Experimental evidence for bifurcation angles control on abandoned channel fill geometry

The nature of abandoned channels' sedimentary fills has a significant influence on the development and evolution of floodplains and ultimately on fluvial reservoir geometry. A control of bifurcation geometry (i.e., bifurcation angle) on channel abandonment dynamics and resulting channel fills, such as sand plugs, has been intuited many times but never quantified. In this study, we present a series of experiments focusing on bedload transport designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angles and the differential channel bottom slopes. We find that disconnection is possible in the case of asymmetrical bifurcations with high diversion angle (≥30∘) and quantify for the first time an inverse relationship between diversion angle and sand plug length and volume. The resulting sand plug formation is initiated in the flow separation zone at the external bank of the mouth of the diverted channel. Sedimentation in this zone induces a feedback loop leading to sand plug growth, discharge decrease and eventually to channel disconnection. Finally, the formation processes and final complex architecture of sand plugs are described, allowing for a better understanding of their geometry. Although our setup lacks some of the complexity of natural rivers, our results seem to apply at larger scales. Taken into account, these new data will improve fluvial (reservoir) models by incorporating more realistic topography and grain size description in abandoned channels.

The Effect of Sand Plug at the Gas Well Bottom-hole on its Productive Characteristics

The reasons that lead to the destruction of the bottomhole zone during the operation of wells with unstable reservoirs are considered. The negative effects of the sand carry-over from the formation into the wellbore are shown. The review of scientific and technical literature on the effect of the sand plug on well productivity is presented. It indicates that the studies have mainly considered oil wells, but the effect of the sand plug on well productivity has not been studied sufficiently. For most gas wells, Darcy's law is primarily violated while filtering gas to the bottom-hole of the well, especially at high gas flow rates. The authors have studied theoretically the effect of a sand plug at the bottom of a gas well with permeability variations on the productive characteristics. According to the results of theoretical studies, the authors construct and analyze graphical dependencies in the form of the ratio of gas flow with and without a sand plug to the ratio of the reservoir permeability and the plug, on condition that the plug blocks the reservoir completely and the plug height exceeds the reservoir thickness. The scientists work a formula for indicating the flow rate of a gas well with a sand plug at the well bottom, on condition that the plug blocks the reservoir completely and the plug height exceeds the reservoir thickness while filtering gas in the reservoir according to the binominal law. On the basis of the research, it is established that on condition of the identical height of the sand plug and the identical ratio of reservoir permeability and the plug, the productivity of the gas well with the sand plug increases with the increase of the reservoir permeability. If the height of the plug exceeds the reservoir thickness, the flow rate of the gas well will be the smaller, the greater is the height of the sand plug compared to the reservoir thickness.

Peak punch-through capacity of spudcan in sand with interbedded clay: numerical and analytical modelling

The presence of a thin soft clay layer inside a bed of sand may significantly reduce the bearing capacity of the sand layer, imposing a risk of punch-through failure. In this paper, finite element (FE) simulations are reported using a hardening soil (HS) model for sand. The FE model has been verified against centrifuge tests involving loose and dense sand layers overlying clay soil. The effects of sand stiffness, foundation roughness, sand friction angle, undrained clay strength, clay strength nonhomogeneity, and sand and clay layer geometries on the foundation peak capacities have been studied. Punch-through failure is initiated with an inclined sand plug being sheared and pushed into the underlying soft clay. During punch-through, the clay layer fails due to significant radial squeezing. Existing analytical models do not capture the combined failure mechanism of sand shearing and clay radial squeezing. A new analytical model is developed to estimate the peak punch-through capacity of a spudcan in sand with an interbedded clay layer, showing improved performance over the current industry guidelines.