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Copyright 2012, Offshore Technology Conference This paper was prepared for presentation at the Arctic Technology Conference held in Houston, Texas, USA, 3-5 December 2012. This paper was selected for presentation by an ATC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright. In many cases, the hydrocarbon reservoirs are known to be overlain by a massive permafrost interval that extends over depths of up to 700 m below the surface active layer. These conditions create unique design and operational challenges for production and injection wells from the perspective of ensuring that well integrity will not be compromised by the inevitable thaw subsidence of the permafrost soil layers. These changes will result in soil deformations (including both vertical settlements (subsidence) and horizontal displacements) which can, in turn, induce significant well casing strains that need to be considered in selecting the well design and layout. The magnitude of the soil deformations that occur throughout the permafrost interval are highly dependent on the deposition history, insitu temperature and the physical and mechanical properties of the individual soil layers. Therefore, in order to accurately predict the soil deformations and resultant localized casing strain levels, it is essential to obtain reliable data to properly characterize the lithology (soil types) within the permafrost interval, as well as the frozen state and the relevant mechanical and thermal properties (both frozen and thawed) of individual soil layers.
We investigate whether atypical wave phenomena (multiples, guided waves, large attenuation) can be used to map the shallow structure and the state of permafrost. We analyze the data from a seismic survey acquired in the Beaufort Sea over degrading permafrost. Using viscoelastic fullwave modeling, we compare observed shots with simulated data over simple, yet representative seismic models of permafrost. Most of the arrivals found in the seismic shots of the Beaufort Sea dataset can be explained by the interaction of high velocity layers and the free surface, lateral velocity variations and high seismic attenuation within the permafrost. Observable features in the data (multiples, refraction amplitude decay, arrival times) can be linked to the frozen layer thickness, depth, velocity and Qfactor, showing the plausibility of using advanced processing technique such as viscoelastic full waveform inversion to detect changing subsea permafrost conditions. Presentation Date: Tuesday, October 13, 2020 Session Start Time: 8:30 AM Presentation Time: 9:45 AM Location: 362D Presentation Type: Oral
Abstract A layer of permafrost, approximately 500m thick, lies immediately beneath the surface of Arctic Alaska’s Central North Slope. This frozen layer of unusually high seismic velocity overlies more than twenty producing fields containing over 60 Billion in place barrels of light and heavy oil. Multiple reservoir levels ranging in depth from 1000m to 3000m are nominally imaged using conventional surface 3D seismic technologies. Natural arctic surface features and man-made sub-surface activities produce local bodies of unfrozen sediment with anomalously low velocity in the surrounding high velocity permafrost. Abrupt lateral and vertical velocity variations detrimentally affect the conventional surface seismic image at reservoir depths, thereby negatively impacting our ability to describe the reservoirs and maximize production of this vast resource. BP-Alaska, as operator of many of these fields, is motivated to apply optimal seismic acquisition and processing techniques to minimize permafrost-related image distortions. We are therefore developing a permafrost velocity characterization of both the offshore and onshore environments on the Arctic Central North Slope to guide the appraisal of available geophysical techniques. Our characterization is underpinned by: 1) physical properties of frozen and unfrozen sediments taken from the literature, 2) regional and local morphologies of the permafrost based on well log measurements, 3) the application of numerous geophysical techniques for the widespread measurement of shallow (upper 600m) velocities, and 4) a dynamic model for thawing of the permafrost caused by either naturally occurring or man-made circumstances to help predict the extent, shape, and depth of local velocity anomalies. BP-Alaska is making progress in understanding the permafrost velocity field on the Central North Slope; but gaps in our ability to characterize a large portion of the permafrost interval still persist. Some techniques show promise but may not be economic in a brown field business environment located in an arctic acquisition environment.