Magnitude Of Minimum Principal Stress Research Articles

Earthquake impact on the stress magnitude at the area of shear zones

Both tectonic activity and geological environment such as faulting and shearing in the rock mass influences the in-situ stress condition in the rock mass. The large-scale earthquakes cause both accumulation and sudden release of strain energy resulting the changes on the magnitude and direction of the in-situ stresses. This is the case in the Himalayan region, where the tectonic movement is active and periodic dynamic earthquakes are common. This manuscript assesses the influence of local shear faults on the in-situ stress condition along the pressure tunnel of Upper Tamakoshi Hydroelectric Project in Nepal. For this a 3D numerical modeling is carried out to assess potential changes in the magnitude of in-situ stresses, in particular the magnitude of minimum principal stress, due to dynamic loading (earthquake).

Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress for Unlined Pressure Tunnels: A Field Experiment Using the Rapid Step-Rate Test

This paper describes an experimental campaign where a simplified test method. The Rapid Step-Rate Test (RSRT) has been investigated in a full-scale field setting. The test, originally developed to assess rock stresses for pressure tunnel considerations, had in an earlier laboratory study demonstrated a promising ability to assess the magnitude of fracture normal stress. The field campaign presented herein was therefore aimed at assessing the field applicability of this test. The field experiments were conducted inside the pressure tunnel of the Løkjelsvatn Hydroelectric Power Plant in Norway, under field conditions identical to those for which the test originally was developed. A total of 29 individual tests were conducted in 7 boreholes, providing 20 test cycles with interpretable test results. The stress estimates made from the test cycles not affected by the near-field stress of the tunnel showed good correlation with the magnitude of minimum principal stress found from a preceding hydraulic fracturing and over-coring stress measurement campaign. The experience from the field campaign suggests that the RSRT can represent an efficient and reliable method to assess the magnitude of minimum principal stress, particularly useful for the final design of unlined pressure tunnels.

ENHANCED TECHNIQUE FOR HYDRAULIC FRACTURING PROCEDURE IN FRACTURED ROCK MASS

Notwithstanding the broad hypothetical work about hydraulic fracturing technique for estimation of stresses in rock mass, the experimental work has so far been restricted, particularly on account of its legitimacy in fractured rocks. Much work has been completed about the induced fractures created by hydraulic fracturing tests, however not on the impact of previously existing fractures and joints and their impact. When a fracture has been originated at the borehole wall, the fluid infiltrates the splitting of the rocks, and the pressure is applied to the walls of the fracture. In this way, the least downhole infusion pressure expected to hold open and expand a crack is somewhat higher than the normal to the plane of the fracture. To overcome the above issue, a sequence of hydraulic fracturing tests was performed on fractured rock masses inside the tunnel of one of the hydroelectric projects. The applied pressures in these tests were stretched out past the standard reaches (6 to 8 l/min), coming to up to 20 MPa with a flow capacity of up to 16 l/min. It was seen that by expanding or diminishing the pumping pressure for each cycle, the fracture opening pressure declined consequently after specific augmentation. It is deciphered that, since the entire fractured rock mass is exposed to the flow of water; these events are because of the opening of fractures at various spatial positions. When high pressure gradients exist, especially when the permeability increases abruptly as the effective pressure approaches zero, a sharp pressure front develops in the region, moving outward from the borehole. If the permeability upsurges with increasing pore pressure, a steep pressure gradient will tend to grow. After shutoff the pump, rapid shut-in pressure is obtained to get the normal stress across the fracture and to calculate the minimum principal stress magnitude and direction. This is the state-of-the-art technique established by the authors for determination of the stresses in fractured ground environments. KEYWORDS: Hydraulic fracturing; fractured rocks; high flow rate; normal stress.

Fluid Flow and Leakage Assessment Through an Unlined/Shotcrete Lined Pressure Tunnel: A Case from Nepal Himalaya

The use of unlined/shotcrete lined pressure tunnels and shafts are cost-effective solutions for a hydropower project and are being implemented worldwide. To implement this concept, the ground conditions at the area of concern should be favorable regarding minimum principal stress magnitude, which should be higher than hydrostatic water head acting on the tunnel periphery. In addition, the rock mass should be relatively unjointed or joints in the rock mass should be relatively tight. Among the most important issues in the design of unlined/shotcrete lined pressure tunnels is the extent of hydraulic jacking and water leakage out of the tunnel during operation. This manuscript first presents fluid flow and potential hydraulic jacking assessment of two selected locations of the headrace tunnel of Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal using the UDEC. It is noted here that the 7960 m long headrace tunnel will experience a hydrostatic water head that will vary from 2.9 to 11.5 bars (0.29–1.15 MPa). The headrace tunnel is supported by sprayed concrete (shotcrete) in combination with systematic rock bolts in the tunnel walls and crown. The invert of the tunnel and few hundred meters downstream end (at surge shaft area) of the headrace tunnel is being concrete lined after the completion of all other works. The qualitative fluid flow assessment carried out using UDEC indicated considerable pressure built-up in the joint systems suggesting potential hydraulic jacking. This was especially the case at the downstream segment (downstream from chainage 7100 m) of the headrace tunnel. The manuscript further presents the quantitative results of water leakage estimation from the headrace tunnel carried out using Panthi (Panthi KK (2006) Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD Thesis, NTNU, Trondheim, Norway;Panthi, Note on estimating specific leakage using Panthi’s approach, NTNU, Trondheim, 2010;) approach. The leakage assessment carried out indicated an average specific leakage of about 2.5 l/min/m tunnel, which may result in over 210 l/s leakage from the headrace tunnel. The evaluation also indicated that the outer reach (860 m downstream segment) of the headrace tunnel after chainage 7100 m seems extremely vulnerable and over 80 l/s water leakage may occur only from this headrace tunnel segment during operation of the hydropower plant.

Hydromechanical insight of fracture opening and closure during in-situ hydraulic fracturing in crystalline rock

Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressure-controlled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixed-mode deformation processes triggered by high-pressure injection.

Evaluation on the Minimum Principal Stress State and Potential Hydraulic Jacking from the Shotcrete-Lined Pressure Tunnel: A Case from Nepal

Reliable estimation of in situ stress state is very important in implementing unlined/shotcrete-lined pressure tunnels and shafts. The topography, local tectonic setting and geological environment greatly influence the magnitude of in situ stress level. This paper aims to evaluate in situ stress state at the Upper Tamakoshi Hydroelectric Project (UTHP), where unlined/shotcrete-lined headrace tunnel with considerable hydrostatic head is being implemented. Initially measured minimum principal stress indicated much lower values than the hydrostatic pressure at the downstream end of the headrace tunnel, which led to shift the alignment at the upper elevation with reduced hydrostatic pressure. In order to explore the reason behind much lower stress level as expected, a comprehensive assessment is carried out by developing a full rock stress model so that the minimum principal stress along the unlined pressure tunnel is evaluated. To address the complex geotectonic and topographic environment of the UTHP project area, a final rock stress model (FRSM) concept as suggested by Stephansson and Zang (2012) has been utilized. The FRSM concept considers stepwise evaluation of the in situ stress state analysis integrating the best estimate stress model (BESM), stress measurement methods (SMM) and integrated stress determination methods (ISD). The analysis carried out revealed that the in situ stress state at the project area has high degree of spatial variation even at the similar overburden due to the presence of complex topography and the presence of local shear and weakness zones. The analysis further demonstrates that a presence of local shear/weakness zone has considerable de-stressing effect, which leads to the reduction of in situ minimum principal stress magnitude. The reduction in the minimum principal stress along the pressure tunnel increases the risk for the potential hydraulic jacking and leakage if static water pressure is higher than the magnitude of minimum principal stress.

Extremely weak fault planes: An estimate of focal mechanisms from stationary seismic activity in the San'in district, Japan

In this paper, using data from dense seismic observations in and around the seismic belt of the San'in district, Japan, we describe our analysis of focal mechanisms of stationary seismic activity, estimated stress states, and pore fluid pressures. We found these focal mechanisms to be described by the estimated stress field, suggesting that the stress field can be treated as uniform in each sub-region of the study area. We also found that events occurred even on unfavorably oriented fault planes with small shear stress. Further, we inferred that pore fluid pressures of approximately 20% of the faults analyzed are greater than the magnitude of minimum principal stress, when we assume μ=0.6. A possible explanation is localized high pore fluid pressure anomalies; another is the coefficient of friction of at least a part of faults analyzed in the study area being substantially smaller than 0.6, which we view as more plausible, since it is difficult to maintain pore fluid pressures higher than the magnitude of minimum principal stress for long periods of time.

Simulation of paleotectonic stress fields and quantitative prediction of multi-period fractures in shale reservoirs: A case study of the Niutitang Formation in the Lower Cambrian in the Cen'gong block, South China

Reservoirs where tectonic fractures significantly impact fluid flow are widespread. Industrial-level shale gas production has been established from the Lower Cambrian Niutitang Formation in the Cen'gong block, South China; the practice of exploration and development of shale gas in the Cen'gong block shows that the abundance of gas in different layers and wells is closely related to the degree of development of fractures. In this study, the data obtained from outcrop, cores, and logs were used to determine the developmental characteristics of such tectonic fractures. By doing an analysis of structural evolution, acoustic emission, burial history, logging evaluation, seismic inversion, and rock mechanics tests, 3-D heterogeneous geomechanical models were established by using a finite element method (FEM) stress analysis approach to simulate paleotectonic stress fields during the Late Hercynian—Early Indo-Chinese and Middle-Late Yanshanian periods. The effects of faulting, folding, and variations of mechanical parameters on the development of fractures could then be identified. A fracture density calculation model was established to determine the quantitative development of fractures in different stages and layers. Favorable areas for shale gas exploration were determined by examining the relationship between fracture density and gas content of three wells. The simulation results indicate the magnitude of minimum principal stress during the Late Hercynian — Early Indo-Chinese period within the Cen'gong block is −100 ∼ −110 MPa with a direction of SE-NW (140°–320°), and the magnitude of the maximum principal stress during the Middle-Late Yanshanian period within the Cen'gong block is 150–170 MPa with a direction of NNW-SSE (345°–165°). During the Late Hercynian — Early Indo-Chinese period, the mechanical parameters and faults play an important role in the development of fractures, and fractures at the downthrown side of the fault are more developed than those at the uplifted side; folding plays an important role in the development of fractures in the Middle-Late Yanshanian period, and faulting is a secondary control. This 3-D heterogeneous geomechanical modelling method and fracture density calculation modelling are not only significant for prediction of shale fractures in complex structural areas, but also have a practical significance for the prediction of other reservoir fractures.

The relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks

Closure pressures measured during injection tests such as mini-fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude. This paper presents stress, strength and image log data from the Australian Cooper Basin, which suggests that in reservoirs with high in situ stress, high tensile strength and weak geological fabrics, interpreted closure pressures may be significantly greater than the minimum principal stress. Closure pressures interpreted from mini-frac injection tests in the Cooper Basin, suggest the minimum principal stress varies from 12.4–27.2 MPa/km (0.55–1.2 psi/ft). To better understand the reasons for this variation in closure pressure, image logs and mini-frac data from 13 treatment zones, and core from seven of these treatment zones, were analysed. The analysis revealed that treatment zones with high measured closure pressures (⩾18.1 MPa/km; 0.8 psi/ft), high treating pressures (>31.6 MPa/km; 1.4 psi/ft) and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength (>7 MPa; 1015 psi) and geological fabrics (planes of weakness) including natural fractures. Conversely, treatment zones with lower measured closure stress (⩽19 MPa/km; 0.84 psi/ft) and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and/or no geological fabrics. We suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the Cooper Basin where they are associated with hydraulic fracture complexity. Rather, they reflect the normal stress incident on pre-existing weaknesses that are exploited by hydraulic fluid during the mini-frac injection.

How close to failure is a granite rock mass at a 5 km depth?

We discuss in this paper the mechanical significance of microseismicity induced by pore pressure variations. Particular attention is given to identifying whether microseismic events reflect only small effective stress perturbations, in a manner somewhat similar to the so-called Kaiser effect, or whether they outline the onset of large-scale failure. This issue is addressed in the context of the development of microseismic activity observed during large-scale water injections conducted in the 5000 m deep experimental geothermal reservoir at Soultz (France). Results from large hydraulic tests together with analysis of borehole images and induced seismicity are integrated to provide a well-constrained characterization of the complete stress field down to 5 km. It is shown that, for this site, pore pressure increments larger than 10% of the natural minimum principal stress magnitude are required for inducing large-scale shear failures that are characterized by a clear structuring of the microseismic events occurrence. But the onset of induced seismicity occurs for much smaller pore pressure variations that only reflect the “elastic” response to changes in local effective stresses. It is concluded that the linear variation with depth of principal stress magnitudes observed in this granite depends on the long-term rheology of this formation rather than on the frictional characteristics of the main faults that affect the massif.

Electrical imaging and hydraulic testing for a complete stress determination

A complete stress determination has been performed in Lodève (southern France) by conducting hydraulic tests in a 200 m deep borehole, with a probe combining electrical imaging with an inflatable straddle packer. Both classical hydraulic fracturing and hydraulic tests on pre-existing fractures have been conducted. The normal stress acting on the tested fracture planes was determined from the simultaneous analysis of both hydraulic and electrical signals observed during shut-in phases. Values derived from quasi-static reopening pressure tests are systematically larger than those derived from shut-in and have been discarded. Hydraulic fractures provide information on the principal stress directions orientation as well as on the minimum principal stress magnitude. HTPF tests provide information on the vertical and the maximum horizontal principal stress components magnitude and are unaffected by any pore pressure effect. At a depth of z=165 m , the vertical stress component is principal and is given by S v =4.0±0.5 MPa . The maximum horizontal principal component is oriented N155°±10° and is given by S H =10.0±0.2 MPa . It is almost twice the value found for S h , S h =5.8±0.1 MPa . These results are consistent with the results obtained nearby. They show that hydraulic properties of faults (discontinuities with length larger than 1 km and offsets larger than 100 m ) do not depend simply on the relative orientation of the fault with respect to the principal stress directions.

Adverse effects of pore-pressure drainage on stress measurements performed in deep tunnels: an example from the Lower Kihansi hydroelectric power project, Tanzania

We describe a campaign to measure stress within a gneissic ridge near the edge of the Tanzanian craton . The measurements were conducted to determine whether the level of minimum principal stress along the trajectory of a hydropower pressure tunnel attained the level of 10 MPa, as required to leave the tunnel unlined. Extrapolation of early hydrofracture and overcoring measurements conducted mostly in boreholes drilled behind face suggested that the requisite level was not quite met. However, the minimum principal stress estimates from the instantaneous shut-in pressures (ISIPs) of the hydrofracture tests generally showed large scatter and were sometimes less than the estimated pre-disturbance pore pressure . It was suspected that the stress tests were affected by stress alteration around the tunnel due to pore pressure drainage. To test this, hydrofracture and hydro-jacking tests were conducted in two long, horizontal holes drilled ahead of the excavation face into relatively undrained rock, and a third vertical hole drilled behind the face. All were near the critical location where the pressure tunnel enters the powerhouse. The minimum principal stress estimates in the ahead-of-face holes were found to be much higher than those from the behind-face hole, consistent with the hypothesis. Estimates of the drawdown in pore pressure about the tunnel and test holes suggested a coupling factor between drawdown and minimum principal stress magnitude of approximately unity. This is slightly higher than predicted from an hydromechanical model that explains the coupling between the pore-pressure and stress fields in terms of fracture compliance and poro-elasticity of intact blocks. Nonetheless, both results demonstrate the importance of accounting for drawdown when conducting stress measurements in boreholes drilled from tunnels. We also note some instances where hydrofracture and hydro-jacking tests yielded different estimates for the minimum principal stress magnitude.

Analysis of heterogeneously connected rock masses by forward modeling of fractional dimension flow behavior

An injection experiment was conducted to investigate the pressure domain within which hydromechanical coupling significantly influences the hydrologic behavior of a rock mass. The study site is located in the village of Le Mayet de Montagne in central France, where the local granitic terrain has been extensively characterized. The test well is 840 m deep, with an open-hole diameter of about 16 cm. A heat-pulse flowmeter was initially used to delineate the natural fluid circulation in the well and water was then injected from the surface through a tube anchored on an inflatable packer set at a depth of 275 m. This packer location was chosen in order to reach a depth where the minimum principal stress shifts from vertical to horizontal, thereby avoiding extraneous surface effects. The injection rate was progressively increased from 20 to 780 L/min while fluid velocity in the well was simultaneously recorded as a function of depth with flowmeters; corresponding well-head pressures gradually increased from 0.3 to 6.4 MPa. Fluid-intake zones were identified and their transmissivities were monitored as a function of varying effective stress. When pore pressure remained less than about 2/3 of the minimum principal stress, the hydrologic system responded linearly and Darcy's Law applied. However, as pore pressure approached or exceeded the minimum principal stress magnitude, several new fluid-intake zones emerged. Meanwhile, other intervals exhibited either slight enhancements or reductions in transmissivity apparently due to changes in the far-field pressure conditions. The borehole was capable of supporting hydraulic pressures nearly equal to the magnitude of the far-field maximum principal stress. Results show that transmissive fractures form a dynamic and interdependent network; individual fracture zones cannot be adequately modeled as independent equivalent continua once hydromechanical coupling becomes significant.

Evaluation of hydromechanical coupling in a granite rock mass from a high-volume, high-pressure injection experiment: Le Mayet de Montagne, France

An injection experiment was conducted to investigate the pressure domain within which hydromechanical coupling significantly influences the hydrologic behavior of a rock mass. The study site is located in the village of Le Mayet de Montagne in central France, where the local granitic terrain has been extensively characterized. The test well is 840 m deep, with an open-hole diameter of about 16 cm. A heat-pulse flowmeter was initially used to delineate the natural fluid circulation in the well and water was then injected from the surface through a tube anchored on an inflatable packer set at a depth of 275 m. This packer location was chosen in order to reach a depth where the minimum principal stress shifts from vertical to horizontal, thereby avoiding extraneous surface effects. The injection rate was progressively increased from 20 to 780 L/min while fluid velocity in the well was simultaneously recorded as a function of depth with flowmeters; corresponding well-head pressures gradually increased from 0.3 to 6.4 MPa. Fluid-intake zones were identified and their transmissivities were monitored as a function of varying effective stress. When pore pressure remained less than about 2/3 of the minimum principal stress, the hydrologic system responded linearly and Darcy's Law applied. However, as pore pressure approached or exceeded the minimum principal stress magnitude, several new fluid-intake zones emerged. Meanwhile, other intervals exhibited either slight enhancements or reductions in transmissivity apparently due to changes in the far-field pressure conditions. The borehole was capable of supporting hydraulic pressures nearly equal to the magnitude of the far-field maximum principal stress. Results show that transmissive fractures form a dynamic and interdependent network; individual fracture zones cannot be adequately modeled as independent equivalent continua once hydromechanical coupling becomes significant.