Injection in Shale: Review of 15 Years Experience on the Norwegian Continental Shelf (NCS) and Implications for the Stimulation of Unconventional Reservoirs

Abstract 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. The paper indicates how safe, secured and controlled injection in shale can be achieved, as opposed to the unconfined cases of the early history on the Norwegian Continental Shelf. It also shows that while maintaining safety and vertical-confinement, massive increase of permeability of nano-Darcy shale rock-masses can be achieved. Both results are extremely important when it comes to cuttings injection in shale. Even more importantly, the paper discusses the implications of these results to the hydraulic stimulation of shale reservoirs in terms of both safety and efficiency. First, the permeability increases observed during cutting re-injection in the Ekofisk area can be set as realistically-achievable goals during shale reservoir stimulation.Second, the identification of the mechanisms having led to this development can be used to adapt current stimulation techniques to reach such permeability-increase goals.Finally, the techniques presented in the paper can be used to ensure the vertical confinement of future stimulation jobs.

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