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Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites. Our coupled fluid-flow and geomechanical model describes the physical processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that prolonged accumulation of poroelastic energy and pore pressure along a fault can nucleate seismic events larger than Mw3 even after terminating well operations. In particular the possibility of large seismic events can be increased by multiple-well operations with alternate injection and extraction that can enhance the degree of pore-pressure diffusion and subsequent stress transfer through a rigid and low-permeability rock to the fault. This study demonstrates that the proper mechanistic model and optimal well operations need to be accounted for to mitigate unexpected seismic hazards in the presence of the site-specific uncertainty such as hidden/undetected faults and stress regime.

The mechanical behavior of frozen soils plays a central role in the design, construction, and maintenance of buildings and infrastructure in cold regions. We present and discuss the main results of an experimental campaign focused on the mechanical behavior of frozen soils from Alaska. The data set is unique because it involves natural frozen soils and covers a wide range of temperatures (i.e., from −6 °C up to −26 °C) and confinements (i.e., from unconfined conditions up to a cell pressure of 54.6 MPa). An elastoplastic model for frozen soils is presented and validated based on the test results. The elastoplastic model accounts for the effects of temperature and cryogenic suction on key properties associated with frozen soil behavior, namely, increase of preconsolidation pressure, stiffness, and strength with the decrease of temperature. Other aspects such as the volumetric behavior of frozen soils at different temperatures and the smooth transition between elastic and plastic states are also qualitatively well captured by the model. We implemented the constitutive model in a fully coupled thermo-hydro-mechanical simulator and we analyzed the behavior of a foundation in a thawing ground. The proposed approach is able to capture the main feature of frozen soil behavior.

A helium leakage detection system was modified to measure gas permeability on extracted cores of nearly impermeable rock. The Helium-Mass-Spectrometry-Permeameter (HMSP) is duplicating the classic Darcy’s experiment with a constant pressure differential and steady-state flow through a sample using helium gas. Under triaxial stress condition, the newly developed HMSP can measure hydraulic permeability of rocks and geomaterials down to the nanoDarcy scale (10−21 m2). The extension of measuring the lower end of the permeability scale may help answer important questions regarding the permeability of rock at great depth where fractures may close completely under high lithostatic stress.

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.

A set of 10 complete hydraulic fracturing stress measurements in an anorthosite was conducted as part of the site investigation of the SM3 hydroelectric project, located some 800 km northeast of Montreal, Quebec. These were the first such measurements in that region of Canada. The vertical test hole was N-size, 260 m deep and adjacent to the future powerhouse. Within the relatively short depth range tested two apparent stress regimes were detected. In the 160–220 m range all three principal in situ stresses approach being lithostatic, suggesting stress relief probably induced by natural fracturing in the rock as evidence by considerable jointing the extracted core. At depths comparable to those of the planned powerhouse (220–255 m) the least horizontal stress is quite consistent and averages 11.0 (± 1.4) MPa, the maximum horizontal stress averages 17.7 (± 3.1) MPa and acts in a NE-SW direction, and the vertical stress is 7.5 (± 0.4) MPa. Thus, ratios of 1.6:1 between the two horizontal principal stresses and of 2.5:1 between the maximum horizontal and vertical stresses, can be expected, and should be carefully considered in design, as should the fact that both horizontal stresses are substantially larger than the vertical stress. The northeastern direction of the maximum horizontal stress, as well as the compressive nature of the measured stresses are in accord with most measurements in eastern North America.

The determination of material parameters for rocks which display anisotropic behaviour has become more important in light of the development of powerful and economical analytical modeling techniques. The study presented herein is based on a testing framework aimed at determining the material parameters required to describe the behaviour of a transverse isotropic solid under axisymmetric loading conditions.

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.

A suite of laboratory triaxial compression and triaxial steady-state creep tests provide quasi-static elastic constants and damage criteria for bedded rock salt and dolomite extracted from Cavern Well No.1 of the Tioga field in northern Pennsylvania. The elastic constants, quasi-static damage criteria, and creep parameters of host rocks provides information for evaluating a proposed cavern field for gas storage near Tioga, Pennsylvania. The Young's modulus of the dolomite was determined to be 6.4 ({+-}1.0) x 10{sup 6} psi, with a Poisson's ratio of 0.26 ({+-}0.04). The elastic Young's modulus was obtained from the slope of the unloading-reloading portion of the stress-strain plots as 7.8 ({+-}0.9) x 10{sup 6} psi. The damage criterion of the dolomite based on the peak load was determined to be J{sub 2}{sup 0.5} (psi) = 3113 + 0.34 I{sub 1} (psi) where I{sub 1} and J{sub 2} are first and second invariants respectively. Using the dilation limit as a threshold level for damage, the damage criterion was conservatively estimated as J{sub 2}{sup 0.5} (psi) = 2614 + 0.30 I{sub 1} (psi). The Young's modulus of the rock salt, which will host the storage cavern, was determined to be 2.4 ({+-}0.65) x 10{sup 6} psi, with a Poisson's ratio of 0.24 ({+-}0.07). The elastic Young's modulus was determined to be 5.0 ({+-}0.46) x 10{sup 6} psi. Unlike the dolomite specimens under triaxial compression, rock salt specimens did not show shear failure with peak axial load. Instead, most specimens showed distinct dilatancy as an indication of internal damage. Based on dilation limit, the damage criterion for the rock salt was estimated as J{sub 2}{sup 0.5} (psi) = 704 + 0.17 I{sub 1} (psi). In order to determine the time dependent deformation of the rock salt, we conducted five triaxial creep tests. The creep deformation of the Tioga rock salt was modeled based on the following three-parameter power law as {var_epsilon}{sub s} = 1.2 x 10{sup -17} {sigma}{sup 4.75} exp(-6161/T), where {var_epsilon}{sub s} is the steady state strain rate in s{sup -1}, {sigma} is the applied axial stress difference in psi, and T is the temperature in Kelvin.

Data from two series of hydraulic fracturing experiments at the Carwynnen test site were used to estimate the state of in situ stress in the Cornwall granite. We discuss several techniques of pressure record interpretation, and alternate methods of in situ stress calculation.

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.