Page 216 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

becomes high enough to allow a second fracture system to cient of each perforation. After only 10,000 Ib of proppant,
accept fluid. If this scenario happens during a pad stage, the the pressure drop across the perforations will be drastically
proppant placement may be unsuccessful because of insuffi- reduced. Therefore, diversion of the pad fluids may be suc-
cient pad. If a second zone opens during the proppant stages, cessful, but diversion of the proppant-laden stages may be
the second zone will quickly screen out. Fracture initiation in unsuccessful. After the perforations have been eroded, one
the screened-out second zone may not happen even when a zone is likely to accept most of the fluid.
new pad fluid is started after the diverter stage. An accurate stress profile of the wellbore is necessary to
Using diverter stages to control placement of fracturing design a successful limited-entry treatment. Each zone will
fluids will usually result in uneven fracture geometries, poor have a different fracture gradient and therefore will break
conductivities near the wellbore and overall poor well perfor- down and fracture at different pressures. If great contrast
mance. Relying on diverters to place multiple fracturing exists between the fracture gradient of individual zones, the
stages should be avoided unless no other isolation methods perforation scheme must be designed to reflect this difference.
are practically feasible. Limited-entry designs often do not consider the net pres-
sure effects of the fracture. It is not uncommon for a fractur-
P-6.2.6: Limited Entry ing treatment to create more than 500 psi in net pressure. An
Limited-entry treatments are designed to place fracturing imbalance in net pressures between zones can effectively
fluids into multiple zones simultaneously. The limited-entry negate the perforation pressure drop. Fracture height and
technique uses the pressure drop created across the perfora- Young’s modulus are two parameters having a major effect
tions during pumping to divert the fracturing fluid into several on net pressure. Both parameters should be closely evaluated
different perforated intervals. Generally, a 500- to 1000-psi prior to the design of a limited-entry treatment.
pressure differential is considered necessary to provide ad- The net pressure in the fracture is inversely proportional to
equate control over fluid placement. Equation P-2 can be the gross fracture height. Large zones will have smaller net
used to calculate the differential pressure across the perfora- pressures and therefore tend to accept a disproportionate
tions. The total flow of fluid entering into a given zone is amount of fracturing fluid. Very small zones will most likely
restricted by controlling the size and number of perforations remain unstimulated because they rapidly build very high net
in that interval. The high pressure drop at the perforations pressures and do not accept significant volumes of fracturing
forces fluid to go to another zone. This diversion technique fluids. The global Young’s modulus of the zone has a similar
has proved popular because of its simplicity and economics. effect: The larger the Young’s modulus, the narrower the
The diversion does not require expensive tools; it does not fracture and the higher the net pressure.
require the running and retrieving of tools or making cleanout One final parameter having significant impact on the
trips. The only cost for applying this type of diversion is the successful placement of fractures via the limited-entry tech-
excess HHP needed to pump the treatment at higher pressure. nique is fluid leakoff. The size of the zone and the rate the
However, if this treatment is not applied correctly, each fluid is pumped into the zone directly impact the leakoff
producing zone may not receive adequate treatment. Several volume. With several zones accepting fluid at one time, the
factors must be considered when designing a limited-entry total pump rate into any one interval may be quite low. Zones
treatment. The number and size of perforations are calculated with the lowest pump rates will generally have poor fluid
to divert the pad fluid. Smaller zones will not need as much efficiency, which may result in an early screenout.
fluid or proppant and therefore require fewer perforations. Accurately placing proppant into multiple zones by using
Some zones may require less than five perforations to control limited entry is extremely difficult. Fracture penetration and
flow into that section. With a limited number of perforations width will most likely be very irregular between zones.
available, the importance of the breakdown procedure becomes Smaller zones may not accept any fluid. The increased pro-
obvious. The loss of one or two perforations can significantly ducing capabilities of several stimulated zones should be
alter the flow distribution into all the zones. carefully examined and weighed against the economic ad-
Introducing sand into the fracturing fluid quickly erodes vantages of a limited-entry treatment before this fracture
the perforations and changes the corresonding flow coeffi- procedure is used.

P-55