7. Main Treatment

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Main Treatment

Stage Design

The productivity ratio that results from a fracture treatment depends on the finaldistribution pattern of the proppant In a packed vertical fracture this final

distribution pattern depends primarily on

bull the fracturing fluid viscosity and type (ie drop-out type versus transport-type)

bull the fluid flow velocity

bull the proppant size density and concentration

We may calculate the settling velocity of an individual proppant particle using thegeneralization of Stokesrsquo law for shear thinning fluids f the settling velocity multiplied y the

total time availale for settling is of the order of the fracture height the fluid is dropout type f

this product is much less the fluid is transport-type

For a dropout type fluid it is very difficult to find a reasonable proppant schedule

When the more viscous sand-transport type of fluid is used the proppant

particles settle only slightly during pumping Ideally the final proppant pack will

be evenly distributed along the whole fracture in both the lateral and vertical

directions To achieve this the proppant concentration in the inected fluid has to

be gradually increased throughout the pumping period because proppant stages

pumped early lose more fluid than the ones pumped at the end of the treatment

In addition a considerable volume of fluid has to be pumped without any

proppant before the proppant carrying stages That fluid volume is called pad

The subseuent step-by-step increase of proppant concentration is called a

ramped proppant schedule

If the proppant schedule is designed correctly the proppant concentration at the

end of pumping is uniform within the whole fracture and eual to the proppant

concentration of the slurry inected at the last moment The width of the pack

after closure on the proppant will be more uniform essentially being similar to

the hydraulically induced width profile The proppant schedule can be continuous

but it is reasonable to divide the treatment into several discrete stages say from

to $ and approampimate the continuous schedule by stairs or ramps

Screenout Problems

(egardless of the type of fluid used fluid loss will cause the proppant

concentration in a small segment of the slurry to gradually increase as it moves

away from the wellbore



When a particular segment of the slurry becomes very concentrated or the

fracture width at a given lateral location is too small to allow the proppant to be

displaced further a screenout will occur ) screenout condition may happen at any

point within the fracture or in the worst case even within the wellbore ) tip

screenout which takes place at or near the eamptremity of the fracture results in amuch slower pressure increase than a screenout near the wellbore ) tip

screenout may be a desirable condition especially in high permeability fracturing

ear-wellbore or wellbore screenouts on the other hand should be avoided by

any means

+ampcessive leakoff ie higher than that used in the design model is the most

common cause of screenouts ther factors responsible for screenouts include

sudden height growth perforation problems multiple strands near-wellbore

tortuosity and insufficient proppant-carrying capacity of the fluid or insufficient

inection rate

ost of the time we may prevent screenouts from occurring by using a sufficient

volume of pad fluid We should pump enough pad fluid to create a fracture width

at the leading edge of the proppant slurry that is adeuate to allow two or three

proppant particles to be carried side by side verdesigned pad volumes should

be avoided however because they increase the load on the formation and lead to

additional fluid and pumping costs

When a tip screenout occurs pumping may be continued The additional proppant

will inflate the width of the already created fracture at least in soft formations

+ven in hard rock the pressure may increase slowly enough so that we can pump

enough solids-free flush fluid to clear the wellbore before reaching the maampimum

pressure limit

When a wellbore screenout occurs we must shut down the pumping operation

immediately because of the surface limitation on pressure If the fracture void

that has been formed at this time does not already contain enough proppant to

satisfactorily prop the fracture the resulting well performance will be very poor

The subseuent cleanup not to mention the transportation or disposal costs ofthe unused material is eamppensive

)s a general rule we should not overflush the proppant-carrying stages nor

should we stop and restart proppant addition during a fracturing operation

especially when using a frac fluid that has eampcellent proppant-suspension

properties This could cause the creation of a proppant-free section within the

fracture which could heal in a fashion similar to that shown in Figure $ Effect

of overflushing proppant thereby effectively forming a choke in the fracture and

severely restricting flow



Figure 1

Treatment Monitoring

onitoring euipment is available that allows continuous real-time recording of

all relevant treatment information in a mobile command post at the well site The

standard monitoring euipment records the total inection rates and surface

treating pressures ome specialty units continuously monitor and record the

proppant concentration and the rheological properties of the frac fluid in addition

to the inection rate and the surface pressures for both the tubing and casing

ince on-line bottomhole pressures are rarely available by far the most important

information during the treatment is the surface treating pressure ) log-log plot

of the treating pressure is often called a Nolte-Smith plot ) ualitative

interpretation of the log-log plot is based mostly on the log-log slope ) steady

positive slope on the order of 01 is interpreted as unrestricted normal fracture

propagation )n abrupt increase in the fracture surface eg due to fast height

growth into another layer is diagnosed if the slope changes to a negative value

)n increasing slope approaching the value of unity is considered a sign of

restricted tip propagation )n even larger than unit slope indicates the fast fill-up

of the near-wellbore region and the wellbore itself The fracturing engineer uses

this information to detect a screenout situation and to determine if it is happening

at the fracture tip or near the wellbore 2epending on the location of the

screenout the type of formation hard or soft and the established surface

pressure limit the engineer may intervene and if necessary prematurely stop

the treatment

Shut-in and Clean-up

Following the treatment the well is shut in for several hours to allow the fracture

to close and the fluid viscosity to break Fractures particularly in tight reservoirs



may reuire long periods to close and during the shut-in period eampcessive near-

wellbore proppant settling may occur uch proppant settling causes a choke

effect and should be avoided

If proppant settling is a potential problem a techniue called forced closure isapplied It consists of flowing back the well uickly and hence trapping the

proppant near the wellbore before settling may occur )s a conseuence the

proppant bridges behind the perforations and a reverse packing occurs

Immediate flowback has the additional benefit of producing back a substantial

part of the gel even in underpressured wells The buildup of pressure produced in

the formation by the fracturing treatment also helps to clean up the well

)ggressively flowing back a well reuires caution because proppant may be

carried out through the perforations +ampperience shows that overbreaking of the

fracturing gel can be more detrimental because of the near-wellbore settling of

proppant )ggressive flow back is indicated for energized fluid and foam

treatments to take advantage of the energy stored in the compressed gas ome

31 treatments however are allowed to soak for several days prior to flowback

in order to gain additional advantage from the penetration of 31 into the

formation

In individual cases additional actions might be necessary to get the well on

production For instance it might be advantageous to blow high-pressure gas if

available through the created fracture

Wells containing proppant after the treatment should be cleaned out 3oiled

tubing with nitrogen foams is often used for this purpose ) cheaper but still

effective method is to break up proppant with a notched collar and reverse out

with brine



When a particular segment of the slurry becomes very concentrated or the

fracture width at a given lateral location is too small to allow the proppant to be

displaced further a screenout will occur ) screenout condition may happen at any

point within the fracture or in the worst case even within the wellbore ) tip

screenout which takes place at or near the eamptremity of the fracture results in amuch slower pressure increase than a screenout near the wellbore ) tip

screenout may be a desirable condition especially in high permeability fracturing

ear-wellbore or wellbore screenouts on the other hand should be avoided by

any means

+ampcessive leakoff ie higher than that used in the design model is the most

common cause of screenouts ther factors responsible for screenouts include

sudden height growth perforation problems multiple strands near-wellbore

tortuosity and insufficient proppant-carrying capacity of the fluid or insufficient

inection rate

ost of the time we may prevent screenouts from occurring by using a sufficient

volume of pad fluid We should pump enough pad fluid to create a fracture width

at the leading edge of the proppant slurry that is adeuate to allow two or three

proppant particles to be carried side by side verdesigned pad volumes should

be avoided however because they increase the load on the formation and lead to

additional fluid and pumping costs

When a tip screenout occurs pumping may be continued The additional proppant

will inflate the width of the already created fracture at least in soft formations

+ven in hard rock the pressure may increase slowly enough so that we can pump

enough solids-free flush fluid to clear the wellbore before reaching the maampimum

pressure limit

When a wellbore screenout occurs we must shut down the pumping operation

immediately because of the surface limitation on pressure If the fracture void

that has been formed at this time does not already contain enough proppant to

satisfactorily prop the fracture the resulting well performance will be very poor

The subseuent cleanup not to mention the transportation or disposal costs ofthe unused material is eamppensive

)s a general rule we should not overflush the proppant-carrying stages nor

should we stop and restart proppant addition during a fracturing operation

especially when using a frac fluid that has eampcellent proppant-suspension

properties This could cause the creation of a proppant-free section within the

fracture which could heal in a fashion similar to that shown in Figure $ Effect

of overflushing proppant thereby effectively forming a choke in the fracture and

severely restricting flow



Figure 1






















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formation







with brine



Figure 1






















the treatment






















formation







with brine



















formation







with brine

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7. Main Treatment