Chandong Chang

True triaxial strength and deformability of the German Continental Deep Drilling Program (KTB) deep hole amphibolite

We designed and fabricated a true triaxial loading system and used it to determine deformational and strength characteristics of the amphibolite penetrated by the superdeep hole drilled in the Bohemian massif of southeastern Germany under the German Continental Deep Drilling Program (KTB). Amphibolite is found between 3200 and 7300 m and thus the dominant rock in this 9100-m boring. Our loading system enables the application of three unequal principal stresses to a rectangular prismatic rock specimen. During a test we maintained the least principal (σ3) and the intermediate (σ2) stresses constant and increased the major principal stress (σ1) until brittle failure occurred, in the form of a fracture steeply dipping in the σ3 direction. Typically, for the same σ3 level the amphibolite compressive strength increased substantially with the magnitude of σ2, demonstrating the inadequacy of Mohr-like failure criteria that ignore the effect of the intermediate principal stress on rock strength. We found that a general criterion for the amphibolite could be expressed in the form of a power function relating the octahedral shear stress at failure to the mean normal stress acting on the plane containing the fracture. With respect to deformation, we established that for the same σ3 the onset of dilatancy increases significantly with the magnitude of σ2. Thus the intermediate principal stress appears to extend the elastic range of the stress-strain behavior for a given σ3 and hence to retard the onset of the failure process. Scanning electron microscopy observations of the failure process reveal that microcracks develop mainly parallel to σ2 direction, as the intermediate stress grows beyond σ3, localizing in close proximity of the eventual main fracture.

In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures

We constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures (borehole breakouts and drilling-induced tensile fractures) detected in four vertical boreholes (C0002, C0001, C0004, and C0006 from NW to SE) drilled in the Nankai accretionary wedge. The directions of the maximum horizontal principal stress (SHmax), indicated by the azimuths of borehole wall failures, are consistent in individual holes, but those in C0002 (margin-parallel SHmax) are nearly perpendicular to those in all other holes (margin-normal SHmax). Constrained stress magnitudes in C0001 and C0002, using logged breakout widths combined with empirical rock strength derived from sonic velocity, as well as the presence of the drilling-induced tensile fractures, suggest that the stress state in the shallow portion of the wedge (fore-arc basin and slope sediment formations) is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike-slip faulting or possible reverse faulting. Thus, the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults. The margin-perpendicular tectonic stress components in the two adjacent sites, C0001 and C0002, are different, suggesting that tectonic force driven by the plate pushing of the Philippine Sea plate does not uniformly propagate. Rather, the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation-driven extension in the fore arc and thrusting and bending within individual geologic domains.

Growth of borehole breakouts with time after drilling: Implications for state of stress, NanTroSEIZE transect, SW Japan

Resistivity at the bit tools typically provide images of wellbore breakouts only a few minutes after the hole is drilled. In certain cases images are taken tens of minutes to days after drilling of the borehole. The sonic caliper can also image borehole geometry. We present four examples comparing imaging a few minutes after drilling to imaging from about 30 min to 3 days after drilling. In all cases the borehole breakouts widen with time. The tendency to widen with time is most pronounced within a few hundred meters below the seafloor (mbsf), but may occur at depths greater than 600 mbsf. In one example the widening may be due to reduced borehole fluid pressure that would enhance borehole failure. In the three other cases, significant decreases in fluid pressure during temporal evolution of breakouts are unlikely. The latter examples may be explained by time-dependent failure of porous sediments that are in an overconsolidated state due to drilling of the borehole. This time-dependent failure could be a consequence of dilational deformation, decrease of pore fluid pressure, and maintenance of sediment strength until migrating pore fluids weaken shear surfaces and allow spallation into the borehole. Breakout orientations, and thus estimates of stress orientations, remain consistent during widening in all four cases. In vertical boreholes, breakouts wider than those initially estimated by resistivity imaging would result in higher estimates of horizontal stress magnitudes. Because the vertical overburden stress is fixed, higher estimated horizontal stresses would favor strike-slip or thrust faulting over normal faulting.

A Failure Criterion for Rocks Based on True Triaxial Testing

Mean effective normal stress acting on thefailure planec Cohesion/ Angle of internal frictionA, n, a, b Material constants1 DescriptionThe failure criterion based on true triaxial testing considersthe effect of all three principal stresses on rock compres-sive strength, and is entirely based on true triaxial testsconducted on rectangular prismatic specimens subjected tothree independent principal stresses. The failure criterion iscommonly expressed in terms of the octahedral shear stressas a monotonically increasing function of the mean effec-tive normal stress acting on the plane of failure. In testsconducted thus far the function best fitting experimentaldata is the one obeying the power law. This criterion wasfirst derived by Mogi (1971) and confirmed for severalother rocks by Haimson and Chang (2000), Chang andHaimson (2000), Oku et al. (2007) and Lee and Haimson(2011).2 BackgroundThe significant observation by Murrell (1963) and Handinet al. (1967) that rock compressive strength in experi-ments conducted in conventional triaxial extension(r

Time-dependent subsidence associated with drainage-induced compaction in Gulf of Mexico shales bounding a severely depleted gas reservoir

Production from ubiquitous oil and gas fields in coastal Louisiana and consequent reservoir compaction has been proposed as an important process contributing to coastal subsidence and land loss in this region. As revealed by three consecutive leveling surveys (in 1965, 1982, and 1993), an unexpected aspect of the subsidence is that the rate of subsidence actually increased after the cessation of production. To explain the accelerated postdepletion subsidence, we propose a mechanism involving time-dependent drainage and compaction in the overlying and underlying shales after depletion. We show that the shale compaction is induced by slow drainage of pore fluid from the shale to the depleted reservoir. We estimate the significance of postdepletion compaction in the bounding shale using a relatively simple analytic model in which time-dependent shale compaction is driven by pore pressure diffusion with two sets of rheological constitutive equations: one accounting for poroelastic effects and one accounting for viscoplastic deformation of the shale matrix. Our modeling shows that despite its very low permeability, after about 10 years, vertical compaction due to pressure drainage in the shale exceeds that due to depletion and compaction of the sand reservoir. Consequently, the calculated subsidence rate due to the shale compaction is higher than the subsidence induced by reservoir depletion, thus demonstrating that postdepletion compaction in the reservoir-surrounding shale may explain the observed acceleration of subsidence after depletion.

Present‐day stress states underneath the Kumano basin to 2 km below seafloor based on borehole wall failures at IODP Site C0002, Nankai accretionary wedge

We constrain the state of stress to 2 km below seafloor in the Nankai accretionary prism at the Integrated Ocean Drilling Program (IODP) site C0002F, southwest Japan, based on borehole wall failures and rock strengths. The logging-while-drilling resistivity images from 872.5 to 2005.5 meters below seafloor show that drilling-mud control in riser drilling worked properly to minimize borehole wall failures. Available breakouts indicate a consistent maximum compression orientation subparallel to the subducting plate margin. Breakout analysis with drill logs suggests that breakouts occurred only when borehole pressure was slightly lowered and time lag between hole cutting and image logging was several hours. This indicates that the observed breakouts are not immediate stress-induced failure, but brought up into shape gradually with time due to other mechanisms. Laboratory investigations on deformation and failure of the cores suggest that the time-delayed breakout might be a result of progressive rock spall-out in borehole wall damage zones that occur at a stress level close to failure condition. We constrain stress magnitudes assuming that the stress state is sufficient to bring about the damage zones at the borehole wall. An integrated method utilizing breakouts, drilling-induced tensile fractures, and a leak-off test suggests that the stress states are on the boundary between strike-slip faulting and normal faulting stress regimes, and somewhat variable depending on depth. The stress magnitudes in the accretionary wedge appear to be controlled by frictional strength of the rock, such that the differential stresses are constrained by the laboratory determined frictional coefficients. This article is protected by copyright. All rights reserved.

In situ stress conditions at IODP Site C0002 reflecting the tectonic evolution of the sedimentary system near the seaward edge of the Kumano basin, offshore from SW Japan

This paper presents a complete set of in situ stress calculations for depths of 200–1400 mbsf (meters below seafloor) at IODP Site C0002, near the seaward margin of the Kumano forearc basin, offshore from southwest Japan. The vertical stress component was obtained by integrating bulk density calculations from moisture and density logging data, and the two horizontal components were stochastically optimized by minimizing misfits between a probabilistic model and measured breakout widths for every 30 m vertical segment of the wellbore. Our stochastic optimization process reveals that the in situ stress regime is decoupled across an unconformity between an accretionary complex and the overlying Kumano forearc basin. The stress condition above the unconformity is close to the critical condition for normal faulting, while below the unconformity the geologic system is stable in a normal to strike-slip fault stress regime. The critical state of stress demonstrates that the tectonic evolution of the sedimentary system has been achieved mainly by the regionally continuous action of a major out-of-sequence thrust fault during sedimentation in the forearc basin. The stable stress condition in the accretionary prism is interpreted to have resulted from mechanical decoupling by the accommodation of large displacement along the megasplay fault.

Reply to Comments on the ISRM Suggested Method “A Failure Criterion for Rocks Based on True Triaxial Testing”

The linearization of the faiure criterion is only mentioned because it has been suggested by others. We do not support that simplification of the criterion, because it assumes that the angle of fiction / is a constant of the material. This is contrary to the findings in true triaxial experiments, which have consistently shown that the slope of the failure plane (which is directly related to /) is far from being constant, and in fact varies with the intermediate principal stress for constant minimum principal stress (Mogi 1971, 1972; Chang and Haimson 2000; Oku et al. 2007). Hence, the entire discussion here is irrelevant.

Comparison of Tensile Strengths in Granite Using Brazilian Tests and Hollow Cylinder Tests for Hydraulic Fracturing Test Interpretation

We conducted hollow cylinder tensile strength tests and Brazilian tests in Seokmo granite to measure tensile strength necessary for estimating the magnitude of the maximum horizontal principal stress in hydraulic fracturing stress measurements. Two different pressurization rates were used in hollow cylinder tests. Tensile strengths were determined to be higher at higher pressurization rate, which suggests that tensile strength should be measurement at the same rate used in actual in situ hydraulic fracturing tests. Considering the effect of pressurization rate and specimen size on tensile strength, the hollow cylinder tests and Brazilian tests yield similar results each other. This demonstrates that Brazilian tests can be utilized to produce representative tensile strengths for interpretation of hydraulic fracturing test results.

Relationships between Gas Hydrate Occurrence Types and Sediment Characteristics in the Ulleung Basin, East Sea

During the 2nd Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) in 2010, gas-hydrate-bearing sediment cores were recovered at 10 drill sites. Base, on Infrared (IR) thermal image and grain-size analysis of the cores, three distinct types of gas hydrate are classified: Type I (fracture-filling in mud layers), Type II (disseminated in mud layers), and Type III (pore-filling in sand layers). Types I and II gas hydrates occur in mud as discrete veins, nodules or disseminated particles. Type III fills the pore spaces of the sand layers encased in mud layers. In this case, the sand content of hosting sediments shows a general linear relationship with gas hydrate saturation. The degrees of temperature anomalies () from IR images generally increase with gas hydrate saturation regardless of gas hydrate occurrence types. Type I is dominantly found in the sites where seismic profiles delineate chimney structures, whereas Type II where the drill cores are composed almost of mud layers. Type III was mainly recovered from the sites where hemipelagic muds are frequently intercalated with turbidite sand layers. Our results indicate that gas hydrate occurrence is closely related to sedimentological characteristic of gas hydrate-bearing sediments, that is, grain size distribution.