Coiled Tubing Operations from a Floating Anchored Vessel
Authors: Arangath R. , Falxa P. , Ackers M. , Ramsey M. 
Published: December 2007

Many production platforms are equipped with small cranes that do not allow the lifting of equipment required to perform well interventions, e.g., wellbore maintenance, matrix stimulation, water control, or through-tubing sand control. Treatments in the past have been performed with different techniques such as spooling the coiled tubing from a reel on a supply vessel to a second reel on the platform or the use of a modular crane. All these techniques require significant logistical support and an extensive amount of time to prepare all the equipment for the operation itself.

We have investigated the possibility of performing coiled tubing operations from an anchored vessel, because this method offers many economical and logistical advantages over conventional well intervention methods. Before implementing this technique at field level, we had to consider both the technical and the safety aspects of operating in an environment with waves up to 5 m height and strong marine currents.

A system was developed that was capable of both compensating for the wave motion and initiating an emergency shutdown if the vessel lost its anchors. This paper provides an overview of the justifications, technicalities, considerations, and constraints of performing a coiled tubing operation from an anchored vessel. The 15 treatments performed up to the present have been analyzed, highlighting the lessons learned, technical achievements, future developments, and main points that differentiate this operation from a standard coiled tubing well intervention operation.

Background

The offshore activity in the West African state of Gabon is concentrated from Cap Lopez to the southern border. (Figure 1). Drilling activity in these waters dates back to the early 1960s, and about 50 offshore platforms have been erected. Exploration activity is still ongoing in these waters, spurred by the more refined seismic data interpretation now possible with recent developments in subsalt layer analysis.

These offshore platforms are in water depths ranging from 9 m to 95 m. Most are four-legged platforms with limited deck size. The platforms have 3 decks: a boat landing deck at 2 m above the sea level, the wellhead deck at 8 m, and the helicopter deck at 13 m. Deck sizes depend on the number of wells located on the platform. Some of the platforms are constituted with a reinforced mono-conductor support system. These platforms are quite small (6.5 × 7.5 m) and are unmanned.

The producing wells are draining mature reservoirs. They require a relatively large amount of well intervention work such as matrix acidizing, water control, scale removal, and sand consolidation to maintain an economic production rate. Because the producing intervals are typically long, the volumes involved in these operations are not negligible and the requirements of fluid mixing and storage tanks are major considerations in equipment selection. Coiled tubing is routinely used for performing these well interventions, while nitrogen is used only in certain cases because most of the wells are equipped with gas lift system. In addition to the coiled tubing and pumping equipment, additional equipment is required, such as well testing and slickline. On certain wells, an electric line unit is required for performing some pretreatment diagnostic work (reservoir monitoring or production logging), reperforating certain sections to drain bypassed oil, or even post-treatment diagnostic work (production logging).

Most of the platform cranes lifting capacities are limited to 6 metric tones or less. On certain production platforms there are cranes with bigger lifting capacities; however, these cranes' relative position to the wellhead platforms do not allow the equipment to be placed on the wellhead deck.