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Recently, significant oil discoveries have been made in the shallower pay zones of the Jurassic Badaowan Formation (J1b) in the Mahu Sag, Junggar Basin, Northwest China. However, little work has been done on the geochemical characteristics and origins of the oil in the J1b reservoir. This study analyzes 44 oil and 14 source rock samples from the area in order to reveal their organic geochemical characteristics and the origins of the oils. The J1b oils are characterized by a low Pr/Ph ratio and high β-carotene and gammacerane indices, which indicate that they were mainly generated from source rocks deposited in a hypersaline environment. The oils are also extremely enhanced in C29 regular steranes, possibly derived from halophilic algae. Oil-source correlation shows that the oils were derived from the Lower Permian Fengcheng Formation (P1f) source rocks, which were deposited in a strongly stratified and highly saline water column with a predominance of algal/bacterial input in the organic matter. The source rocks of the Middle Permian lower-Wuerhe Formation (P2w), which were deposited in fresh to slightly saline water conditions with a greater input of terrigenous organic matter, make only a minor contribution to the J1b oils. The reconstruction of the oil accumulation process shows that the J1b oil reservoir may have been twice charged during Late Jurassic–Early Cretaceous and the Paleogene–Neogene, respectively. A large amount volume of hydrocarbons generated in the P1f source rock and leaked from T1b oil reservoirs migrated along faults connecting source beds and shallow-buried secondary faults into Jurassic traps, resulting in large-scale accumulations in J1b. These results are crucial for understanding the petroleum system of the Mahu Sag and will provide valuable guidance for petroleum exploration in the shallower formations in the slope area of the sag. [ABSTRACT FROM AUTHOR]

Residual oil zones (ROZs) have large potential for CO2 enhanced oil recovery (EOR) and geologic storage. During CO2 injection, the migration of CO2 in ROZs controls the performance of both EOR and storage. However, it has not been clearly visualized and understood that how geological heterogeneity factors control the transport of CO2 in ROZs. In this study, the oil recovery performance and geologic storage potential during continuous CO2 injection in a representative ROZ are studied based on geostatistical modelling and high-fidelity three-phase flow simulation. We examined the influence of autocorrelation length of permeability, global heterogeneity (Dykstra–Parsons coefficient), and permeability anisotropy on cumulative oil recovery and CO2 retention fraction. Simulation results indicate that, as the permeability autocorrelation length increases, the cumulative oil recovery and CO2 storage efficiency decrease. This results from the accelerated migration of CO2 along high permeability zones (i.e., gas channeling). The increase in global heterogeneity and permeability anisotropies can lead to low oil recovery and poor CO2 sequestration performance, depending on the degree of CO2 channeling. The net utilization ratio of CO2 (CO2 retained/oil produced) unfavorably increases with both autocorrelation length and Dykstra–Parsons coefficient, but decreases with the increase in kv/kh. Such a decrease is attributed to enlarged swept volume induced by gravity override. The study provides important implications for field-scale CO2 EOR and storage applications in ROZs. [ABSTRACT FROM AUTHOR]

Spontaneous imbibition is an important phenomenon in tight reservoirs. The existence of a large number of fractures and micro-nano pores is the key factor affecting the spontaneous imbibition of tight reservoirs. In this study, based on high-pressure mercury injection and nuclear magnetic resonance experiments, the pore distribution of tight sandstone is described. The influence of fractures, core porosity and permeability, and surfactants on the spontaneous imbibition of tight sandstone are studied by physical fracturing, interfacial tension test, wettability test and imbibition experiments. The results show that: the pore radius of tight sandstone is concentrated in 0.01e1 mm. Fractures can effectively reduce the oil drop adsorption on the core surface, enhancing the imbibition recovery of the tight sandstone with an increase of about 10%. As the number of fractures increases, the number of oil droplets adsorbed on the core surface decrease and the imbibition rate increases. The imbibition recovery increases with the increase in pore connectivity, while the imbibition rate increases with the increases in core porosity and permeability. The surfactant can improve the core water wettability and reduce the oil-water interfacial tension, reducing the adsorption of oil droplets on the core surface, and improving the core imbibition recovery with an increase of about 15%. In a word, the existence of fractures and surfactants can enhance the pore connectivity of the reservoir, reduce the adsorption of oil droplets on the core surface, and improve the imbibition rate and recovery rate of the tight oil reservoir. [ABSTRACT FROM AUTHOR]

To solve the problems of rock strength increase caused by high in-situ stress, the stress release method with rock slot in the bottom hole by an ultra-high-pressure water jet is proposed. The stress conditions of bottom hole rock, before and after slotting are analyzed and the stress release mechanism of slotting is clarified. The results show that the stress release by slotting is due to the coupling of three factors: the relief of horizontal stress, the stress concentration zone distancing away from the cutting face, and the increase of pore pressure caused by rock mass expansion; The stress concentration increases the effective stress of rock along the radial distance from 0.6R to 1R (R is the radius of the well), and the presence of groove completely releases the stress, it also allows the stress concentration zone to be pushed away from the cutting face, while significantly lowering the value of stresses in the area the drilling bit acting, the maximum stress release efficiency can reach 80%. The effect of slotting characteristics on release efficiency is obvious when the groove location is near the borehole wall. With the increase of groove depth, the stress release efficiency is significantly increased, and the release range of effective stress is enlarged along the axial direction. Therefore, the stress release method and results of simulations in this paper have a guiding significance for best-improving rock-breaking efficiency and further understanding the technique. [ABSTRACT FROM AUTHOR]