Page 213 - A Practical Companion to Reservoir Stimulation

PRACTICAL COMPANION TO RESERVOIR STIMULATION

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P-6.2: Controlling Fracture Placement
Zonal Isolation
The most reliable method of controlling the placement of
There are often several potential producing zones penetrated fracturing fluids is to limit perforations to a single zone.
by a wellbore that must be hydraulically fractured. To ensure When several zones of a well are to be stimulated, the
each zone is stimulated effectively, these intervals must be individual zones must be isolated from one another during
isolated from one another, and individual treatments must be pumping. This is best accomplished through progressive
designed and pumped for each one. Several isolation meth- perforation and stimulation. After a fracturing treatment has
ods have proved to be effective. These methods can be used been placed in the first zone, it is isolated, and another zone of
only when the various formations and intervals are isolated interest is perforated and treated in another single stage. Of
from each other behind the casing by cement. course,’this methodology works best when the deepest zone
is completed first and subsequent zones are individually
P-6.1: Importance of the Cement Sheath stimulated by working uphole.
The cement sheath must provide zonal isolation during both P-6.2.1: Mechanical Bridge Plugs
production and stimulation operations. For a producing well,
the cement seal between pipe and formation must be rela- Several mechanical methods are available to provide ad-
tively tight to prevent fluids from flowing through the annu- equate isolation. The most reliable method is the use of
lar area. The permeability of a set Portland cement of normal mechanical bridge plugs. The bridge plug can be run on
density is in the low microdarcy range. However, if the tubulars or on a wireline. Bridge plugs that are run on tubulars
cement does not bond perfectly to either the pipe or formation are retrievable and can be moved and reset several times.
and a small channel remains, the effective cement permeabil- Wireline bridge plugs cannot be moved once set and generally
ity can be drastically increased. However, very large need to be milled out after the treatment. Wireline bridge
permeabilities may result from channel widths that are quite plugs are often used when several treatments are attempted in
small (see Chapter 1 in Well Cementing, Prentice Hall, 1990). one day or when a rig is not scheduled to be over the hole
For example, a channel width of only 1.4 x in. is suffi- during the treatment. They can be run in the hole very
cient to create an effective cement permeability of 1000 md. quickly, and cleanout trips are not required between stages.
Channel permeabilities of this order may allow significant The retrievable bridge plugs are used when zones are indi-
crossflow between zones. vidually tested before another zone is opened. Any excess
However, during hydraulic fracturing these small chan- proppant must be circulated out of the hole before the tool is
nels, or microannuli, are relatively insignificant. The effective moved in order to prevent the proppant from sticking the tool.
cement permeability, given above, does not create a high A typical treatment would involve perforation of the bot-
leakoff risk for the fracturing fluid within the annulus. A tom zone, a hydraulic fracture treatment and a zonal isolation
leakoff rate for this channel is less than 1 gal/min. As long as by setting a bridge plug immediately above the perforated
major channels within the cement can be avoided, containment interval. The next zone would then be perforated and fracture
of a fracturing treatment should be possible. stimulated. Subsequent zones would be stimulated following
The effects of fracturing pressures on the adhesion tension the same procedure (Fig. P-59).
between cement and casing or cement and formation are not P-6.2.2: Sand Plugs
clearly understood. Data on this phenomenon are lacking.
Consequently, the resulting condition of the cement sheath A similar method of isolation can be achieved by using sand
following hydraulic fracturing is difficult to predict. Sonic plugs after the fracturing treatment. The volume of sand
logs run after fracturing treatments often indicate that the necessary to cover the perforated interval is added to the
cement bond across the fractured interval is destroyed, but casing. The sand plug is then tested by pressuring up on the
the bond further uphole remains intact. The loss of cement casing. The next zone is then perforated and stimulated. Once
bond across the fractured interval probably does not affect the all zones have been fracture stimulated, the sand can be
placement or containment of the fracturing treatment. Cement circulated out of the wellbore by using either conventional or
of relatively low compressive strength should prevent the coiled tubing. The amount of sand needed above the top
fracture from migrating between the casing and the formation. perforations is generally quite small and can be calculated by
However, the alteration of the bond may occur after the applying Darcy’s law to linear flow:
treatment. Any small loss of adhesion may result in a 1622.88A kAp
microannulus that would lead to crossflow of reservoir fluids. q. = (P-7)
PL

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