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Discrete fracture network model developed from a high resolution LIDAR outcrop survey of a naturally fractured unconventional Niobrara reservoir
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... Two joint sets tend to remain consistent in orientation throughout the formation (both laterally in two pits and vertically across multiple benches), while the dip angle and strike of the conjugate shear fracture sets have higher variability compared to the orientation of joint sets. While the fracture orientations remain relatively consistent throughout the Niobrara Formation, the spacing within individual fracture sets varies depending on lithologies and structural complexity (Grechishnikova 2016a). The brittle nature of chalk interbeds results in additional clusters of lithofacies-bound fractures. ...
... Listric faults within the negative flower structures observed at the quarry ( Figure 13) were found to affect lithofacies-bound and throughgoing fracture spacing at a distance of up to 9.1 m (30 ft) (Grechishnikova, 2016a). ...
... LIDAR dataset representation of A Marl. Top to bottom: point cloud, photographic view; final mesh colored on dip angle; final mesh colored on strike; d) extracted fracture plane(Grechishnikova, 2016a). ...
Accurate characterization of natural fracture networks is an important component in developing 3D geological models that predict the pathways for the interaction of surface waters and groundwater resources at mining sites. Overall mine planning and blasting design can also benefit from the improved understanding of fracture patterns. Natural resources managers dealing with source water protection, watershed management, and environmental compliance monitoring can benefit from the integration of new technologies to map discontinuities (fractures, joints, etc.) in geological reservoirs. To better understand the character of natural fracture networks at an active quarry research site located in a structurally complex setting we collected a digital dataset comprised of fracture plane orientations, fracture intensity variations, geologic framework, and lithofacies. The application of unmanned aerial vehicles (UAVs), LIDAR (Light Detection and Ranging) and photogrammetry allowed for collection of a high fidelity, high confidence geotechnical dataset. The purpose of this research was to use an outcrop derived dataset to propagate fracture networks into the subsurface and analyze potential zones of high permeability contributing to water discharges.
... Collins and Sonnenberg, 2013 .14: En echelon pattern of shear faults associated with the wrench fault zones (modified from Woodcock and Fischer, 1986 (Grechishnikova, 2016a .20: Fracture intensities: total (colored on fracture intensity), and for each individual fracture set (J 1red, J 2green, S 3orange, S 4blue) for scan lines within different representative benches, left to right: C Marl, D Chalk, D Chalk near the listric fault, and the Fort Hays (Grechishnikova, 2016a (Grechishnikova, 2016a .14: ...
... En echelon pattern of shear faults associated with the wrench fault zones (modified from Woodcock and Fischer, 1986 (Grechishnikova, 2016a .20: Fracture intensities: total (colored on fracture intensity), and for each individual fracture set (J 1red, J 2green, S 3orange, S 4blue) for scan lines within different representative benches, left to right: C Marl, D Chalk, D Chalk near the listric fault, and the Fort Hays (Grechishnikova, 2016a (Grechishnikova, 2016a .14: Complex faulting styles in the structural framework exhibited by a distance from the fault property. ...
... En echelon pattern of shear faults associated with the wrench fault zones (modified from Woodcock and Fischer, 1986 (Grechishnikova, 2016a .20: Fracture intensities: total (colored on fracture intensity), and for each individual fracture set (J 1red, J 2green, S 3orange, S 4blue) for scan lines within different representative benches, left to right: C Marl, D Chalk, D Chalk near the listric fault, and the Fort Hays (Grechishnikova, 2016a (Grechishnikova, 2016a .14: Complex faulting styles in the structural framework exhibited by a distance from the fault property. ...
... The outcrop analysis revealed the complexity of fracture sets and distributions allowing the differentiation of four major fracture and joint sets including J1 compressional joints, J2 extensional joints, and S3 and S4 conjugate shear fracture sets (Grechishnikova, 2016). Subsurface fracture interpretations of image logs, core, and microseismic data showed the presence of similar fracture sets (Table 1). ...
Heterogeneity of an unconventional reservoir is one of the main factors affecting production. Well performance depends on the size and efficiency of the interconnected fracture “plumbing system”, as influenced by multistage hydraulic fracturing. A complex, interconnected natural fracture network can significantly increase the size of stimulated reservoir volume, provide additional surface area contact and enhance permeability. The purpose of this study was to characterize the natural fracture patterns occurring in the unconventional Niobrara reservoir and to determine the drivers that influenced fracture trends and distributions. Highly fractured areas/fracture swarm corridors were identified and integrated into a reservoir model though DFN (Discrete Fracture Network) application for further prediction of reservoir performance using reservoir simulations. The predictive capability of DFN models can aid in improved reservoir performance and hydrocarbon production through optimized well spacing, re-frac stage locations planning for existing wells as well as completion strategies design for new wells.
Underground projects require a realistic understanding of geological, structural and mechanical characteristics of the rock mass. This includes a comprehensive rock mass characterization, assessment to all existing site information and an accurate geological model. However, due to the lack of truly three-dimensional subsurface fracture geometry data, this reality is rendered difficult. Thus, the use of Discrete Fracture Networks (DFNs) arises as an interesting tool. DFN models are statistically generated from discontinuity features mapped in-situ. These deterministic data can be obtained by employing various techniques, and more specifically with the use of laser scanning (i.e. Light Detection and Ranging - LiDAR). Complimentary discontinuity data can be obtained through fiber optic sensors coupled to tendon rock support elements, adding structural information from within the rock mass to the DFN model. This paper presents the methodology associated with the implementation of LiDAR data into DFN models for the purpose of application in underground projects and the use of optical sensors to capture three-dimensional loading of tendon support and, in turn, identify intersecting discontinuity planes. The Distributed Optical Strain Sensing (DOS) technology is demonstrated through double shear laboratory tests, as well as through an in-situ application to monitor rock mass movement during a tunnel excavation.
Unconventional reservoirs, including shale formations and many tight-gas sands, contain natural fractures, fissures, faults, and microfractures that contribute to the rock flow capacity; thus, core-measured permeability, and especially crushed-core permeability measurements, may not be representative of the true reservoir rock flow capacity. Evaluating the in-situ rock permeability, and simulating production, requires sampling not only the matrix, but also the fractures and fissures that contribute to the unconventional rock system permeability. A properly designed well test in an unconventional reservoir will sample a volume of reservoir rock that is representative of the whole. In other words, a well test should sample a representative elementary volume, which is the smallest volume of rock with properties characteristic of the whole.
Diagnostic fracture-injection/falloff tests (DFIT) have been routinely implemented since the late 1990s to understand leakoff mechanisms, identify fracture closure stress, estimate initial reservoir pressure, and determine permeability-thickness in unconventional reservoirs. In almost every unconventional well completed, a DFIT is the only well test that will be completed during the well lifecycle, but historically, the tests have been designed empirically based on analog formations and without considering the volume of rock investigated.
We demonstrate a new method for calculating the volume of rock investigated by a DFIT, and we show how a DFIT design can allow for sampling a representative elementary volume of the reservoir.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1985. Supervised by Herbert H. Einstein. Includes bibliographical references (p. 745-764).
Reflection seismic studies in the Denver basin near Greeley, Colorado, illustrate an association between listric normal faults (which sole out in detachment or decollement zones) and basement-controlled faults. Recurrent movement on basement-controlled faults triggered development of listric normal faults in tectonically sensitive stratigraphic intervals of the Cretaceous. Stratigraphic intervals exhibiting listric normal faults include the Cretaceous Laramie-Fox Hills-Upper Pierre, Middle Pierre Hygiene zone, and the Niobrara-Carlile Greenhorn. Listric normal faults are prevalent on the flank of a fault-bounded basement-controlled paleostructural block termed the Wattenberg block by Weimer and Sonnenberg (1982). Listric normal faults influence Cretaceous reservoir systems in the Hambert field area on the north flank of the Wattenberg-Greeley Lineament Zone.
The Niobrara Shale is a major reservoir in the Denver Basin. To understand the importance of natural fractures in this reservoir a dataset comprised of fracture plane orientations and fracture intensity variations using a ground-based LIDAR and photogrammetry was acquired. The purpose was to use an outcrop as an analog to the subsurface reservoir for improved reservoir characterization. Listric faults associated with negative flower structures show increased fracture intensity near the faults. Fracture sets remain consistent throughout the interbedded chalks, marls, and limestones. However, there is an apparent variability of fracture spacing associated with changes in lithology. Mapping the fracture orientation and intensity into the subsurface using the outcrop analog should give rise to better geomechanical and fluid flow modeling.
Integration of lithologic and wireline log data allows subdivision of the Upper Cretaceous Niobrara Formation into stratigraphic units that provide a framework for predicting both hydrocarbon source and reservoir potential of the formation throughout the Rocky Mountain region of the western United States. Regional mapping of these Niobrara subdivisions also provides a basis for interpreting the nature of deposition in the Western Interior Seaway during a time of relatively high sea level. Water depth, climate, proximity to the thrust belt bordering the seaway on the west, paleobathymetry, current circulation patterns, and sea level fluctuations all played a role in shaping the nature and distribution of Niobrara lithologies. The Niobrara and stratigraphically equivalent formations to the west contain a spectrum of lithologies including chalks, marls, shales, and sandstones. Coccolith-rich fecal pellets, probably formed by pelagic copepods that thrived during times of high sea level as water circulated through the seaway, provide a distinctive feature of the formation. These chalk pellets occur even in some siliciclastic-rich intervals in the western part of the seaway. Hydrocarbon production comes from three major lithologies: microporous and fractured coccolith- and planktonic foraminifer-rich limestones present mainly in the eastern part of the seaway; fractured sandrich facies, mainly in the western and southwestern parts of the seaway; and fractured marls and shales, mainly in the central part of the seaway. Black shales, some of which are quite rich in coccoliths and chalk fecal pellets, provide the major hydrocarbon source rocks. Similar shaly beds also provide sealing facies over more chalky or sandy reservoir intervals. Organic richness in the source intervals generally increases from less than 1% in the siliciclastic-rich western facies to more than 7% in the eastern clastic sedimentstarved facies. Thermal maturity of these source beds varies regionally on the basis of burial depth and local heat flow.
This paper presents a stratigraphic analysis of Paleozoic and Mesozoic strata in the northern Denver basin. The analysis shows that recurrent movement on basement faults influenced sedimentation, especially during major sea level changes. The research integrates surface and subsurface data for an area of approximately 30,000 square miles. Twenty stratigraphic intervals in the Paleozoic and Mesozoic were identified from well data. Based on the results obtained a tectonics and sedimentation model of recurrent fault block movement was derived. This model will aid in the exploration for hydrocarbons in the Denver basin by predicting the distribution of both source and reservoir rocks and by identifying early traps associated with paleostructures. Refs.
Timing and genesis of fractures in the Niobrara formation, northeastern Front Range and Denver Basin, Colorado. Master's thesis
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