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Characteristics of marls: (a) and (b) bioturbation within calcareous marls (lens as scale in a), C org rich shale (16 wt.%) in (c) thin section and (d) outcrop.
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In fractured reservoirs characterized by low matrix permeability, fracture networks control the main fluid flow paths. However, in layered reservoirs, the vertical extension of fractures is often restricted to single layers. In this study, we explored the effect of changing marl/shale thickness on fracture extension using comprehensive field data a...
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... laminae observed in the field or thin sections (Fig. 4b,d). CaCO 3 content of CMs and LCMs ranges from 44 wt.% to 82 wt.%; both are slightly higher than the content of RMs and LMs (25e56 wt.%) ( Table 1). Within some marls, sharply defined color changes, from light grey to dark grey, occur along strongly bioturbated transitions (Chondrites type; Fig. 5a and b). Thin sections reveal that these Table 1 Lithology, average bed thickness (cm), CaCO 3 and C org content (wt.%) of each bed collected in all sections. WBL ¼ well-bedded limestones, SNL ¼ semi-nodular limestones, CM ¼ calcareous marls, LCM ¼ laminated calcareous marls, LM ¼ laminated marls, RM ¼ regular marls. Please note that all ...
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... tubes are ubiquitous within calcareous marls (Fig. 5b). These marls/shales are rich in organic carbon (up to 16 wt.% C org ), as evidenced by the dark color in the outcrop ( Fig. 5c and ...
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Citations
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Hydraulic fracturing technology can be used to jointly exploit unconventional natural gas such as coalbed methane and tight sandstone gas in coal-measure superimposed reservoirs for the enhancement of natural gas production. Hydraulic fracturing usually induces mixed fractures of I and II modes, but existing studies have not considered the influence of reservoir lithology on the stress intensity factor of I/II mixed fractures in coal-measure superimposed reservoirs. This paper develops an analytical stress model and a seepage-mechanical-damage numerical model for the vertical propagation of I/II mixed fractures in coal-measure superimposed reservoirs. The variation of stress intensity factor of I/II mixed fractures is analyzed when the fractures are close to the interface of different lithologic reservoirs and the effects of elastic modulus difference, stress state, fracturing fluid viscosity, shear and tensile failure modes on the vertical propagation of hydraulic fractures are investigated. Finally, the ratio of elastic modulus of adjacent reservoirs is proposed as an evaluation index for the fracture propagation through reservoir interface. These investigations revealed that hydraulic fracture propagation through the reservoir interface is a process of multi-physical interactions and is mainly controlled by the injection pressure and the elastic modulus ratio of adjacent reservoirs. A critical line is formed in the coordinates of elastic modulus ratio and injection pressure. A fracture can propagate through the reservoir interface when the combination of injection pressure and the elastic modulus ratio is in the breakthrough zone. These results can provide theoretical support for the site selection of horizontal wells in coal-measure gas exploitation.
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High resolution borehole resistivity image logs provide valuable bedding and fracture data from subsurface rocks including fracture type, orientation and aperture. The present study shows that common average shape and aperture geometry of uniformly located layer-bound fractures can be inferred from scan-line density and aperture-depth plots using image log data from a deviated well.
The proposed method extends the use of borehole image logs and allows 3-dimensional (3D) modeling of fractures and fractured layers from subsurface data. So far, such 3D fracture studies could only be conducted using outcrop data. The approach is quite robust because correspondence between fracture geometry and image log plots is independent of fracture length and orientation, and applies to any fracture shape and aperture geometry.
Joint interpretation of fracture scan-line density and aperture-depth plots, and bedding picks from borehole images also helps differentiate isolated/dispersed, unbound and multilayer fractures. Abundance of fractures that breach bed boundaries gives an idea about the cohesive bonding strength of bedding contacts and pave the way to construct fracture stratigraphy with several fractured layers that are separated by interfaces of varying bonding strength or by ductile beds. 3D fracture geometry may also provide some information on the type and mode of fractures and geo-mechanical conditions under which the fractures were generated.
