compensated thermal neutron porosity management
Sticking Mechanism Categories and Preventation actions
- Solids Induced Pack-off
a) At the first signs of the drill string torquing up and trying to pack-off, the pump strokes should be reduced by half. This will minimise pressure trapped should the hole pack-off. Excessive pressure applied to a pack-off will aggravate the situation. If the hole cleans up, return flow to the normal rate.
b) If the string packs off, immediately stop the pumps and bleed down the standpipe pressure [NB not possible with a non-ported float valve]. When bleeding pressure down from under a pack-off, control the rate so as not to "U" tube solids into the drill string in case they plug the string.
c) Leave low pressure (<500 psi ) trapped below the pack-off. This will act as an indicator that the situation is improving should the pressure bleed off.
d) Holding a maximum of 500 psi on the standpipe and with the string hanging at its free rotating weight, start cycling the drill string up to maximum make-up torque. At this stage do not work the string up or down.
e) Continue cycling the torque, watching for pressure bleed off and returns at the shakers. If bleed off or partial circulation occurs, slowly increase pump strokes to maintain a maximum of 500 psi standpipe pressure. If circulation improves continue to increase the pump strokes.
f) If circulation cannot be regained, work the pipe between free up and free down weight. DO NOT APPLY EXCESSIVE PULLS AND SET DOWN WEIGHTS AS THIS WILL AGGRAVATE THE SITUATION (50k lb max). Whilst working the string continue to cycle the torque to stall out and maintain a maximum of 500 psi standpipe pressure.
g) DO NOT ATTEMPT TO FIRE THE JARS IN EITHER DIRECTION.
h) If circulation cannot be established increase the standpipe pressure in stages up to 1500 psi and continue to work the pipe and apply torque.
i) If the pipe is not free once full circulation is established, commence jarring operations in the opposite direction to the last pipe movement. Once the pipe is free rotate and clean the hole prior to continuing the trip.
- Unconsolidated formations
Description
An unconsolidated formation falls into the well bore because it is loosely packed
with little or no bonding between particles, pebbles or boulders. Video clip of sand sloughing
The collapse of the formation is caused by removing the supporting rock as the well is drilled. This is very similar to digging a hole in sand on the beach, the faster you dig the faster the hole collapses. It happens in a well bore when little or no filter cake is present. The un-bonded formation (sand, gravel, small river bed boulders etc.) cannot be supported by hydrostatic overbalance as the fluid simply flows into the formation. Sand or gravel then falls into the hole and packs off the drill string. The effect can be a gradual increase in drag over a number of metres, or can be sudden. This mechanism is normally associated with shallow formations. Examples are shallow river bed structures at about 500m in the central North Sea and in surface hole sections of land wells. This mechanism normally occurs:
· While drilling shallow unconsolidated formations.
Preventative Action
These formations need an adequate filter cake to help stabilise the formation. Seepage loss can be minimised with fine lost circulation material. If possible, avoid excessive circulating time with the BHA opposite unconsolidated formations to reduce hydraulic erosion. Spot a gel pill before POOH. Slow down tripping speed when the BHA is opposite unconsolidated formations to avoid mechanical damage.
Start and stop the pumps slowly to avoid pressure surges being applied to unconsolidated formations. Control-drill the suspected zone to allow time for the filter cake to build up, minimise annulus loading and resultant ECD’s. Use sweeps to help keep the hole clean. Be prepared for shaker, desilter and desander overloading. A method successfully used in the North Sea is to drill 10m, pull back to the top of the section and wait 10 minutes. Note
any fill on bottom when returning to drill ahead. If the fill is significant then ensure the process is repeated every 10m. It may be impossible to prevent the hole collapsing. If so let the hole stabilise itself with the BHA up out of harm’s way.
Rig site indications
· Increase in pump pressure.
· Fill on bottom.
· Overpull on connections.
· Shakers blinding.
Freeing
Follow First Actions but be aware that the pressures (i.e. 500 psi, 1500 psi) will probably not be achievable in shallow
formations.
- Mobile Formations
Description
The mobile formation squeezes into the well bore because it is being compressed by the overburden forces. Mobile formations behave in a plastic manner, deforming under pressure. The deformation results in a decrease in the well bore size, causing problems running BHA’s, logging tools and casing. A deformation occurs because the mud weight is not sufficient to prevent the formation squeezing into the well bore. This mechanism normally occurs: · While drilling salt.
Preventative Action
Maintain sufficient mud weight. Select an appropriate mud system that will not aggravate the mobile formation. Plan frequent reaming/wiper trips particularly for this section of the hole. Consider bi-centre PDC bits. Slow trip speed before BHA enters the suspected area. Minimise the open hole exposure time of these formations. With mobile salts consider using a slightly under-saturated mud system to allow a controlled washout.
Rig site indications
· Overpull when moving up, takes weight when running in.
· Sticking occurs with BHA at mobile formation depth.
· Restricted circulation with BHA at mobile formation depth.
Freeing
Spot a fresh water pill if in a salt formation. (Consider the effect on well control and on other open hole formations ). If moving up, apply torque and jar down with maximum trip load. If moving down, jar up with maximum trip load. Torque should not be applied while jarring up.
- Fractured & Faulted Formations
Description
A natural fracture system in the rock can often be found near faults. Rock near faults can be broken into large or small pieces. If they are loose they can fall into the well bore and jam the string in the hole. Even if the pieces are bonded together, impacts from the BHA due to drill string vibration can cause the formation to fall into the well bore. This type of sticking is particularly unusual in that stuck pipe can occur while drilling. When this has happened in the past, the first sign of a problem has been the string torquing up and sticking. There
is a risk of sticking in fractured / faulted formation when drilling through a fault and when drilling through fractured limestone formations.
This mechanism can occur:
· in tectonically active zones.
· in prognosed fractured limestone.
· as the formation is drilled.
Preventative Action
Minimise drill string vibration. Choose an alternative RPM or change the BHA configuration if high shock vibrations are observed. Slow the trip speed before the BHA enters a suspected fractured/faulted area. Generally, fractured formations require time to stabilise. Be prepared to spend time when initially drilling and reaming prior to making significant further
progress. Circulate the hole clean before drilling ahead. Restrict tripping speed when BHA is opposite fractured formations and fault zones. Start / stop the drill string slowly to avoid pressure surges to the well bore. Anticipate reaming during trips. Ream fractured zones cautiously.
Rig site indications.
· Hole fill on connections.
· Possible losses or gains.
· Fault damaged cavings at shakers.
· Sticking can be instantaneous.
Freeing
If packed off while off bottom then follow First Actions. Otherwise JAR UP in an effort to break up formation debris. Use every effort to maintain circulation. Circulate high density viscous sweeps to clean debris. Spot acid if stuck in limestone
- Naturally Over-Pressured Shale Collapse
Description
A naturally over-pressured shale is one with a natural pore pressure greater than the normal hydrostatic pressure gradient. Naturally over-pressured shales are most commonly caused by geological phenomena such as under-compaction, naturally removed overburden (i.e.
weathering ) and uplift. Using insufficient mud weight in these formations will cause the hole to become unstable and collapse.
This mechanism normally occurs in:
· Prognosed rapid depositional shale sequences.
Preventative action
Ensure planned mud weight is adequate. Plan to minimise hole exposure time. Rigorous use of gas levels to detect pore pressure trends. Use of other information to predict pore pressure trends (for example Dexp ). Once the shale has been exposed do not reduce the mud weight. It may also be the case that the mud weight will need to be raised with an increase in inclination See Well bore stability Section of SP KB 1997 Guidelines.
Rig site indications
· Cavings (splintery) at shakers.
· Increased torque and drag.
· Gas levels, D exponent.
· Circulation restricted or impossible.
· Hole fill.
· An increase in ROP.
· Cuttings and cavings are not hydrated or mushy.
Freeing
Follow First Actions.
- Induced Over-Pressured Shale Collapse
Description
Induced over-pressure shale occurs when the shale assumes the hydrostatic pressure of the well bore fluids after a number of days exposure to that pressure. When this is followed by no increase or a reduction in hydrostatic pressure in the well bore, the shale, which now has a higher internal pressure than the well bore, collapses in a similar manner to naturally over-pressured shale. Video clip - Unstable Shale in WBM
This mechanism normally occurs:
· In WBM.
· After a reduction in mud weight or after a long exposure time during which
the mud weight was constant.
· In the casing rat hole.
Preventative action
Non water based muds prevent inducing over-pressure in shale. Do not plan a reduction in mud weight after exposing shale. If cavings occur, utilise good hole cleaning practices. See Hole Cleaning Section
Rig site indications
· Cuttings / cavings show no sign of hydration.
· Cavings (splintery) at shakers.
· Tight hole in casing rat hole.
· Increased torque and drag.
· Circulating restricted or impossible.
· Hole fill.
1.2.6.4 Freeing
Follow First Actions.
- Reactive Formations
Description
A water sensitive shale is drilled with less inhibition than is required. The shale absorbs the water and swells into the well bore. The reaction is ‘time dependent’, as the chemical reaction takes time to occur. However, the time can range from hours to days.
This mechanism normally occurs:
· When using WBM in shales and clays in young formations.
· When drilling with an incorrect mud specification. Particularly, an
insufficient concentration of inhibition additives in OBM and WBM such as
salts (KCl, CaCL), glycol and polymer.
Preventative action
Use an inhibited mud system. Maintain the mud properties as planned. The addition of various salts (potassium, sodium, calcium, etc. ) will reduce the chemical attraction of the water to the shale. Various encapsulating (coating) polymers can be added to WBM mud to reduce water contact with the shale. Monitoring mud properties is the key to detection of this problem. Open hole time in shale should be minimised. Regular wiper trips or reaming trips may help if shales begin to swell. The frequency should be based on exposure time or warning signs of reactive shales. Ensure hole cleaning is adequate to clean excess formation i.e. clay balls, low gravity solids etc.
Rig site indications
· Hydrated or mushy cavings.
· Shakers screens blind off, clay balls form.
· Increase in LGS, filter cake thickness, PV, YP, MBT.
· An increase or fluctuations in pump pressure.
· Generally occurs while POOH.
· Circulation is impossible or highly restricted.
Freeing
POH slowly to prevent swabbing. See First Actions.
- Hole Cleaning
Description
In deviated wells cuttings and cavings settle to the low side of the hole and form layers called solids beds or cuttings beds. The BHA becomes stuck in the solids bed.
OR Cuttings and cavings slide down the annulus when the pumps are turned off and pack-off the drill string. Avalanching can also occur while the pumps are on.
Good hole cleaning means removal of sufficient solids from the well bore to allow the reasonably unhindered passage of the drill string and the casing.
There are several main reasons for solids not being cleaned out of the well bore.
These are:
· A low annular flow rate.
· Inappropriate mud properties.
· Insufficient circulation time.
· Inadequate mechanical agitation.
If any of the above are missing good hole cleaning will be very unlikely. In 40-65 degree wells the cuttings bed will slide down the low side of the hole. This can happen while pumping, not just when the pumps are off. In highly deviated wells of 65 degrees or more cuttings settle very quickly in spite of high flow rates. This is known as
avalanching. A cuttings bed of 10% of the hole diameter (1.75 inches in 17.5 inch hole) looks harmless enough. Add a drill string and the situation looks very different.
Cuttings beds can also increase drag in the well and cause problems with applying WOB in horizontal holes.
Preventative Action
· Maximise the annular velocity.
- Consider the use of a third mud pump.
- Consider using larger drill pipe.
· Ensure circulation times are adequate.
- Consult the hole cleaning charts for confirmation.
- Monitor the cuttings returns at the shakers.
· Maximise mechanical agitation of cuttings beds.
- Rotation.
- Reciprocation.
· Optimise mud properties.
- increase YP in near vertical wells.
Rig site indications· Overpulls increasing while POOH from TD in deviated hole (7-10 stands).
· Erratic pump pressure.
· Poor weight transfer to bit.
· Difficulty orienting toolface.
· Absence of returns at shakers.
· Presence of re-ground cuttings (LGS).
· Overpulls inside casing.
Freeing
See First Actions
Refer to Hole Cleaning section for more information.
- Tectonically Stressed Formations
DescriptionWell bore instability is caused when highly stressed formations are drilled and there exists a significant difference between the near well bore stress and the restraining pressure provided by the drilling fluid density. Tectonic stresses build up in areas where rock is being compressed or stretched due to movement of the earth’s crust. The rock in these areas is
being buckled by the pressure of moving tectonic plates. When a hole is drilled in an area of high tectonic stresses the rock around the well bore will collapse into the well bore and produce splintery cavings similar to those produced by over-pressured shale. In the tectonic stress case the hydrostatic pressure required to stabilise the well bore may be much higher
than the fracture pressure of the other exposed formations
This mechanism usually occurs:
· in or near mountainous regions.
Preventative action
Plan to case off these formations as quickly as possible. Maintain mud weight within planned mud weight window. Well bore instability shows itself as a hole cleaning problem. If possible drill these formations in smaller hole sizes. This will minimise the impact of a hole cleaning problem. Ensure that the circulation system is capable of handling the additional volume of cavings often associated with this mechanism. If hole problems do occur, Ref Hole Cleaning section. Use offset data to establish optimum inclination and azimuth as these are key factors in reducing the extent of the problem. Ref Wellbore Stability section in Rig site indicators
· Pack-offs and bridges may occur.
· Cavings at the shakers (splintery).
· Increase torque and drag.
· If stuck, circulation is likely to be impaired or non-existent.
· Increase in volume of returns at the shakers relative to the hole volume drilled.
Freeing
See First Actions
CATALYTIC CRACKING
INTRODUCTION
exist between the FCC feedstocks and converted FCC products. As oil refining has evolved
over the last 60 years, the FCC process has evolved with it, meeting the challenges of cracking heavier, more contaminated feedstocks, increasing operating flexibility, accommodating environmental legislation, and maximizing reliability.
The FCC unit continuously circulates a fluidized zeolite catalyst that allows rapid
cracking reactions to occur in the vapor phase. The KBR Orthoflow FCC unit (Fig. 3.1.1)
consists of a stacked disengager-regenerator system that minimizes plot space requirements. The cracking reactions are carried out in an up-flowing vertical reactor-riser in
which a liquid oil stream contacts hot powdered catalyst. The oil vaporizes and cracks to
lighter products as it moves up the riser and carries the catalyst along with it. The reactions
are rapid, requiring only a few seconds of contact time. Simultaneously with the desired
reactions, coke, a material having a low ratio of hydrogen to carbon, deposits on the catalyst
and renders it less catalytically active. Catalyst and product vapors separate in a disengaging vessel with the catalyst continuing first through a stripping stage and second
through a regeneration stage where coke is combusted to rejuvenate the catalyst and provide heat for operation of the process. The regenerated catalyst then passes to the bottom of the reactor-riser, where the cycle starts again. Hydrocarbon product vapors flow downstream for separation into individual products.
KBR, through its ancestry in The M.W. Kellogg Company, has been a leader in FCC
technology developments since the inception of the process. In recent years, KBR has
worked with its FCC partner, ExxonMobil, to create and refine FCC technology features
that have led the industry. To date, KBR has licensed more than 120 grassroots FCC
units throughout the world, including 13 grassroots units and more than 120 revamps
since just 1990.
FEEDSTOCKS
FCC’s reputation as one of the most flexible refining processes in use today. Examples of
common feedstocks for conventional distillate feed FCC units are
● Atmospheric gas oils
● Vacuum gas oils
● Coker gas oils
● Thermally cracked gas oils
● Solvent deasphalted oils
● Lube extracts
● Hydrocracker bottoms
Residual FCCU (RFCCU) processes Conradson carbon residue and metals-contaminated
feedstocks such as atmospheric residues or mixtures of vacuum residue and gas oils.
Depending on the level of carbon residue and metallic contaminants (nickel and vanadium),
these feedstocks may be hydrotreated or deasphalted before being fed to an RFCCU.
Feed hydrotreating or deasphalting reduces the carbon residue and metals levels of the
feed, reducing both the coke-making tendency of the feed and catalyst deactivation.
PRODUCTS
Products from the FCC and RFCC processes are typically as follows:● Fuel gas (ethane and lighter hydrocarbons)
● C3 and C4 liquefied petroleum gas (LPG)
● Gasoline
● Light cycle oil (LCO)
● Fractionator bottoms (slurry oil)
● Coke (combusted in regenerator)
● Hydrogen Sulfide (from amine regeneration)
Although gasoline is typically the most desired product from an FCCU or RFCCU, design
and operating variables can be adjusted to maximize other products. The three principal
modes of FCC operation are (1) maximum gasoline production, (2) maximum light cycle
oil production, and (3) maximum light olefin production, often referred to as maximum
LPG operation. These modes of operation are discussed below:
Maximum Gasoline The maximum gasoline mode is characterized by use of an intermediate cracking temperature (510 to 540°C), high catalyst activity, and a high catalyst/oil ratio. Recycle is normally not used since the conversion after a single pass through the riser is already high. Maximization of gasoline yield requires the use of an effective feed injection system, a short-contact-time vertical riser, and efficient riser effluent separation to maximize the cracking selectivity to gasoline in the riser and to prevent secondary reactions from degrading the gasoline after it exits the riser.
Maximum Middle Distillate The maximum middle distillate mode of operation is a low-cracking-severity operation in which the first pass conversion is held to a low level to restrict recracking of light cycle oil formed during initial cracking. Severity is lowered by reducing the riser outlet temperature (below 510°C) and by reducing the catalyst/oil ratio. The lower catalyst/oil ratio is often achieved by the use of a fired feed heater which significantly increases feed temperature. Additionally catalyst activity is sometimes lowered by reducing the fresh catalyst makeup rate or reducing fresh catalyst activity. Since during low-severity operation a substantial portion of the feed remains unconverted in a single pass through the riser, recycle of heavy cycle oil to the riser is used to reduce the yield of lower-value, heavy streams such as slurry product. When middle distillate production is maximized, upstream crude distillation units are operated to minimize middle distillate components in the FCCU feedstock, since these components either degrade in quality or convert to gasoline and lighter products in the FCCU. In addition, while maximizing middle distillate production, the FCCU gasoline endpoint would typically be minimized within middle distillate flash point constraints, shifting gasoline product into LCO.
If it is desirable to increase gasoline octane or increase LPG yield while also maximizing
LCO production, ZSM-5 containing catalyst additives can be used. ZSM-5 selectively
cracks gasoline boiling-range linear molecules and has the effect of increasing gasoline
research and motor octane ratings, decreasing gasoline yield, and increasing C3 and C4
LPG yield. Light cycle oil yield is also reduced slightly.
Maximum Light Olefin Yield
The yields of propylene and butylenes may be increased above that of the maximum gasoline operation by increasing the riser temperature above 540°C and by use of ZSM-5 containing catalyst additives. The FCC unit may also be designed specifically to allowmaximization of propylene as well as ethylene production by incorporation of MAXOFIN
FCC technology, as described more fully in the next section. While traditional FCC operations typically produce less than 6 wt % propylene, the MAXOFIN FCC process can produce as much as 20 wt % or more propylene from traditional FCC feedstocks. The process increases propylene yield relative to that produced by conventional FCC units by combining the effects of MAXOFIN-3 catalyst additive and proprietary hardware, including a second high-severity riser designed to crack surplus naphtha and C4’s into incremental light
olefins. Table 3.1.1 shows the yield flexibility of the MAXOFIN FCC process that can
alternate between maximum propylene and traditional FCC operations.
PROCESS DESCRIPTION
and several other aspects of the process will vary from unit to unit, depending on the
requirements of the application. The following sections provide more detailed descriptions
of the converter, flue gas train, main fractionator, and VRU.
Converter
The KBR Orthoflow FCCU converter shown in Fig. 3.1.2 consists of regenerator, stripper,and disengager vessels, with continuous closed-loop catalyst circulation between the
regenerator and disengager/stripper. The term Orthoflow derives from the in-line stacked
arrangement of the disengager and stripper over the regenerator. This arrangement has the
following operational and cost advantages:
● Essentially all-vertical flow of catalyst in standpipes and risers
● Short regenerated and spent catalyst standpipes allowing robust catalyst circulation
● Uniform distribution of spent catalyst in the stripper and regenerator
● Low overall converter height
● Minimum structural steel and plot area requirements
Preheated fresh feedstock, plus any recycle feed, is charged to the base of the riser reactor.
Upon contact with hot regenerated catalyst, the feedstock is vaporized and converted to lower boiling fractions (light cycle oil, gasoline, C3 and C4 LPG, and dry gas). Product vapors are separated from spent catalyst in the disengager cyclones and flow via the disengager
Coke formed during the cracking reactions is deposited on the catalyst, thereby reducing
its activity. The coked catalyst, which is separated from the reactor products in the
disengager cyclones, flows via the stripper and spent catalyst standpipe to the regenerator.
The discharge rate from the standpipe is controlled by the spent catalyst plug valve.
In the regenerator, coke is removed from the spent catalyst by combustion with air. Air
is supplied to the regenerator air distributors from an air blower. Flue gas from the combustion of coke exits the regenerator through two-stage cyclones which remove all but a
trace of catalyst from the flue gas. Flue gas is collected in an external plenum chamber and
flows to the flue gas train. Regenerated catalyst, with its activity restored, is returned to the
riser via the regenerated catalyst plug valve, completing the cycle.
ATOMAX Feed Injection System
The Orthoflow FCC design employs a regenerated catalyst standpipe, a catalyst plugvalve, and a short inclined lateral to transport regenerated catalyst from the regenerator to
the riser. The catalyst then enters a feed injection cone surrounded by multiple, flat-spray, atomizing feed injection nozzles, as shown in Fig. 3.1.3. The flat, fan-shaped sprays provide
uniform coverage and maximum penetration of feedstock into catalyst, and prevent catalyst
from bypassing feed in the injection zone. Proprietary feed injection nozzles, known
as ATOMAX nozzles, are used to achieve the desired feed atomization and spray pattern
while minimizing feed pressure requirements. The hot regenerated catalyst vaporizes the
oil feed, raises it to reaction temperature, and supplies the necessary heat for cracking.
The cracking reaction proceeds as the catalyst and vapor mixture flow up the riser. The
riser outlet temperature is controlled by the amount of catalyst admitted to the riser by the
catalyst plug valve.
Riser Quench
The riser quench system consists of a series of nozzles uniformly spaced around the uppersection of riser. A portion of the feed or a recycle stream from the main fractionator is
injected through the nozzles into the riser to rapidly reduce the temperature of the riser
contents. The heat required to vaporize the quench is supplied by increased fresh feed preheat or by increased catalyst circulation. This effectively increases the temperature in the
lower section of the riser above that which would be achieved in a nonquenched operation,
thereby increasing the vaporization of heavy feeds, increasing gasoline yield, olefin production, and gasoline octane.
Riser Termination
At the top of the riser, all the selective cracking reactions have been completed. It is important to minimize product vapor residence time in the disengager to prevent unwanted thermal or catalytic cracking reactions which produce dry gas and coke from more valuableproducts. Figure 3.1.4 shows the strong effect of temperature on thermal recracking of
gasoline and distillate to produce predominantly dry gas.
Closed cyclone technology is used to separate product vapors from catalyst with minimum
vapor residence time in the disengager. This system (Fig. 3.1.5) consists of riser
cyclones directly coupled to secondary cyclones housed in the disengager vessel. The riser
cyclones effect a quick separation of the spent catalyst and product vapors exiting the
riser. The vapors flow directly from the outlet of the riser cyclones into the inlets of the
secondary cyclones and then to the main fractionator for rapid quenching. Closed cyclones
almost completely eliminate postriser thermal cracking with its associated dry gas and
butadiene production. Closed cyclone technology is particularly important in operation at
high riser temperatures (say, 538°C or higher), typical of maximum gasoline or maximum
light olefin operations.
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