查看更多>>摘要：Most rocks exhibit time-dependent deformation and failure. Two main mechanisms are generally considered, the progressive growth of cracks and viscoelastic and/or viscoplastic deformation. In this study, cracking process is described by a viscous phase-field method which is coupled with a viscoplastic model. The evolution of crack field is controlled by both elastic and viscoplastic tensile volumetric and deviatoric strains. The threshold of viscoplastic deformation is weakened by the growth of cracks. The efficiency of the proposed model is first assessed by comparing numerical predictions with experimental data in triaxial compression and creep tests. Then, the proposed model is applied to modeling time-dependent deformation and failure process of a high slope section in the left bank of Jinping-I hydropower station in China. Numerical predictions are compared with field measurements.
Goldswain, GarethOlivier, GerritToit, Hendrik J. du
8页
查看更多>>摘要：Distributed acoustic sensing (DAS) is being applied in an increasing number of geoscientific fields. We present herein preliminary results of applying DAS to monitor microseismicity in underground mines. We installed 275 m of fiber optic cable approximately 1500 m below the surface in an operational mine and recorded mine seismicity for two weeks during November of 2018. The first approximately 50 m was tied to the sidewall safety mesh and a further 225 m was grouted into a down-dipping borehole alongside four three-component geophone sensors with 14 Hz high-sensitivity elements. The geophone sensors are located 132 m, 154 m, 176 m and 198 m down the borehole. We show that, for the larger seismic events (MW > -0.1), there is relatively good agreement in the data collected with DAS compared to the geophone data. There is some amplitude variation, especially at higher frequencies, but the phase of the recordings match well. We were able to include the DAS data in processing select events using PhaseNet, a machine-learning based algorithm, to pick phase arrivals and then locate these events using a recently developed interferometric method. In our comparison of the background noise of the DAS and that of the geophones, we found that the DAS as installed in our experiment has a noise floor orders of magnitude higher than the seismic background of the mine. Furthermore we compared the recordings in the different fiber sections (tied to the safety mesh and grouted, respectively) to analyze the feasibility of using existing mining communication infrastructure as sufficiently accurate seismic sensors. Fiber optic communication cables are typically tied to the sidewall mesh in the absence of dedicated conduits, and thus our meshcoupled section was a suitable proxy for this scenario. We found extremely limited utility in the recordings of the mesh coupled fiber.
查看更多>>摘要：Understanding the stress distribution, deformation and fracture behavior around a circular hole or inclusion is critical for the stability assessment of rock structures such as tunnels and backfilled openings. In this study, the analytical solutions of stress and displacement distribution around a circular inclusion of different elastic properties were first derived based on the theory of elasticity. Then prismatic sandstones with a circular hole in different filling modes were prepared and tested in uniaxial compression tests. Progressive deformation and fracturing behavior around holes were analyzed by digital image correlation technique. Moreover, the energy evolution characteristics of specimens were studied to reveal its deformation and failure mechanism. The results show that after filling, the radial and tangential stress and displacement of the inclusion increase, while the stress and displacement around the surrounding rock of the hole decreases, thereby improving the strength and deformation properties. In addition, the crack types and initiation positions around the unfilled and filled holes are quite different due to the stress redistribution. Based on the energy evolution characteristics and the major principal strain fields, the stress-strain behavior is divided into five stages. It is found that the inclusion not only has passive supporting and energy absorption effects on the surrounding rock, but also has a similar crack arrest effect on the initiation and propagation of new cracks. Finally, the effects of inclusion elastic properties and lateral stress coefficient on the stress concentration characteristics around the filled hole were discussed.
Moein, Mohammad Javad AfshariEvans, Keith F.Valley, BenoitBar, Kristian...
14页
查看更多>>摘要：The scaling laws describing the spatial arrangement of fractures along six deep boreholes penetrating the crystalline rocks in the Rhine Graben were derived using a correlation analysis. Five of the wells, two to 5 km depth, were located at the Soultz geothermal site and one well to 5 km depth was located at Basel, some 150 km from Soultz. Five datasets were derived from borehole imaging logs, whilst one stemmed from the analysis of 810 m of continuous core at Soultz. The two differed inasmuch as the core dataset included essentially all fractures, whereas the image log dataset had few fractures narrower than 1-3 mm. The results of the analysis for all image datasets showed that the spatial arrangement of fractures followed fractal behavior at all scales from meters to several hundred meters, the largest scale amenable to assessment, and that the fractal dimensions were confined to the narrow range 0.85-0.9. However, the core dataset showed significant deviation from fractal behavior, the best-fit fractal dimension of 0.8 being somewhat lower than values obtained from imaging logs in neighboring wells. Eliminating fractures with apertures less than 1 mm from the core dataset to improve comparability led to even lower fractal dimension estimates, indicating the discrepancy was not due to imaging log resolution. Analysis of successive depth sections of the core log suggested the discrepancy was due to the presence of a localized zone between 1750 and 2070 m where the fractal organization is disturbed or takes a lower dimension than elsewhere. Aside from this zone, no systematic variation of fractal dimension with depth was observed in any dataset, implying that a single exponent together with intensity adequately describes the arrangement of fractures along the entire length of the boreholes. The results are relevant to the parameterization of DFN models of deep rock masses.
查看更多>>摘要：Wellbore instability is an important consideration during both drilling and production of hydrocarbon reservoirs. The optimal well trajectory must be determined during the design phase to avoid wellbore shear failures. This study makes two fundamental contributions towards improving contemporary wellbore shear models. For the first time, the analytical wellbore shear models are formulated implicitly to significantly improve the compu-tational efficiency and obtainable accuracy. The stability problem is resolved with the bisection method and an optimisation algorithm, where Powell's method and the Nelder-Mead method have been implemented here. The second contribution is to account for the depletion of various formations, especially coal, in the stability models. Three stress models, namely those of Gray, Shi and Durucan, and Cui and Bustin, were used to develop stress paths for a depleted coal reservoir. The results were quantified via the maximum allowable pressure (MAP), which indicates the wellbore pressure required to avoid wellbore failure and thus guide corresponding opera-tional decisions. The results of this work show that implicit methods significantly improve computational efficiency over the conventional iterative method used in the literature and industry. In particular, it was found that Powell's method saves greater than 95% of the computation time for sandstone and coal case studies, respectively. In terms of stability during depletion, a higher depletion pressure resulted in an increased MAP. For a drilling application, this means that a greater overbalance pressure is required. While in a production application, a lower maximum drawdown pressure would be expected. The Gray model indicates the largest impact on stability prediction for depleted coals, and the Cui and Bustin model is the most conservative among the three stress models. The proposed numerical framework provides an efficient tool to determine the optimal well trajectory for different formations (e.g. coal, clastic rock) experiencing depletion before or after drilling.
查看更多>>摘要：A series of cyclic compression tests of marble spheres and scanning electron microscopy (SEM) imaging measurements were performed in this work. The cyclic compression tests of marble spheres reveal three different fracture types: Hertzian cracks, oblique fractures and multi-meridian fractures. The displacement of marble spheres with Hertzian cracks or oblique fractures in cyclic compression tests can be significantly larger than the displacement (0.31 mm) corresponding to sample breakage in monotonic compression tests. The SEM observations reveal that mineral fragmentation, plastic shear sliding and crack closure occur in the local contact area, resulting in obvious residual displacement. Hertzian cracks were produced by the combination of densification and shear strength degradation under cyclic loading. Typical two-stage behaviour of cumulative residual displacement and dynamic secant stiffness evolution with the number of cycles was clearly observed. The occurrence of Hertzian cracks or oblique fractures produced significant cumulative residual displacement. The present laboratory results also show that the increase in amplitude can result in a significantly larger cumulative residual displacement and that the number of cycles of marble sphere failure in meridian fracture significantly increases with an increase in the frequency of cyclic loading. The upper contact force ratio and amplitude ratio significantly influence the dynamic secant stiffness evolution. The dynamic secant stiffness of tests under amplitude ratios of 0.7 and 0.8 shows a clear decreasing trend after the initial decelerating stage, which may be related to the loosening and decohesion of mineral particles caused by local significant fragmentation.
查看更多>>摘要：The deep coal reservoir is in a high in-situ stress regime, and the drop in pore pressure during the coalbed methane extraction can induce significant slippage effect and effective stress change. Both of them play a competitive role in permeability evolution. In our research, coal seepage experiments under variable bulk stress and pore pressure paths are performed to investigate the slippage effect and effective stress effect during gas migration in deep coal seams. The results show that the gas flow pattern changes from effective stress-control to slippage effect-control during the increase of bulk stress. Because of the dual pore structure and rich clay, coal presents microscopically inhomogeneous assemblage in mechanical and seepage properties. It makes the effective stress coefficient (ESC) for permeability, kappa, always greater than 1, up to 1.94-2.64. By substituting the elastic modulus of the bedding for that of the clay in the coal, the clay-particle model can be used to explain the kappa > 1. The linear-Klinkenberg equation is no longer applicable to the correction of gas slippage effect in deep coal reservoir, but conforms to the double-slip-Klinkenberg equation. Slippage effect weakens the stress sensitivity of permeability. As the effective stress increases, the slippage coefficient b increases accordingly, and the slippage effect becomes more pronounced. The gas flow pattern in deep coal reservoir is basically in the region of slippage flow, and the slippage effect cannot be ignored in gas migration. Furthermore, the anisotropy of kappa is weakened in deep coal reservoirs, and the intrinsic permeability of coal, Ic infinity, has the strongest stress sensitivity to sigma 3.
查看更多>>摘要：The main objective in this study is to experimentally investigate how two types of shales different in swelling characteristic and organic carbon content behave in terms of the mechanical and infiltration properties when being subjected to changing osmosis with varying brine salinity under in-situ stress conditions. This research also aims to determine the swelling tendency of thePierre shale and replicate the effect of selection of brine salinity on oil recovery in liquid-rich shales. A series of coupled measurements from the triaxial experiments at reservoir conditions was conducted on Pierre shale core samples to determine rock properties and behavior under fluid pore pressure intrusion.The main use of the measured properties is to demonstrate the impact of fluid transport induced by low salinity water injection accompanied by osmosis, clay swelling, and rock elasticity change on oil recovery from rock matrix. The Young's modulus, bulk modulus, and Poisson's ratio of the Pierre shale were determined from measurements on the two types of Pierre shale samples obtained from an outcrop having high 65 wt% of smectite content and well core having lean clay content and 3.5 wt% TOC (total organic carbon). The first two experiments were performed on the outcrop samples comprising of similar characteristics of 65 wt% smectite with negligible TOC while the last experiment was performed on the well core sample containing negligible smectite content with 3.5 wt% TOC. The membrane efficiencies of the shale samples were experimentally determined to range from 5% to 28% depending on clay content and stress. The outcrop shale showed 1% of swelling that is significantly higher than 0.07% of swelling developed in the well core shale lean in clay content. One of the most important findings of this research is that the membrane coefficient representing the stress sensitivity of membrane efficiency of organic-rich well core shale was found to be 11.5%/kpsi which is significantly higher than that of the swelling shale ranging from 2.2 to 3.5%/kpsi. This is interesting because the approximately 5.5 times stiffer matrix of organic-rich shale shows the significantly higher stress sensitivity in terms of the membrane efficiency. The Young's modulus of swelling shale declined with water saturation as well as the water transfer driven by osmotic pressure with low salinity brine. In contrast, the well core shale showed significant increase in the dynamic Young's modulus.The experimental data were used to evaluate the mass exchange between the fractures and rock matrix using a coupled fluid flow and geomechanics model accounting for shale swelling and osmosis transport to evaluate how fluid and rock interaction affect the oil recovery from the rock matrix of the Pierre shale during low salinity water injection (LSWI). A 25% increase in oil recovery factor was determined after considering the effects of changing membrane efficiency due to clay swelling resulting from LSWI. The modeling results therefore suggest that the variation of membrane efficiency due to swelling can be an important factor affecting the oil recovery from matrix block during LSWI enhanced oil recovery.
Nkosi, Nomqhele Z.Manzi, Musa S. D.Westgate, MichaelRoberts, Dave...
17页
查看更多>>摘要：Petrophysical properties of cylindrical core specimens from three boreholes from the International Continental Scientific Drilling Program, the DSeis project, measured at ambient pressure and room temperature conditions in various laboratories are presented and compared with downhole petrophysical data (sonic and density). The measured properties are from sixty-six rock specimens constituting metasediments, metabasalts and intrusives. Seismic velocities were measured using 0.5 MHz P- and S-wave transducers. To investigate the source of seismic reflectivity observed on the 2D legacy seismic data, we computed synthetic seismograms for adjacent rock units using downhole petrophysical data and compared them with seismic reflections from the reflection seismic profile. The experimental measurements show that the metasediments exhibit lower bulk densities and seismic velocities than the metabasalts and intrusive specimens. The porosity was found to be less than 2% for all the samples. No clear trends emerge when the Poisson's ratio is plotted against the P-wave velocities and porosities of the samples. A positive relationship is observed between the bulk modulus and P-wave velocities of the rock samples. The highest calculated reflection coefficients (RC) are associated with the metasediment-intrusive interfaces in all three boreholes. The intrusive-metabasalt and the metasediment-metabasalt interfaces exhibit low RC. Synthetic seismograms reveal strong reflections that coincide with high RC calculated using the bulk density and velocity data. The synthetic seismograms also revealed additional strong reflections that were not identified using the reflection coefficients calculated from the rock specimens, due to core loss in some lithological units. Successful correlations are carried out between the synthetic seismic data and the real seismic data, enabling us to correlate the stratigraphic sequence drilled in the boreholes to the seismic reflections observed on the legacy 2D reflection seismic data.
NSTL
Elsevier