Research

Seismology

Analysis of the local stress heterogeneity and fault stability along the central part of the Great Sumatran fault

Grant: Kementrian Riset dan Pendidikan Tinggi, Program Riset Unggulan 2017 (PPMB)

Publication: Sahara et al. [2017], Agustina et al. [submitted]

The in-situ stress along the Great Sumatran fault is assumed to be strike-slip, and the maximum principal stress is oriented N15°E. This is confirmed by the World Stress Map data on a regional scale. However, this estimation neglects the local impact of fault branching and material heterogeneity which might have a significant impact on the stress heterogeneity, e.g. San Andreas and the Alps. Often, the local stress heterogeneity overrules the far-field stress in governing the local deformation. Despite its importance, very few studies have been performed on this issue in the Sumatran fault. We intend to analyze significant variations in the direction of the maximum horizontal compressive stress, SHmax, in the central part of the Great Sumatran fault. Waveform data from a join seismic network of 19 stations deployed by the GeoForschungZentrum (GFZ) Germany and the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) is used. The seismic network had a full coverage of the Sumatran fault and recorded the seismicity from 1 January 2010 to 31 December 2015. Due to its high seismicity rate in the study region, a couple of hundreds of good events are expected to be determined. The moment tensor of selected events will be estimated using a nonlinear focal mechanism technique developed in this study. Then, a joint inversion of the principal stress and the fault stability is performed to analyze the local stress heterogeneity as well as the possibility of fault reactivation in each sub-region. A correlation between the fault branching and the local stress heterogeneity in the Sumatran fault is expected to be drawn from the results of this study. We hypothesize that the local stress heterogeneity is also affected by the mechanism of the fault, i.e. creeping or locked section of the fault. Furthermore, a better understanding of the potential hazard along the Sumatran fault is expected from our study results.

Stress drop, earthquake aftershocks and regional stress relation base on static Coulomb failure stress

Grant: Kementrian Riset dan Pendidikan Tinggi, Program Penelitian, Pengabdian kepada Masyarakat dan Inovasi (P3MI 2017)

Publication: Sahara et al. [2017a], Kusumawati et al. [2017], Kusumawati et al. [2017a]

Coulomb failure criterion has been applied widely in the scope of earthquake science to explain earthquake interactions base on stress change, with the well-known method named Coulomb failure stress change (ΔCFS). Preceding studies have showed: increase ΔCFS, depicted as positive stress lobes, has correlation with occurrence of following events. However, in the calculation process, ratio between regional stress and earthquake stress drop would affect stress distribution. Based on preceding researches, earthquake stress drop with similar magnitude to regional stress, would give results positive stress lobes along and at the base of the fault. Those stress distribution, would then help explaining events interaction and mechanism of earthquake. This work carries out synthetic modeling of static ΔCFS upon varying earthquake stress drop and regional stress using COULOMB3.3. In accord with preceding studies, the results show positive ΔCFS along the fault when stress drop is comparable to regional stress. And yet, positive ΔCFS would take place at the top and at the base of the fault, expanding to the center of the fault -where the hypocenter is assumed- as the stress drop reaching regional stress in magnitude. This matter might explain separated clusters of aftershock in different depth for some cases of earthquake.

Geothermal

A study of the physical mechanisms of a geothermal reservoir: from exploration to monitoring

Grant: Kementrian Riset dan Pendidikan Tinggi, Program Penelitian, Pengabdian kepada Masyarakat dan Inovasi (P3MI 2017)

Publication: Natania et al. [2017], Hijriani et al. [2017]

Understanding the physical mechanisms involved prior and after hydraulic stimulation is the key parameter to estimate the fluid flow and permeability increase within the geothermal reservoir. An attempt to infer the mechanical structure and behavior of a geothermal reservoir for exploration and monitoring purposed is performed in this study. Microearthquake data is used as a basis for the monitoring of the percolation of the fluid within reservoir during the injection and production activity. In this study, we aim to determine the 4D seismic velocity structure of a geothermal reservoir using P-and S-wave arrival times of microearthquake data as well as analyze its mechanical changes due to hydraulic injection using the evolution of the microseismicity. A data from a volcanic-type geothermal field in Indonesia is used in this study. In this field, microearthquakes were recorded using 13 and 16 stations deployed before and after the injection, respectively. A total of 2.826 microearthquake were recorded, in which only 135 microearthquakes were recorded before injection and significantly increased to 2,691 events after hydraulic fracturing. This highlights the significant impact of the hydraulic injection in altering the mechanical state within the reservoir. This huge database, with the help of geology and hydraulic data, enabled us to analyze the secondary fracture fracture system, permeability, and changed in steam and fluid content in the reservoir.

Impact of fracture networks on borehole breakout heterogeneities in crystalline rock

Grant: A PhD project supported by the Indonesia Directorate General of Higher Education (DIKTI), the German Academic Exchange Service(DAAD), Energie Baden-Wuerttemberg (EnBW) and the Helmholtz Association of German Research Centres (BMBF)

Publication: Sahara et al. [2015], Sahara [2016]

Breakouts are commonly used as principal indicator of stress orientation. However, variation of breakout orientation with depth, especially in the vicinity of fracture zones, is frequently observed. This study describes a systematic analysis of breakout occurrence, variation of breakout orientation and fracture characteristics. We infer the impact of fracture networks on the development of breakouts from detailed analysis of 1221 borehole elongation pairs in the vicinity of 1871 natural fractures observed in the crystalline section of the GPK4 well of the Soultz-sous-Forêts geothermal field (France). Breakout orientation anomalies are found to concentrate in the immediate vicinity of fault cores and to decrease with distance to the fault core. Patterns of breakout orientation in the vicinity of natural fractures suggest that the breakout rotation, relative to the mean Shmin direction, is strongly influenced by the fracture orientation. Even a direct relationship between fracture and breakout orientations is found in some depth intervals. In highly fractured zones, with different fracture families present, breakout orientations are especially heterogeneous, resulting from the overlapping effects of the fracture network. Additionally, breakouts are typically found to be asymmetrical in zones with high fracture density. Borehole breakout heterogeneities do not seem to be related to the principal stress heterogeneity only, but also to the effect of mechanical heterogeneities like weak zones with different elastic moduli, rock strength and fracture patterns. Consequently, care has to be taken when inferring the principal stress orientation from borehole breakout data observed in fractured rock.

Numerical modeling

Analysis of borehole breakout development using continuum damage mechanics

Grant: A PhD project supported by the Indonesia Directorate General of Higher Education (DIKTI), the German Academic Exchange Service(DAAD), Energie Baden-Wuerttemberg (EnBW) and the Helmholtz Association of German Research Centres (BMBF)

Publication: Schoenball et al. [2014], Sahara et al. [2017]

Damage distribution and evolution have a significant effect on borehole stress concentrations. To model the complex fracturing process and inelastic deformation in the development of the borehole breakout we implement a continuum damage mechanics (CDM) concept that takes tensile and compressive failure mechanisms into account. The proposed approach explicitly models the dissipative behavior of the material due to cracking and its evolution, which leads to an inhomogeneous redistribution of material properties and stresses in the vicinity of the borehole wall. We apply a constitutive plastic model for Berea sandstone and compare our numerical results to laboratory experiments performed on Tablerock sandstone. We are able to reproduce several characteristics of the failure process during the breakout development as observed in experimental tests, e.g. localized crack distribution in the vicinity of the borehole wall, damage evolution, which exhibits a widening process in the beginning followed by subsequent growth in depth, and shear fracturingdominated breakout growth in sandstone. A comparison of our results with laboratory experiments performed on a range of stress conditions shows a good agreement of the size of borehole breakouts. The importance of the constitutive plastic law in defining the failure mechanisms of the damaging processes is discussed. We show that the depth and the width of breakouts are not independent of each other and no single linear relation can be found between the size of breakouts and the magnitude of the applied stress. Consequently, only one far field principal stress component can be estimated from breakout geometry if the other two principal stresses are known and sufficient data on the plastic parameters are available.

A coupled Thermal-Hydro-Mechanical modeling of a fluid injection in a naturally fractured formation

Grant:

Publication: Suhendi et al. [2017], Suhendi [2016]

Injection CO2 into subsurface formation changes in-situ pore pressure and temperature which in turn alters the effective stress condition. An attempt to analyze the geomechanical responses induced by CO2 fluid injection in a sand reservoir is performed on this study. We develop coupling program code that linking two existing and proven programs, TOUGH2 and FLAC3D. TOUGH2 is a numerical simulator that solves fluid flow and transport equation, whilst FLAC3D is a numerical code to simulate geomechanical analysis. Fluid – flow and geomechanical equations are sequentially solved by using finite difference methods. The coupling method used in this paper is two – way coupling where the coupling parameters are transferred from each code in certain time step. Simulation parameters are, then, extracted from geophysical and geological data conducted in the study area. The sensitivity test is performed by varying the injection and reservoir geomechanical parameters. The distribution of the injected CO2 plumme and the possible rock deformation induced by injection is shown for several injection case. Furthermore, a correlation between the injection strategy and the reservoir stability is drawn from the results of this study. This model could be applied to other field and serve as a basis information for the injection strategy.