Wells – Processes – Cementing – plugging or consolidating
Patent
1996-03-20
1997-12-16
Tsay, Frank
Wells
Processes
Cementing, plugging or consolidating
166295, E21B 3313
Patent
active
056974418
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of selectively isolating zones of oil wells or the like in which a liner or casing is situated in the well but is not continuously cemented to the wellbore wall so as to be isolated therefrom.
In conventional well completion a tubular liner or casing is run into the well after it is drilled, and cement is pumped between the casing and the wellbore wall and allowed to set. This isolates the various zones of the well from each other, and so prevents, for example, oil from entering aquifers which might be at lower pressure and providing drinking water. Where it is desired to allow formation fluids such as oil or gas to enter the well, the lining and cement are perforated by an explosive charge in order to provide a channel for the fluid to enter the lining so that it can pass more sensitive zones without causing damage or pollution.
This technique generally works well in wells which are more or less vertical, since it is straightforward to run the liner into the well, to ensure it is centralised, and to ensure that cement is placed evenly around the liner and bonds to both the liner and the wellbore. However, when the well is deviated from vertical--particularly when highly deviated, even horizontal--certain problems can occur. In particular, it is often difficult to ensure that the liner remains central in the well before the cement is placed. There is a tendency for the liner to lie on the lower side of the well such that when the cement is pumped it flows easily along the high side of the liner, but little if any penetrates around the low side; this can lead to bonding problems. Also, the tools used to perforate the liner and cement can become eccentred, and then do not operate efficiently.
The horizontal parts of deviated wells are often arranged so as to remain within a producing formation, and therefore in these sections good communication between the formation and the liner is required whereas in the vertical sections leading to the surface good zonal isolation is required. One technique which is used in such horizontal situations is to provide a perforate liner--for instance, a slotted, perforated, or predrilled liner, or a screen or a pre-packed screen--in the horizontal section of the well without any cementing to bond the liner to the wellbore, the annular gap between the liner and the wellbore either being left "empty" or (as may be preferable in certain circumstances) being packed with suitably-sized gravel. However, problems can arise if it is desired to perform a selective well treatment on, or produce selectively from, one zone in the horizontal section since inside a perforate liner there is no way of isolating the zone in question from the remainder of the well by using packers as is done with a conventional liner, because fluid can by-pass the plug by exiting the liner into the annular space therearound. One technique which has been proposed to overcome this problem is to provide one or more packers around the outside of the liner (external casing packers) which contact the wellbore and provide localised restriction to communication outside the liner; this then allows packers inside the liner to be used to isolate a portion of the well. Unfortunately, there are also problems associated with the use of external casing packers; they must be sealed against the wellbore, they must be positioned in advance, and they substantially increase the cost of the completion.
U.S. Pat. No. 5,197,543 discloses a method of isolating regions of a horizontal wellbore in an unconsolidated formation involving placement of alternating blank (unperforated) liner sections such as 32a, 32b and screen sections, eg slotted liners, such as 30a 30b, 30c. Internal plugs 42, 44 are positioned in blank liner sections adjacent the region to be plugged, and plugging liquid is pumped through the intervening screen section into the surrounding formation. The technique is only described in connection with treatment of unconsolidated formation, where the surrounding formation has collap
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Davies Stephen Nigel
Maitland Geoffrey C.
Pafitis Demosthenis Georgeou
Poyet Jean Pierre
Vercaemer Claude Joseph
Dowell, a division of Schlumberger Technology Corporation
Tsay Frank
Vick, Jr. John E.
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