In-situ stress is critical for wellbore stability, selection of injection and production well location as well as performance of hydraulic stimulation for Enhanced Geothermal System (EGS). This study introduces the integrated in situ stress estimation method and result applied to the Pohang EGS research site, which is the first of its kind in Korea.
The in-situ stress estimation analysis was conducted at an exploration borehole, EXP-1, which is about 1 km in depth. Three hydraulic fracturing stress measurement tests were conducted at around 700m depth. Several borehole breakouts and drilling-induced fractures were observed at similar depth from the borehole televiewer. The analysis combining the hydraulic fracturing results and borehole observation was carried out to make an integrated in-situ stress model at EXP-1 hole. The rock mechanical property data are based on the laboratory experiments on rock core samples extracted from EXP-1.
Instead of simply averaging the principal stress and orientation data from independent measurement methods, we took an average after transforming the measured in situ stress data to an identical axis. A few examples with synthetic data shows that this method can be more accurate since it truly treat the in situ stress as 2nd order tensor. The suggested in-situ stress model is also compared with results from previous in situ stress measurements in the region in order to investigate the appropriateness of the suggested model.
In-situ stress measurement and its application have been regarded as critical factors in the geomechanics field. Especially for Enhanced Geothermal System (EGS), in-situ stress estimation and stress model composition is essential for wellbore stability, selection of injection and production well location as well as performance of hydraulic stimulation.
A 3-D P-wave upper-mantle tomography model under eastern Tibet is inferred using a large number of high-quality arrival-time data collected from seismic stations in southwestern China. The results show that high-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and Ordos and Alashan blocks, whereas low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, Songpan-Ganzi, Qiangtang, Lhasa, and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are obstructed by the Sichuan basin, Ordos and Alashan blocks, and they could be extruded through the Qinling fold zone and Chuan-Dian block to eastern China. In the mantle transition zone (MTZ), broad high-V anomalies are visible, and they are connected upward with the Wadati-Benioff seismic zone beneath Burma arc. Some large earthquakes occurred on the boundaries of high- and low-V anomalies in the upper mantle, suggesting that the crustal large earthquakes of the region are related to the upper-mantle structure. These results could be helpful to better understand the rock stress around the source areas.
The Tibetan plateau, being called the third pole of the world, is characterized by a highly deformed zone with a very broad area, due to the Indo-Eurasian collision since 50-60 Ma (e.g., Tapponnier et al., 1981; England & Houseman, 1986; Yin & Harrison, 2000). The present study region, the eastern Tibetan plateau, contains the stable Sichuan and Qaidam basins and Ordos and Alashan blocks as well as several large active faults, such as the Longmenshan fault zone and the Red-River fault (Figure 1). Along these fault zones, many moderate-to-strong earthquakes have occurred so far, the recent ones are the 12 May 2008 Wenchuan (Ms 8.0), the 14 April 2010 Yushu (Ms 7.1), the 20 April2013 Lushan (Ms 7.0), the 22 July 2013 Minxian-Zhangxian (Ms 6.6), and the 3 August 2014 Ludian (Ms 6.5) earthquakes. Although the Ludian earthquake is of a moderate size, it is the largest and damaged one in Yunnan since 2000, which killed over 589 people and caused at least 2,400 injuries by 6 August 2014 (www.cea.gov.cn). Although in recent years many researchers have conducted several seismic tomographic models, it is still unclear if these strong earthquakes are related to the upper mantle structure.
The construction of the Romaine-3 surface powerhouse requires an important excavation of approximately 98 m (left to right bank direction) by 64 m (upstream-downstream direction) and 80 m deep in the rock mass. The water intake tunnel as a length of approximately 1.8 km including the lined portion and sits at an average depth of 150 m below the natural ground level. The unlined portion of the power tunnel is located in a rock mass that can sustain a hydraulic pressure of about 1.7 MPa as to prevent hydraulic jacking of existing joints. During the design phase, in-situ stresses have been measured with hydraulic jacking tests done in several boreholes. These boreholes are aligned and distributed along the axis of the future lined tunnel, starting from the powerhouse site to a distance of 350 m upstream. The tests were conducted in an undisturbed rock mass at variable depths corresponding to about 20 m above and below the future tunnel elevation. To acquire a better understanding of the stress relief caused by the powerhouse large excavation, a new testing program, with 2 boreholes, was conducted after the completion of the excavation. The first borehole was located about 0.6 m beside a previously tested borehole. Also, with a borehole televiewer, it was possible to identify, in this new borehole, the same joints sets that were tested in the existing borehole. The test intervals elevation has been adjusted in order to test these specific joints so as to compare the stress conditions pre and post-excavation. A second borehole was drilled at about 44 m upstream of the powerhouse to evaluate if the effect of excavation on in-situ stresses was observed closer to the excavation. Based on these hydraulic jacking results, it is shown that no substantial stress relief has been measured in the rock mass at 56 m upstream of the powerhouse after its excavation.
Rantatunneli is a project of moving highway 12 into a tunnel in the city of Tampere. The excavation work was finished in the summer 2015. The tunnel will be operational in 2017. The twin tunnel will be the longest highway tunnel in Finland with the length of 2.3 kilometres. The project included in-situ stress measurement campaigns, stability analyses and rock displacement monitoring during the excavation phase. The stress measurements were conducted with hydraulic fracturing method. In this paper, the test results and their analysis are presented. Comparison of the predicted and the observed displacement behaviour of the rock mass are discussed.
The city of Tampere, which is the third largest city in Finland, is located on an isthmus between two lakes and thus space is limited. Tampere is also a national traffic nexus as it houses key rail and road connections including three national highways and Helsinki-Oulu railway. All these have originally run through the city center, but two highways have so far been partially rerouted to a ring road running south of the central isthmus. The Rantatunneli project aims to transfer the third highway, national Highway 12, into a tunnel under the city centre. After completion, the tunnel will free up the land on central Tampere northern areas for urban development. The excavated rock is used to fill in the water body for a new park in the future Ranta-Tampella district.
Rantatunneli is built as 2.3 km long twin tunnel which will be the longest highway tunnel in Finland. The tunnel starts from Santalahti in west and ends at Naistenlahti in east. A short concrete tunnel section is built at Santalahti. The tunnel passes under Tammerkoski rapids at midway, and houses a space reservation for a possible underground junction. The span of a single tunnel is 13 ... 15 m and height 9,2 m (Figures 1 and 2). The thickness of rock cover varies from 8 to 27 m (Figure 3). The total excavated volume for the two tunnel tubes is 650.000 m3. The project was realised as an Alliance scheme between the key organisations, and the design work started in July 2012. An overall cost target was set in mutual understanding. The construction works started in autumn 2013 with excavation of the access tunnels. Tunnel excavations were finished by Midsummer 2015. The construction work is currently in progress, and the tunnel is scheduled to open to traffic in November 2016, few months ahead of schedule. The work will continue after this with construction of a new multilevel junction at Santalahti, landscaping and finishing works. The project is to be completed in 2017.
In situ stress measurements based on the recovery principle, commonly called overcoring techniques, consist in calculating stresses under strain gauge rosettes using recovered strains at borehole walls or ends. These stresses are then combined through models which account for the stress-strain relationships of the rock and the influence of the borehole on the in situ stress tensor. Up until1994, only strains resulting from complete stress relief were used in the calculation of stresses using such techniques. The RPR method developed by Corthésy et al. (1994), for the modified doorstopper technique allowed using transient strains on the recovery curve to increase the number of equations available for obtaining more stress tensor components from a single measurement. In order to increase the confidence and quality of the stresses estimated from the modified doorstopper technique, a new data logger is presented along with a new approach for calculating the stresses. It is shown how using the complete strain recovery curve and estimating the far field stress tensor with the inverse problem allows to dramatically minimize the adverse effects of noise on the strain recovery curve in isotropic or anisotropic rocks.
After more than 35 years of stress measurement field experience in varied and difficult geomechanical settings all over the world, the authors have come to consider in situ stress measurements as a sampling problem, in which the sampled data are multivariate regionalized random variables. Because of the heterogeneities and discontinuities found in the rock mass, stress variations in both magnitude and direction are present at various scales which further complicates the sampling and data analysis process (Corthésy et al., 1991, 1993a). Although this problem is complex, the first requirement in any sampling procedure is to have access to samples which are minimally biased, as non-biased rock stress measurements is something which has not yet been achieved because of the complex nature of rocks and rock masses and the aggressive environment in which such measurements are taken. In the following sections, sources of error at certain stages of the stress measuring and data reduction procedures will be covered and means of alleviating these errors or biases will be presented for the modified doorstopper technique.
Posiva Oy is responsible for implementing the final disposal program for spent nuclear fuel of its owners, Teollisuuden Voima Oy and Fortum Power & Heat. The concept for geological disposal is based on a multibarrier methodology. Due to the geological concept, the knowledge of the properties of the host rock, such as in situ stress conditions, excavation damaged zone and hydraulic conductivity are highly important for the long term safety analysis. For the repository site characterization, an underground rock facility called ONKALO has been excavated at Olkiluoto Island.
Recent research indicates that the excavation damage zone (EDZ) can be meaningfully separated in to construction induced (EDZCI) and stress induced (EDZSI) Excavation Damage Zones. The EDZ has been thoroughly investigated in ONKALO at the ONK-TKU-3620 niche located at -345 meter depth. In the studies that used various geophysical methods and hydraulic conductivity testing, a horizontal fracture without the characteristics of natural fracture was observed. In ground penetrating radar investigations fairly large reflectors occurred at the depth around 50 - 70 em and Mise-a-Ia-Masse surveys gave indications of electrically highly conductive area. The characteristics of the fracture were confirmed and further observed when the investigation area was cut off using wire sawing. Rock mechanics modelling was used to confirm that the excavation of the niche causes a stress redistribution that can trigger stress induced plasticity or fracture growth beneath the excavated floor.
In the research of Siren et al. (2015) a terminology was established for the Excavation Damage Zone (EDZ) in relation to the birth mechanism for both construction induced (EDZCI) and stress induced (EDZSI) Excavation Damage Zones. Understanding the properties of the EDZ are important for the nuclear waste management industry, which has two rock laboratories in Scandinavia, Äspö Hard Rock Laboratory (HRL) in Sweden and ONKALO in Western Finland. The research by Siren et al. (2015) mostly focused on studying EDZCI solely at Äspö HRL, while also the general EDZ studies in ONKALO were presented. A study area for EDZ in ONKALO is located at ONK-TKU-3620, at which signs of stress induced EDZSI fractures have been detected.
Theoretical calculation methods generally exaggerate the extent of the EDZ and used formulas do not apply to most rock types. This study comprises of correlating the theoretical to the realized extend of the EDZ on drill and blast (D&B) excavation surface. Generally when excavation is designed, applied theoretical EDZ extend origins from explosive manufactures material to fulfill the EDZ requirements of the blasted surface. Well known theories and formulas have been used to calculate the failure of the rock.
In this work actual EDZ extend was defined and correlated to the theoretical extend when using Kemiitti 810 bulk emulsion explosive. The results were compared to realization using nitroglycerin based pipe charge F-pipe 17 x 500. All together 20 m of tunnel was excavated in crystalline granodiorite in Tampere test mine and numbers of small granite blocks were blasted in Kuru dimensional stone quarry. Realized EDZ extend definition was done visually on core samples and saw cut surfaces. Definition of theoretical EDZ extend requires rock mechanical testing of the site rock as well as borehole pressure data on used charges. Required and laboratory defined rock mechanical measures were the unconfined compressive strength, dynamic Young's modulus and Poisson's ratio. Also borehole pressure was defined in series of tests. Measured pressure values were compared to the calculated values based on ideal detonation, though it is known that in commercial explosives detonation is nonideal. The larger the charge diameter is, the more ideal explosion comes. When calculated borehole pressures were compared to the measured values it was noticed that the constant applied in the formula should vary along the charge diameter. Revised formula demonstrates how the EDZ extend can be calculated in crystalline rock when using explosives charges less than 600 g/m. The result is more realistic EDZ extend values.
We constrain the state of tectonic stress in the Nankai accretionary wedge at the Integrated Ocean Drilling Program (IODP) site C0002, southwest Japan, based on borehole wall failures such as breakouts and drilling-induced tensile fractures (DITFs) observed during IODP Expedition 338. The borehole drilled is 2005.5 meter below seafloor (mbsf) deep and exhibits a limited number of borehole wall failures because of optimal drilling-mud pressure control under riser drilling. The breakouts occurred only when the borehole pressure was slightly lowered and the time lag between hole cutting and image logging was at least several hours due to drill-pipe extension, which suggests that the observed breakouts are not entirely stress-induced as typically assumed, but rather brought up into shape with time due to other mechanisms (e.g., hydraulic erosion). However, based on the consistent breakout azimuths throughout the drilled depths, it is inferred that the breakouts are stress-dependent such that the stress-induced premature rock damage zones are spalled out progressively with time. Thus, breakout azimuths can be used to estimate stress orientation, but breakout geometry (width) should not be used to estimate stress magnitudes. The orientation of the maximum horizontal principal stress (SHmax) is determined to be NE-SW to -2000 mbsf, subparallel to the subduction margin between Philippine Sea plate and Eurasian plate. We attempt to estimate stress magnitudes assuming that the stress state is sufficient to bring about rock damage zones at the borehole wall, and that it is on the verge of the creating borehole wall failures based on laboratory triaxial compression experiments in cores. An integrated method that utilizes breakouts and DITFs as well as the result from a leak-off test yields the SHmax values slightly higher than the vertical stress (Sv) and the minimum horizontal principal stress (Shmin) possibly lower than Sv, suggesting that the stress state is predominantly in favor of strikeslip faulting.
We derived a tectonic stress field model in the vicinity of the source area of the Mw9.0 Northeastern Japan Earthquake from modeling seismicity and analyzing earthquake focal mechanisms before and after the mainshock. There are 5 large earthquakes (Mj>7.0) occurred in the source region of Mw9.0 Tohoku-Oki Earthquake from 2003 to 2011. We performed temporal and spatial analysis to the stresses before and after the mainshock from static stress change and focal mechanism solutions. The results show that the stress field in source region barely had any change before and after each large earthquake prior to the Mw9.0 mainshock, and it might indicate that the background stress had been accumulating and a series of progressive failure processes happened prior to the mainshock. The stress state did not change in the places where are distant away from the rupture center of the Mw9.0 mainshock during its occurrence, with the azimuth of the maximum principal stress to be 290“ and dip angle to be 19°. In comparison, the static stress has significant change in the central part of the source region after mainshock, with the occurrence of numerous normal fault type events. It turns out that there exists an area with negative stress change in the source area of the Mw9.0 earthquake after the mainshock, with the P axis rotated from horizontal direction to vertical direction, which triggered the occurrence of numerous normal faulting events, where was dominated by thrust faulting prior to the mainshock. This pattern indicates a complete stress and strain release or locally stresses regime change. Our results provide a constrain to the spatial distribution of the stress release pattern spatially and temporally for the source region of 2011 Tohoku-Oki earthquake.
The 2011 Mw9.0 Tohoku-Oki earthquake occurred along the plate boundary between the subducting Pacific plate and the overlying Okhotsk plate. This earthquake is the largest one even recorded in Japan Trench Subduction Zone (JTSZ). The dynamic background and mechanics of this event is one of the hottest research topics in geodynamics and seismology currently.
One of the most common ways to measure deformation and strength characteristics of rocks in-situ is the pressuremeter test. This method involves loading of the wellbore wall with the stress through the impermeable membrane. Lame's elastic solution for thick-walled cylinder subjected to high internal pressure is used to estimate Young's modulus of the rock mass at a certain Poisson's ratio, which is taken from the reference book, measured on the core material in the laboratory or calculated from the elastic wave velocities. The disadvantages of this approach include a significant difference in the estimates of the elasticity modulus depending on the chosen method of research that results in incorrect calculations of the rock massif deformation characteristics. This work is aimed at improving the reliability of the borehole measurements of elastic modulus in the rocks through the pressuremeter and hydraulic fracturing data sharing. The idea of the developed method is to use two different and independent data sets: the first one is obtained from standard deformation studies in well while the second one describes the behaviour of the same well with fracture formed in the direction of the maximum compressive stress. The difference in the geometrical arrangement of loading systems leads to different dependencies between the involved characteristics; joint processing of the results will give the values of Young's modulus and Poisson's ratio of the enclosing rocks. To implement this method a new type of downhole equipment is developed. II consists of an automatic delivery system that allows carrying out measurements in long inseam boreholes at a considerable distance from the underground workings without usage of drill rods.
The widely used method of studying the deformation properties of rocks and soils in situ is pressuremeter test (Clarke, 1996; Amadei et al., 1995). The results are used in calculations of the stability and designing of underground constructions, predicting of geodynamic events.