Overpressure and gas charging in tight sandstone:Xujiahe Formation,northeastern Sichuan Basin

Overpressure is a key factor for oil and gas charging in tight reservoirs,but it is still a challenge to evaluate the overpressure evolution and its control on oil and gas charging.Taking Xujiahe Formation in the northeastern Sichuan Basin as an example,this paper presented a method for evaluating overpressure and its effect on natural gas charging in tight sandstone in compressional basin.The abnormally high pressure and its causes were analyzed by measured data and logging evaluation.Theoretical calculation and PVT simulation were used to investigate the amounts of overpressure resulted from hydrocarbon generation and tecto nic compression,respectively.Then the source rock-reservoir pressu re differences were calculated and the characteristics of natural gas charging during the natural gas charging periods were analyzed.It was revealed that hydrocarbon generation and tectonic compression were the main causes of the overpressure.The overpressure of both source rocks and reservoir exhibited a gradually increasing trend from Middle Jurassic to Early Cretaceous(J2-K1),then decreased since Later Cretaceous(K2),and some of that preserved to now.The contributions of the hydrocarbon generation and tectonic compression to overpressure were different in different periods.The residual pressure difference between the source rocks and the reservoir is the major driving force for tight sandstone gas charging.The main hydrocarbon generating area of the source rocks and the area of high driving force were major natural gas enrichment areas,and the driving force determined the natural gas charging space in the pore throat system of the reservoir.This research helps evaluate the overpressure and pressure difference between source rocks and reservoir in compressed basin,as well as investigate the effective pore throat space of tight gas charging by the driver of overpressure.

Water blocking effect caused by the use of hydraulic methods for permeability enhancement in coal seams and methods for its removal

To research techniques for removing the water blocking effect caused by hydraulic applications in coal seams,the use of surfactants is proposed,based on the mechanics of the water blocking effect.Centrifugal experiments were used to validate the effects of using surfactants;the results show that after dealing with vacuum saturation with water,the volume of micropores decreases,which results in a larger average pore size,and the volume of transitional pores,mesopores,macropores and total pores increases.Based on the distribution of pore size,the operation mode of ‘‘water infusion after gas extraction,then continuing gas extraction" is recommended to improve the volume of coal mine gas drainage.When the reflectance of vitrinite in coal samples is less than 1,using the surfactants Fast T,1631,APG,BS can mitigate the damage caused by the water blocking effect.But when the reflectance of vitrinite is larger than 1.4,the damage caused by the water blocking effect can be increased.When the surfactant CMC is used in hydraulic applications,the capillary forces of coal samples are almost negative,which means the capillary force is in the same direction as the gas extraction.The direction of capillary forces benefits the gas flow.So,using CMC can play an active role in removing the water blocking effect.Centrifugal experiments confirm that using CMC can effectively remove the water blocking effect,which has a beneficial effect on improving the gas drainage volume.

Research on 1:2 subharmonic resonance and bifurcation of nonlinear rotor-seal system

The 1：2 subharmonic resonance of the labyrinth seals-rotor system is inves- tigated, where the low-frequency vibration of steam turbines can be caused by the gas exciting force. The empirical parameters of gas exciting force of the Muszynska model are obtained by using the results of computational fluid dynamics （CFD）. Based on the multiple scale method, the 1：2 subharmonic resonance response of the dynamic system is gained by truncating the system with three orders. The transition sets and the local bifurcations diagrams of the dynamics system are presented by employing the singular theory analysis. Meanwhile, the existence conditions of subharmonic resonance non-zero solutions of the dynamic system are obtained, which provides a new theoretical basis in recognizing and protecting the rotor from the subharmonic resonant failure in the turbine machinery.

Effects of aeration method and aeration rate on greenhouse gas emissions during composting of pig feces in pilot scale

The aim of this study was to uncover ways to mitigate greenhouse gas（GHG） emissions and reduce energy consumption during the composting process. We assessed the effects of different aeration rates（0, 0.18, 0.36, and 0.54 L/（kg dry matter（dm）·min）） and methods（continuous and intermittent） on GHG emissions. Pig feces and corn stalks were mixed at a ratio of 7：1. The composting process lasted for 10 weeks, and the compost was turned approximately every 2 weeks. Results showed that both aeration rate and method significantly affected GHG emissions. Higher aeration rates increased NH3 and N2O losses,but reduced CH4 emissions. The exception is that the CH4 emission of the passive aeration treatment was lower than that of the low aeration rate treatment. Without forced aeration,the CH4 diffusion rates in the center of the piles were very low and part of the CH4 was oxidized in the surface layer. Intermittent aeration reduced NH3 and CH4 losses, but significantly increased N2 O production during the maturing periods. Intermittent aeration increased the nitrification/denitrification alternation and thus enhanced the N2 O production. Forced aeration treatments had higher GHG emission rates than the passive aeration treatment. Forced aeration accelerated the maturing process, but could not improve the quality of the end product. Compared with continuous aeration, intermittent aeration could increase the O2 supply efficiency and reduced the total GHG emission by 17.8%, and this reduction increased to 47.4% when composting was ended after 36 days.

Adsorption and desorption of SO_2, NO and chlorobenzene on activated carbon

Activated carbon（AC） is very effective for multi-pollutant removal; however, the complicated components in flue gas can influence each other＇s adsorption. A series of adsorption experiments for multicomponents, including SO_2, NO, chlorobenzene and H2 O,on AC were performed in a fixed-bed reactor. For single-component adsorption, the adsorption amount for chlorobenzene was larger than for SO_2 and NO on the AC. In the multi-component atmosphere, the adsorption amount decreased by 27.6% for chlorobenzene and decreased by 95.6% for NO, whereas it increased by a factor of two for SO_2,demonstrating that a complex atmosphere is unfavorable for chlorobenzene adsorption and inhibits NO adsorption. In contrast, it is very beneficial for SO_2 adsorption. The temperature-programmed desorption（TPD） results indicated that the binding strength between the gas adsorbates and the AC follows the order of SO_2〉 chlorobenzene 〉 NO. The adsorption amount is independent of the binding strength. The presence of H2 O enhanced the component effects, while it weakened the binding force between the gas adsorbates and the AC. AC oxygen functional groups were analyzed using TPD and X-ray photoelectron spectroscopy（XPS） measurements. The results reveal the reason why the chlorobenzene adsorption is less affected by the presence of other components. Lactone groups partly transform into carbonyl and quinone groups after chlorobenzene desorption. The chlorobenzene adsorption increases the number of C = O groups, which explains the positive effect of chlorobenzene on SO_2 adsorption and the strong NO adsorption.