U-Shaped Re-Entry Drilling Revitalizes Depleted Reservoirs Offshore Congo
Authors: Carpenter Chris 
Published: May 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191436, “Extended-Reach U-Shaped Re-Entry Drilling in Shallow, Depleted Fractured Reservoirs in Congo’s Oldest Offshore Oil Field,” by Mike Tony Chembou, Bruno Valeri, Lois de Crevoisier, Michel Akue, and Vincent Rodet, Perenco Congo, and Gabriele Ghia, Achille Mouamba, and Ali Lazzem, Schlumberger, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed.

Against the background of a low-oil-price environment, a redevelopment project was launched to give a second life to a shallow, depleted, mature offshore Congo oil field with viscous oil (22 °API) in a cost-effective manner. The solution selected was to drill U-shaped side tracks (inclination at total depth=115°) from the original boreholes on an existing platform at a water depth of 60 m. The objective was to create a second drainage area. The project team used innovative, low-cost techniques to overcome many challenges.

Geological Context and Objectives

Under 27 platforms (four to five well-heads each), an accumulation of heavy, viscous oil is trapped in three reservoir layers. The field was produced with a combination of cold mode followed by a period of hot mode from steam injection. The consequence is a depleted (0.3-sg) and highly steam-fractured reservoir where fluid loss presents one of the primary challenges for infill development.

The pilot project consists of re-entering the current producers and then deepening them in a U shape to access the same shallow reservoirs more than 200 m away from the initial production leg. All reservoirs share the same water table identified below an impermeable formation underneath the target layers. To avoid water breakthrough, the trajectories were planned with up to 18°/30 m to prevent the well from entering the water-bearing layer, with minimum 30-m true vertical depth standoff from the oil/water contact.

Well Design and Architecture

The well architecture is selected to match the existing mother-well construction. All selected re-entry candidates are producing wells showing the following well architecture:

9 5/8-in. casing set as intermediate casing at approximately 100 m measured depth (MD) below the rotary table

A mixed string composed of cemented blank 7-in. casing and uncemented 7-in. slotted liner in front of the pay zones. An external casing packer (ECP) was installed between the two strings, and a cementing job was performed above the ECP. The float shoe at the bottom, therefore, was uncemented and prone to be untorqued during re-entry

The completion system consisted of progressive-cavity-pump or sucker-rod-pump strings, depending on the wells.

To fulfill the geological and reservoir objectives, the wells were re-entered by drilling a 6-in. hole section from a 7-in. float shoe to the U-shaped total depth before a 4½-in. liner was run. The sand-face completion evolved slightly from the first two wells (blank perforated) to the last three wells (slotted liner).