Sidetracking in a Single Trip
Authors: Miller Greg , Dewey Charles , Saylor James 
Published: December 2007

IADC Members

Historically, sidetracking through existing casing requires multiple trips to effectively mill a useable window for the next directional drilling assembly. The number of trips needed depends upon the sidetracking tools capability and performance and a comfort level of the drilling engineer in charge.

This paper presents a true one-trip system for sidetracking operations. The paper will discuss design criteria, engineering analysis and validation of the design by detailed test data from field tests including photographs of actual window profiles filmed by a downhole video camera.

The objective of the one-trip system is to accomplish the following seven steps in a single trip:run assembly;orient whipface (with MWD);set (hydraulically) whip-stock assembly;separate the mill from the whip-stock;mill window;drill rat hole;pull out of hole.

The result is a full-size window with a minimum of milling time and a full-size rat hole for a directional drilling assembly.

The paper will discuss the technical innovation incorporated in the four major components of the system, namely:flow switch, operationally manipulated by pump pressure, providing on/off mode for flow required for MWD orientation, circulation and anchor setting;running tool, to hydraulically set the integral packer and anchor;milling tool, a 3-in-1 mill, each with different objectives and dressed with preformed and crushed carbide and finally;the whip-stock which includes a multi-ramp design to guide the mill effectively and expediently (Fig. 1).

Also discussed will be contributions made by computer programs for bottom hole assembly and hydraulic analysis of the one-trip sidetracking system.

Introduction

It is apparent that one of the major problems in sidetracking casing is achieving the initial cut into the casing. Producing this cut out is normally done using the starter mill. Since the purpose is to achieve the sidetrack in one trip, the use of a starter mill was not an option. Typically, the starter mill has an extension below the milling portion that is used to push the cutting area off the whipface and into the casing wall. The next mill into the hole then picks up at the cut out, and continues milling the window. The mill used in a one trip system must be able to efficiently cut out of the casing as well as mill the rest of the window and drill the desired rat hole. Other systems may use a wedge, ramp, or block to initiate cut-out while protecting the whipface from the mill. The ramp device may be configured in any number of ways, but usually has to be milled away at some point during the operation.

With the system being presented here, no such ramps are needed. The special design of the mill and the whip-stock ensure that the two components work together to achieve the sidetrack.

Design Criteria

The major design criteria was to sidetrack a wellbore in one trip in 9-5/8 inch casing. It was broken into multiple design tasks as follows:–Make-up the one trip assembly on the rig floor with minimum time and run in the hole to desired depth.–Orient and hydraulically set the anchor.–Shear-off at a predetermined value.–Create a full size window (size to be the same as the drift of the casing, 8.50 inches).–Drill a full gage rat hole (same diameter as casing drift, 8.50 inches). The rat hole should also be allowed to be as short as possible.–Pull the milling assembly out of the hole without resistance. - Make-up and run the directional drilling assembly for 8.50 inch hole and drill further.–Pull out of the hole with the directional drilling assembly.

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