formation pressure should be zero. the hydrostatic pressure depends on the mud weight which, in turn, depends on the type of solids added to the fluid making up the mud and the density of the continuous phase. In practice, an overbalance,(Where the pressure in the wellbore in higher than the pressure in the formation), 100 to 200 psi (trip margin) is normally used to provide an adequate safe guard against well kick. The pressure overbalance sometimes referred to as chip hold down pressure (CHDP), and its value directly influences penetration rate. In general, penetration rate decreases as (CHDP) increases. When an abnormally pressured formation is encountered, the (CHDP) becomes negative and sudden increase in penetration rate is observed. This is normally taken as an indication of a well kick.
4. Carry cuttings out of the hole: for effective drilling, cuttings generated by the bit must be removed immediately. The drilling mud carries these cuttings up the hole and to the surface, to be separated from the mud. The removal of cuttings depends on the viscous properties called "Yield Point" which influences the carrying capacity of the flowing mud and "gels" which help to keep the cuttings in suspension when the mud is static to prevent them from accumulating on the bottom of the hole and causing pipe sticking. The flow rate of mud is also critical in cleaning the hole.
5. Stabilize the wellbore and prevent it from caving in: the formation of a good mud cake helps to stabilize the walls of plaster to interior walls (like plastering a room walls to keep them from flaking). The pressure differential between hydrostatic pressure of mud and that of the wellbore stable. Shale stability is largely dependent on the type of mud used To minimise the swelling stresses caused by the reaction of the mud with the shale formations. This reaction can cause hole erosion or cavings resulting in an unstable wellbore. Minimisation of wellbore instability is provided by the "inhibition" character of the drilling mud.. At last it should be noted that the best way to keep a hole stable is to reduce time during which the hole is kept open.
6. Helps in the evaluation and interpretation of well logs: wire line logs are run in mud-fills holes in order to ascertain the existence and size of hydrocarbons zones. Open hole logs are also run to determine porosity, boundaries between formations, location of geopressured (or abnormally pressured) formations and the site for the next well. Hence, the drilling mud must possess such properties that it will aid the production of good logs (Log response may be enhanced through selection of specific fluids and conversely, use of a given fluid may eliminate a log from use. Drilling fluids must be evaluated to assure compatibility with the logging program).
7. Limiting the corrosion of drilling equipment: the drilling mud in most cases will have water that contains dissolved salts as its base liquid. This serves as a medium in which corrosion takes place. If corrosion is suspected, then the cause should be determined and steps taken to prevent damage of the equipment. It has been found that in muds containing oil as the continuous phase, little or no corrosion occurs.
8. Transmit Hydraulic Horsepower to Bit: Hydraulic horsepower generated at the bit is the result of flow volume and pressure drop through the bit nozzles. This energy is converted into mechanical energy which removes cuttings from the bottom of the hole and improves the rate of penetration.

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CLASSES OF LOST CIRCULATION (Mud Losses)

Lost circulation or mud losses when oil gas drilling can be grouped into four classes:

1. Seepage losses: From 1-10 bbl/hr and lost while circulating at the normal drilling circulateng rate

2. Partial losses: From 10-50 bbl/hr and lost while circulating at the normal drilling circulating rate

3. Severe losses: Greater than 50 bbl/hr and lost while circulating at the normal drilling circulating rate. In some cases, no losses may be seen if pumping stops indicating that the ECD is the cause of lost circulation.

4. Total losses: When the mud level in the annulus can not be seen or the hole can not be filled

through the annulus. Total losses usually occur in cavernous formations.

Read More → CLASSES OF LOST CIRCULATION (Mud Losses)

What worker doing during Drilling Operation?

During drilling, the personnel and equipment must be protected against unexpected pressure surges in the wellbore. In oil and gas drilling, these surges can come from hydrocarbon fluids trapped under impermeable rock which holds them at pressures higher than the static head of the fluid column in the wellbore, and in geothermal operations the surges come from hot formations which heat the pore or wellbore fluids above the saturation temperature at the static wellbore pressure. In either case, the first line of control is the weight of the fluid column in the wellbore. 

With a gas column, this weight is negligible, but with mud the liquid density will range from slightly greater than water (-8.5 pounds per gallon) to almost three times that. In addition to the clays and additives which raise the viscosity of the mud to improve hole cleaning, weighting materials such as barite are often added to increase the mud's density and enable it to control higher downhole pressures.

The pressure surge cannot immediately be controlled with fluid weight, the wellbore can be mechanically sealed at the surface with BOPS, or blow-out preventers. There are three principal types of BOP: blind rams, which are sliding plates that come together across the wellbore when the drill string is not in the hole; pipe rams, which are like blind rams except that the sliding plates are cut out in the center so the rams can seal around the drill pipe; and an annular preventer, which is an inflatable bladder that seals around drill collars, stabilizers, or other off-size or irregularly shaped tools.

Read More → What worker doing during Drilling Operation?

DRILLMEC PRESENTS HOD, NEW SYSTEM FOR CONTINUOUS FLUID CIRCULATION

Continuous flow of drilling fluids offers many advantages, including well-bottom well pressure control combined with improved blade cleanliness and stability. In the current scenario of the oil market, these features become key requirements for personnel safety, operational efficiency and cost reduction, particularly in drilling environments with very narrow margins with ever-increasing water depths , and well situations characterized by high pressure and high temperature.

HoD (Heart of Drilling) technology is an advanced system for continuous circulation, developed and patented by Drillmec, where an automated control system provides for switching of sludge circulation between the drive head (top drive ) and a lateral opening integrated into each valve (sub) mounted at the top of the drilling lengths before starting the drilling step. A completely self- locking, remote-controlled key locks the opening and closing of the integrated side door with an operation that does not involve any manual action.

When a new length is added to the drill string, a sub mounted on the perforated length, and in the well, is positioned at the rotary table and the automatic key, which links a lateral flow line, is engagement with the sub. Once the key lock hydraulic clamps are securely connected to the sub, the probe staff can move away from the most vulnerable area, delimited by a red perimeter, and handle the rest of the operating sequence by a control panel remote. Acting on the key controls from the control panel, the operator opens the outer cover of the sub side opening, which remains inside the key throughout the operating sequence. When the control system confirms the opening state of the outer cap, the drilling mud stream can be directed by the top drive to the side opening in the sub before unscrewing the top drive from the drill string; after adding a new drilling length, the mudflow can be redirected to the top drive. The flow rate of drilling sludge to the shaft remains constant throughout the entire connection sequence, thus maintaining a dynamic shaft condition characterized by a constant bottom well pressure and continuous drilling of the BHA (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. the drilling mud stream can be directed by the top drive to the side opening in the sub before unscrewing the top drive from the drill string; after adding a new drilling length, the mudflow can be redirected to the top drive. The flow rate of drilling sludge to the shaft remains constant throughout the entire connection sequence, thus maintaining a dynamic shaft condition characterized by a constant bottom well pressure and continuous drilling of the BHA (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. the drilling mud stream can be directed by the top drive to the side opening in the sub before unscrewing the top drive from the drill string; after adding a new drilling length, the mudflow can be redirected to the top drive. The flow rate of drilling sludge to the shaft remains constant throughout the entire connection sequence, thus maintaining a dynamic shaft condition characterized by a constant bottom well pressure and continuous drilling of the BHA (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. after adding a new drilling length, the mudflow can be redirected to the top drive. The flow rate of drilling sludge to the shaft remains constant throughout the entire connection sequence, thus maintaining a dynamic shaft condition characterized by a constant bottom well pressure and continuous drilling of the BHA (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. after adding a new drilling length, the mudflow can be redirected to the top drive. The flow rate of drilling sludge to the shaft remains constant throughout the entire connection sequence, thus maintaining a dynamic shaft condition characterized by a constant bottom well pressure and continuous drilling of the BHA (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. thus maintaining a dynamic shaft condition characterized by a constant bottom drain pressure and continuous BHA drilling (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases. thus maintaining a dynamic shaft condition characterized by a constant bottom drain pressure and continuous BHA drilling (Bottom Hole Assembly). The continuous circulation system HoD can be used both during the drilling phases and in the maneuvering phases.

Some important considerations during the design process have given rise to high safety standards for staff and equipment, ease of integration into the drilling rigs in operation, and the ability to minimize downtime by integrating a maintenance management system in control systems.

All components of the HoD system are designed according to applicable APIs for a working pressure of 7500 psi and a maximum flow rate of 1000 gpm during connection. The side side opening design guarantees a double safety barrier between the pressure inside the drill and the outside during connection to the probe and in the well. Both barriers are independent and have been tested at one and a half times the exercise pressure.

The system is designed to be integrated into ground and sea systems, with the manifold running the sludge flow, the hydraulic unit and the control system integrated in the same frame with a small footprint. This feature provides complete flexibility during installation, safe and fast assembly operations without the need for expensive modifications to the sludge circuit of the drilling rig. The typical installation layout isolates the manifold from the pumping system during the drilling phases. Consequently, the load losses added to the mud circuit are minimized and the duration of the valves in the sludge manifold can be drastically increased. In addition, with the non-pressurized sludge manifold during drilling,

Operations during connection are completely controlled through a secure area on the probe plane or directly from the perforator cabin. Human intervention is only required to engage and remove the automatic key, but the key itself and the associated hinge are not pressurized during such operations. For newly conceived Drillmec systems, where the HoD Continuous Circulating System can be integrated directly into the mud system, a fully automated keypad handling system has also been developed.

Management software provides complete remote control of operating sequences, as well as providing real-time status of each component of the system on the remote control panel. The connection sequence can be performed with a fully automated or semi-automatic routine. In both cases, the control system acquires and processes signals from integrated sensors into the main components of the system, reducing human errors with text messages and alarms. The software also includes a Computerized Maintenance Management System (CMMS) that helps maintain a historical database of operating parameters for each component of the system, plan and monitor maintenance activities, and manage transaction reports.

After successfully completing rigorous hydrostatic and functional testing programs, the HoD Continuous Circuit System has recently completed field application in a deep pit for the confinement of a ground field in Europe. In particular, the HoD® Continuous Circulating System has been used to perform 12-inch and 1/4 phase drilling with the objective of maintaining constant ECD ("equivalent circulation density") density during connections, improve drill and hole battery cleaning and stability during drilling and drum extraction from the well. For this application, "ad hoc" designed and built for acid environments containing H2S,

The entire HoD package showed excellent results in terms of functionality and reliability of components in extreme working conditions and characterized by high specific weight sludges and high hydraulic parameters. Continuous circular connections were carried out in complete safety with a maximum pressure of the plant probe manifold of 4,200 psi and a maximum flow rate of 750 gpm.

Read More → DRILLMEC PRESENTS HOD, NEW SYSTEM FOR CONTINUOUS FLUID CIRCULATION

Drill String

A drilling battery is a tool that is used to drill deep hole holes in the ground in order to locate and extract oil or other resources. The construction of this device allows for rapid drilling and at the same time to extract large amounts of rock and mineral from a digging site. Mud is also injected down through the drilling battery to cool the tip while it is moving and to soften the surface that is boring through, reducing the likelihood of an improper cutting and increasing the overall bit time. A medium drilling string extends 15,000 feet (4,572 m) into the ground once mounted on the ground and up to 30,000 feet (9,144 m) or more when built offshore,

Within the drill battery assembly, there are four main components: lower hole mounting (BHA), transition tube, drilling rods and drill bit subs. The BHA is the stabilizing system that consists of the same tip and massive heavy rods that apply enormous amount of force down to facilitate drilling. A passage pipe connects the heavy rods to the actual drill pipe, and together these two components provide the necessary stability to ensure that the tip remains solid at such drastic depths. Drilling rods are also the majority of the length inside a drilling column, so they must be constructed using specific chemical compositions and forged at extreme temperatures.

Most components within a drill column are constructed at 31 or 46 feet (9.4 or 14 m) intervals, and two to four of them are combined to do what is termed a stand. Each substrate is then lowered into the ground before drilling initiated, in order to ensure that the drill always stays within perfect alignment. Similarly, they are removed from the ground before the drill is extracted.

Sometimes, the stands can get stuck and become difficult to remove, and specialized recovery tools called drill string jars and resonant vibrators are used to remedy this otherwise difficult situation. These methods are normally implemented by experienced oil companies. Technological advances discovered during the mid-20th century have made drilling strings much easier to handle.

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Mud Gun

A mud gun is a device used to shake the mud on a drilling rig. More than a nozzle of a gun, a mud gun is connected to a pump to circulate the drilling mud and prevent solids from settling into the mud tank. The mud mixture is sucked into a tube by a powerful pump and ejected into the sludge tank to be pushed through the mud gun nozzle. The process is similar to how a jet of water works on a power washer.

Perforating mud is a damp substance that resembles a thick clay and is used to cool the drilling head and to carry the surface drilling remains in drilling well and oil or gas drilling operations. The mud is recycled by passing through a series of filters or screens as it is circulated in the hole and back into the mud tanks on the drilling. To keep the mud as fluid as possible, a mud gun is used to keep the tank agitated and mixed. Mud is made by mixing water or oil with a clay substance that also contains many other chemicals in a large reservoir called the mud tank on a drilling rig.

The type of mud used during drilling depends on the type of drilling that is performed. The mud gun is the same for any kind of mud used. Often made by adding a nozzle type on a piece of pipe, mud gun works like positioning an inch above the end of a water pipe. This is pressurized mud in the tank, making it a mixing action. Exhausting the mixture also helps in removing debris from the mixture as it is filtered through the various screens in the tank.

The viscosity of the mud is very critical, with a thin blend being able to keep the cuttings from the suspended drilling head until it reaches the surface and can be shielded from the blend. A blend that is too thick can waste profits and can also slow down drilling productivity. Another function of the mud gun is to keep the suspension of the puncture cuttings so that they can be removed at the first pass through the screens. Talees that are not removed at first pass through the screens can break into smaller pieces that are more difficult to remove from the mix.

Read More → Mud Gun

Mud Tank

A mud tank is a large container used to contain drilling fluid reserves, also known as drilling mud, for a drilling rig. Drilling fluid is used to reduce the friction on the drilling components to allow them to work faster and faster with less risk of breaking. Many companies produce and restructure mud tanks of various shapes and sizes for industrial use, and entrepreneurs specializing in cleaning tanks and other drilling equipment are also available. The cost for tanks and associated services varies greatly, especially when personal designs are involved.

Historically, wells in the soil near a well have been used to contain sludge, and mud tanks are sometimes referred to as mud wells in a reference to this. A modern mud tank is a large container, usually open over and divided into different compartments. In some situations, a plan can be used to reduce the risk of worker accidents, with a parapet and a gangway, allowing people to look into the tank to control the level and consistency of the drilling fluid.

New fluid can be periodically added, and components can be mixed in to modify the formulation if it is deemed necessary. Perforation mud acts as a lubricant and coolant and the demands placed on it are very high. It is essential to maintain a constant flow in a puncture site to prevent stoppages. If a plant runs out of fluid, closing it temporarily can be extremely expensive.

Several drilling fluid blends are used, depending on the type of drilling, the geology, and the equipment in use. The fluid tank to pump mud on and through the drill. Mud baths can be set to receive recycled drilling fluid, a common practice in many sites. In these situations, the fluid is pumped from the puncture site, passed through a series of tanks to separate the fluid from rocks and other debris, and then routed back into a mud tank.

These tanks can eventually become in-crusted with drilling mud and can be corroded by fluid components. For this reason, waste companies and periodically clean their tanks with high pressure cleaners and other equipment. A large company run their own mud tank cleaning and maintenance, while smaller companies can call a company to clean their mud tanks and prepare them for continuous service in the field.

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Mud Weight in Drilling Operation

Mud weight is a term used to express the amount of drilling fluids used in sinking wells, especially in the exploration and extraction of crude oil industry. The weight of the mud of a drilling fluid is generally expressed in pounds per gallon (ppg), even if more than one unit of measurement is used, including kilograms per cubic meter (kg / m 3). The drilling fluids are used to cool the tips, to remove drilling debris from the shaft, and to avoid collision wrap. A mud scale, consisting of a level sliding scale, is generally used to determine the weight of the mud.

Considering the rugged environment that generally surrounds pit drilling, the process is even more complex and delicate. Tips work at great depths and are subject to extreme conditions, like the other components involved in the process. One of the elements used to reduce the voltage on these components is the drilling fluid within which the tip operates. These sludges, as are commonly known, cool the tip, and help in the removal of drilling debris. They also suspend cuts during breaks in the drilling process and hydrostatic pressure control inside the well.

Several means are used as drilling sludges, including water, oil, and gas-based fluids. The type of drilling mud used in a particular drilling site is carefully formulated to meet specific environmental conditions with different wells, rarely using the same mixture of mud. One of the most important variables in formulating drilling fluids is mud or fluid density. Wrong mud weight values ​​can cause several serious problems, such as circulation leakage. The density of these fluids is controlled by the addition of barites or, less frequently, halite and calcium carbonate.

A specially designed sliding scale known as a mud equilibrium is used to calculate the weight of mud drilling fluids. This instrument consists of a cursor equilibrium beam equipped with a type bubble leveling system. A sealed container is attached to one end of the beam where the sample of the drilling fluid is placed. The slider is moved along the bar to determine the density of the fluid.

Mud weight values ​​are generally expressed in pounds per gallon or ppg. Other units are used if, including kg per cubic meter (kg / m 3) and grams per cubic centimeter (g / cm 3). The weighing and test procedures used to measure sludge weight values ​​are set out in a set of globally recognized standards published by the American Petroleum Institute.

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CONVENTIONAL OIL

Definition Oil is a hydrocarbon formed over thousands of years from the decomposition of dead plants and organisms. Intense heat and pressure on this material triggers a reaction, which leads to the creation of oil

Conventional oil is a term used to describe oil that can be produced (extracted from the ground) using traditional drilling methods.  It is liquid at atmospheric temperature and pressure conditions, and therefore flows without additional stimulation.  This is opposed to unconventional oil, which requires advanced production methods due to its geologic formations and/or is heavy and does not flow on its own. 

You may have heard of these terms used to distinguish different types of oil:

​Light vs. Heavy - this refers to the density of oil and its ability to flow.  Lighter oil can be refined with minimal processing due to higher fractions of light hydrocarbons.

Sweet vs. Sour - this refers to the sulphur content of the oil, sulphur must be removed prior to refining.  When oil has sulphur greater than 0.5% it is referred to as "sour."

Because of these variations, oil quality is a spectrum and the distinction between conventional and unconventional is not always black and white. Generally, however, if traditional drilling techniques are used in the oil production it is considered conventional regardless of its physical properties.

Conventional oil is produced using drilling technologies that utilize the natural pressure of an underground reservoir.  Production of a conventional oil well has four main phases[2]:

Exploration: Geological exploration is a series of technologies that are used by geologists and geophysicists to predict the location and extent of underground oil reservoirs.

Drilling: Once a reservoir has been located with sufficient certainty, a drilling rig is used to bore a hole from the surface to the oil reservoir.  Piping is then inserted, allowing the oil to be brought to the surface.  Some of the oil in the reservoir will be produced using the natural pressure of the reservoir.  

Pumping: Gradually the pressure of the well will decrease as oil is produced. At this point a pump will be connected to allow the remaining oil to be extracted.

Abandoning: After all the economically viable oil has been extracted from the well, the well is filled with cement to prevent any hydrocarbons from escaping and a special cap is placed over it to protect the area[3].

Context

Conventional oil tends to be less expensive and complex to extract than unconventional oil due to the routine nature of the production techniques.  This oil is also the most valuable in global markets because it requires the smallest amount of processing prior to refining to create value-added products. Consequently, many of our global conventional oil supplies have already been extracted, limiting the availability of these source for future extraction[2].

Generally, drilling and well abandonment are well-understood and regulated processes but there are always risks with such industrial operations. In drilling, pressure must be regulated carefully to avoid accidents and immediate environmental impacts like land disturbance must be carefully monitored.  After abandonment, well leaks can occur if improper procedures were taken.  

As with all fossil fuel production, there are also concerns with greenhouse gas emissions from their combustion 

Read More → CONVENTIONAL OIL

Steerable Downhole Mud Motor - Directional Drilling

Steerable Downhole Mud Motor (SDMM) commonly referred to as Mud Motor or Drilling Motor acts much as a positive displacement motor which provides additional rpm to the drill bit from the flow of drilling fluid (mud).

This drilling motor is far different from an electrical motor in it's working principle and operation.

(A lot of people get confused initially)

Since its introduction, the positive displacement motor has undergone revolutionary changes and improvements. Downhole drilling motors have proven to be successful in the most rigorous of drilling environments. From the time of its inceptions, the mud motors have gone extensive improvements that has enhanced its performance, operational and economical reliability. 

Today there are numerous players in the industry providing mud motors for different operational requirements. Few to name are National Oil Varco (NOV), Schlumberger, Halliburton, Baker Hughes, Weatherford, Cavo, Bico, Jaguar, APS, etc. Different mud motors provided by different companies vary a little from each other but, there basic operating principle remains the same. 

Mud Motors have extensively wide range of applications and few of them are listed below:

Conventional Directional Drilling

Side-Tracking

Performance Drilling

Short/Medium/Ultra-short Radius Wells

Air/Foam or Under-balanced Drilling 

ERD Wells

HP/HT Wells

Coiled Tubing Drilling

Vertical Drilling

Casing Drilling

Milling

Coring

Slim Hole Drilling

Working Principle

Mud motors converts the flow energy of drilling fluid (mud) in rotational motion that's utilized in rotating drill bits at a much higher rpm. 

It's imperative that flow rate can be used to control the rpm of the drill bit as per operational requirements. Flow rates for muds are provided by the mud pumps.

Bit RPM = {Flow rate (in GPM) x RPG (Revolutions Per Gallon)} + Rotary RPM 

Note: 

RPG is defined as the revolutions made by bit box and in turn bit, when one gallon of mud flows through it & is mentioned by the manufacturer for each type of SDMM.

While sliding rotary rpm will be zero.

Parts of SDMM:

Simple classification of SDMM parts can be categorized as: 

Top Sub Options

Power Section

Drive Shaft Assembly

Adjustable Bent Housing Assembly

Bearing Assembly

Bit Box

Top Sub options

Top Sub: 

Top sub is simply a cross over housing at the top end of the motor. The lower connection uses a thread that connects to the upper box of the stator housing.

Dump Sub:

It contains a Dump Valve Assembly. This allows the mud to fill or drain from the drill string while tripping.

To avoid the ingress of solids from the annulus when the pumps are off, it’s normal to run a float sub as close to the motor as possible.

The motor will function perfectly without a dump valve - It can be laid down and replaced by a sub having the same connections or run with the ports blanked-off. 

Failure of the dump valve assembly can cause sometimes serious troubles.

Motor Catch & Rotor Catch Top Subs:

The rotor catch system is designed to retrieve the motor in case of a housing fracture. It will retrieve the motor from the upper stator box connection down to the drill bit. The motor catch system has the additional feature of an integral catch flange within the top sub. It will retrieve the motor from the top sub down to the drill bit.

Power Section

Positive Displacement Motors (commonly called a PDM) are reverse applications of a Moineau pump or screw pump. 

It mainly consists of Rotor & Stator.  

Rotor is chrome-plated alloy steel of spiral-helix shape. 

Stator is a hollow steel housing, lined with a molded-in-place elastomer rubber compound. 

A spiral-shaped cavity is produced in the stator during manufacturing. The rotor is produced with matching lobe profile and similar helical pitch to the stator, but with one lobe less. The rotor can therefore be matched to and inserted inside the stator. When assembled, the rotor and stator form a continuous seal along their matching contact points. Fluid is pumped into the motor’s progressive cavities. The force of the fluid movement causes the shaft to rotate within the stator. Thus, it is a positive displacement motor. The rotational force is then transmitted through the connecting rod and drive shaft to the bit.

  

Stage is the distance measured parallel to the axis between two corresponding points of the same spiral lobe. This distance is commonly referred to as the lead of the stator. A slight interference fit between rotor OD and stator ID controls motor power. 

Mud motors are divided into slow-speed, medium-speed and high-speed types. This is done by changing the pitch of the motor stages, by the number of "lobes" and resultant cavities of the stator. 

The greater the number of lobes, the higher the motor torque and the lower the output RPM. 

Increasing the flow rate through a given power section directly increases the output speed. To increase the output speed of a power section without changing the flow rate, the cavity size is changed. A high speed power section will require a larger fluid inlet area (cavity) to allow more fluid throughput into the cavity.

The torque generated by the power section is proportional to the differential pressure applied across the power section and is independent of fluid flow. Generally, the more weight applied to the bit, the higher the torque needed to keep the bit turning, so the higher the differential pressure across the Power Section.

The maximum recommended differential pressure is limited by the stator elastomer. If pressure increases beyond the limits of the elastomer, the stator elastomer will deform, breaking the cavity seal so the mud flow leaks past the rotor and rotation stops – this is commonly known as a stalled motor.

Drive Shaft Assembly

The drive shaft assembly converts the eccentric motion of the rotor into concentric rotation for the bearing assembly via a connecting rod attached to the lower end of the rotor. It transmits the torque and rotational speed from the rotor to the drive shaft and bit. Universal joints convert the eccentric motion of the rotor into concentric motion at the drive shaft. 

It also accommodates any angle set on the adjustable bent housing (or fixed bend housing) and carries the thrust load from the rotor caused by the pressure drop across the power section.

Adjustable Bent Housing

ABH connects stator to the bearing assembly and also houses drive shaft assembly. It has a field adjustable angle-setting to produce a wide range of build rates.

Angle setting may be set to zero for vertical drilling or may be set to any other angle setting as desired. Once the angle is set for the mud motor, it can't be changed when it's down hole and has to be pulled out of the hole to change the angle-setting.

Higher rotary rpm could be used at low angle-setting as compared to a high angle-setting.

Drilling at a higher rotary rpm provides a drill bit with more torsional force provided by the entire rotating drill string as compared to the torsional force provided alone by the mud motor.

(That's the reason why ROP in rotary mode > ROP in sliding mode)

Bearing Assembly

The drive shaft assembly is supported within the bearing housing by radial and axial thrust bearings. It transmits the rotation of the drive shaft assembly to the drill bit and the compressive thrust load created by the weight of the collars and drill string to the rotating bit box & supports the radial and bending loads developed while directional drilling.  

It also carries the tensile off-bottom thrust load produced by the pressure drops across the rotor and the drill bit, as well as any load caused during back reaming. The high capacity radial bearings readily withstand side loads caused by drilling with a deflection device or uneven cutting action along the drill bit periphery. The tungsten carbide radial bearings and angular contact bearing section supports the radial loads along the full length of the bearing assembly, creating a very stiff, strong assembly

Types of Bearing Assembly-

Mud Lubricated Bearing Assembly

Oil Sealed Bearing Assembly

Mud Lubricated Bearing Assembly regulate the flow of mud through the bearing assembly. This diverted mud (usually 4 - 10%) is used to cool and lubricate the shaft, radial and thrust bearings. It exits to the annulus directly above the bit sub. The exact percentage of mud diverted is determined by the condition of the bearings and the pressure drop across the bit. Mud lubricated bearing assemblies can be used in the hottest holes with the lowest aniline point drilling fluids, as there are no elastomeric seals.

Oil Sealed Bearing Assembly is an alternative to the mud-lubricated bearing. A sealed bearing would be recommended where corrosive muds are used, where a lot of LCM of various sizes is pumped or where there is a requirement for a very low pressure drop across the bit (Pbit).

Bit Sub

At Bit sub the drill bit is make up with the motor and it's the only moving external part of the motor.

  

Note: 

In addition to above, different manufacturers can have more or less parts.

The operating conditions and parameters for the mud motors may vary for different manufacturers.

Read More → Steerable Downhole Mud Motor - Directional Drilling

9 Distinct Mud Systems

For mud to manage its many tasks, a broad range of different fluid systems have been developed. 9 distinct mud systems are defined here. 

The first seven are water-based, while the eighth is oil-based. The ninth category is a specialized one in which air or gas is the continuous fluid. 

The 9 categories are:

1. Non dispersed. These may consist of spud muds, natural muds and other lightly treated systemsgenerally used for shallow wells or top-hole drilling.

2. Dispersed. At greater depths or where hole-conditions may be problematic, muds are often dispersed, typically by means of lignosulphonates or other deflocculants. These and similar products are also effective filtrate reducers.

3. Calcium treated. Divalent cations such as calcium and magnesium, when added to a mud, inhibit the swelling of formation clays and shale, and are therefore added to control sloughing shale, hole enlargement and to prevent formation damage. Hydrated lime, gypsum (calcium sulphate) and calcium chloride are principal ingredients of calcium systems. Gyp systems (note: Gyp = gypsum) usually have a pH of 9.5 to 10.5 and an excess gyp concentration of 2 to 4 lb/ bbl; Lime systems have an excess lime concentration of 1 to 15 lb/bbl and a pH of 11.5 to 12.0.

4. Polymer. Muds incorporating long-chain, high-molecular-weight chemicals are effective in increasing viscosity, flocculating muds, reducing filtrate loss and stabilizing the formation. Various types of polymers are available for this purpose, including Bentonite extenders. Bio polymers and cross-linked polymers are also used and have good shear-thinning properties at low concentrations.

5. Low solids. This includes systems in which the amount and type of solids are controlled. Total solids should not range higher than about 6% to 10% by volume (and clay < 3% by volume). One primary advantage of low-solids systems is that they significantly improve the rate of penetration.

6. Saturated salt. Include several groups: Saturated salt systems have a chloride ion concentration of 189 000 ppm. Saltwater systems have a chloride content from 6 000 to 189 000 ppm, and at its lower level are usually referred to as brackish or seawater systems.

7. Workover. Completion and workover fluids are specialized systems designed to minimize formation damage, and be compatible with acidizing and fracturing operations (acid soluble) and capable of inhibiting swelling clays that reduce formation permeability. Density is obtained  through dissolved salt to avoid long term settling.

8. Oil/synthetic. Oil-based fluids are used for high temperature wells, deviated holes and wells where pipe sticking and hole stabilization is a problem.

 They consist of two types of systems:

1) Invert emulsion muds are water-in-oil fluids and have water as the dispersed phase and oil as the continuous phase. They may contain up to 50% water in the liquid phase. Emulsifier (commonly fatty acids amine derivatives, high-molecular-weight soaps), and water concentrations are varied to control rheological and electrical stability;

2) Synthetic fluids are designed to duplicate the performance of oil-based muds, without the environmental hazards. Primary types of synthetic fluids are esters, poly alpha olefins and food grade paraffin. They are environmentally friendly, can be discharged offshore and are non-sheening and biodegradable.

9. Air, mist, foam and gas. Four basic operations are included in this specialized category according

to the IADC. These include:

1) Dry air drilling, which involves injecting dry air or gas into the wellbore at rates capable of achieving annular velocities that will remove cuttings;

2) Mist drilling involves injecting a foaming agent into the air stream, which mixes with produced water and lifts drill cuttings;

3) Stable foam uses chemical detergents and polymers and a foam generator to carry cuttings in fast-moving air stream;

4) Aerated fluids rely on mud with injected air (which reduces the hydrostatic head) to remove drilled solids from the wellbore.

Read More → 9 Distinct Mud Systems