Through Tubing Rotary Drilling (TTRD) provides a low cost means of accessing near-wellbore reserves. To date most TTRD wells have been drilled from either land locations or fixed offshore platform installations. In recent years, subsea TTRD (SSTTRD) has been coming to the fore as a means of maximising production from bypassed or attic oil in brown fields. Conventional subsea operations are already very expensive, therefore operational efficiency is critical to drilling, completion and intervention operations cost and economics. Against this background, Operators must realise that there are significant differences (operational, technical and logistical) between subsea and platform drilled TTRD wells. There are those situations where modifications will have to be made to the chosen rig or surface equipment to accommodate TTRD operations. Therefore, it is important that in deciding to embark of a subsea TTRD project, a project team with the appropriate mix of personnel and expertise is constituted to manage the planning and execution of the project.
Planning and Operations
Some of the planning and operational challenges associated with subsea TTRD centre on the management of rig dynamics (heave, roll and pitch), hole cleaning inside the marine riser, effect of heave on swab and surge pressures, design of appropriate drilling fluid, well control contingency planning, striking a balance between BHA design and formation evaluation requirements, tubing wear and management of integrity, equipment design and qualification for subsea operations among other things.
Drill string design is a key area determining not only the required
pipe diameter, (largely governed by existing completion constraints
with 3 ½ “or 2 7/8” pipe the most commonly used), but the mechanical,
hydraulic and metallurgical properties. The most demanding aspect of
planning and actually drilling a SSTTRD well is to avoid damaging
critical completion components and/or creating excessive
tubing/accessories wear to the point where post-drilling remaining
tubing wear is incapable of supporting expected production case load.
Therefore, given high intervention and/or workover cost, any SSTTRD
planning must ensure that the post-drilling intervention phase leaves
the well in a fit-for-purpose manner to meet production obligations.
As a ‘brown’, ‘mature’ field technique aimed at increased oil recovery,
TTRD will almost certainly drill through zone(s) of significant
depletion. Where this is the case, the drilling operating window is
narrow and requires focused and detailed engineering to successfully
execute a TTRD well.
Our Experience
LEA has in depth experience from our operations in the UKCS an NCS
areas where we programmed the worlds first SSTTRD for Norsk Hydro in
Norway. Our specialist-drilling engineers can create a development plan
from candidate well screening selection to conceptual planning, design
and execution stage. LEA staff will also help manage the design and
development of project-specific policies, procedures, and equipment and
vendor selection necessary to execute the project. This
‘cradle-to-grave’ approach has been employed in all our projects to
date.