Abstract
Enhancing oil recovery in onshore mature fields is one of the important aspects of Total Gabon's activity. This involves workover or horizontal sidetracks on existing wells located on Total's various fields within Gabon.
For these operations, Total Gabon faces several challenges. Economical: these operations usually target relatively small oil reserves, therefore the drilling unit choice is a key factor in the operational economics.
Technical: variety of the well candidates is considerable with reservoir TVD ranging between 800 to 2500 meters.
Rig availability: over the past few years, increasingly fewer onshore drilling units were available in Gabon and none of them could fulfill both the economical and technical criteria.
The use of Hydraulic Workover Unit (HWU) was considered as a possible alternative. Fewer and lighter loads would have to be moved between locations. These units have a smaller foot print requiring less civil works and have almost similar capabilities as a light drilling unit.
A feasibility study was carried out and preliminary technical discussions were held with various contractors. It was concluded that this approach was indeed feasible, provided that some modifications were done to existing units: installation of an adequate mud system, improvement on existing rotary heads, reinforcement of the hydraulic power supply to allow 24 hours operations instead of the usual 12 hours.
The HWU campaign started in February 2004 and included three workovers plus four re-entries (horizontal sidetrack after milling casing).
Beside the increase in oil production potential of about 14000 bbls/day, the potential cost saving is about 3.5 Million US$ for the entire campaign compared to a conventional rig operation.
The project provided a valuable learning experience. The use of HWU for re-entry operations is now regarded as a proven technique
