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Introduction: Contrary to the traditional recognition that CO2 has large injectivity (the ratio of CO2 injection volume to pressure drop) in low permeability oil reservoirs to keep the formation pressure at a high level, the CO2 injection rate usually cannot attain the set value. It is essential to study the factors that influence CO2 injectivity and propose the optimal strategies to improve the CO2 injectivity.Methods: Therefore, in this study, several core samples collected from low permeability oil reservoirs are used to experimentally investigate the influences of CO2 injection rate, formation permeability, pressure and water saturation on CO2 injectivity, and the corresponding pressure drop, oil and gas production are examined. To determine the primary factor that influences the CO2 injectivity, orthogonal experimental design (ODE) and numerical simulations are utilized. In addition, to improve CO2 injectivity, the techniques of mini-fracturing and radial perforation are presented, and the threshold values for these two parameters are determined.Result and discussion: The results demonstrate that according to the magnitude of the extent that influences CO2 injectivity, the rank for the above factors is CO2 injection rate, reservoir pressure, formation water saturation and permeability. The oil recovery is mainly influenced by CO2 injection rate and formation permeability, and the influences of reservoir pressure and water saturation on oil recovery are smaller. The threshold values for radial perforation and fracture half-length are 25 m and 50m, respectively, which can provide some guidance for the strategies that should be taken to improve the CO2 injectivity and recover more oil from low permeability oil reservoirs.

Steam flooding is an important thermal recovery method for heavy oil reservoirs, and convective heating technology is used to fracture oil shale reservoirs with good results. This paper reviews the main prediction methods, optimization approaches for steam flooding performance, and its application in fractured oil shale reservoirs. The prediction methods include experimental, numerical simulation, and statistical models. These provide insights into steam override, heat transfer, and production dynamics. To optimize steam flooding, parameters like quality, temperature, injection rate, and allocation need to be coordinated based on reservoir conditions and monitoring data. Real-time injection control, economic analysis, and sweep efficiency improvements should also be considered in optimization workflows. Although progress has been made, more field studies are needed to establish systematic optimization practices utilizing advanced technologies. This review summarizes the key developments in steam flooding modeling and optimization, providing a reference for further research and field applications.

Introduction: Contrary to the traditional recognition that CO2 has large injectivity (the ratio of CO2 injection volume to pressure drop) in low permeability oil reservoirs to keep the formation pressure at a high level, the CO2 injection rate usually cannot attain the set value. It is essential to study the factors that influence CO2 injectivity and propose the optimal strategies to improve the CO2 injectivity. Methods: Therefore, in this study, several core samples collected from low permeability oil reservoirs are used to experimentally investigate the influences of CO2 injection rate, formation permeability, pressure and water saturation on CO2 injectivity, and the corresponding pressure drop, oil and gas production are examined. To determine the primary factor that influences the CO2 injectivity, orthogonal experimental design (ODE) and numerical simulations are utilized. In addition, to improve CO2 injectivity, the techniques of mini-fracturing and radial perforation are presented, and the threshold values for these two parameters are determined. Result and discussion: The results demonstrate that according to the magnitude of the extent that influences CO2 injectivity, the rank for the above factors is CO2 injection rate, reservoir pressure, formation water saturation and permeability. The oil recovery is mainly influenced by CO2 injection rate and formation permeability, and the influences of reservoir pressure and water saturation on oil recovery are smaller. The threshold values for radial perforation and fracture half-length are 25 m and 50m, respectively, which can provide some guidance for the strategies that should be taken to improve the CO2 injectivity and recover more oil from low permeability oil reservoirs. [ABSTRACT FROM AUTHOR]

The study of heterogeneous and complex oil and gas reservoirs poses a significant challenge due to the problem of unstable seepage flow. This research investigates the non-linear seepage flow patterns in such reservoirs, with a particular focus on ultra-low permeability tight gas reservoirs, using fractal theory. By analysing the scale invariance in the seepage flow of ultra-low permeability tight gas reservoirs, we have derived a fractal geometric expression based on the capillary pressure curve. This expression was integrated with mercury intrusion porosimetry to study the fractal dimension of ultra-low permeability reservoirs. Our experimental results indicate the presence of fractal characteristics in ultra-low permeability tight gas reservoirs. Based on this, we have investigated the fractal nature of matrix porosity and fractures and their effect on the stress sensitivity of ultra-low permeability tight gas reservoirs. This led to the development of a new fractal model for seepage flow in these reservoirs, which takes into account the non-Darcy flow behaviour of gas in nanoscale pores. The mathematical model was simplified using point source solutions and regular perturbation methods. The analysis shows that the fractal parameters of the matrix and fractures, together with the adsorption coefficients and the permeability modulus, have a significant influence on the dynamic pressure behaviour. The effects of slip and diffusion coefficients and stress sensitivity on the dynamics were also investigated. The pressure dynamics curves of ultra-low permeability tight gas reservoirs show distinct differences from those of homogeneous reservoirs and conventional dual porosity reservoirs. This study not only reveals the unique dynamic pressure characteristics of ultra-low permeability tight gas reservoirs, but also shows that the traditional reservoir models are, under certain conditions, special cases of the model proposed in this paper. [ABSTRACT FROM AUTHOR]