Directional Drilling with Casing

Summary

Casing while drilling (CwD) has proven to be an effective method of reducingdrilling costs and solving drilling problems.Most of the current CwDactivity focuses on drilling vertical wells, but interest in directional wellsis increasing as CwD benefits in straight holes are demonstrated.

A directional CwD system has been run sufficiently to prove that directionaldrilling with casing is practical with casing sizes from 7 to 13? in.Thesystem uses a wireline-retrievable-directional-drilling assembly (positioned atthe lower end of the casing) to replace the conventional directional tools usedwhen drilling with drillpipe.These tools have been used to drill toinclinations greater than 90° and have been retrieved and rerun at inclinationsranging from vertical to horizontal.

Directional CwD can be used for a broad range of directional applications tocapture proven advantages demonstrated in vertical wells.

Introduction

Growing commercial activity shows that drilling with casing is increasinglyaccepted as a practical method of reducing drilling costs and solving drillingproblems.1-5 This activity includes both onshore applications in which theentire well is drilled with casing and offshore applications in the Gulf ofMexico and Gulf of Thailand, in which only the first hole section or two aredrilled with casing.

Most CwD activity has been focused on drilling vertical intervals, butinterest in drilling with casing in directional wells is increasing as theprocesses for drilling straight holes become proven, CwD benefits aredemonstrated, and more versatile tools become available.

Vertical wells may be drilled with casing, using a simple system consistingprimarily of a special bit attached to the casing that can be drilled out torun subsequent casing strings.But when there is a need to drill with amotor without rotating the casing, or the section cannot confidently be drilledwith a single bit, then a retrievable drilling assembly that can be recoveredand rerun is required.Even some sections that can be drilled with adrillout bit may be more cost-effectively drilled with a retrievablesystem.

Retrievable CwD equipment is required for directional wells because of theneed to recover the expensive directional drilling and guidance tools, the needto have the capability to replace failed equipment before reaching the casingpoint, and the need for quick and cost-effective access to the formations belowthe casing shoe.

Versatile CwD tools required for successful directional-drilling operationshave become available recently.A wireline-retrievabledirectional-drilling assembly, positioned in the lower end of the casing,replaces the directional tools used in a conventional bottomhole assembly(BHA).

This directional CwD system has been used with 5½-, 7-, 9?-, and 13?-in.casing to drill deviated wells with relatively low inclinations.Thesystem also has been used with 7- and 5½-in. casings to drill several testwells with inclinations that approached or exceeded horizontal.

Successful directional operations require more than simply havingdirectional tools available that can be run below the casing.BHA responsemay be quite different when drilling with casing, compared to drilling withconventional systems.Torque and drag must be managed through selectingthe casing connections, stabilization, and operational practices at thewellsite. Special surface-handling equipment is often required to makethe CwD process efficient.

The following paragraphs explain the processes that are used todirectionally drill with casing, highlight some of the issues that must beaddressed when planning these operations, and discuss some of the testing andfield applications in which the system has been used.