Breakouts observed in a vertical borehole (C0002A) drilled through two major tectonic sedimentary formations consisting of forearc basin (upper) and accretionary prism (lower) sediments in the Nankai ...accretionary wedge, Japan, exhibit distinctive geometric features in respective formations. Breakouts in the lower accretionary prism sediments are markedly wider than those in the forearc basin sediments, and breakout azimuths in the two units are horizontally rotated relative to one another. Breakout azimuths are widely used as a proxy for the determination of principal stress directions. However, strength anisotropies related to the presence of bedding planes may affect both breakout azimuths and widths, which can result in misleading in situ stress interpretations. While thinly bedded mudstones are the dominant lithology in both the forearc basin and accretionary prism sediments, bedding planes in the accretionary prism sediments are relatively steeper than those in the forearc basin sediments, with possible implications for breakout geometry and interpretations of principal stress directions. To investigate the effects of bedding planes on breakout geometry (azimuth and width), we conducted borehole wall failure analyses using a weak-plane failure model that incorporates material strength anisotropies. The model results show that breakout widths and orientations are strongly affected by steeply dipping (>40°) bedding planes in cases where dip directions are unaligned with the principal stress orientation. Our theoretical results suggest that variations in breakout azimuths in the C0002A site may be associated with bedding plane related strength anisotropy, and not associated with the rotation of stress field. That is, stress orientation is consistent throughout the borehole (down to the bottom-hole depth of 1495 m below sea floor). In addition, disregarding the effects of bedding planes tends to yield an overestimation of in situ stress magnitude.
•We model borehole breakouts considering anisotropic rock strength due to beddings.•Strength anisotropy affects significantly on breakout geometry as bedding is steep.•Ignoring anisotropy tends to yield an overestimation of stress magnitude.
We investigate the effects of possible fault activation and slip during geological CO2 storage operations numerically. To this end we develop a numerical simulator by coupling a multiphase flow ...simulator with an existing geomechanics simulator in order to model fault slip and seismic events induced by reservoir pressurization. After validation and verification of the developed coupled simulator, we apply it to a pilot-scale site of geological CO2 storage located in the Janggi Basin, South Korea. We design a fault structure with a fault core and surrounded damage zones, assuming the fault to be activated along the fault core only. Zero-thickness interface elements are employed in the numerical model to represent the fault core. We perform numerical tests with different injection scenarios to evaluate the impact of pore pressure on fault activation and its post-failure behavior. We model occurrence and the magnitude of seismic events, identifying sudden changes of the moment magnitude at the onset of fault activation. Specifically, we take CO2 injection with 100 , 200, 400 ton/day for 20 years, from which the cumulative moment magnitudes (Mw’s) of 1.46, 2.36 and 3.11 are estimated, respectively. However, the maximum instantaneous changes of Mw occur at the onset of fault activation for all the injection cases (i.e., approximately 0.1, 0.36, 0.75, respectively). As the injection rate increases, fault activation occurs early and the magnitude of induced seismicity becomes large, but substantial reduction of the injection rate can weaken following induced seismicity. Interestingly, the location of maximum surface lift does not correspond to the injection well due to heterogeneity of geology and geomechanics. We thus expect relatively low risk of induced seismicity overall within the range of injection rates considered in this study, but monitoring surface uplift and seismic signals is still necessary to prevent any unexpected events due to uncertainty of parameters.
Shale formations, due to the presence of laminations and weak planes, exhibit directional strength characteristics. In most conventional wellbore stability analyses, rock formations are typically ...assumed to have isotropic strength. This may cause erroneous results in anisotropic formations such as shales which show strength variations with changing loading directions with respect to the plane of weakness. Therefore a more complex wellbore stability model is required. We have developed such a model in which the anisotropic rock strength characteristic is incorporated. Applying this model to two case studies shows that shear failures occur either along or across the bedding planes depending on the relative orientation between the wellbore trajectories and the bedding planes. Additionally, the extent of failure region around the wellbore and the safe mud weights are significantly affected by the wellbore orientation with respect to the directions of bedding plane and in-situ stress field.
► Developed is a wellbore stability model in which anisotropic strength and two shear failure modes are considered. ► Wellbore failure depends on the relative orientation between well paths and weak planes. ► Safe mud weight also depends on their relative orientation. ► Generally an up-dip drilling is more stable than other drilling directions.
► Two series of true triaxial tests conducted on the SAFOD granodiorite. ► In both tests rock strength increases with raising
σ
2 for a given
σ
3. But unjacketed rock strength is 50% lower than ...jacketed one for the same
σ
2 and
σ
3. ► Jacketed sample failed typically by forming a through-going shear fracture. But unjacketed sample failed by forming extensile cracks parallel to the
σ
3 faces.
At great depths, where borehole-based field stress measurements such as hydraulic fracturing are challenging due to difficult downhole conditions or prohibitive costs, in situ stresses can be ...indirectly estimated using wellbore failures such as borehole breakouts and/or drilling-induced tensile failures detected by an image log. As part of such efforts, a statistical method has been developed in which borehole breakouts detected on an image log are used for this purpose (Song et al. in Proceedings on the 7th international symposium on in situ rock stress,
2016
; Song and Chang in J Geophys Res Solid Earth 122:4033–4052,
2017
). The method employs a grid-searching algorithm in which the least and maximum horizontal principal stresses (
S
h
and
S
H
) are varied, and the corresponding simulated depth-related breakout width distribution as a function of the breakout angle (
θ
B
= 90° − half of breakout width) is compared to that observed along the borehole to determine a set of
S
h
and
S
H
having the lowest misfit between them. An important advantage of the method is that
S
h
and
S
H
can be estimated simultaneously in vertical wells. To validate the statistical approach, the method is applied to a vertical hole where a set of field hydraulic fracturing tests have been carried out. The stress estimations using the proposed method were found to be in good agreement with the results interpreted from the hydraulic fracturing test measurements.
A small scale injection project into an offshore aquifer near shoreline has begun which is seeking the connection with a capture project in Korea. The Pohang basin was selected as a site for a ...small-scale pilot project and detailed site characterization has been undertaken to establish a geological model for the site since 2013. A potential reservoir for CO2 injection was found at a depth more than 730 meters in the southern offshore part of the Pohang basin from seismic surveys and one exploration well. The seismic interpretation revealed the reservoir is a steeply dip and horizontally fault bounded aquifer. In this study, numerical reservoir simulations were carried out for assessing CO2 storage capacity especially in the dipping reservoir regarding the reactivation pressure of the bounded faults. At the same time, to investigate the effect of impurity in CO2 stream was one of our goals. Captured CO2 streams contain various levels of impurity which vary depending on the combustion technology and CO2 sources. Specifically for geological storage, the non-condensable gases in CO2 streams are not favorable because they can decrease density of the injected CO2 stream and can affect buoyancy of the plume. The effect of the non-condensable gases on storage capacity, injectivity and plume migrations were tested in the reservoir.
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