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As a part of the U.S. Department of Energy (DOE) SubTER (Subsurface Technology and Engineering Research, Development and Demonstration) initiative, University of Wisconsin- Madison, Sandia National Laboratories, and Lawrence Berkeley National Laboratory conducted the Permeability (k) and Induced Seismicity Management for Energy Technologies (kISMET) project. The objectives of the project are to define the in situ status of stress in the Sanford Underground Research Facility (SURF) in Lead, South Dakota and to establish the relations between in situ stress and induced fracture through hydraulically stimulating the fracture. (SURF) in Lead, South Dakota. In situ tests are conducted in a 7.6 cm diameter and 100 long vertical borehole located in the 4850 Level West Access Drift near Davies Campus of SURF (Figure 1). The borehole is located in the zone of Precambrian Metamorphic Schist.

Simulation results of near-wellbore failure phenomena are presented from a joint experimental-numerical study directed at developing a robust numerical simulation capability for the exploration and prediction of near-wellbore mechanics. An experimental procedure was developed for the laboratory simulation of slurry injection. A true-triaxial vessel, which applied realistic, three-dimensional stress conditions, was used to perform slurry injection into Berea sandstone. Under anisotropic horizontal stress conditions, vertical hydraulic fractures initiated and propagated in the direction of the maximum horizontal stress. Under isotropic horizontal stress conditions, multiple vertical fractures were induced and propagated in random orientation. A computationally efficient numerical model based on the discrete element method (DEM) is described and applied to simulate various wellbore phenomena. Radially graded, two-dimensional DEM models of the near-wellbore region were created of bonded disk elements. Source ...

We have conducted five hydraulic fracturing stress measurement campaigns in Korea, involving 13 test holes ranging in depth from 30 to 250 m, at locations from North Seoul to the southern coast of the peninsula. The measurements reveal consistent crustal stress magnitudes and directions that suggest persistence throughout western and southern Korea. The maximum horizontal stress σ H is oriented between ENE–WSW and E–W, in accord with plate movement and deformation, and with directions indicated by both focal mechanism solutions from earthquakes inland and offshore as well as borehole breakouts in mainland China close to its eastern coast. With respect to magnitudes, the vertical stress is the overall minimum stress at all tested locations, suggesting a thrust faulting regime within the relatively shallow depths reached by our tests. Typically, such a stress regime becomes one favoring strike-slip at greater depths, as is also indicated by the focal mechanism solutions around Korea.

We conducted 9 complete hydraulic fracturing in situ stress measurements in vertical borehole HF1 at the 420 Level of AECL's Underground Research Laboratory (URL), near Pinawa, Manitoba. The tests did not result in vertical fractures. Thus, a generalized least-squares criterion was employed to compute the estimated principal horizontal in situ stresses. The large number of redundant test results enabled us to estimate the stress condition with confidence, even though the hydraulic fractures were only gently inclined. The estimated in situ stress regime within the range between 25 and 95 m below the 420 Level (445–515 m below the surface) is S v = 12–14 MPA ; S h = 36(±16) MPA ; S H = 54(±13) MPa at 120°(±32°). The results are consistent with other independently conducted measurements at the URL.

We subjected boreholes drilled vertically in the center of cubical specimens of Lac du Bonnet granite to constant vertical (S v ) and least horizontal (S h ) farfield stresses, and increased the maximum horizontal stress (S H ) until breakouts occurred. A thin section study revealed that extensile cracking behind the borehole in two zones aligned with S h preceded the development of breakouts. Progressive detachment of thin long flakes bounded by these densely spaced subparallel cracks led to ‘dog ear’ shaped breakouts. Breakout mechanism and appearance match those found in the field. Results also suggest the potential for using breakout occurrence to estimate S H if S h is known or vice versa.

Statistical analysis of hydraulic fracturing field data enhances the objectivity of determining shut-in and fracture reopening pressures and fracture orientations. Using nonlinear regression analysis (NLRA), we isolate the closed-fracture segment of the pressure-time curve by fitting it to an exponential decay model. The shut-in pressure (Ps) is expected to be in the range between Psepd−1 (the onset of the post fracture-closure segment) and Psepd−u (the pressure level of the best-fitting exponential curve extrapolated to the time of pump shut-off). We employ bilinear regression analysis to improve the objectivity of selecting Ps from the pressure-decay rate vs pressure plot. The same analysis helps to identify Ps in the pressure vs flowrate record obtained during steprate pressurization of the hydrofractured interval. We identify unambiguously the fracture reopening pressure (Pr) by superposing the ascending portion of the pressure-time curves in the fracture-inducing cycle and a subsequent cycle. Pr is the onset of deviation between the two curves, defined as the point where the difference in pressures is larger than a statistical “reference threshold”, provided the difference continues to increase from there on. Circular statistics and a sinusoidal curve-fitting method provide the means of objectively delineating the complete trace of induced hydrofractures from fragmented or splaying traces on the impression packer or on the borehole televiewer photograph. Examples of field hydraulic fracturing data and statistically determined pressures and fracture orientations are presented. A startling discovery was made in the course of determining Pr, with our new technique: it appears to be very nearly equal to the estimated Ps. A review of the literature shows that in a great number of cases Pr and Ps are indeed close to each other, which raises the question whether we are in fact reading the correct fracture reopening pressure.