Fracture Dip Angle Research Articles

Double-porosity medium

A new algorithm for P-wave reverse time migration (RTM) in a fracture-induced TTI double-porosity medium is proposed in this paper, by combining RTM with the method of P-and S-wave field separation in high-order staggered grid finitedifference scheme. Simulation results show that for the fracture-induced TTI double-porosity medium the conventional migration with a background velocity without fractures would cause the imaging position to be under the true interface. To study the influence of fracture parameters, the phase velocity formulae are derived from Christoffel equations, and the effects of fracture angles and density are analyzed. In the 2D case, the imaging position gets closer to the real interface along with increasing fracture dip angles, while a bigger fracture density can cause much more deviation.

Determination of in situ stresses normal to a fracture around an internal pressure tunnel by hydrojacking testing

When designing an internally pressurized tunnel, the normal stress required to reopen the natural fractures should be a major parameter for its stability. Also the relationship between the fracture opening and the fluid pressure should be pre-interpreted for the stability of structures, such as the spillway tunnel of a dam. Apart from the general hydraulic fracturing in situ stress measurements, the stress normal to the natural fracture plane should be identified with respect to the dip direction and the dip angle of the pre-existing fracture. Hydrojacking test can be adopted for this purpose, and is used to determine the mechanical and hydraulic behaviors of pre-existing fractures around an internal pressure tunnel site. This paper summarizes the results and evaluation of the hydrojacking test conducted in the internally pressured spillway tunnel site of Juam dam, located 300km south of Seoul, near Suncheon, South Korea. The test intervals were selected after inspecting borehole televiewer images and rock cores, and the jacking pressure and the normal stress to the fracture plane have been determined.Considering that the new spillway tunnel will be adjacent to the existing drainage tunnel next to the main dam, the mechanical and hydraulic behaviors of the natural fractures in the rock mass should be thoroughly interpreted because the spillway tunnel will be pressurized internally. A proper determination of the minimum principal stress should be related to the applied safety factors for hydraulic confinement. The tests indicated that the hydrojacking test is a useful method for determining the hydro-mechanical behaviors of fractures in an unlined pressure tunnel.

Azimuthal anisotropy from OBS observations in Mahanadi offshore, India

We have carried out an ocean bottom seismometer (OBS) survey in a grid along with multichannel seismic survey for gas hydrate exploration in the Mahanadi offshore, India. Here, we report on some interesting observations in seismic waveform data and their interpretations. These include sudden amplitude dimming in the multichannel data that is azimuth- and space-dependent and a clear manifestation of seismic anisotropy in the region. We observe significant patterns of shear wave splitting in the azimuthal gathers in the OBS data, clearly isolating the fast (S1) and slow (S2) axes of propagation in the radial azimuthal gathers. Further, amplitude nulls and amplitude maxima are observed in the transverse azimuthal gathers. These two features are diagnostic of the existence and orientation of anisotropy which is also modeled by generating full waveform synthetic seismograms. We interpret the occurrence of anisotropy to be due to the presence of fractures. The strike of this fracture set is inferred to be [Formula: see text] from the S1 and S2 orientation and variation in the P-wave amplitude with azimuth. The density of fracture network is estimated by full wave modeling of the OBS data. A good match between the synthetic and observed data is noticed for a near vertical fracture (dip angle of about 85°). The seismic image obtained from the 2D high-resolution multichannel profiles correlate well with the OBS results. Based on these analyses, we are able to delineate a fracture zone, which is linked to the near vertical faulting in the gas hydrate layers.

ANALYSIS ON CRACKING MECHANISM OF FRACTURED ROCK MASSES UNDER UNIAXIAL COMPRESSION BASED ON THE TWO CRACKING CRITERIA

In this paper, with two cracking criteria, i.e., Rankine’s maximum tensile stress theory and Mohr-Coulomb shear failure theory for tensile and shear crack initiation respectively, the cracking mechanism of fractured rock masses under uniaxial compression was analyzed. Based on the analytical solution to stress field of single inclined elliptic fracture in an infinite rock mass under far-field load, the factors of the principal stresses concentration surrounding the fracture and the cracking functions, cracking positions and the crack initiation loads of the two cracking mechanism were calculated for different ratio of the smaller to the larger semi-axis k of the elliptical fracture and fracture dip angle α (the angle from the larger semi-axis of elliptic fracture to direction of the load) through a Matlab program. For rock masses containing multiple elliptic fractures, elastic stress fields were calculated by ABAQUS and cracking mechanisms were analyzed. It is concluded that: 1) With the increase of the number of fractures, the factors of the principal stresses concentration in the rock matrix increase slightly and the crack initiation loads decrease slightly, while the cracking positions are the same. 2) With the increase of fracture dip angle, the tensile stress concentration of the rock matrix changes from a small zone around fracture tip to a large zone surrounding the fracture, and vice versa for compressive stress, 3) There exists an critical fracture dip angle α0 which is around 45°; When 0<α≤α0, i.e., the fracture dip angle is small, the tensile stress and shear stress are both concentrated in the vicinity of fracture tip and the tensile crack initiation load are close to the shear crack initiation load with the increase of the ratio of the smaller to the larger semi-axis k, which means that tensile cracks and shear cracks will appear simultaneously; While for α0<α≤90°, i.e., the fracture dip angle is large, shear cracks or crushing zones will firstly initiate at the small region of the fracture tip due to high compressive stress concentration, and then large amount of tensile cracks will occur at the middle or tip of the pre-existing fractures due to high tensile stress concentration in this zone. The above analyses coincide with the experimental results.

Fractures of lacustrine shale reservoirs, the Zhanhua Depression in the Bohai Bay Basin, eastern China

Fractures play a vital role in the exploration and development of shale oil and gas by providing effective space for shale reservoirs and significantly improving the fluid flow capability. Core observations, microscopic analyses of thin sections, scanning electron microscopy, and Formation MicroScanner Imaging (FMI) were used to determine the types, causes of formation, and development characteristics of the fractures in lacustrine shale reservoirs in the lower part of the Paleogene Shahejie Formation (Es3) in the Zhanhua Depression, Bohai Bay Basin, eastern China. X-ray diffraction (XRD) analysis, total organic carbon (TOC) measurements, and porosity and permeability measurements were used to study the controlling factors of the fractures in the shale reservoirs, and to analyze the impact of the fractures on the shale reservoirs properties and subsequent exploration and development. The studied shale reservoir mainly displays tectonic fractures as well as various types of non-tectonic fractures. The non-tectonic fractures mainly include over-pressure fractures, diagenetic fractures, inter-layer bedding fractures, and fractures of mixed origins. In the study area, the tectonic fractures which were formed under the combined action of tensile and shear stress display the following characteristics. The dip angle of the tectonic fractures varies significantly. Unfilled or half-filled effective fractures have a high proportion. These fractures are mainly oriented in the NE–SW, NNE–SSW, and WNW–ESE directions, with fractures in the NE–SW direction accounting for the highest proportion. The tectonic and non-tectonic fracture development is affected by multiple types of factors such as the presence of faults, mineral composition, lithology, abnormal pressure and organic matter content. Abnormally high pore pressure is a very important factor in the development of non-tectonic fracture. It is inferred that the over-pressure is mainly related to hydrocarbon generation during thermal evolution. Fractures effectively improve the porosity and permeability of the shale reservoirs, and the enhancement of permeability is particularly significant. The current stress field affects the fluid flow capability of the fracture reservoirs, and the present maximum principal stress in Zhanhua Depression is oriented in the NEE–SWW direction, which has a small angle with fractures in NE–SW direction. We propose that the fractures in this direction have the greatest connectivity and thus are a high-priority target for petroleum exploration and development.

An Experimental Investigation of the Effect of Fracture Dip Angle on Oil Recovery and Drainage Rate in Free Fall Gravity Drainage in Fractured Reservoirs Using a Glass Micromodel (A Pore Level Investigation)

Gravity drainage is the main production mechanism in the gas invaded zone in naturally fractured reservoirs. However, there are large ambiguities and complexities, resulting from the dynamic of oil depletion from matrix blocks toward the fracture network. Visualization of drained oil at pore scale using glass micromodels provides the opportunity to better understand the effects of different parameters which might affect oil recovery from fractured reservoirs. In this work a micromodel apparatus generated by laser etching is used to perform some gravity drainage tests on the network patterns. The experiments were performed on double block systems using crude oil. The block to block interactions and the formation and changes of capillary bridges between matrix blocks were visually investigated. A suitable empirical model of gravity drainage is used for the prediction and evaluation of experimental data. The empirical model will be used for sensitivity analysis of fracture dip angle which might affect oil recovery efficiency and oil drainage rate, and as seen in experiments this parameter has a direct relationship to and positive effect on oil recovery. We also found the important rule of the block to block effect on the recovery of blocks and the higher recovery of upper matrix blocks than lower ones shows this reality; also it was claimed that the threshold height of fractured reservoir blocks doesn't show the true matrix capillary pressure of matrices, because of the block to block effect that helps the gravity forces in the matrix draining procedure. By increasing the block tilting angle up to α = 90 – tan – 1(y/x) as y and x are the 2-dimensional block sides lengths, the recovery of the block will increase but its initial drainage rate will decrease. It's a result of an increase in effective block height and increasing the capillary bridges stability which increases recovery, and the decrease of rate is due to capillary bridges frequency decreasing, which decreases the area open to flow.

Analysis of RVE size based on three-dimensional fracture numerical network modelling and stochastic mathematics

A new method to study representative volume element (RVE) size is developed based on three-dimensional (3D) fracture numerical network modelling and a stochastic mathematical model. A 3D fracture numerical network is established, and the network factors that affect RVE size, such as fracture density, diameter, strike, and dip angle, are collected. This paper utilizes these four factors to obtain the RVE size based on the contingency table method. It is concluded that the side length of the RVE is 2–8 times larger than the average trace length. Fracture frequency is chosen to validate the RVE size through 3D fracture numerical network modelling. RVE size calculated through fracture frequency is consistent with the contingency table method. It is proven that the method for calculating RVE size through the contingency table method is feasible and credible.

Characterizing and modeling natural fracture networks in a tight carbonate reservoir in the Middle East: A methodology

Fractured reservoirs are challenging to handle because of their high level of heterogeneity. In particular, natural fractures have a significant impact on well performance and water production. Therefore, understanding their significance through fracture characterization is helpful in well placement and field development. This paper presents a best practice methodology for building a 3D stochastic fracture model using a Middle Eastern tight carbonate field example. This model is generated through the analysis and integration of data including cores, borehole images (BHI), logs, mud losses, production logs, well test data and 3D seismic data. The impact of lithology on fracture occurrence was quantified based on rock-typing. Rock-types are distributed in a 3D geological model using a high resolution sequence stratigraphic framework. The length, dip angle and orientation of fractures, together with the shale content of the facies where they occur, were defined to sort the tectonic fractures from the non-tectonic (diagenetic) ones. It was found that multiple sets of tectonic diffuse fractures are generally associated with cleaner limestone units. Altogether, three sets of diffuse fractures were identified from BHI data: NE-SW, EW and NW-SE. In addition, large-scale fracture corridors, including sub-seismic faults identified from seismic analysis, were detected and calibrated with cores and BHI. The final model incorporates two scales of tectonic fractures with a direct bearing on field production behavior: diffuse fractures and large fracture corridors. Fracture calibration was performed using production logs and well production data. Permeability at wells was computed in the 3D fracture model and matched with real build-up data. These data were then used to propagate 3D fracture properties (fracture porosity, fracture permeability and equivalent block size) in the upscaled geological model, for constructing a full-field reservoir simulation model. Few changes of the fracture properties were needed to obtain a good history match, indicating that the fracture model produced is robust.

Characteristics and interpretation of fracture-filled gas hydrate – An example from the Ulleung Basin, East Sea of Korea

Through the use of 2-D and 3-D seismic data, a total of thirteen sites were selected and drilled in the East Sea of Korea in 2010. A suite of logging-while-drilling (LWD) logs was acquired at each site. LWD logs from the UBGH2-3A well indicate significant gas hydrate in clay-bearing sediments including several zones with massive gas hydrate with a bulk density less than 1.0 g/m3 for depths between 5 and 103 m below the sea floor. The UBGH2-3A well was drilled on a seismically identified chimney structure with a mound feature at the sea floor. Average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and P-wave velocity logs are 80 ± 13% and 47 ± 16%, respectively, whereas they are 46 ± 17% and 45 ± 16%, respectively from the anisotropic analysis. Modeling indicates that the upper part of chimney (between 5 and 45 m below sea floor [mbsf]) is characterized by gas hydrate filling near horizontal fractures (7° dip) and the lower part of chimney (between 45 and 103 mbsf) is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and P-wave velocity. The anisotropic analysis using P40H resistivity (phase shift resistivity at 32 mHz with 40 inch spacing) and the P-wave velocity yields a gas hydrate saturation of 46 ± 15% and 46 ± 15% respectively, similar to those estimated using ring resistivity and P-wave velocity, but with quite different fracture dip angles. Differences in vertical resolution, depth of investigation, and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy. Forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts.

Relationship between fracture dip angle, aperture and fluid flow in the fractured rock masses

Most of the Earth's crust contains fluids, and fractures are common throughout the upper part. They exist at a wide range of scales from micro-fractures within grains to major faults and shear zones that traverse the crust. In this paper, the stress-dependent permeability in fractured rock masses have been investigated considering the effects of nonlinear normal deformation and shear dilation of fractures using a twodimensional distinct element method program, UDEC. A new analytical and numerical model was proposed to determine the relationship between fracture dip angle, aperture and permeability. The numerical work were conducted in two ways: (1) increasing the overall stresses with a fixed ratio of horizontal to vertical stresses components; and (2) increasing the differential stresses (i.e., the difference between the horizontal and vertical stresses) while keeping the magnitude of vertical stress constant. The results showed that at the stress ratio of 1 the significant shear dilation occurs at an approximately low stress and mean fracture angles. For the differential stresses case, the shearing process can result in breakage of the asperities, resulting in the decrease of the dilation rate and strain softening of the fracture.

신보광산 주변지역의 지질구조와 우라늄 지화학 이상대

본 논문은 전라북도 진안군 신보광산의 동부지역에서 고품위 우라늄 지화학 이상대와 지질구조 사이의 상관관계를 규명하고 지하수 흐름에 대한 기본적인 정보를 제공하기 위해 암상구분에 의한 상세한 지질도를 작성하고 이들 구성암류에 발달하는 연성 및 취성 구조요소의 특성을 파악하였다. 그 결과, 이 지역은 주로 동북동-서남서 방향으로 대상 분포를 보이는 선캠브리아기 규암, 변성이질암, 변성사질암과 이를 관입하는 시대미상의 페그마타이트와 백악기 반암류 등으로 구분되고, 추정과 달리 이들 암상을 부정합으로 피복하는 백악기 진안층군은 관찰되지 않았다. 선캠브리아기 변성퇴적암류의 주요 연성 변형구조는 적어도 세 번의 변형단계 [동북동 방향성의 광역엽리 (D1) -> 동북동 내지 동서 방향성의 파랑엽리 (D2) -> 서북서 내지 동서 방향성의 개방, 밀착, 킹크 습곡 (D3)]를 걸쳐 형성되었고, S1 광역엽리의 방향성은 선캠브리아기 변성퇴적암류의 대상 분포 방향과 유사한 동북동 주향에 남쪽으로 경사하는 것이 우세하게 나타난다. 지하수의 유동과 밀접한 관련성 있는 취성 고각 단열 (경사각<TEX>${\geq}45^{\circ}$</TEX>)의 방향성[동북동 (출현빈도: 24.3%), 남북 (23.9%), (북)북서 (18.8%), 서북서 (16.9%), 북동 (16.1%) 단열조 순]은 변성퇴적암류의 대상 분포 방향성 및 S1 광역엽리의 우세 방향성과 일치하는 동북동 주향에 남쪽으로 경사하는 것이 가장 우세하게 나타나고 남북 주향에 동서 방향으로 경사하는 것이 다음으로 우세하게 나타난다. 상기된 선캠브리아 변성퇴적암류의 대상 분포 방향성과 S1 광역엽리 및 고각 단열의 우세 방향성 등으로부터 신보광산과 그 동부지역의 우라늄 지화학 이상대의 형성요인을 고찰해 볼 때 이들 지역에서 기대되는 우라늄의 초생적인 근원암은 기존연구 결과와 같이 페그마타이트이며 광산지역의 우라늄 지화학 이상대는 이를 대수층으로 하는 지하수가 광산지역으로 통과하면서 이차적으로 부화되었을 가능성이 높다고 판단된다. This paper examined the characteristics of ductile and brittle structural elements with detailed mapping by lithofacies classification to clarify the relationship between the geological structure and the geochemical high-grade uranium anormal zone and to provide the basic information on the flow of groundwater in the eastern area of Shinbo mine, Jinan-gun, Jeollabuk-do, Korea. It indicates that this area is mainly composed of Precambrian quartzite, metapelite, metapsammite, which show a zonal distribution of mainly ENE-WSW trend, and age unknown pegmatite and Cretaceous porphyry which intrude them. But the Cretaceous Jinan Group which unconformably covers them, contrary to assumption, could not be observed. The main ductile deformation structures of Precambrian metasedimentary rocks were formed at least through three phases of deformation [ENE striking regional foliation (D1) -> ENE or EW striking crenulation foliation (D2) -> WNW or EW trending open, tight, kink folds (D3)]. The predominant orientation of S1 regional foliation strikes ENE and dips south, being similar to the zonal distribution of Precambrian metasedimentary rocks. Most predominant orientation of high-angled brittle fracture (dip angle <TEX>${\geq}45^{\circ}$</TEX>) [ENE (frequency: 24.3%) > NS (23.9%) > (N)NW (18.8%) > WNW (16.9%) > NE (16.1%) fracture sets in descending frequency order], which is closely related to the flow of groundwater, strikes ENE and dips south. It also agrees with the zonal distribution of metasedimentary rocks and the predominant orientation of S1 regional foliation. The next one strikes NS and dips east or west. Considering the controlling factor of the geochemical uranium anormal zone in the Shinbo mine and its eastern areas from the above structural data. the uranium source rock in these areas might be pegmatite and the geochemical uranium anormal zone in the Sinbo mine area could be formed by an secondary enrichment through the flow of pegmatite aquifer's groundwater into the Sinbo mine area like the previous research's result.

Numerical Simulation of Effect of Fracture Distribution on Failure Mechanism of Tunnel

In order to study the effect of rock fracture distributions on stability of tunnel, the effect of joints with different strike dip, the size and density was considered by using probability and statistics method. Based on the Monte Carlo random simulation, the statistical network model of 2D jointed rockmass was established to describe and characterize jointed rock mass. On the basis of this, the numerical code RFPA2D is employed to analyze effect of fracture dip angle on the tunnel stability. Numerical results show that the fracture distribution has a little effect on tunnels failure mechanism and failure type of tunnel because the failure of tunnel in the jointed rockmass is similar to the one without joints and fissures. However with the change of the fracture dip angle, the loading capacity of the tunnel in the jointed rockmass varies accordingly.

Numerical Simulation of Effect of Fracture Distribution on Failure Mechanism of Tunnel

In order to study the effect of rock fracture distributions on stability of tunnel, the effect of joints with different strike dip, the size and density was considered by using probability and statistics method. Based on the Monte Carlo random simulation, the statistical network model of 2D jointed rockmass was established to describe and characterize jointed rock mass. On the basis of this, the numerical code RFPA2D is employed to analyze effect of fracture dip angle on the tunnel stability. Numerical results show that the fracture distribution has a little effect on tunnels failure mechanism and failure type of tunnel because the failure of tunnel in the jointed rockmass is similar to the one without joints and fissures. However with the change of the fracture dip angle, the loading capacity of the tunnel in the jointed rockmass varies accordingly.

Determination of Geometrical and Structural Representative Volume Elements at the Baihetan Dam Site

A case study at the Baihetan dam site was undertaken to obtain a representative volume element (RVE) size. Two-dimensional fracture information in an exploration tunnel was used to generate a large three-dimensional fracture network. By dividing the entire modeled rock mass into cubes, the volumetric fracture density (P32) value of each cube was determined. The size effect can be determined by changing the cube size. The RVE was determined using P32 calculation and statistical tests, including Kolmogorov-Smirnov (KS) and Wilcoxon rank-sum tests. The P32 value depends on the geometrical parameters of fracture density and size; in this study, this value is called the geometrical RVE. P32 is dependent on fracture density and size, and cannot appropriately reflect certain data such as fracture dip or dip angle. Therefore, we propose a structural RVE (SRVE) considering fracture dip direction, dip angle, density, and size, which together constitute the available information about the fracture network. Therefore, the SRVE is more applicable for use in solving geological problems than the RVE. In this analysis, the KS and Wilcoxon rank-sum tests were used to determine the SRVE size.

Detailed DEM analysis of a rockslide scar to characterize the basal sliding surface of active rockslides

The basal sliding surfaces in large rockslides are often composed of several surfaces and possess a complex geometry. The exact morphology and location in three dimensions of the sliding surface remains generally unknown, in spite of extensive field and subsurface investigations, such as those at the Åknes rockslide (western Norway). This knowledge is crucial for volume estimations, failure mechanisms, and numerical slope stability modeling. This paper focuses on the geomorphologic characterization of the basal sliding surface of a postglacial rockslide scar in the vicinity of Åknes. This scar displays a stepped basal sliding surface formed by dip slopes of the gneiss foliation linked together by steeply dipping fractures. A detailed characterization of the rockslide scar by means of high‐resolution digital elevation models permits statistical parameters of dip angle, spacing, persistence, and roughness of foliation surfaces and step fractures to be obtained. The characteristics are used for stochastic simulations of stepped basal sliding surfaces at the Åknes rockslide. These findings are compared with previous models based on geophysical investigations. This study discusses the investigation of rockslide scars and rock outcrops for a better understanding of potential rockslides. This work identifies possible basal sliding surface locations, which is a valuable input for volume estimates, design and location of monitoring instrumentation, and numerical slope stability modeling.

Characteristics of the nuclear magnetic resonance logging response in fracture oil and gas reservoirs

Fracture oil and gas reservoirs exist in large numbers. The accurate logging evaluation of fracture reservoirs has puzzled petroleum geologists for a long time. Nuclear magnetic resonance (NMR) logging is an effective new technology for borehole measurement and formation evaluation. It has been widely applied in non-fracture reservoirs, and good results have been obtained. But its application in fracture reservoirs has rarely been reported in the literature. This paper studies systematically the impact of fracture parameters (width, number, angle, etc), the instrument parameter (antenna length) and the borehole condition (type of drilling fluid) on NMR logging by establishing the equation of the NMR logging response in fracture reservoirs. First, the relationship between the transverse relaxation time of fluid-saturated fracture and fracture aperture in the condition of different transverse surface relaxation rates was analyzed; then, the impact of the fracture aperture, dip angle, length of two kinds of antennas and mud type was calculated through forward modeling and inversion. The results show that the existence of fractures affects the NMR logging; the characteristics of the NMR logging response become more obvious with increasing fracture aperture and number of fractures. It is also found that T2 distribution from the fracture reservoir will be affected by echo spacing, type of drilling fluids and length of antennas. A long echo spacing is more sensitive to the type of drilling fluid. A short antenna is more effective for identifying fractures. In addition, the impact of fracture dip angle on NMR logging is affected by the antenna length.

Outcrop fracture characterization using terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas

Determination of fracture orientation can be an important aspect of structural analysis in reservoir characterization. The availability of ground-based laser scanner systems opens up new possibilities for the determination of fracture surface orientation in rock outcrops. Scanners are available in low-sample-density, low-accuracy, and fast, high-sample-density, high-accuracy models. These automatic laser scanner systems produce enormous volumes or “clouds” of point data at an instrument-dependent accuracy and resolution, which can be at the millimeter level. This huge volume of data calls for an automated and objective method of analysis. We have developed a surface classification algorithm based on a multipass partitioning of the point cloud. The method makes use of both spatial proximity and the orientation of an initial coarse-grained model of the point cloud. Unsupervised classification of surface sets is demonstrated herein using the new algorithm. Both previously mentioned types of scanners have been used to map the Jackfork sandstone outcrop at Big Rock Quarry in Little Rock, Arkansas. We apply the surface classification algorithm to these data to extract fracture surface orientations from the point cloud. The effectiveness of these new technologies when applied to fracture analysis is clearly demonstrated in this example. It is also shown that the low-density, low-resolution type of scanner is adequate to define general geomorphology but is inadequate for fracture definition. The surface classification algorithm can be used to reliably extract fracture and bedding strike and dip angles from the three-dimensional point locations acquired using centimeter-accurate, high-density laser scanner systems.

Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

Abstract— Bosumtwi is a very well‐preserved 1.07 Myr old, complex terrestrial impact crater located in south‐central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures.We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p‐wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta‐graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north‐south.

Estimation of the apertures of water‐saturated fractures by nuclear magnetic resonance well logging

ABSTRACTFracture aperture is an important transport property in subsurface hydrology because it influences well productivity and the volume of the water resource. Nuclear magnetic resonance (NMR) well logging measures the hydrogen‐bearing fluid molecules in porous or fractured strata, and the NMR signal intensity increases with the amount of fluid in the sensed region of the NMR sonde. Fluid confined in a large fracture of &gt;&gt;0.2 mm in aperture has T2 (i.e. spin‐spin relaxation time) values as long as those of the bulk fluid. The bulk‐fluid porosity (i.e. porosity calculated using this long T2 component in a T2 histogram data) increases linearly with aperture. Therefore, NMR logging enables quantitative estimation of fracture apertures of &gt;&gt;0.2 mm using the bulk‐fluid porosity data if the calibration of the NMR sonde is performed adequately. We applied NMR logging to a borehole in a Holocene andesite lava at Sumikawa, Japan, to estimate the aperture of open fractures within the lava. A test well of 100 m depth and 20 cm diameter, filled with bentonite drilling mud, was scanned with an NMR sonde to obtain a profile of the porosity and the T2 histogram of the andesite. The bulk‐fluid porosity was calculated from the T2 histogram data, as the porosity at which the T2 value is larger than or equal to a threshold T2 of bulk bentonite mud. The bulk‐fluid porosity of a specific inclined fracture responsible for the total loss of circulation at 61.2 m depth during drilling was calculated assuming a threshold or T2 cut‐off of 33 ms, and again for a cut‐off of 100 ms. Calibration of the NMR sensor in a laboratory and measurement of the fracture dip angle by electrical microimaging logging enabled us to estimate the fracture aperture as 1.7 cm, assuming a T2 cut‐off of 33 ms, or 1.6 cm for a T2 cut‐off of 100 ms. The method of aperture determination described in this study is independent of fluid species and lithology, and is applicable to various hydrogen‐bearing borehole fluids (clean water, mud and oil) and geological settings.

Brittle fracture in two crystalline rocks under true triaxial compressive stresses

Abstract We employed our new polyaxial cell to carry out true triaxial compression tests on dry (jacketed) rectangular prisms of two crystalline rocks, in which different magnitudes of the least and intermediate principal stresses σ 3 and σ 2 were maintained constant, and the maximum stress σ 1 was increased to its peak level in strain control. Both Westerly granite (Rhode Island, USA) and KTB amphibolite (Bohemian Massif, Germany) revealed similar mechanical behaviour, much of which is missed in conventional triaxial tests in which σ 2 = σ 3 . Compressive failure in both took the form of a main shear fracture, or fault, steeply dipping in the σ 3 direction. Compressive strength rose significantly with the magnitude of σ 2 , suggesting that the commonly used Mohr-type strength criteria, which ignore the σ 2 effect, predict only the lower limit of rock strength. The true triaxial strength criterion for each of the crystalline rocks can be expressed as the octahedral shear stress at failure as a function of the mean normal stress acting on the fault plane. We found that the onset of dilatancy increases considerably for higher σ 2 . Thus, σ 2 extends the elastic range for a given σ 3 and, hence, retards the onset of the failure process. The main fracture dip angle was found to increase as σ 2 rises, providing additional confirmation of the strengthening effect of σ 2 . SEM inspection of the micromechanics leading to specimen failure showed a multitude of stress-induced microcracks localized on both sides of the through-going fault. Here too the effect of σ 2 is noted, in that microcracks gradually align themselves with the σ 1 -σ 2 plane as the magnitude of σ 2 is raised.