Engineering Approach Uses Underbalanced Coiled-Tubing Drilling
Authors: Carpenter Chris 
Published: July 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 210978, “Novel Engineering Approach Using Underbalanced Coiled-Tubing Drilling Solutions for Tight Sour Gas Carbonate,” by Mohamed Osama Abd El Meguid, Ayman El Shahat, and Ali Sulaiman Bin Sumaida, ADNOC, et al. The paper has not been peer reviewed.

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Because of declining reservoir pressures in some of its onshore gas carbonate fields, the operator initiated a three-well underbalanced coiled-tubing drilling (UBCTD) campaign in its onshore Asab and Bab fields, with two wells to be drilled in Asab and one in Bab. Because of the high hydrogen sulfide (H2S) content, a closed-loop system—the first one successfully implemented in the Middle East, according to the authors—was implemented. The complete paper discusses the design and planning involved in the drilling of these three wells, the operational challenges encountered, and mitigations used.

Introduction
One of the operator’s main objectives was to increase production while safely maintaining or minimizing operating costs. One method of achieving these goals is the use of coiled tubing (CT) to drill formations in underbalanced mode.
The Asab and Bab fields are southwest of Abu Dhabi. Both fields have high H2S and CO2 content, with a bottomhole temperature of approximately 260–270°F. The candidate wells were drilled conventionally until the top of the reservoir section; then, the UBCTD package was moved to drill the reservoir section in underbalanced mode.
The typical well completions, from top to bottom, include the following:
- 5.5-in. tubing
- 4.5-in. tubing
- 7-in. liner
- 100 ft of 6-in. openhole

Project Objectives
Because of safety and logistical constraints, the project would use a closed-loop system and recirculate the drilling fluid, thus avoiding high water consumption. Also, the system needed to be designed to safely handle H2S in return fluids. UBCTD using a closed-loop system has been performed elsewhere but not in a high-H2S and high-CO2 environment.

Trajectory Design and Tubing Forces
Trajectory design is an important part of drilling operations; reservoir targets given by geologists and reservoir engineers must be met. To expose more drilling footage in producing formations and to increase overall productivity, the geologists provide formation-top coordinates so that the drilling engineer knows how to proceed as drilling progresses through the reservoir subunits.
When the well trajectory was designed in the discussed case, the CT engineer input planned well surveys into software so that the CT forces could be analyzed. Because these predrilled wells had never had CT interventions, the friction coefficient was unknown and the outcome of the software could have been very different from that observed while drilling.
Friction coefficients from other conventional CT operations within the UAE, as well as those from other CT drilling operations in the region, were reviewed and then used for analysis. With the lowest friction coefficient of 0.3 for open hole, the target drilling length could not be achieved. Project planning required a solution to enable the lateral to meet the proposed objectives.