Effect of pore structure on slippage effect in unsaturated tight formation using pore network model

ACKNOWLEDGMENTS We acknowledge the Beijing Natural Science Foundation of China (No. 2204093), Science Foundation of China University of Petroleum, Beijing (No.2462018YJRC033) and financial support from China Scholarship Council ((No. 201906440134)Peer reviewedPostprin

Stress arch effect on the productivity of the vertical fractured well

Rock permeability impacts by effective stress. Permeability modulus is used to evaluate the level of permeability reduction due to effective stress change. And the permeability modulus is always obtained by the experiment which assumes that the overburden pressure is constant during production. Actually, the overburden pressure reduces during production due to stress arch effect and it is easy to form a stress arch in the overburden when the reservoir is small and soft compared with surrounding’s rock. Based on the definition of the permeability modulus, we obtain an expression between permeability modulus bγ considering stress arch effect and permeability modulus b0 without stress arch. There lies a linear ship between bγ and b0, which is also proved by the experiment data. Based on the relationship between bγ and b0, a delivery equation for vertical fractured well is established. Compared with the absolute open flow with stress arch ratio of 0, the absolute open flow increases by 2.87 %, 6.79 %, 12.32 %, 20.12 % and 25.44 % for the stress arch ratio of 0.12, 0.28, 0.5, 0.8 and 1, respectively, with permeability modulus b0 of 0.0397 MPa-1. And it increases by 7.31 %, 18.1 %, 34.88 %, 61.02 % and 79.97 % for the stress arch ratio of 0.12, 0.28, 0.5, 0.8 and 1, respectively, when b0= 1. So absolute open flow with high permeability modulus b0 is more sensitive to stress arch ratio. Stress arch also impacts the optimum fracture half-length. Vertical well has the maximum absolute open flow when it has the optimum fracture half-length. The maximum absolute open flow increases with the increasing of stress arch ratio, while optimum fracture half-length decreases with increasing of stress arch ratio for the same permeability modulus b0. Compared with case with no stress arch, the optimum fracture half-length reduces by 2.86 %, 5.7 %, 11.43 %, 17.14 % and 22.86 % for the stress arch ratio of 0.12, 0.28, 0.5, 0.8 and 1 respectively when b0 equals to 0.0397 MPa-1. While the maximum absolute open flow increases by 1.6 %, 3.8 %, 7.16 %, 12.02 % and 15.60 % for the stress arch ratio of 0.12, 0.28, 0.5, 0.8 and 1 respectively. Thus, vertical well considering stress arch needs smaller fracture half-length than that with no stress arch. Meanwhile, the maximum absolute open flow and optimum fracture conductivity both increase as stress arch ratio increases. Compared with the case without stress arch, the optimum fracture conductivity increases by 50 %, while the maximum absolute open flow increases by 21.40 % with stress arch ratio of 0.5 when b0 equals to 0.0397 MPa-1. The stress arch greatly impacts on the stress sensitive permeability, permeability modulus and well performance, which can’t be neglected especially in the low and ultra-low permeability reservoir

Effect of multi-arc current on the microstructure and properties of TiAlSiN coating on zircaloy-4 alloy

TiAlSiN coatings are deposited on zircaloy-4 by multi-arc ion plating at 50 A, 60 A and 70 A, respectively. The macro-morphology, micro-morphology, chemical composition and phases of the prepared coatings are observed and analyzed, and the high temperature oxidation behavior and adhesion strength of coatings are tested. The results demonstrate that the quantity of large particles on TiAlSiN coating increases gradually as the arc current increases, the coating becomes denser and denser with the porosity decreases gradually, and the content of Al elements in the coating increases gradually with the increase of multi-arc current. The phases of TiAlSiN coatings deposited at different multi-arc currents before oxidation mainly includes Ti3AlN, AlN, Ti2N. Meanwhile, the high temperature oxidation behavior of the coating is also improved gradually and the adhesion strength increases first and then decreases. When the arc current is 60 A, the adhesion strength is up to 23 N which is relatively large. The coatings deposited at 70 A shows the best high temperature oxidation resistance behavior, mainly due to the generation of a large amount of Al2O3 oxide in the coating after oxidation

A Model for Multiple Transport Mechanisms Through Nanopores of Shale Gas Reservoirs with Real Gas Effect-Adsorption-Mechanic Coupling

Multiple transport mechanisms coexist in nanopores of shale gas reservoirs with complex pore size distribution and different gas-storage processes, including continuum flow, slip flow and transition flow of bulk gas and surface diffusion for adsorbed gas. The force between gas molecules and the volume of the gas molecules themselves cannot be negligible in shale gas reservoirs with high pressure and nanoscale pores, influences gas transport and must be taken into account as a real gas effect. During depressurization development of shale gas reservoirs, the adsorbed gas desorption and a decrease in an adsorption layer influence gas transport. Meanwhile, due to the stress dependence, decreases in intrinsic permeability, porosity and a pore diameter also influence gas transport. In this work, a unified model for gas transport in organic nanopores of shale gas reservoirs is presented, accounting for the effects of coupling the real gas effect, stress dependence and an adsorption layer on gas transport. This unified model is developed by coupling a bulk gas transport model and an adsorbed gas surface diffusion model. The bulk gas transport model is validated with published molecular simulation data, and the adsorbed gas surface diffusion model is validated with published experimental data. The results show that (1) in comparison with the previous models, the bulk gas transport model developed on the basis of a weighted superposition of slip flow and Knudsen diffusion can more reasonably describe bulk gas transport, (2) surface diffusion is an important transport mechanism, and its contribution cannot be negligible and even dominates in nanopores with less than 2 nm in diameter, and (3) the effect of stress dependence on fluid flow in shale gas reservoirs is significantly different from that in conventional gas reservoirs, and is related to not only the shale matrix mechanical properties and the effective stress but also the gas transport mechanisms

Transport mechanism of desorbed gas in coalbed methane reservoirs

The gas-liquid two-phase flow and mass transfer principle shows that the diffusion caused by concentration difference only happens in a single-phase fluid; gas-liquid two-phase diffluent solution happens in the way of dissolution; and gas-liquid two-phase insoluble or semi- soluble solution flows under differential pressure driving. These facts demonstrate that the transport of desorbed gas through matrix pores is the flow, and it doesn't conform to Fick law. The dissolution, diffusion, nucleation and bubble processes of desorbed gas through depressurization are studied, and the nonlinear flow model of free gas from matrix pores to the cleat and fracture system is established based on force analyses of the gas bubble and the gas column. Research shows that a small amount of desorbed gas diffuses by dissolution; most of them becomes nucleation and bubble, and then flows to the coal cleat and fracture system under the pressure difference driving; considering the existence of the pressure difference between the matrix pores and cleats, the pressure in coal matrix will reduce more slowly, the investigated radius will be shorter, and the outflow lag phenomenon of desorbed gas will appear. The dynamic reserve should be calculated not by using cleat pressure but by the pressure in coal matrix. The mechanism of enhanced methane recovery by CO2 injection is not only replacement but displacement. Improved methane recovery can be obtained by optimizing the production pressure difference, it is not reasonable that the lower formation pressure gives higher methane recovery. Key words: coalbed methane, diffusion, desorption, percolation, developmen

Homologous recombination occurs frequently at innate GT microsatellites in normal somatic and germ cells in vivo

Abstract Background In somatic cells, homologous recombination (HR) is a rare event caused by eventual DNA double-strand breaks (DSBs). In contrast, germ cells show high frequency of HR caused by programmed DSBs. Microsatellites are prone to DSBs during genome replication and, thereby, capable of promoting HR. It remains unclear whether HR occurs frequently at microsatellites both in normal somatic cells and germ cells in a similar manner. Results By examining the linkage pattern of multiple paternal and maternal markers flanking innate GT microsatellites, we measured HR at the GT microsatellites in various somatic cells and germ cells in a goldfish intraspecific heterozygote. During embryogenesis, the HR products accumulate gradually with the increase of the number of cell divisions. The frequency of HR at the GT microsatellites in advanced embryos, adult tissues and germ cells is surprisingly high. The type of exchanges between the homologous chromosomes is similar in normal advanced embryos and germ cells. Furthermore, a long GT microsatellite is more active than a short one in promoting HR in both somatic and germ cells. Conclusions HR occurs frequently at innate GT microsatellites in normal somatic cells and germ cells in a similar manner