Calculation Model of Relative Permeability in Tight Sandstone Gas Reservoir with Stress Sensitivity

During the development of tight gas reservoir, the irreducible water saturation, rock permeability, and relative permeability change with formation pressure, which has a significant impact on well production. Based on capillary bundle model and fractal theory, the irreducible water saturation model, permeability model, and relative permeability model are constructed considering the influence of water film and stress sensitivity at the same time. The accuracy of this model is verified by results of nuclear magnetic experiment and comparison with previous models. The effects of some factors on irreducible water saturation, permeability, and relative permeability curves are discussed. The results show that the stress sensitivity will obviously reduce the formation permeability and increase the irreducible water saturation, and the existence of water film will reduce the permeability of gas phase. The increase of elastic modulus weakens the stress sensitivity of reservoir. The irreducible water saturation increases, and the relative permeability curve changes little with the increase of effective stress. When the minimum pore radius is constant, the ratio of maximum pore radius to minimum pore radius increases, the permeability increases, the irreducible water saturation decreases obviously, and the two-phase flow interval of relative permeability curve increases. When the displacement pressure increases, the irreducible water saturation decreases, and the interval of two-phase flow increases. These models can calculate the irreducible water saturation, permeability and relative permeability curves under any pressure in the development of tight gas reservoir. The findings of this study can help for better understanding of the productivity evaluation and performance prediction of tight sandstone gas reservoirs

Meso-Mechanical Characteristics of Granite with Natural Cracks after Mud Acid Corrosion

Most of the discovered high-temperature geothermal energy systems are often related with granite that is characterized by natural faults, fractures and cracks of different size. However, the porosity and permeability of the granite matrix is very low, greatly limiting the efficiency of heat extraction in granitic rock. Chemical stimulation, which is regarded as one of the most important means of reservoir stimulation, has consequently received more and more attention. In this paper, a Triassic granite obtained from the eastern region of Liaoning Province in China was reacted with three different concentration of mud acid solution (8% HCl + 1% HF, 10% HCl + 2% HF, 12% HCl + 3% HF) and the resulting microstructure changes studied by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). The experimental results show that the number of micropores in the granite increases after chemical corrosion by mud acid solution. A higher mud acid solution concentration results in a much higher pore volume. Triaxial compression tests on the granite before and after chemical corrosion were carried out to study the effect of acidification on the mechanical characteristics of granite, showing that the peak stress and elastic modulus of granite decreases 25.7% and 16.5%, respectively, after exposure to mud acid solution (12% HCl + 3% HF) corrosion for three weeks at room temperature. The particle flow program PFC2D based on discrete element method was used to investigate the mechanical response before and after the chemical corrosion. Considering that the granite is rich in microcracks, the study is simplified by considering them all grouped into one main closed fracture. The influences of main crack inclination angle, crack length, friction coefficient and confining pressure on the mechanical response were investigated. Under the triaxial compression loading state, wing cracks appear at the initial crack tip, then secondary cracks begin to appear. The sensitivity analysis shows that three characteristic strengths (crack initiation strength, damage strength and peak strength) are strongly correlated with crack length, crack inclination angle, crack surface friction coefficient and confining pressure. These three characteristic strengths decrease 60%, 59% and 53%, respectively, compared with their initial values with the increase of main crack length from 6 mm to 22 mm, while they present positive correlation with the fracture friction coefficient from 0 to 1.0 and confining pressure from 10 to 50 MPa. There is a critical inclination angle of the main crack (i.e., 45°), meaning that these three characteristic strengths of granite decrease with inclination angles smaller than 45°, while they increase with an inclination angle larger than 45°. After the corrosion effect of mud acid solution on granite, the pore structure was changed and mechanical properties was damaged, which further affect the failure mode and failure process of granite samples affected by mud acid solutions. This paper provides a theoretical reference for evaluating the effect of chemical stimulation technology on the mechanical characteristics of granite, serving for the continuous hydraulic stimulation design after the chemical stimulation

Potential evaluation on CO2-EGR in tight and low-permeability reservoirs

CO2-EGR, i.e. enhanced gas recovery by injecting CO2, is to displace natural gas by injecting CO2 in the supercritical phase. It can both enhance the recovery of gas reservoirs and realize CO2 storage. Currently, this technique is still at its exploring stage. The effect of CO2-EGR is not clarified, the geologic conditions for CO2-EGR are not definite, and the rational working system for CO2-EGR is not available. In this paper, the long-core experiment was conducted to determine whether and how much the recovery of low-permeability reservoirs can be enhanced by injecting CO2. According to the experimental results, the recovery can be enhanced by 12% when CO2 content in produced gas is more than 10%. Moreover, the multi-component seepage mathematical model was built for displacing natural gas by injecting supercritical CO2, and the model accuracy was verified using laboratory data. With this mathematical model, the influence factors for displacing natural gas by injecting supercritical CO2 were analyzed in order to define the conditions for selecting favorable zones. The Well DK13 area in the Daniudi gas field, Ordos Basin, was selected for potential evaluation of CO2-EGR. As indicated by the numerical simulation results, when CO2 content of producing wells in the Well DK13 area is 10% (with a lower cost for corrosion prevention), the ratio of CO2-EGR is 8.0–9.5%, and 31.1% HCPV(hydrocarbon pores volume) of CO2 storage can be realized. It is thus concluded that the CO2-EGR technique can enhance the recovery of gas reservoirs and also store CO2 underground, contributing to the increase of both social and economic benefits

Lanosterol synthase deficiency promotes tumor progression by orchestrating PDL1‐dependent tumor immunosuppressive microenvironment

Abstract Lipid metabolic reprogramming is closely related to tumor progression with the mechanism not fully elucidated. Here, we report the immune‐regulated role of lanosterol synthase (LSS), an essential enzyme in cholesterol synthesis. Database analysis and clinical sample experiments suggest that LSS was lowly expressed in colon and breast cancer tissues, which indicates poor prognosis. The biological activity of tumor cell lines and tumor progression in NOD scid gamma (NSG) mice were not affected after LSS knockdown, whereas LSS deficiency obviously aggravated tumor burden in fully immunized mice. Flow cytometry analysis showed that LSS knockdown significantly promoted the formation of tumor immunosuppressive microenvironment, characterized by the increase in M2 macrophages and polymorphonuclear myeloid‐derived suppressor cells (PMN‐MDSCs), as well as the decrease in anti‐tumoral T lymphocytes. With the inhibition of myeloid infiltration or loss function of T lymphocytes, the propulsive effect of LSS knockdown on tumor progression disappeared. Mechanistically, LSS knockdown increased programmed death ligand 1 (PDL1) protein stability by 2,3‐oxidosqualene (OS) binding to PDL1 protein. Anti‐PDL1 therapy abolished LSS deficiency‐induced immunosuppressive microenvironment and cancer progression. In conclusion, our results show that LSS deficiency promotes tumor progression by establishing an OS–PDL1 axis‐dependent immunosuppressive microenvironment, indicative of LSS or OS as a potential hallmark of response to immune checkpoint blockade