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Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forets, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress S H . A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forets (where the magnitude of S H is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of S H from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of S H based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.

The orientations of the greatest horizontal principal stresses (SH) derived from borehole breakouts are presented for 30 boreholes located in the Central Graben, the Viking Graben, the More-Voring region, and the Barents Sea area. Additional borehole breakout data from 20 other wells for which information about breakout length was available have been used to calculate the mean azimuth of SH for the four areas of investigation. The mean azimuth of SH is calculated from the length-weighted distribution of the breakout orientations. In the Viking Graben and the Central Graben a SH orientation of about N 78° and N 97° was found, respectively. For the More-Voring region a mean SH orientation of N 127° has been evaluated. Borehole breakout data from the Barents Sea area indicate a SH orientation of about N 177°. The SH orientations presented in this study are consistent with the results from previous studies which suggests that the direction of the maximum compressive horizontal stress SH differs off the coast of Norway markedly from the well documented NW-SE trend of SH in Western Europe. The observed SH orientation changes significantly across the investigated regions which may have to be attributed to their complicated geomechanical setting.

For a borehole of the Gullfaks field in the northern North Sea 60 days borehole pressure data of two successive registrations (hereafter parts I, II) were analyzed. From correlation with theoretical ocean tides a loading efficiency γ of 0.35-0.40 and of 0.16-0.19 were obtained for parts I and II, respectively. No influence of air pressure or earth tides could be resolved. Assuming an undrained Poisson ratio vuranging between 0.30 and 0.36 and assuming that the vertical loading results in an areal strain εa of the same magnitude as the vertical strain εV a Skempton ratio B of about 0.45 for part I and of about 0.20 for part II were derived. B values are somewhat higher assuming smaller εa. The notably different values for γ and thus, B can be explained by a significant change in reservoir properties, which was induced by several injections in the borehole in the time between the two registrations.

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

This paper presents field data from pump-in/flowback tests at different locations offshore Norway, illustrating that standard extended leak-off tests (XLOTs) in tight formations may overestimate the minimum horizontal in-situ stress by at least 20 bar, corresponding to 0.1 s.g. at 2000m. The pump-in/flowback test is a simple variant of an XLOT. After the shut-in period, fluid is allowed to flow back over a fixed choke, and the pressure and the returned volume are recorded as a function of time. We compare results from several pump-in/flowback tests to the results of the conventional pump-in/shut-in method used for XLOTs. For the tests presented here, performed in tight formations, the true fracture closure pressure is shown to be up to 20 bar lower than what would be inferred from a traditional interpretation of the standard XLOT. It is argued that the pump-in/flowback test in a majority of cases will be straightforward to interpret, and that it should hence be the preferred method for measuring the minimum horizontal in-situ stress.

Abstract Borehole failures like drilling-induced fractures and breakouts observed inimage logs offer the possibility to derive high quality stress orientations. Image logs from 16 wells in the Northern North Sea are analysed for theoccurrence of borehole failures. While in wells to the west of the Viking Graben very few stress related borehole failures are observed abundantdrilling-induced fractures are detected in all of the image logs in wells to the east of the graben. Also the orientation of SHappears to be slightly different for the western and the eastern part of the investigated area. An SHorientation of approximately NI000E is found in the western part, while the SHorientation is found to be approximately N800E for the wells in the eastern part. Investigating the conditions for fracture initiation in three of the wells allows to constrain the possible magnitude of SH. For all three wells the occurrence of drilling-induced fractures can only be explained if theSHmagnitude is at least slightly greater than the vertical stress, which indicates a strike-slip tectonic regime(Sh Introduction There is increasing awareness for the state of stress being an important parameter for engineering tasks in reservoir development, and for the geoscientific understanding of fault seal and hydrocarbon migration. While quite sophisticated tools for e.g. prediction of wellbore stability and sand production are available, the knowledge of the present-day in-situ stress field is still often based on scarce and poor quality data. Formerly, stress orientations were commonly derived from breakout orientations determined from the analysis of four-arm caliper data. As this tool provides only two diameters of the borehole cross section, the accuracy of derived stress orientations is generally low, and special care has to be taken to differ key seats from breakouts. This has in many former analyses lead to a significant scatter in the orientation of SHwhich often was explained by local influence of topography, faults, or salt layers, while it may be due to the immanent weaknesses of the four-arm caliper analysis.

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling-induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near-hydrostatic pore pressure.