Unlocking Tight Chalk Potential in Mature North Sea Oilfield Through Effective Acid Fracturing

Abstract The Mungo Field is a medium-sized oil field with a primary gas cap located in the Central North Sea. It has been developed as part of the Eastern Trough Area Project (ETAP) via a Normally Unmanned Installation (NUI) which is positioned directly above the field, with production tied back ca. 20 km to the ETAP Central Processing Facility (CPF). Hydrocarbons are trapped in a pierced four-way dip closure against the Mungo salt diapir. The principal reservoir of the Mungo field is the steeply-dipping Palaeocene sands of the Sele, Lista and Maureen Formations, which overlie the Ekofisk, Tor and Hod Formations of the Chalk Group. The Palaeocene sandstone reservoir has been developed under combined water and gas injection since 1998. The underlying tight Chalk reservoir contains a poorly understood but potentially very substantial oil resource (estimated at 30 – 300 MM.bbls STOIIP). Direct development of the Chalk within the Mungo field has been very limited to date, with little offtake and few completions. There have been a number of key challenges to overcome in order to demonstrate that the Chalk can be efficiently developed on Mungo. These challenges include the ability to achieve economic well production rates combined with demonstrable recovery of oil from the low permeability chalk matrix, given that there is very little evidence of natural fracturing. This is most accurately described in the definition and selection of an appropriate and efficient completion approach; a process and case history which this paper will fully detail. There are several features that make development of the Mungo Chalk both appealing and compelling, including the proximity/connection to existing infrastructure (with reducing throughput) and penetration of the Chalk by some existing Mungo development wells. This combination offers a unique potential suite of opportunities for low cost intervention, appraisal and subsequent development via recompletion. In 2015, BP performed a Chalk appraisal test in an existing Mungo producer, W169 (22/20-A19), including the pumping of two distinct acid fracturing stages, a flow-back/clean-up, stable rate well-test and a long term shut in for a PBU. The W169 well was selected as the candidate well, as it intersected 220 m of the chalk sequence, including the Hod, Tor and Ekofisk formations. This paper presents the full sequence and details of this Mungo Chalk reservoir evaluation process. The information provided will describe the approaches taken to the design, the planning and the execution of cost-effective stacked, multi-zone acid-fracturing operations. Finally, the paper will close-out with the results of the operations and post job analysis, and provide an overview of future potential that has been unlocked by this sequence of operations.