Completion and intervention costs across the North Sea are at an all-time high and continue to challenge development economics; this has been exacerbated recently by a deteriorating oil price making costeffective completion and intervention operations even more challenging. Typically at times such as these, the industry turns to the fracturing and stimulation process to maximise the return from existing assets and new wellbores. While this method continues to be feasible, increasingly this can only be achieved through challenging existing conventions and applying increasingly innovative approaches to the principal aspects and assumptions applied to the operations themselves; namely the deployment and the effectiveness of such operations in the field.As the cost of a fracturing and stimulation execution mounts, due to numerous factors, including the efficiency of the service itself, available/economic well intervention capability, provision of sufficient accommodation (somewhat surprisingly/unhelpfully) and an ability to flow-back and clean-up in an efficient and effective manner. This paper will provide examples and case histories, across a number of our North Sea operations, which challenge and optimise each of these areas and demonstrate a number of mechanisms and approaches that have successfully been applied in order to reduce overall costs.Some of the examples include a case history of the use of a boat (stimulation vessel) to boat (intervention vessel) stimulation approach, in order to eliminate secondary rig costs and thereby maximise the economics. Consideration of the use of a rig-based stimulation capability vs. extensive boat operations, efficient and cost effective well recompletion approaches and a suite of incremental improvements born out of continued North American solution development and deployment, such as the use of expandable packers and ball-drop matrix stimulations. Increasing the effectiveness of fracturing and stimulation (and maximising the productivity of each and every individual wellbore) is an obvious approach to improving economics and can be achieved in any number of ways. Again this paper will present a suite of examples and case histories of where this has already been achieved, across a number of our North Sea operations.The current 'perfect storm', of a low oil-price environment combined with high industry contractual costs, is forcing all operators to become increasingly innovative in their pursuit of economic resource development across the North Sea. This paper presents a number of examples of innovation and cost-saving approaches that have been employed in order to achieve such economic intervention and provide further insight into the additional development potential and opportunity that may exist across this varied asset base.

Matrix stimulation is commonly utilized to increase well productivity and it is a process in which fluid selection plays a key role in treatment success. However, in offshore fields, the acid formulation must not only be effective in damage removal but it also has to meet stricter HSE regulations while safeguarding expensive and complex installations from corrosion impact. In high temperature environments or where sensitive metallurgy is deployed, higher doses of corrosion inhibitors are required as well as additional additives in order to avoid unwanted reactions, further complicating the handling and HSE aspects of such acids. The environmentally friendly chelate, glutamic acid N,N-diacetic acid GLDA, has been examined as an alternative for acidizing and descaling treatments, demonstrating good field performance in terms of productivity and injectivity increase. All this achieved while providing a safe and convenient system for handling, due its low toxicity, fewer required additives and biodegradability. Numerous laboratory tests have measured and confirmed considerably less corrosion risk, across a wide range of conditions, when compared to conventional formulations based on both HCl and organic acids. Within this paper, the field performance of the GLDA system will be evaluated under even more challenging conditions, endured during the acidizing of an offshore well in the North Sea. The wellwork programme for the low rate gas producer consisted of performing two new perforation runs, followed by the injection of the GLDA treatment, which was then bullheaded into the formation with a nitrogen assist. The treatment formulation consisted of GLDA diluted in fresh water with trace amounts of surfactant and mutual solvent to aid in the flow-back of the spent acid. Due to an unexpected power failure on the platform, the treatment remained stagnant in the tubing for some 28 hrs at 300°F. As this was the first GLDA treatment in this field, this situation appropriately raised potential well integrity concerns; as would most certainly have been the case with a conventional HCl acid package. However, once the operation was restarted the acid was successfully bullheaded into the formation and no issues resulted with the low carbon and CRA based metallurgy of the completion even after this extensive unplanned exposure. Additionally the treatment resulted in a significant productivity increase. These operations and results demonstrated not only the gentle nature of GLDA for integrity considerations; but also an effective cleanout of perforations and near wellbore area as required from a replacement system. The success of the treatment proved the intrinsic value and reduced risk that can be accessed by use of such systems.

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.