Fracture Density Distributions and Well Yields in Coastal Maine

Understanding the distribution of water-bearing fractures in crystalline rock is an important component in evaluating the availability and vulnerability of water resources throughout the northeastern U.S. The State of Maine requests well drillers to report estimates of fracture depths and fracture yields for all bedrock wells drilled in the state. Using these data we analyze fracture-depth and fracture-yield data from 227 bedrock wells in coastal Maine in order to understand how fracture locations and yields are distributed with depth. Numerical simulations and statistical tests show that it is not possible to infer how fractures are distributed with depth: fracture depths are consistent with several distributions, including uniform fracture density with depth. In order to understand how fracture yield varies with depth, we group yield data into 50 foot depth intervals and compare distributions in each interval using nonparametric statistical tests. These tests show that the distribution of fracture yield in different depth intervals are statistically equivalent. These results imply that there is no empirical justification for limiting well depth when drilling for water resources in fractured bedrock in coastal Maine.     

Identifying Aquifer Type in Fractured Rock Aquifers using Harmonic Analysis

Determining aquifer type, unconfined, semi‐confined, or confined, by drilling or performing pumping tests has inherent problems (i.e., cost and complex field issues) while sometimes yielding inconclusive results. An improved method to cost‐effectively determine aquifer type would be beneficial for hydraulic mapping of complex aquifer systems like fractured rock aquifers. Earth tides are known to influence water levels in wells penetrating confined aquifers or unconfined thick, low‐porosity aquifers. Water‐level fluctuations in wells tapping confined and unconfined aquifers are also influenced by changes in barometric pressure. Harmonic analyses of water‐level fluctuations of a thick (～1000 m) carbonate aquifer located in south‐central Oklahoma (Arbuckle‐Simpson aquifer) were utilized in nine wells to identify aquifer type by evaluating the influence of earth tides and barometric‐pressure variations using signal identification. On the basis of the results, portions of the aquifer responded hydraulically as each type of aquifer even though there was no significant variation in lithostratigraphy. The aquifer type was depth dependent with confined conditions becoming more prevalent with depth. The results demonstrate that harmonic analysis is an accurate and low‐cost method to determine aquifer type.     

Experimental Measurement of P-wave Attenuation Due to Fractures Over the 100 to 300 kHz Bandwidth

— A series of experiments was conducted where rock core specimens were cyclically loaded to 50?MPa uniaxial stress while P waveforms were pulsed along the core axis. Some of these specimens were intact while others were prepared with a single through-going tensile fracture oriented perpendicular to the core axis. Recorded data was processed to determine Q, velocity and fracture closure. Results indicated that Q was constant over the studied bandwidth and Q for the fractured specimens decreased relative to the intact specimens as fracture stress decreased. Observed variations in static fracture stiffness among the tested specimens did not result in corresponding variations in Q. Velocity results showed similar trends. This work was done to provide comparative data for related field studies examining the feasibility of using attenuation measurements in comparable frequency bands to indicate the potential for roof failure during excavation in fractured rock masses.     

差应力条件下岩石Q值测定方法的改进

本文指出了脉冲传播和谱振幅比法测定岩石破裂孕育过程中Q值变化矛盾的原因,提出用能量法测定Q值,并以大理岩,大姚铜矿砂岩为例给出了真三轴压缩时岩石破裂孕育过程中P波总能量的变化及不同方法所测Q值的变化特征。其中能量法所测Q值与P波总能量变化特征一致,即主断裂前它们都出现了“下降—回升”变化及变化的各向异性。     

Heavy tailed probability distributions for non-Gaussian simulations with higher-order cumulant parameters predicted from sample data

Forecasting of extreme events and phenomena that respond to non-Gaussian heavy-tailed distributions (e.g., extreme environmental events, rock permeability, rock fracture intensity, earthquake magnitudes) is essential to environmental and geoscience risk analysis. In this paper, new parametric heavy-tailed distributions are devised starting from the exponential power probability density function (pdf) which is modified by explicitly including higher-order “cumulant parameters” into the pdf. Instead of dealing with whole power random variables, novel “residual” random variables are proposed to reconstruct the cumulant generating function. The expected value of a residual random variable with the corresponding pdf for order G, gives the input higher-order cumulant parameter. Thus, each parametric pdf is used to simulate a random variable containing residuals that yield, in average, the expected cumulant parameter. The cumulant parameters allow the formulation of heavy-tailed skewed pdfs beyond the lognormal to handle extreme events. Monte Carlo simulation of heavy-tailed distributions with higher-order parameters is demonstrated with a simple example for permeability.     

Experimental and numerical study on supercritical CO2/brine transport in a fractured rock: Implications of mass transfer,capillary pressure and storage capacity

This study presents the impact of fractures on CO₂ transport, capillary pressure and storage capacity by conducting both experimental and numerical studies. A series of laboratory experiment tests was designed with both a homogeneous and a fractured core under CO₂ storage conditions. The experimental results reveal a piston-like brine displacement with gravity override effects in the homogeneous core regardless of CO₂ injection rates. In the fractured core, however, two distinctive types of brine displacements were observed; one showing brine displacement only in the fracture whereas the other shows brine displacement both in the fracture and matrix with different rates, which were dependent on the magnitude of the pressure build-up in the matrix. The injectivity in the fractured core was twice of the homogeneous core, while the amount of calculated CO₂ in the homogeneous core was over 1.5 times greater than the fractured core. Salt precipitation, which is likely to occur near injection wells, was observed in the experiments; X-ray images enabled the observation of salt-precipitation during CO₂-flooding tests. Finally, numerical simulations predict free-phase CO₂ transfer between fracture and matrix in a fracture-matrix system. Pressure gradients between the fracture and matrix enforced CO₂ to transfer from the fracture into matrix at the front of the CO₂ plume, whereas, the reversal of pressure gradients at the rear zone of the CO₂ plume reversed the transfer process. The variation of CO₂ saturation within the fracture was caused by fracture aperture variations, and local variations of fracture permeability control the free-phase CO₂ transfer between the fracture and matrix.     

Experience in registration of variations caused by strong earthquakes in the level and physicochemical parameters of ground waters in the piezometric wells: the case of Kamchatka

The hardware complex that was produced by OOO Polynom, Khabarovsk, for registration of the level, temperature, and electrical conductivity of ground water in wells and meteorological parameters (atmospheric pressure, air temperature) at a measurement frequency from 5 min to 1 h is described. The equipment is installed in the wells of Kamchatka and has been used for several years to register variations caused by earthquakes in the parameters of ground waters. Different variations in measured parameters of ground waters due to strong earthquakes of February 28, 2013, M _W = 6.8 and May 24, 2013, M _W = 8.3 are registered with this equipment in wells YuZ-5 and E-1. The registered variations and their systematization are described taking into account the mechanisms of a seismic impact on the state of the well–water-saturated rock system.     

Groundwater Resources Potential in Hard Rock Terrain: A Multivariate Approach

Groundwater resources are limited and difficult to predict in crystalline bedrock due to heterogeneity and anisotropy in rock fracture systems. Municipal‐level governments often lack the resources for traditional hydrogeological tests when planning for sustainable use of water resources. A new methodology for assessing groundwater resources potential (GRP) based on geological and topographical factors using principal component analysis (PCA) and analysis of variance (ANOVA) was developed and tested. ANOVA results demonstrated statistically significant differences in classed variable groups as well as in classed GRP scores with regard to hydrogeological indicators, such as specific capacity (SC) and transmissivity. Results of PCA were used to govern the weight of the variables used in the prediction maps. GRP scores were able to identify 79% of wells in a verification dataset, which had SC values less than the total dataset median. GRP values showed statistically significant correlations using both parametric (using transformed datasets) and non‐parametric methods. The method shows promise for municipal or regional level planning in crystalline terrains with high levels of heterogeneity and anisotropy as a hydrogeologically and statistically based tool to assist in assessing groundwater resources. The methodology is executed in a geographic information systems environment, and uses often readily available data, such as geological maps, feature maps and topography, and thus does not require expensive and time‐consuming aquifer tests.     

Fluid Pressure Variation in a Sedimentary Geothermal Reservoir in the North German Basin: Case Study Groß Schönebeck

The Rotliegend of the North German basin is the target reservoir of an interdisciplinary investigation program to develop a technology for the generation of geothermal electricity from low-enthalpy reservoirs. An in situ downhole laboratory was established in the 4.3 km deep well Groβ Schönebeck with the purpose of developing appropriate stimulation methods to increase permeability of deep aquifers by enhancing or creating secondary porosity and flow paths. The goal is to learn how to enhance the inflow performance of a well from a variety of rock types in low permeable geothermal reservoirs. A change in effective stress due to fluid pressure was observed to be one of the key parameters influencing flow properties both downhole and in laboratory experiments on reservoir rocks. Fluid pressure variation was induced using proppant-gel-frac techniques as well as waterfrac techniques in several different new experiments in the borehole. A pressure step test indicates generation and extension of multiple fractures with closure pressures between 6 and 8.4 MPa above formation pressure. In a 24-hour production test 859 m³ water was produced from depth indicating an increase of productivity in comparison with former tests. Different depth sections and transmissibility values were observed in the borehole depending on fluid pressure. In addition, laboratory experiments were performed on core samples from the sandstone reservoir under uniaxial strain conditions, i.e., no lateral strain, constant axial load. The experiments on the borehole and the laboratory scale were realized on the same rock types under comparable stress conditions with similar pore pressure variations. Nevertheless, stress dependences of permeability are not easy to compare from scale to scale. Laboratory investigations reflect permeability variations due to microstructural heterogeneities and the behavior in the borehole is dominated by the generation of connections to large-scale structural patterns.     

四川汶川8．0级地震前山西昔阳地震台体应变特征和同震效应研究

该文对2008年5月12日四川汶川8．0级地震前山西昔阳地震台体积应变仪出现的异常变化进行了初步分析,结果表明,从3月29日出现的异常变化完全符合在实验研究岩石破裂时,主破裂前和破裂时显示出的突变变化特征。     

Experimental investigation of the strength and failure behavior of layered sandstone under uniaxial compression and Brazilian testing

As a typical inherently anisotropic rock, layered sandstones can differ from each other in several aspects, including grain size, type of material, type of cementation, and degree of compaction. An experimental study is essential to obtain and convictive evidence to characterize the mechanical behavior of such rock. In this paper, the mechanical behavior of a layered sandstone from Xuzhou, China, is investigated under uniaxial compression and Brazilian test conditions. The loading tests are conducted on 7 sets of bedding inclinations, which are defined as the angle between the bedding plane and horizontal direction. The uniaxial compression strength (UCS) and elastic modulus values show an undulatory variation when the bedding inclination increases. The overall trend of the UCS and elastic modulus values with bedding inclination is decreasing. The BTS value decreases with respect to the bedding inclination and the overall trend of it is approximating a linear variation. The 3D digital high-speed camera images reveal that the failure and fracture of a specimen are related to the surface deformation. Layered sandstone tested under uniaxial compression does not show a typical failure mode, although shear slip along the bedding plane occurs at high bedding inclinations. Strain gauge readings during the Brazilian tests indicate that the normal stress on the bedding plane transforms from compression to tension as the bedding inclination increases. The stress parallel to the bedding plane in a rock material transforms from tension to compression and agrees well with the fracture patterns; “central fractures” occur at bedding inclinations of 0°–75°, “layer activation” occurs at high bedding inclinations of 75°–90°, and a combination of the two occurs at 75°.     

远场和超远场前兆的模拟实验研究

在岩石试件、传压块体（包括岩石和钢块）和压机架上布设应变观测点，观测研究它们在岩石试样破裂孕育过程中的变化特征。结果发现，岩样临破坏前除岩样上各个测点都出现异常变化外，同时较远处的测力元件、传压块体和压机架上的应变也出现了突变或扰动前兆异常。岩石试样外部测点的应变特征变化在极临近主破裂时才有反映。而岩样上的测点除极临近主破裂时有突变和扰动外在离主破裂较早时已有反映。岩石试样外的测点距岩样约25－90cm，约为破裂尺度的3－10（多）倍。这表明在孕震区以外很远的地方都可能出现前兆异常。并初步探讨了形成远场前兆的原因。     

慢地震慢前兆的机制研究

通过岩石高温高压破裂实验与有机玻璃破裂试验的应力—应变曲线对比、地震波形记录(观测资料)与试样破裂波形(实验资料)对比、地震前后定点应变与主破裂前后应变形态变化的对比，认为慢地震是临界或亚临界破裂或预先滑动所致，是低频应变波动。它是材料屈服、弱化或塑性变化的必然结果。而慢前兆则是临近大地震(大破裂)前出现的诸如形变、低频地震波(破裂弹性波)等的短期及临震前兆现象，它比较可靠。但由于许多岩石主破裂前并无明显的临界破裂或预滑动现象，也并非所有岩石都一定出现明显屈服，所以也并非每个地震前都有慢地震，因此，也就不一定都出现低频波动。从而，也并不一定都出现相同的短期、临震前兆现象。     

Efficient testing of earthquake forecasting models

Computationally efficient alternatives are proposed to the likelihood-based tests employed by the Collaboratory for the Study of Earthquake Predictability for assessing the performance of earthquake likelihood models in the earthquake forecast testing centers. For the conditional L-test, which tests the consistency of the earthquake catalogue with a model, an exact test using convolutions of distributions is available when the number of earthquakes in the test period is small, and the central limit theorem provides an approximate test when the number of earthquakes is large. Similar methods are available for the R-test, which compares the likelihoods of two competing models. However, the R-test, like the N-test and L-test, is fundamentally a test of consistency of data with a model. We propose an alternative test, based on the classical paired t-test, to more directly compare the likelihoods of two models. Although approximate and predicated on a normality assumption, this new T-test is not computer-intensive, is easier to interpret than the R-test, and becomes increasingly dependable as the number of earthquakes increases.     

Burst strain disturbance — A new test study on fracture precursor

A multi-functional high-frequency seismometer, a new instrumental system, is used for recording multi-point strain variations of rock samples in the process of fracture development. The results indicate that strain in each point appears burst disturbance synchronously besides a tendency variation before main fracture. The burst disturbance could probably be one of the short-impending precursors before great earthquake.     

The role of primary nonuniformity of mountain massifs in the reconstruction of tectonic stresses according to geological indicators

Features of the distribution of planes with traces of tectonic dislocations on gliding planes are examined, and the local stress conditions are restored on them, according to O.I. Gushchenko’s method [1979]. Most of the tectonic stresses were determined in the rock-crystal-bearing area of the Pripolyarnii Ural in the rocks of the Proterosoic and Lower Ordovician age. The rock crystal was of Late Permian age. The distributions of the planes with gliding trails depend on the primary anisotropy of the rocks and on the type of stress condition. The specific type of stress condition named variation was marked out. The variation of the type of the stress condition (VSCT) is strictly confined to rock crystal pockets. The already existing planes of nonuniformity in the rocks, which often do not coincide with shear planes, are used in case of a deformation in the last tectonic stress fields. The criteria for distinguishing tectonic stress classes, based on the models of the distribution of tectonic stresses in the surroundings of faults, are suggested.     

Matrix Diffusion Effects on Contaminant Migration from an Injection Well in Fractured Sandstone

Deep-well injection into fractured sandstone is an option for the disposal of contaminated mine dewatering discharge from an open pit uranium mine. As part of the assessment of potential contaminant migration from deep-well injection, the effect of matrix diffusion was evaluated. An analytical mathematical model was developed for the simulation of the radial movement of a contaminant front away from an injection point under steady flow conditions in a planar fracture with uniform properties. The model includes the effects of advection in the fracture, diffusion of contaminants from the fracture into the rock matrix, and equilibrium adsorption on the fracture surface as well as in the rock matrix. Effective diffusion coefficients obtained from laboratory experiments on 11 intact core samples varied from 3.4 × 10⁻⁸ to 3.2 × 10⁻⁷ cm²/s. Model simulations were made with diffusion coefficient values in this range and with single-fracture injection rates estimated from fracture frequencies in boreholes, and from bulk hydraulic conductivity values obtained from field tests. Because of matrix diffusion, the rate of outward movement of the front of the nonreactive contaminants from the injection well is much slower than the rate of water flow in the fractures. Simulations of the movement of contaminants that undergo adsorption indicate that even a small distribution coefficient for the rock matrix causes the contaminants to remain very close to the injection well during the one-year period. The results of the simplified model demonstrate that matrix diffusion is an important process that cannot be neglected in the assessment of a waste disposal scheme located in fractured porous rock. However, in order to make a definitive assessment of the capability of matrix diffusion and associated matrix adsorption to significantly limit the extent of contaminant migration around injection wells, it would be necessary to conduct field tests such as a preliminary or experimental injection.     

Burst strain disturbance—A new test study on fracture precursor

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Experimental Study of the Earthquake Recurrence Period and the Trend of Post-seismic Development

In order to study the earthquake recurrence and the characteristics of earthquake series, rupture tests of rock samples and plexiglass samples were made. On rock samples, a number of acoustic emission (AE) and strain measuring points were deployed; the load was one side direct shear. The variation characteristics of AE and strain at different detecting points around the extra large fracture were observed and studied. On plexiglass samples, a series of inclined cracks were prefabricated by a small-scale compressive testing machine. The samples were then loaded on a shockproof platen, when the samples were loaded, the stress intensity factor (SIF) was determined by the laser interferometric technique and shadow optical method of caustics. The fracture conditions such as material toughness around the extra large fracture were also studied. From those experimental results and the theory of fracture mechanics, the earthquake recurrence period and the trend of post-seismic development were studied.     

Constructing a discrete fracture network constrained by seismic inversion data

Rock fractures are of great practical importance to petroleum reservoir engineering because they provide pathways for fluid flow, especially in reservoirs with low matrix permeability, where they constitute the primary flow conduits. Understanding the spatial distribution of natural fracture networks is thus key to optimising production. The impact of fracture systems on fluid flow patterns can be predicted using discrete fracture network models, which allow not only the 6 independent components of the second‐rank permeability tensor to be estimated, but also the 21 independent components of the fully anisotropic fourth‐rank elastic stiffness tensor, from which the elastic and seismic properties of the fractured rock medium can be predicted. As they are stochastically generated, discrete fracture network realisations are inherently non‐unique. It is thus important to constrain their construction, so as to reduce their range of variability and, hence, the uncertainty of fractured rock properties derived from them. This paper presents the underlying theory and implementation of a method for constructing a geologically realistic discrete fracture network, constrained by seismic amplitude variation with offset and azimuth data. Several different formulations are described, depending on the type of seismic data and prior geologic information available, and the relative strengths and weaknesses of each approach are compared. Potential applications of the method are numerous, including the prediction of fluid flow, elastic and seismic properties of fractured reservoirs, model‐based inversion of seismic amplitude variation with offset and azimuth data, and the optimal placement and orientation of infill wells to maximise production.