Page 207 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

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difficult, if not meaningless (see Fig. 9- 15 in Well Cemenfing.
Effects of Perforations
Prentice Hall, 1990).
Perforations are the communication link between the produc- P-5.2: Breakdown of Perforations
ing reservoir and the wellbore. The number of perforations Wells are often “broken down” after perforating and prior to
required to perform this function is determined by two factors. pumping the fracturing treatment. These treatments are often
First, the producing capability of the well should be consid- performed several days before the actual fracture treatment to
ered. A mechanical skin can be created if too much produc- allow testing of the formation. They are performed in cased
tion is forced through a limited number of perforations, wells to ensure that the perforations have been effectively
thereby reducing the productivity of the formation. The other shot through the casing and are “open,” creating a commu-
factor controlling the number of required perforations is the nication path between the reservoir and the wellbore. Even if
rate at which the stimulation treatment is to be pumped. holes have been shot through the casing, it is very common
Frictional pressure drop through perforations becomes sig- for the perforation tunnel to be extensively damaged, especially
nificant once the fluid rate through a given perforation ex- when wells are perforated in an overbalanced condition (i.e.,
ceeds 0.5 BPM. Therefore, to avoid excessive pressure drop the hydrostatic pressure of the well is greater than the reser-
and the resulting high treating pressure, at least two effective voir pressure). This damage is often severe enough to prevent
perforations are needed for each BPM planned for the treat- the flow of stimulation fluids from adequately entering the
ment. formation. A proper breakdown procedure effectively removes
P-5.1: Calculation of Number of Effective Perforations perforation debris and eliminates most of the damage. At the
The following procedure can be used to calculate the number same time, this small treatment can confirm whether it is
possible to pump into the formation without excess pressure
of effective perforations. The total frictional pressure loss is before an entire stimulation crew is mobilized to the wellsite.
estimated by shutting down the pumping units and measuring This is also a very convenient time to ensure that the
an ISIP (instantaneous shut-in pressure). The pressure drop at fracture gradient used in the hydraulic fracture design is
the wellhead can be accounted for by the loss of friction in accurate. If the estimation is in gross error, the fracture
both the casing and through the perforations. To separate treatment can be redesigned so that surface pressure limita-
these values, the friction pressure drop of the fracturing fluid tions are not exceeded. The fracture gradient can be estimated
must first be estimated as discussed in Section P-3.5. This by taking an ISIP and applying the following equation:
value is subtracted from the total friction loss, leaving the
remaining friction to be accounted for by perforational fric- ISIP + pH ISIP + pH
tion drop. g.f = H H 3 3
g.f =
where pH is the hydrostatic pressure.
Performing a breakdown treatment is very important when
The remaining frictional losses can be attributed to perfo- the interval has been shot with a limited number of perfora-
ration friction drop. The number of effective perforations can tions or when perforations are spread over an interval exceed-
then be calculated using the following equation: ing 100 ft. If a limited number of perforations have been shot
and a breakdown has not been performed, excessive treating
pressures may be encountered because of an insufficient
number of perforations accepting fluid. When a large interval
has been perforated, some of the perforations may not accept
where qi is the pump rate (BPM), p is the density of the fluid and that part of the interval may not be adequately
fracturing fluid (gkm’), n is the number of perforations, and treated.
dpcrfis the diameter of the perforations (in.). For quick refer- However, when a relatively small interval has been shot
ence, this equation is often used in nomograph form and is with a high perforation density (four shots per foot over = 25
shown in Fig. P-57. ft), there may be no advantage in performing a separate
This method of estimating the number of perforations is of breakdown treatment. In this case, enough perforations are
limited use if proppant-laden fluid has been pumped through present so that a fracture treatment can be started without
the perforations. Data show that after only 10,000 Ib of experiencing excessive pressures.
proppant, the perforation face can become significantly eroded. When a breakdown treatment is pumped, ball sealers are
As the perforation face becomes rounded, the choke effect of required to divert the breakdown fluid into all the perfora-
the perforation changes, making this type of calculation very tions. It is common practice to use at least 50 to 100% more

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