Schlumberger coiled-tubing drilling (CTD) initiative was implemented in west Venezuela (Lake Maracaibo) in 1993 from a CTD Barge. This purpose-built barge was assembled to minimize the risk to personnel and equipment while drilling superficial zones for a Venezuelan operator where the presence of shallow-gas sands makes safety a priority (Adamson S.J. 1994). To date, this project has drilled over 300 wells.
Nowadays, the operator has new challenges: to continue draining the remaining oil reserves, new process and designs are required to successfully achieve their expectations.
The new CTD challenge is focused on the Field C and Field D and more specifically the Eocene primary target zone and, as a secondary target zone, the Miocene. These zones are separated by a discordance1 . From the discordance and around the upper Eocene, formation pressure is significantly reduced (around 400 psi lower) with respect to the Miocene age reservoir pressure. This issue represents a serious challenge during the drilling phase because fluid losses might be present when crossing the discordance zone. To address this situation, an intermediate casing can be placed at the end of Miocene; however, this will limit the subsequent completion by reducing the final tubing size and therefore, affecting the potential well production.
The proposed solution is based on drilling a larger surface hole, 14¾ in., and setting larger surface casing, 10¾ in., allowing the operator to run larger completions (intermediate and/or production casings). If isolation is required across the discordance, this design will provide an optimal completion without compromising the well's oil production.
Several studies and analysis were implemented to adapt the design and execution phase with the new requirement from the operator and the operating capabilities of CTD Barge.
To date, four wellbores have been successfully enlarged to 14¾ in., completed with 10¾-in. casing, and cemented. Subsequently, a conventional drilling rig / barge has been used to complete these wells to their production phase, and represent an incremental production gain, estimated at over 150 bbl/day of oil for the four wells.