Advancements in whipstock orientation tools and casing mills have reduced the time required to complete a sidetrack in cased hole. A specialized valve allows a whipstock to be oriented quickly with a measurement-while-drilling (MWD) tool. Newly designed mills can cut a sidetrack window through casing in only two assembly trips, as compared with three or more trips required with previous technologies. Applying unique mill dressing materials and milling parameters significantly increases milling efficiency, and provides better mill gauge protection.

Time savings are quantified by comparing Amoco Norway's results with the new whipstock orientation and milling technologies in the North Sea to a database of conventional North Sea sidetracking jobs. Results from 14 other North Sea applications of the new technology also are compared to the database of conventional sidetracking jobs.


The recent rise in cased hole sidetracking has been driven by platform age, slot availability, better reservoir knowledge, and the improved reliability and economics of casing whipstock and milling systems. Combined field use of several new technologies has reduced the time required to complete a sidetrack through casing by close to 50%.

While casing whipstock systems, mechanical wedges that provide positive deflection from the original casing axis, have been used since the inception of directional drilling, they have proven reliable only in the last decade. Further, significant improvement has been achieved in the last 18 months with new downhole whipstock orientation technology.

Concurrently, advanced mill designs and improved mill dressings have been introduced. These have improved milling performance in the sidetracking process, as well.

A casing sidetrack is accomplished by anchoring a whipstock at a prescribed well depth and orientation and, using this wedge, directing a milling tool through the side of the casing. The milling system then completes a prescribed "window" through the casing. The time required to complete these steps varies according to whipstock anchor design, whipstock orientation method, and milling program used.

Casing Whipstock Systems

Casing whipstock system configurations vary according to design of whipstock and means of whipstock anchor actuation.

Configurations. Casing whipstocks are available in one- and two-piece designs. Two-piece systems require the anchor to be set on an initial wireline or drillstring trip into the hole. Next, a wireline survey is run to determine the precise orientation of the anchor's locator lug. The whipstock is then attached to the anchor during another drillstring trip.

One-piece anchor/whipstocks are simply run to sidetracking depth on the drillstring, oriented, and set. Milling can commence with a single drillstring trip.

Actuation methods. Casing whipstock anchors differ according to how setting force is applied and whether an abandonment packer is integral to the anchor. Two-piece whipstock systems, which typically utilize modified production packers, are primarily set hydraulically using wireline or drillpipe setting tools. The anchors in one-piece whipstock systems can be set mechanically or hydraulically, depending upon design.

Mechanically-actuated anchors require a cement plug or bridge plug to be placed in the well prior to tripping into the hole with the anchor/whipstock. When the anchor mechanically contacts the plug, pins are sheared and the resulting applied force sets the whipstock.

With this approach, actual applied forces at the whipstock cannot be assured due to varying well profiles and drillstring drag. Also, interaction with anything in the well prior to reaching sidetracking depth can activate the anchor prematurely. The quality of anchor/whipstock placement is less than certain.

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