The Planning and Execution of the Tortue High Rate Gas Well DST in Offshore Deep Water Mauritania

Abstract

Abstract The Greater Tortue Ahmeyim (GTA) field offshore Mauritania and Senegal is a large, deep-water gas complex with two main reservoir sequences, the Lower Cenomanian and the Albian. The area was discovered by Kosmos Energy (Kosmos) in April 2015 by the Tortue-1 discovery well. Kosmos has since successfully confirmed three major fairways of the Senegal River Trend within the broader Mauritania and Senegal basin with the Guembeul-1A, Marsouin-1, Ahmeyim-2, Teranga-1, and Yakaar-1 wells. The outboard Cretaceous petroleum system offshore Mauritania and Senegal is potentially one of the largest petroleum systems ever opened along the Atlantic Margin. Kosmos entered into a joint venture partnership with BP in respect of its interests in Mauritania and Senegal in December 2016. Dynamic production data was desired to further appraise the Lower Cenomanian reservoirs. A fast-track Drill Stem Test (DST) was conceived, planned, and conducted on the Tortue-1 discovery well beginning in June 2017 to assist in validating reservoir connectivity, productivity, original gas-in-place, and fluid quality. The DST, operated by Kosmos on behalf of the partnership, targeted two distinct reservoirs within the Lower Cenomanian sequence. The key information extracted from the well test has been used to support the Front End Engineering Design (FEED) currently underway, with a corresponding Final Investment Decision (FID) planned in late 2018. This paper presents the objectives and design methodology, as well as the technical and operational challenges while conducting a dual-zone, high rate gas well test in ultra deep water within a compressed time schedule. This paper also presents the technology that Kosmos and BP selected to isolate the test intervals and gather pressure and temperature data during the extended flowing and buildup periods. Finally, the key results of the DST are highlighted that underpin the development concept for the Tortue field.