Pressure effect on the characteristics of oil and gas produced by in-situ heating of shale:a case study of the third submember of Member 7 of Yanchang Formation in Ordos Basin
The shale in Member 7 of Triassic Yanchang Formation(Chang-7)in Ordos Basin has a giant potential to produce oil and gas by in-situ heating,which is attributed to its low maturity,high organic carbon content and good type of organic matter.To ex-plore the impact of pressure during in-situ heating on the characteristics of oil and gas from shale reservoirs and provide a reliable ba-sis for the development of Chang-7 shale oil and gas by in-situ heating,a study was performed on the third submember of Member 7 of Yanchang Formation,and in-situ heating simulation experiments were conducted on typical samples at the heating rate of 20 ℃/d and different pressures of 0MPa,2 MPa,4MPa and 8MPa,respectively.The experimental results show that under the same heating conditions,pressure changes have an effect on the maximum oil production from Chang-7 shale and also the oil characteristics,and a greater effect on the composition of the gas produced.Under four different pressures,the maximum production of Chang-7 shale by in-situ heating is 36.97-41.47kg/t,and the oil production decreases with the increasing of pressure.As pressure increases,the light components of the produced oil increase,and the oil quality becomes better,which is attributed to a high-degree cracking of heavy oil generated by organic matters under high pressure.When heated up to 450 ℃,the total gas production is 14.28-21.05 m3/t,and increases with pressure.Moreover,the gas-oil ratios of shale oil and gas generated in four experimental sequences ranges from 289.70m3/m3 to 427.04m3/m3,and increase with pressure.However,there is a great difference in the production of various gas components that in-crease with pressure,i.e.,the total production of hydrocarbon gases and CO2 increases with pressure,while H2 production obvious-ly declines and H2 S production changes little with pressure.The production of different gas components presents different changing laws with the increasing pressure,which has relation with many factors,such as the formation mechanism and shale adsorption ca-pacity of different gaseous components,and the secondary reaction among various phase products within the certain range of heating temperature and pressure.According to the oil production,quality of produced oil,yields of different gas components,current oil-gas prices as well as the processing costs on harmful products,it is suggested that a relatively low bottom-hole pressure in production well should be hold during shale oil-gas development in Chang-7 by in-situ heating,which is of great reference significance for deter-mining the development plan for shale oil and gas from Chang-7 shale by in-situ heating.
Ordos BasinYanchang Formationshalein-situ heatingpressureshale oil and gas yield
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