Anisotropy and beyond: Geologic perspectives on geophysical prospecting for natural fractures (original) (raw)
Related papers
Deep borehole log evidence for fractal distribution of fractures in crystalline rock
Geophysical Journal International, 1991
Sonic velocity and electrical resistivity logs run to a depth of 3.5 km in crystalline rock near the San Andreas fault at Cajon Pass in southern California correlate over scale-lengths both small (sub-metre) and large (tens to hundreds of metres). No such correlations are seen with the more lithologically sensitive natural gamma intensity log. The correlation between the sonic velocity and electrical resistivity logs suggests that a non-lithologic property of the crystalline rock controls fluctuations. In situ fracture intensity is a logical candidate for the controlling rock property. The fluctuations of the individual sonic velocity and electrical resistivity logs are examined with the Hurst rescaled range parameter over borehole log intervals 1.5 m < L < 1500 m. For log fluctuations arising from a scale-invariant physical process the Hurst rescaled range scales with data interval as L H , 0 < H < 1. A purely random sequence of in situ fractures produces a scaling exponent H = 0.50. Fluctuations in the Cajon Pass sonic velocity and electrical resistivity logs yield H-0.70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals. The tendency for fracture clustering over log intervals 1.5 m < L < 1500 m suggests that fracture formation is a fractal process independent of length-scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals. Seismic reflectivity derived from the borehole sonic velocity log is also scale independent over the range of data intervals 1.5 m < L < 1500 m with a Hurst exponent H = 0.21. If we associate fracture clustering with crustal fault formation, the Cajon Pass borehole sonic velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension D-2.30. The equivalent 6-value for earthquake size distribution is b-D / 2-1.15. On this hypothesis observed b-values <1.15 indicate a tendency for earthquakes to cluster on existing (weak?) faults. A scale-independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws, cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting. Thus in crustal regions of active but low strain, fracture anisotropy is observed to be aligned with the inferred maximum principal stress, while in actively faulted crust, fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process.
Limits to crack density: The state of fractures in crustal rocks
SEG Technical Program Expanded Abstracts 1993, 1993
Shear-wave splitting in sedimentary basins and above small earthquakes in a wide range of geological and tectonic domains typically displays evidence for azimuthal shear-wave velocity anisotropy of between 1% and 5%. Interpreted as the effects of parallel vertical fractures, microcracks, and preferentially oriented pore-space, these percentages of anisotropy are equivalent to crack densities of = 0.01 and 0.05 with normalized mean crack diameters of 0.43 and 0.74, respectively. The only exceptions are percentages of anisotropy exceeding 10% > 0.1) observed in near-surface rocks where there is pronounced jointing.
Journal of Geophysical Research, 1992
This study was conducted in order to characterize the frequency, orientation, and aperture of macroscopic fractures in the crust and their effect on physical properties over an appreciable depth interval (1829-3450 m). The following are our major findings: (1) Over the range of apparent apertures measured with confidence, the frequency of fractures with a given aperture decreases as aperture increases. With applied corrections for sampling bias, the observed distribution of fracture aperture has a power law form providing evidence of the self-similar nature of fractures in the crystalline crust. Fractal analysis of the fracture aperture data yields a fractal dimension of 1.4 over the range of reliable aperture measurements in this study from 15 to 100 mm. (2) Fracture frequency does not systematically decrease with depth in the study interval. (3) No significant correlation was found between fracture occurrence and lithology, and both fracture spacing and aperture are uncorrelated with fracture orientation or depth. (4) The majority of fractures encountered in the well strike NNW-SSE and dip steeply to the west. One set of steeply dipping fractures appears to be related to the NW striking San Andreas fault and appears to be related to steeply dipping, NW striking shear fractures observed in nearby outcrops that are characterized by laumontitic alteration. (5) The fractures bear no obvious relation to the current northeast direction of maximum horizontal compression but do correlate with anomalies in physical properties measurements of compressional and shear velocity, porosity, and resistivity. (6) Macroscopic fractures strike in a direction nearly orthogonal to the fast propagation direction of seismic wave anisotropy determined from vertical seismic profiling experiments in the well. These fractures appear to be unrelated to the observed seismic anisotropy. (7) Hydraulically conductive fractures and major faults indicate that fluid-conducting fractures are a subset of the overall statistically significant population and not related to the San Andreas fault or to the orientation of SHmax in an obvious way. Field experimentation has documented the response of seismic velocities to macroscopic fractures. In situ velocities depend on fracture spacing, stress state, fracture roughness, and degree of saturation. In situ velocities are generally lower than velocities measured in the laboratory, and this velocity difference is correlated with fracture frequency [
Distribution of natural fractures and joints at depth in crystalline rock
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1983
This paper presents the results of studies of the natural fracture distribution encountered in 10 test wells drilled in three areas of the United States. Seven of the wells were drilled to depths of 200-250 m, while three were drilled to depths of about 1 kin. Using an ultrasonic borehole televiewer, fracture depths, strikes, and dips were determined. Steeply dipping fractures were found throughout each of the wells, and in general, few horizontal fractures were observed. Statistically significant fracture pole concentrations were found for each well which were basically invariant with depth, although some variation of fracture orientation with depth was found in two wells. The significant fracture orientations were not found to be the same in wells only several kilometers apart in a given region. In none of the wells did the number of observable fractures decrease markedly with increasing depth. No simple relationship of fracture orientation or fracture density with major structural features such as the San Andreas fault were observed, and no simple relation between the significant fracture orientations and either past or present regional stress fields could be determined.
Natural Fracture Patterns in Layered Rocks: Initiation and Propagation Mechanisms
2000
Important contributors to reservoir permeability are fracture systems common in low porosity reservoirs. Although the conditions of fracturing in reservoirs on anticlines, in tectonically deformed areas, are relatively well identified, the formation of regional fractures in flat-lying reservoirs is still unclear. Such regional fractures have a great economic significance and are known to form at depth as a result of
Frontiers in Earth Science, 2022
We use observations of hydraulic fractures in core, outcrop attributes of natural hydraulic fractures, and analogue models, to address how hydraulic fracture networks evolve. A slant core from the Wolfcamp Formation-an unconventional shale hydrocarbon reservoir in the Permian Basin of West Texas-collected within 18 m and 30 m of two hydraulically stimulated horizontal wells, provided an opportunity to examine hydraulic fractures directly. In approximately 183 m of core, 309 calcite-sealed natural opening-mode fractures and 375 hydraulic fractures were identified. Many hydraulic fractures in the core show complex morphology, including twist-hackle segmentation, diversion, and bifurcation; these structures most commonly develop at lithological bed boundaries and mechanical heterogeneities such as natural fractures and concretions. An outcrop of bed-parallel pavements in the Cretaceous Boquillas Formation in West Texas contains opening-mode fractures that likely formed by natural hydraulic fracturing. Fracture traces provide evidence of twist-hackle segmentation, and are typically associated with bed boundaries and preexisting bed-parallel stylolites. A laboratory study of hydraulic fracturing of 33 synthetic blocks of gypsum and hydrostone revealed fracture steps, diversions, twist hackles, and multiple overlapping fractures together with information on fracture growth directions. These complexities in the fracture network were dominantly nucleated at inclusions used to simulate pre-existing fractures, and as a result of mechanical heterogeneity introduced by the wellbore and perforations. Collectively, our results show that complex fracture networks are produced in hydraulic fracturing of selfsourced reservoir strata. Mechanical stratigraphic boundaries and other heterogeneities are likely to enhance fracture network complexity through the processes of segmentation, diversion, and bifurcation. These processes create multiple fracture strands, resulting in an increased number of hydraulic fractures over those initiated, thereby increasing total fracture surface area. Our study provides insight into hydraulic fracture network propagation, and has applications for evaluation, completion, production, and fracture modeling of unconventional reservoirs.
Rock Fractures in Geological Processes
2009
Rock fractures largely control many of the Earth's dynamic processes. Examples include plate-boundary formation and development, tectonic earthquakes, volcanic eruptions, and fluid transport in the crust. How rock fractures form and develop is of fundamental importance in many theoretical and applied fields of earth sciences and engineering, such as volcanology, seismology, hydrogeology, petroleum geology, natural hazards, and engineering geology. An understanding of rock fractures is essential for effective exploitation of many of the Earth's natural resources including ground water, geothermal water, and petroleum. This book combines results from fracture mechanics, materials science, rock mechanics, structural geology, hydrogeology, and fluid mechanics to explore and explain fracture processes and fluid transport in the crust. Basic concepts are developed from first principles and are illustrated with numerous worked examples that link models of geological processes to real field observations and measurements. Calculations in the worked examples are presented in detail with simple steps that are easy to follow-providing the readers with the skills to formulate and quantitatively test their own models, and to practise their new skills using real data in a range of applications. Review questions and numerical exercises are given at the end of each chapter, and further homework problems are available at www.cambridge.org/gudmundsson. Solutions to all numerical exercises are available to instructors online. Rock Fractures in Geological Processes is designed for courses at the advancedundergraduate and beginning-graduate level, but also forms a vital resource for researchers and industry professionals concerned with fractures and fluid transport in the Earth's crust.
Scale effects in natural fracture networks
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1997
The geometry of fracture systems was analysed at various scales in the western Arabian Plate from regional to outcrop. In the first instance, a ground analysis provided a better understanding of joint and fault relationships associated with Red Sea geodynamics. The fracture datasets were then subjected to statistical and geostatistical analyses (a) of orientation, length, spacing and density, and (b) of the variability in these factor quantities with space and scale. Finally, a multifractal study was carried out and complemented by a local wavelet analysis. The main conclusion is that fracturing is not a self-similar process from centimetre scale up to 1000-km scale. The fracture systems can be regarded as being made up of a number of individual attributes such as orientation, length, spacing and density, each of which can be scale independent, but which combine throughout the scales to render the organization hierarchical in nature. Different power-law distributions hold separately in distinct limited ranges that are separated by characteristic length scales revealed by the different techniques. A one-to-one correspondance is seen between the characteristic length scales and the thickness of the different rheological layers in the study area, which range from the sedimentary bed, through formations and basins, up to the finite width of the seismogenic crust.
The fracture criticality of crustal rocks
Geophysical Journal International, 1994
Preface. It is a pleasure and a honour to be included in this issue commemorating the centenary of Robert Stoneley's birth. I was, I believe, the last of the small number of research students supervised by Stoneley, and it gives me great pleasure that most of my research has been, by chance, in a field he initiated-(azimuthal) seismic anisotropy. Bob Stoneley was one of the first seismologists to consider azimuthally anisotropic seismic waves (specifically surface waves) in cubic and orthorhombic symmetries . Although, he considered these papers 'essentially as a development in the theory of elasticity', they were invaluable references to me when I first began to calculate surface waves in an anisotropic Earth. Bob would have been gently amused that a large part of the Earth is now recognized as having orthorhombic anisotropic symmetry.