Three production and injection examples from the Permian basin demonstrate how advanced technology identifies problems that can be treated with foamed cement. The process involves computer simulation (modeling), log a...
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Three production and injection examples from the Permian basin demonstrate how advanced technology identifies problems that can be treated with foamed cement. The process involves computer simulation (modeling), log analysis, and tracer surveys. Operators of these wells identified conformance-control problems caused by fractures, interwell communication, and flooding fluid breakthrough. After circulating the open hole clean, the operator noted that the open hole section had been totally consolidated with its caliper being bit-sized. The well was placed on injection and began taking fluid at the required daily injection rate below fracture pressure. The final injection test was after follow-up perforating gained entry into selected portions of the open hole.
An abstract model neuron, simple enough to be used in analytical studies of neural networks, is defined. It incorporates neuronal adaptation, i.e. the coupling between neuronal activity and excitability, which can con...
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An abstract model neuron, simple enough to be used in analytical studies of neural networks, is defined. It incorporates neuronal adaptation, i.e. the coupling between neuronal activity and excitability, which can control the complexity of neural network dynamics. A generalized neuronal activation function is defined as the output activity, i.e. the firing rate of action potentials, as a function of an input and an excitability variable. For a biological neuron, the intracellular concentration of Ca ions is shown to be a useful excitability variable. The principal features of the generalized activation function are derived by means of a multicompartment model neuron with ionic currents described by Hodgkin-Huxley-type equations. As a relevant neuron type for investigations of cortical associative memory, a neocortical pyramidal cell of the regularly-spiking type is selected. An approximate analytical consideration suggests a simple form of independent variable of the generalized activation function. The complete numerical treatment verifies this form and displays a threshold character of the activation function. The derivation thereby provides an interpretation of dynamic threshold models of neuronal adaptation in terms of ionic mechanisms. The threshold with respect to the input has well-defined dynamics directly given by the intracellular Ca dynamics. A comparison of the response of the Hodgkin-Huxley-type and reduced-model neurons to a varying input variable indicates that the reduced model is a useful approximation even for the detailed behaviour of the firing rate. The generalized activation function also accounts for the dependence on neuromodulation via an adaptability parameter. An abstract model neural network of units thus defined has the capacity to describe complex network dynamics and the simplicity to allow elucidation of mechanisms and applications to large-scale systems.
In a recurrent artificial neural network, the units active in an attractor state typically reach their maximum activity value while the others are quiescent. In contrast, recordings of cortical cell activity in vivo r...
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In a recurrent artificial neural network, the units active in an attractor state typically reach their maximum activity value while the others are quiescent. In contrast, recordings of cortical cell activity in vivo rarely reveal cells firing at their maximum rate. This discrepancy has been one of the main arguments against using attractor networks as models of cortical associative memory. In this study we show that low-rate sustained after-activity can be obtained in a simulated network of mutually exciting pyramidal cells. This is achieved by assuming that the synapses in the network are of a saturating type. When the application of a monoamine neuromodulator is simulated, after-activity with firing rates around 60?s?1 can be produced. The firing pattern of the network was found to be similar to that of the experimentally most comparable system, the disinhibited hippocampal slice. The results obtained are robust against simulated biological variation and background noise.
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