Edwards, Stephen Tean; (1997) A study of in-situ stress magnitudes in the North Sea basin from borehole measurements. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The stress field in the crust is a fundamental first order geophysical property that is intimately linked to the dynamic behavior of the Earth. This project has concentrated on crustal stress magnitudes as these are generally less well understood than stress orientations. Leak-off tests have been evaluated as a potential method of stress magnitude estimation, and the most suitable uses of leak-off data to estimate stress magnitudes have been defined. Vertical stresses have been estimated using geophysical logs. The variations of stress with depth, geographic domain, lithology and pore pressure have been has been studied in order to investigate the origins of crustal stress in the North Sea basin. The most reliable method of stress magnitude determination is the hydraulic fracturing (hydro-frac) method, however, hydro-frac data is rare. By contrast the leak-off test is performed routinely by the oil industry, with several tests in each hole drilled. An extensive 3-dimensional dataset of leak-off pressures therefore exists for the North Sea. Datasets have been obtained for the southern North Sea, and also from onshore boreholes drilled by UK Nirex, where leak-off tests and hydro-fracs have been performed in the same holes. This has enabled the leak-off test to be evaluated as a possible stress determination method. From these datasets, it is concluded that the trends of leak-off pressure with depth reflect changes in the minimum horizontal stress magnitude (σh) with depth. Where leak-off test pressure records are available, it is seen that the shape of many leak-off test pressure/volume plots resemble those of hydro-frac re-opening plots, and that in these leak-off tests, the leak-off pressure is very close to the hydro-frac determined σh. Furthermore, when leak-off tests are conducted carefully, a slightly extended test procedure can yield even better estimates of σh. Over 3,000 leak-off test results have been obtained from throughout the North Sea. The trends of the leak-off test pressures with depth are investigated and from this data it is inferred that σh varies with geographic location within the North Sea, with lithology, and with formation pore pressure. These variations are investigated within the context of models of stress with depth applicable to sedimentary basins. It is concluded that although a large part of σh is due to the weight of the overburden, a component of tectonic stress is also present. This component of tectonic stress is significant at all depths in the southern North Sea but increases with depth in the northern and central North Sea to become significant only at depths of several thousand feet and more. This is consistent with the pattern of borehole breakouts seen from previous basinwide studies in the North Sea. Where possible, estimates of σh have been combined with borehole breakout measurements from 4-arm dipmeters. This integrated data has been analysed, within the framework of a suitable rock failure criterion, to estimate a lower bound for the magnitude of σH. Results indicates that the stress regime in the North Sea is predominantly a strike-slip regime. The trend of σh with depth established for the central/northern North Sea in this study is compared to trends established by other workers in a variety of geological settings from around the world. It is seen that at shallow depths, σh in the North Sea (and other relatively young sedimentary basins) is low compared to other regions. However, at depths of several thousand feet and greater, the level of σh is equal to or greater than many other regions around the world. The high levels of σh at depth in the North Sea could be partly due, at least in part, to the effects time dependent of creep process.

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