The stress-dependent permeability of ultra-low permeability rocks has a significant impact on the seepage. A number of experimental studies on the variation of permeability with stress change in low permeability reservoir have been done, but few are involved in ultra-low permeability cores. In this study, first, we fabricated the shear-stress microfracture cores and then the stress sensitive experiments were conducted. Second, a fracture deformation model was introduced in order to depict the stress sensitivity features of fracture media theoretically. Finally, a comparison of the theoretical curves with the experimental data had been done. The results show that the fabricated microfracture cores can be used in the permeability stress sensitive experiments. The permeability stress sensitive hysteresis of the microfracture cores is not obvious and the recovery degree of stress sensitive hysteresis is high in the unloading cycle for the grains were compacted in the process of shearing. The theoretical curves have the desired behavior which is consistent with the experimental data well. It indicates that the method of fabricating microfractures can similarly reveal the stress sensitivity features of the real microfracture ultralow permeability reservoir. This study do great benefit to reveal the stress sensitivity features of ultra-low permeability reservoir.
Stress-sensitivity effects have been recognized to have impact on the pressure/rate transient behavior of wells in several reservoirs. Although the effects of stress-sensitivity have been considered in well testing theory in the past thirty years, little has been done to determine their influence on rate decline behavior. This paper presents a single phase flow model considering stress-sensitive formation permeability to investigate the characteristic of production rate decline of a vertical well. The stress-sensitive permeability is considered as an exponential form. The permeability changes with pressure drop are described by a permeability modulus. By introducing two pseudo functions, the equations of the mathematical model are linearized and approximate semi-analytical solutions are obtained. The analytical solutions are carefully verified through numerical simulation. Two sets of new decline type curves are diagramed on a log-log plot for constant rate case and constant bottomhole pressure case respectively. The influence of stress-sensitive permeability on decline curves are analyzed and compared. From this work, we recognized that the rate decline characteristics of stress-sensitive reservoir under constant rate and constant bottomhole producing condition are different. New analysis method should be developed to analyze field variable rate/variable pressure drop data.
The purpose of frac fluid flowback is to outflow the fracturing fluid and prevent the proppant backflow, which determines the effect of fracturing fluid flowback. The fracturing treatment in field lacks relevant theoretical models for prediction. Therefore, this work focuses on establishing the flowback model of hydraulic fracturing for multilayer fractured wells. Then the wellhead pressure and fluid flowback volume are predicted based on the actual parameters of fracturing treatment in the process of flowback and are compared with the field data of frac fluid flowback. Results show that the wellhead pressure and fluid flowback volume calculated by the model are consistent with the field data. The sand production ratio of the three wells during the flowback process is lower than 1% indicating that the optimized parameters achieve the purpose of flowback operations and reduce the flowback volume of the proppant.
Because the development of ultra-low permeability reservoir is relative to fracture system, suitable well pattern arrangement is very significant for effective flooding management. There were three kinds of well pattern used to waterflood in Changqing oilfield: square inverted nine-spot, rhombus inverted nine-spot and rectangular five-spot pattern, according to the degree of fracture growth. In view of the defects of these well patterns in the development of ultra-low permeability reservoirs, a new well spacing concept is developed. Numerical simulations are carried out to illustrate the adaptability and strong points of this new well pattern. For this well pattern, on the one hand, the distance between producers and injectors along the fracture direction is widened and thus massive fracturing can be conducted to enhance oil productivity and water injection, and on the other hand, a high producer/injector ratio ensures high oil recovery rate in early stage. Besides, this new well pattern has a great ability of adjustment. Field application showed a remarkably well producing performance.
Reflective polyimide films were prepared by incorporation of silver ions in to the pre-hydrolytic layer of polyimide films, and subsequent treatment in hydrazine hydrate solution led to the reduction of silver ions to silver nanoparticles, which aggregated and formed continuous silver layers on both sides of polyimide substrate. The maximum reflectivity could be detected up to 80% on the downside and 90% on the upside surface, respectively. It was found that more silver loadings in the downside modified layers resulted in the two-sides difference in properties.
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