Abstract

Through-Tubing Rotary Drilling (TTRD) is a slim-hole sidetracking technique successfully employed from North Sea platforms to economically access marginal targets in mature fields from existing wellbores. TTRD typically uses 2–7/8" standard production tubing with high-torque threaded connections, run from the standard platform drilling rig, to drill-out through the existing completion via whipstock.

TTRD sidetracks in the North Sea have been less than 3500 feet in length, and completed with 2–7/8" or 3–1/2" flush-jointed liners. As a contingency, the 2–7/8" drillstring can be cemented in place as a liner.

Since

  1. TTRD does not require the mobilization of specialized equipment, and

  2. the drillstring may be rotated to reduce differential sticking and improve cuttings returns, and

  3. the drillstring will tolerate high overpulls, TTRD has proven consistently more economical than Coiled Tubing Drilling (CTD) in similar applications.

The objective of this paper is to review and discuss string design and applications issues for Through-Tubing Rotary Drilling. Topics will include:

  • Drill-Floor Handling Issues

  • Drilling Torque and Over-Torque

  • Downhole OD Wear of Tubing Upsets

  • Fluid Dynamics, especially Equivalent Circulating Density (ECD) Issues

  • Hole Cleaning

  • Fatigue Life Estimation

  • Repeated Make & Breaks with Production Tubing

  • Subsequent Use of the Drill String as Production Tubing

  • Cementing and Completions

Overview of Through-Tubing Drilling (TRD)

Through-Tubing Drilling (TRD) is a slim-hole side-tracking technique currently used on offshore wells to inexpensively drill marginal targets with a KOP in the existing production liner or completion tubing, usually with the goal of prolonging the economic life of a mature field. TRD eliminates the need to recover completion tubulars to perform a sidetrack.

There are two versions of Through-Tubing Drilling: Coiled-Tubing Drilling (CTD) and Through-Tubing Rotary Drilling (TTRD) using production tubing or slender drillpipe as the workstring. CTD has a generally-accepted drilling envelope of about 2500 feet from the whipstock (usually limited by sliding friction), while the envelope for TTRD is generally accepted to be about 3500 feet from KOP.

Both techniques typically use whipstocks within the existing completion, relatively simple BHAs with orienting tools and usually some simple form of LWD (e.g. resistivity and annular pressure measurement or APWD), and a PDM driving a bi-centered bit.

The selection of CTD versus TTRD is often an economic rather than technical decision, and a large body of knowledge has developed over the last few years concerning this economic decision (e.g. see Refs. 1–3 and 6).

Coiled Tubing Drilling (CTD)

Coiled Tubing Drilling is often the best alternative if:

  • The platform is not normally manned, has no resident main drilling rig, or the main drilling rig has been decommissioned.

  • The main rig is in operation, but is fully engaged in development drilling, so simultaneous operations are desirable.

  • Well plan calls for abandonment and setting whipstock on-line, or many round-trips are expected.

  • Differential sticking and hole cleaning risks are low.

  • CT drilling rig are locally available and easily mobilized (e.g. proper rig cranes available).

  • Real-time LWD is required (best done with e-line in CT), or the well must be drilled underbalanced.

  • The target accumulation is less than 2500 feet from KOP

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