This paper discusses a field case review of the processes used to identify, characterize, design and execute a solution for a waterflood conformance problem in the Ekofisk Field that developed in late 2012. The Ekofisk Field is a highly-fractured Maastrichtian chalk reservoir located in the Norwegian sector of the North Sea. Large scale water injection in the field began in 1987 and overall the field has responded well to waterflood operations. However, fault reactivation coupled with extensive natural fractures and rock dissolution has resulted in some challenging conformance issues. In late 2014, a solution was executed to control this problem. Details of the diagnostic efforts and how this data was used to identify, characterize and mitigate an injector/producer connection through a void space conduit (VSC) will be outlined and discussed. These diagnostics include pressure transient analysis (PTA), interwell tracers, injection profiles, seismic mapping, fluid rate analysis, fluid composition and temperature monitoring. The importance of this data analysis is the key element necessary to select an effective solution. The selected approach involved pumping a large tapered nitrified cement treatment into the offending injector, which is believed to be the single largest nitrified cement operation ever pumped within the oil industry. Because of extremely rapid communication with an offset producer, a protective gel was used to reduce the risk of cement entry into that producer. A brief review of alternative mitigation options and the reasons for selecting the nitrified cement treatment will be discussed. Additionally, a complete review of the shutoff technique, product, damage mitigation strategy, and complications associated with timing and coordination in an offshore environment will also be discussed. Finally, a summary of lessons learned, job execution observations, post-treatment performance results over the past three years, and forward plans will be presented. Based on these results it is believed that there are a number of opportunities to add strong value through conformance engineering.

Injection in shale with matrix permeability in the nano-Darcy range and without the presence of any permeable layers has been performed for over 15 years on the Norwegian Continental Shelf, for the purpose of cuttings and waste fluids re-injection. The review of this experience shows that, in the early days, many cases of unconfined injection occurred through the vertical propagation of hydraulic fractures over 3000 feet distance that sometimes led to leakages to the seafloor after only a few thousands of barrels had been injected.For avoidance of this, some Operators -e.g. ConocoPhillips Norway -developed techniques allowing wells to dispose of several million barrels into individual shale domains, in safe confined-conditions, with vertical propagation of the disposal domain less than 1000 feet above the injection point. In the case of Ekofisk, this confinement is established by a careful selection of the injection interval and location, a detailed analysis of the injection records and dedicated monitoring programs. Recently, usage of frequent 4D interpretations of seismic surveys shot over a permanent sensor array placed on the seafloor above the field allowed a detailed domain-mapping and independent dynamic-monitoring.The paper focuses on the detailed analysis of those later cases and demonstrates their success from the use of comprehensive field data, which were obtained by the creation of a massively-fractured domain around the injection point -i.e. a conjunction of induced fractures and the opening of pre-existing natural fractures. The analysis of hundreds of pressure fall-off examples shows that the permeability height product (kh) of the shale rock-mass around the injection point reaches several Darcy feet -i.e. orders of magnitude more than during shale reservoir stimulation.One of the cases presented in the paper is used to show how the injection confinement was achieved and how the permeability around the injection zone developed as a function of the injected volume. It shows that after injecting about 10 thousand barrels, the permeability height product (kh) around the injection zone already reached several Darcy feet. The mechanisms responsible for the blockage of the propagation of the primary hydraulic fracture and the diversion of the fluid into secondary and tertiary fractures are clearly indentified and quantified.