Fracture conductivity reduces significantly in shale formations due to the high embedment of proppants. Hydraulic fracture networks with required fracture conductivities are decisive for the cost-effective production from unconventional shale reservoirs. Optimum conductivity is essential for hydraulic fracturing due to its significant role in maintaining productivity.
Hydraulic fracturing optimization is therefore fundamentally dependent upon uncertainty, and a change of approach toward the pursuit of design diversity can have a profound impact on efficiency and risk, and in turn cost and environmental impacts, in oil and gas development. For the case of oil/gas wells, diversified designs of portfolios can improve 60% more stimulation efficiency and halve the corresponding risk. This approach can substantially increase return and decrease risk when designing systems where outcome desirability is definable as a smoothly-varying return and where risk is related to the variance of outcomes tied to uncertain spatio-temporal variability of the design environment. We propose an expansion of the principle of design diversity wherein the focus becomes developing optimal portfolio combinations of multiple designs rather than repeated application of a single design. However, as with many engineering systems, the impact of variability and uncertainty (in this case of reservoir properties) is not accounted in these deterministic approaches. The history of oil and gas well stimulation through hydraulic fracturing is characterized by a pursuit of optimal designs tailored to reservoir properties. Finally, a summary with speculation on future development trends is given. The theoretical background and current status of DEM are introduced first, and the principles, applications, and advantages and disadvantages of different fluid flow/DEM coupling methods are discussed. Given their importance, the availability or unavailability of best practice guidelines is outlined. The purpose of this paper is to give a comprehensive review of fluid flow/DEM coupling methods and relevant research. For researchers and engineers, the key to solve a specific problem is to select the most appropriate fluid/DEM coupling method among these modeling technologies. Although these coupling methods have been successfully applied in various engineering fields, no single fluid/DEM coupling method is universal due to the complexity of engineering problems and the limitations of the numerical methods. Many coupling techniques have been developed to include the effects of fluid flow in the discrete element method (DEM), and the techniques have been applied to a variety of geomechanical problems. The past decade has witnessed the substantial growth in research interests and progress on the subject of coupled hydro-mechanical processes in rocks and soils, driven mainly by the surge of research in unconventional hydrocarbon reservoirs and associated hazards. Except for Duncan–Chang model parameters fitting in this work, more experimental and numerical researches are expected to improve the performance in predicting post-failure behavior. Experimental and numerical results indicate that (1) triaxial compression strength increases with confining stress and hydrate saturation (2) stress–strain curve becomes smooth at a higher hydrate saturation thanks to the stability enhancement of MHBS structure (3) heterogeneous distribution of hydrates leads to local instability with non-bonded hydrate particles (4) grading properties (uniformity coefficient and mean particle diameter) non-apparent influence on compressive strength and dilatancy due to particles re-distribution and (5) MHBS presents mechanical behavior of brittleness or plasticity in undrained tests rather than strain softening in drained tests. Triaxial drained and undrained numerical tests are carried out to investigate effects of hydrate saturation, confining stress, heterogeneity and grading properties on mechanical behavior of pore-filling hydrate sediment. To describe nonlinear mechanical behavior, Duncan–Chang model is embedded into DEM model and verified with experimental results. A discrete element method (DEM) model is developed to examine mechanical responses of MHBS by considering real MHBS-based microstructure and particles contact. In this paper, MHBS is synthesized in laboratory and triaxial compressive tests are carried out to capture mechanical response. Mechanical behavior of MHBS is critical issue to analyze geomechanical hazards. Interests appear in investigating methane-hydrate-bearing sands (MHBS) to address engineering problems, such as foundation instability of man-made permafrost facilities, wellbore instability and sanding during production.
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