Preferred Orientation Of Fractures Research Articles

Characterisation of fracture evolution of a single cemented brittle grain using in-situ X-ray computed tomography

In this paper, the diametrical fragmentation of a single brittle glass bead cemented by ductile epoxy resin is studied using in-situ X-ray computed tomography, focusing on the fractures within the grain that underpin its crushability. Key quantities of interest include the evolution of fracture regions, paths and orientations, as well as the evolution of fracture statistics within the grain. The complex nature of the fracture process is revealed by qualitative and quantitative analysis based on both continuous X-ray radiography and discrete tomography. A series of image processing techniques are used and developed to obtain the statistical details of fragment volume, surface area, morphological geometry, fracture spatial locality and fracture preferential orientations. The single cemented grain failure consists of an initial splitting fracture process creating large and irregular glass fragments, followed by a further rapid propagation of multiple fractures through the cleavage fragments in the form of severe comminution that tends to generate smaller and more regular fragments. Despite the whole fragmentation process shows a shifting trend of the overall median anisotropy of fragments and a decreasing trend of conversion rate between fracture energy and input energy, it still maintains a constant fragment surface area power law distribution and appears to remain scale-invariant over time. The central region of the glass bead is markedly more susceptible to fracture with a constant preferred fracture orientation along the loading direction, indicating that the tensile-activated fracture is the dominant fracture mechanism during diametrical fragmentation.

Effect of excavation stress condition on hydraulic fracture behaviour

The fracture behaviour of single- and multi-stage hydraulic fracturing (HF) under varying excavation stress conditions was studied using a flow-coupled discrete element method (DEM). Conventional HF theories and analytical solutions for excavation stress redistribution are used to verify the numerical model. The numerical simulation results indicated that for the single-stage HF, both the in situ stresses and injection location have major influences on the HF propagation path, caused by the redistributions of the local maximum and minimum principal stresses under excavation conditions. Fractures generally propagate perpendicular to the local minimum principal stress (σmin) and also propagate preferentially towards the area with lower σmin. Hydraulic fractures propagation is dominated by tensile failure, and the generation of fracture branches is disfavored because of the high local compressive stress field induced by the HF process itself. As for multi-stage HF, different excavation stress conditions may cause different fracture development performances because of alterations of the preferred fracture orientation, blunting of propagating fractures, or creating a more complex fracture geometry in the affected zone. Therefore, when using high-pressure HF for preconditioning the rock mass to assist mechanical excavation in hard rock mining, performance can be enhanced by understanding the impact of stress changes and careful selection of appropriate HF methods depending on the in situ stress.

Completion Optimization Provides Step Changes in Horizontal Well Performance in the Oooguruk Field, Alaska

Summary The Oooguruk Unit is on a man-made gravel island in the Beaufort Sea, five miles offshore the Alaskan North Slope (ANS) in Harrison Bay. The field produces from the Kuparuk, Torok, and Nuiqsut reservoirs. The focus of this paper is the Nuiqsut sandstone, which is currently undergoing water and lean-gas injection for secondary recovery. The wells are completed as 6,000- to 7,000-ft horizontal laterals aligned parallel with the preferred fracture orientation in a line-drive waterflood pattern. Recent optimizations in mechanical-diversion fracturing in these laterals have provided significant improvements in production rates, including several recent wells with initial production of more than 7,000 BOPD. This paper will document the completion and fracturing-design evolution over several vintages of wells, as well as the use of preinstalled tracer systems to verify production uniformity and diversion success. The reservoir ranges in thickness from 60 to 120 ft and is divided into several producing sand and shale intervals. The initial phase (I) of completions planned for this reservoir was to use 8,000-ft-long undulating openhole horizontal laterals. However, these were quickly abandoned after the first well collapsed in a shale section. The second phase (II) used undulating wellbores for producing wells, but were completed with preperforated pups spaced evenly throughout the uncemented liner in the horizontal section. These wells were also stimulated with dynamic-diversion fracturing treatments that used ball sealers. Because of the logistical difficulties and expense in fracturing operations on a gravel island in the Beaufort Sea, two wells were also completed during this phase as unfractured dual-laterals, but resulted in productivity similar to dynamic diversion fracturing in one well and significantly less in the second well. During these phases of increased well productivity, modifications were required to increase waterflood well injectivity, which was accomplished by implementing a system of high-pressure breakdown (HPBD) stimulations, as well as fully mechanical diversion-fracture treatments. These changes in injection-well completions will also be described in the paper. Phase II wells resulted in production improvements of nearly 100% over the Phase I completions. This led to the third phase of development, which used mechanical-diversion techniques, implemented in relatively flat horizontal laterals. This completion type allowed mechanical-diversion fracturing treatments that placed more than three times the low-density ceramic (LDC) proppant and generated wells with initial production of more than 7,000 BOPD (an additional 100% increase over Phase II completions). All future producing wells are now planned to be completed with mechanical-diversion equipment. The completion-optimization evolution described in this paper will be useful to completion and development engineers of other conventional reservoirs, and the lessons learned are already being successfully applied to another nearby ANS development.

Compressional and EM wave velocity anisotropy in a temperate glacier due to basal crevasses, and implications for water content estimation

AbstractWe have conducted a series of experiments designed to investigate elastic and electromagnetic (EM) velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south-central Alaska, USA. Measurements include a three-dimensional compressional wave (P-wave) seismic reflection survey conducted over a 300 m x 300 m survey patch, with uniform source grid and static checkerboard receiver pattern. Additionally, we acquired a multi-azimuth, multi-offset, polarimetric ground-penetrating radar (GPR) reflection experiment in a wagon-wheel geometry with 94° of azimuthal coverage. Results show azimuthal variation in the P-wave normal-moveout velocity of >3% (3765 and 3630 m s–1 in the fast and slow directions respectively) and difference of nearly 5% between the fast (0.164 m ns–1) and slow (0.156 m ns–1) EM velocities. Fracture orientations estimated from the GPR and seismic velocity data are consistent and indicate a preferred fracture orientation that is 30-45° oblique to glacier flow; these measurements agree with borehole observations. Anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 0.73 ±0.11%. We conclude that meaningful estimates of physical properties in glaciers based on EM or seismic velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary.

VP/VS ratio and shear-wave splitting in the Nankai Trough seismogenic zone: Insights into effective stress, pore pressure, and sediment consolidation

To estimate variation of stress state and sediment consolidation in the Nankai plate subduction zone off southwest Japan, we measured the P-wave to S-wave velocity ratio (VP/VS) and S-wave splitting along the seismic line extending from the trench to the seismogenic zone. For this purpose, we used active-source seismic data recorded by multicomponent ocean bottom seismometers (OBS). Because it is difficult to identify the PS-converted reflection waveforms for each of the geological boundaries in this deep offshore region, we focused on the more easily identified PPS-refracted waveforms that register the conversion of the up-going P-waves to S-waves at the igneous crust surface. We estimated the average VP/VS ratio within the sedimentary section by using the time lag between the P-refracted waves and PPS-converted waves. This VP/VS ratio changes abruptly at the trough axis (i.e., the deformation front of the accretionary prism) arguably because of compaction associated with the accretion process. We observed relatively high VP/VS around the seismogenic megasplay fault, which may partially indicate the abnormal pore pressure and intensive fractures associated with the fault. To estimate the stress-induced fracture orientation and stress magnitude, we computed the fast S-wave polarization direction and estimated S-wave velocity anisotropy by applying the crosscorrelation method to the PPS-converted waves. To improve signal-to-noise ratio of the waveform for S-wave splitting analysis, we stacked PPS-converted waveforms on receiver gather. These anisotropic characteristics change at the seismogenic megasplay fault: the fast polarization direction is nearly parallel to the subduction direction seaward of the megasplay fault and is perpendicular to the subduction direction landward of the megasplay fault. This velocity anisotropy is especially strong around the megasplay fault. These results imply that the preferred fracture orientation, as well as the principal stress orientation, is oblique to the direction of plate subduction near the megasplay fault.

Satellite imaging to monitor CO2 movement at Krechba, Algeria

Abstract The Krechba field is one of several gas fields located in the Algerian Sahara desert, and was set in operation in August 2004 as part of a joint venture with BP, Sonatrach and StatoilHydro. The natural gas in the fields contains up to 10% CO 2 , which has to be reduced to 0.3% before the gas is sold, resulting in the production of around 1 million tonnes/year CO 2 . Rather than vent the CO 2 to the atmosphere (business as usual), it is re-injected into the water leg of the Krechba Carboniferous Sandstone gas producing reservoir (20 m thick) via three horizontal wells at a depth of around 1,900 metres. CO 2 injection started in August 2004 and to date nearly 2.5 million tonnes of CO 2 have been injected, amounting to approximately 25% of the gas extracted from the Krechba field over the same period. A number of key technologies to monitor the injection, and the subsurface movement and storage of CO 2 have been, and will continue to be, deployed to provide long term assurance of sequestration. Time lapse satellite images (using PSInSARTM Technology) which measure ground deformation to assess the movement of CO 2 in the subsurface have proven to be much more successful than initially thought, despite the depth of injection and the low voidage replacement rate (25%). Satellite images collected since start of injection show clear increases in ground elevation of up to 30mm around the three injectors while subsidence is also apparent in the area of maximum gas production. The images have also confirmed the CO 2 is moving in the direction of preferred fracture orientation at reservoir level. Recent downhole pressure measurements in one of the injectors also indicates that the CO 2 is being contained within the injection horizon. Work is ongoing to integrate the satellite images with geomechanical and seismic data to better understand how these images can be used for monitoring of CO 2 movement in the subsurface. A key part of the forward monitoring programme will be the acquistion of time-lapse 3D seismic, deployment of tiltmeters and GPS to confirm and calibrate the satellite imagery data, microseismic detectors in shallow boreholes to assess the degree of rock strain and use of shallow wells to monitor the water chemistry in the vadose zone. These will supplement the ongoing tracer, wellhead sampling and satellite imagery data acquistion. This paper shares the latest results of the ongoing CO 2 monitoring project.

Pit crater formation on Kilauea volcano, Hawaii.

Most of the known pit craters in Hawaii occur along the East and Southwest Rift Zones of Kilauea volcano. The pit craters typically are either astride a single rift zone fracture or between a pair of rift zone fractures. These fractures are prominent in the pit crater walls. The pit craters are elliptical in plan view, with their major diameters ranging from 8 to 1140 m. They range in depth from 6 m to 186 m. They typically develop with initially steep, locally overhanging walls, but as the walls collapse, the craters fill with talus and become shaped like inverted elliptical cones. None of the craters apparently formed as eruptive vents, although some have been subsequently filled by lava. Devil's Throat is the best-exposed pit crater along the East Rift Zone. It is sited at a `waist' between two east-striking zones of ground cracks; the spacing between the crack zones decreases towards Devil's Throat. East-striking fractures are also prominent in the pit crater walls. Pit craters along the Southwest Rift Zone typically are elongate in plan view along the direction of the rift, have large caves at their bases along the long axes of the craters, and are smaller than those of the East Rift Zone. Some closely spaced pits there have coalesced to form a trough. Based on our observations and mechanical considerations, we infer that pit craters form by stoping over an underlying large-aperture rift zone fracture, and not by piston-like collapse over broad magma bodies or voids. Flow of magma along the underlying fracture may remove stoped blocks and prevent the fracture from being choked with debris. This mechanism is consistent with pit crater location, ground crack patterns, the preferred orientation of fractures in pit crater walls, and pit crater geometry (both in map view and cross-section). The mechanism also fits with observations of stoping into a gaping rift fracture that conducted lava from Kilauea caldera during the 1920s. Additionally, the ratio of pit crater width to depth of 0.5 to 2 is consistent with pit craters forming over a nearly vertical opening mode fracture.

Seismic traveltime analysis for azimuthally anisotropic media: Theory and experiment

Natural fractures in reservoirs, and in the caprock overlying the reservoir, play an important role in determining fluid flow during production. The density and orientation of sets of fractures is therefore of great interest. Rocks possessing an anisotropic fabric and a preferred orientation of fractures display both polar and azimuthal anisotropy. Sedimentary rocks containing several sets of vertical fractures may be approximated as having monoclinic symmetry with symmetry plane parallel to the layers if, in the absence of fractures, the rock is transversely isotropic with symmetry axis perpendicular to the bedding plane. A nonhyperbolic traveltime equation, which can be used in the presence of azimuthally anisotropic layered media, can be obtained from an expansion of the inverse‐squared ray velocity in spherical harmonics. For a single set of aligned fractures, application of this equation to traveltime data acquired at a sufficient number of azimuths allows the strike of the fractures to be estimated. Analysis of the traveltimes measured in a physical model simulation of a reverse vertical seismic profile in an azimuthally anisotropic medium shows the medium to be orthorhombic with principal axes in agreement with those given by an independent shear‐wave experiment. In contrast to previous work, no knowledge of the orientation of the symmetry planes is required. The method is therefore applicable to P‐wave data collected at multiple azimuths using multiple offset vertical seismic profiling (VSP) techniques.

Permeability of the oceanic crust based on experimental studies of basalt permeability at elevated pressures

Permeabilities have been measured as a function of confining pressure at constant temperature for basalt dredge samples from the Juan de Fuca and Tonga-Kermadec regions. Permeabilities at 40 MPa are 1.1 × 10 −19 m 2 for the Juan de Fuca basalt and 7.4 × 10 −19 m 2 for the higher porosity Tonga-Kermadec basalt. Permeabilities decrease significantly with increasing confining pressure. The measured values of permeability are considerably lower than the estimated 10 −14 to 10 −15 m 2 permeabilities of the upper few hundred meters of basement rocks of the oceanic crust, thereby supporting oceanic crustal convection models in which seawater convection occurs largely through macrocracks at relatively shallow depths. Based on our laboratory measurements it is estimated that permeabilities of layer 2C and layer 3 are generally less than 1.0 × 10 −19 m 2. Permeabilities of layers 2A and 2B in young oceanic crust are expected to be anisotropic due to the preferred orientation of fractures. An anisotropic upper crustal model is presented in which layer-2 permeabilities are low parallel to spreading directions.

Directional Drilling Equipment and Techniques for Deep, Hot Granite Wells

Summary Conventional directional drilling technology has been extended and modified to drill the first well of a subsurface geothermal energy extraction system at the Fenton Hill, NM, hot dry rock (HDR) experimental site: Ambitious borehole geometries, extremely hard and abrasive granite rock, and high formation temperatures combined to provide a challenging environment for directional drilling tools and instrumentation. Introduction Completing the first of a two-wellbore HDR system has resulted in the definition of operational limitations of many conventional directional drilling tools, types of instrumentation, and techniques. The successful completion of the first wellbore, Energy Extraction Well 2 (EE-2), to a measured depth of 15,300 ft (4.7 km) in granite reservoir rock with a bottomhole temperature of 600°F (315°C) required the development of a new high-temperature downhole drilling motor and modification of existing wireline-conveyed steering tool systems. Conventional rotary-driven directional assemblies were modified successfully to accommodate the very hard and abrasive crystalline rocks encountered during drilling of a nearly 8,500-ft (2.6-km) directional hole to a final inclination of 35° from vertical at a controlled azimuthal orientation. Drilling Objectives The HDR geothermal resource is derived from a subsurface region that exhibits a relatively high geothermal gradient. At the Fenton Hill site, granitic basement rock is encountered at a depth of 2,400 ft (730 m) and exhibits a static geothermal temperature of 600°F (315°C) at a true vertical depth (TVD) of 14,500 ft (4.42 km). Hydraulic fractures in the granitic rock are vertical and preferentially oriented in a northwesterly direction. The rock matrix is slightly porous (<1%) but has very low permeability (from 1.0 to 0.0 µd). The method of heat extraction experiments currently under way at the Fenton Hill site requires that two boreholes - one injection well and one production well - be drilled to a depth exhibiting an economically attractive reservoir temperature. To enhance reservoir production objectives, the two wells will be inclined 35° from the vertical through the reservoir region at an azimuthal direction normal to the preferred fracture orientation. The wells will be drilled vertically coplanar with a constant separation of 1,200 ft (370 m) between the underlying injection well and the overlying producer. Fig. 1 illustrates the already described geometry of the Well EE-2/Well EE-3 extraction system in the 11,000- to 14,500-ft (3.35- to 4.42-km) reservoir region. The sequentially formed interconnecting fracture system will be inflated hydraulically, and water will be circulated at a total flow rate of approximately 1,500 U.S. gal/min (95 dm3/s). Drilling Problems The implications for drilling-related problems1 during the construction of such a system are significant. The flow capacity requirements of the system require a minimum production drilled-hole diameter of 8 3/4 in. (22.2 cm) and a minimum intermediate drilled-hole diameter of 12 1/4 in. (31.1 cm). The extremely hard and abrasive rock requires that all roller cone bits have a tungsten carbide cutting structure. All drilling tools, bottomhole assembly components, and the drill string are subjected to severe abrasive wear that limits useful life.

Joints, tensile strength and preferred fracture orientation in sandstones. New Brunswick and Prince Edward Island, Canada

The orientations of tensile fractures in Upper Devonian, Carboniferous and Lower Permian sandstones of southern New Brunswick and Prince Edward Island show correlation at the submicroscopic scale of microcrack alignments, the macroscopic scale of joints, and the megascopic scale of airphoto lineaments. Planes of minimum tensile strength and planes of preferred fracture, induced in visibly unfractured sandstone test-samples by line-loading and point-loading tests, are identified as microcrack alignments. The joints and airphoto lineaments have microcrack alignments parallel to them, but not all microcrack alignments are represented at the macroscopic and megascopic scales.
 
 Deformed Carboniferous sandstones east of Saint John have two prominent orthogonal Joint systems with microcrack alignments parallel to the four joint sets. The airphoto lineaments are parallel to only three joint sets. Southeast and northeast striking joints in one orthogonal system are tentatively interpreted as planes of extension and release respectively, related to the direction of compression during the Variscan-Appalachian orogeny. The other orthogonal system, with joint sets striking close to 010° and 100°, is interpreted as post-Triassic in age, possibly related to the presently acting crustal stresses in eastern North America.
 
 RÉSUMÉ
 Il y a correspondance dans les grès du Dévonien supérieur, du Carbonifère et du Permien inférieur du aud du Nouveau-Brunswick et de l'ile du Prince-Edouard entre l'orientation des fractures de tension à diverses échelles: microscopique — alignement des microfissures; macroscopique — dis-clases; et megascopique — linéaments observes à partir de photographies aériennes. On reconnait comrae alignements de microfissures les plans de force de traction minimale et les plans de fracture préférentlelle occasionnés par des essais de charges linéaires et ponctuelles sur des échantillons types de grès préalablement exempts de fractures apparentes. Bien que parallèlement aux diaclases et aux linéaments on retrouve des alignements de microfissures, ce ne sont pas tous les alignements de microfissures qui s'expriment aux échelles macroscopique et megascopique.
 
 À l'est de Saint-Jean, les grès déformes du Carbonifère possèdent deux systèmes distincts de diaclases orthogonales accompagnes d'alignements de microfissures parallèles aux quatre ensembles de diaclases. Les linéaments observes sur photographié aérienne ne sont parallèles qu'a trois ensembles de diaclases. On interprète provisoirement les diaclases de direction sud-est et nord-est dans l'un des systèmes orthogonaux comme, respectlvement, des plans d'extension et de relâchement reliés à la direction de compression durant l'orogenèse varisque-appalachienne. Des orientations se rapprochant de 010" et 100° caractérisent l'autre système orthogonal qui serait d'Sge post-triasique et peut être relié aux forces de tension contemporaines agissant sur l’écorce terrestre dans l'est de l'Amèrique du Nord.
 
 [Traduit par le journal]

Prediction of massive hydraulic fracturing from analyses of oriented cores : Logan, J M; Teufel, L W Proc 19th US Symposium on Rock Mechanics, Stateline, Nevada, 1–3 May 1978, V1, P340–344. Publ Reno: University of Nevada, 1978

An attempt has been made to predict the orientation of massive hydraulic fractures stimulated between depths of 2316 and 2464 M within the Muddy J formation of the Wattenberg Gas Field, Colorado. Predictions were based upon a study of the properties of 3 oriented cores and 2 unoriented cores. A preferred orientation of fractures induced in point-load tests is found for all 5 wells. The second phase involves investigation of the composition, grain size, microfracture orientation, crystallographic orientation, grain dimensional orientation, and residual strain. It is concluded that the fractures oriented at 335 on the east and west sides of the field are primarily controlled by the dimensional orientation of quartz grains, while those in the center of the field having an orientation of 110 are controlled primarily by the residual strain. Subsurface geology shows a buried fault trending northeast through the center of the field, postulated to have influenced the distribution of residual strain. In the eastern and central wells, good agreement was found with the predictions from the core analysis. 10 references.